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Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review
Applied Energy xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review S.M. Sohel Murshed ⇑, C.A. Nieto de Castro Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal
h i g h l i g h t s A review on key heat transfer features of ethylene glycol-nanofluids is presented. Nanofluids preparation and stabilization methods are discussed and summarized. Effects of various key factors on their thermal conductivity have been analyzed. Nanofluids exhibit superior heat transfer compared to base conventional fluids. Energy applications and research needs of nanofluids are identified and addressed.
a r t i c l e
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Article history: Received 26 April 2016 Received in revised form 5 November 2016 Accepted 6 November 2016 Available online xxxx Keywords: Nanofluids Thermal conductivity Convection heat transfer Ethylene glycol Nanoparticles
a b s t r a c t This paper presents a state of the art review on the research and development of conduction (thermal conductivity) and convection heat transfer characteristics of ethylene glycol-based nanofluids. Nanofluids preparation and stabilization methods are summarized and discussed. The effects of nanoparticles type, size and concentration as well as temperature on the thermal conductivity of available ethylene glycol and ethylene glycol/water mixture-based nanofluids have been critically and individually analyzed and discussed. Studies on convective heat transfer of these nanofluids have also been reviewed and results from different studies are compared. The review clearly demonstrates that these nanofluids possess considerably higher thermal conductivity and convective heat transfer characteristics compared to their base fluids i.e., ethylene glycol and its aqueous mixture. These thermal features of nanofluids are key factors for their performance in thermal management and energy applications. With the enhanced thermal conductivity and convective heat transfer coefficient nanofluids offer immense potential in energy harvesting and storage as well as to advanced cooling applications. Ó 2016 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofluids preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature survey on thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Overview of EG-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Al2O3/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. TiO2/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. ZnO/EG nanofluids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. CNT/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. CuO/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Cu/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Other EG-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. EG/W mixture-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Studies on convective heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
00 00 00 00 00 00 00 00 00 00 00 00 00
⇑ Corresponding author at: Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail address: [email protected] (S.M.S. Murshed). http://dx.doi.org/10.1016/j.apenergy.2016.11.017 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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5. 6.
Nanofluids in energy applications . Conclusions. . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction Due to an overwhelming amount of attention from the researchers worldwide, nanofluids became one of the most popular research fronts in recent years. Nanofluids, which are the suspensions of nanometer-sized particles (typically <100 nm) in traditional heat transfer fluids such as water (W), ethylene glycol (EG), and engine oil possess superior thermophysical properties than those of their base fluids [1–16]. Besides highly desirable enhanced thermal properties, this new class of fluids can offer tremendous benefits and potential applications in industrial, electronics, and energy fields. Nanofluids can be used for better heat transfer and for other performances of systems or technologies in many engineering areas including transportations (engine cooling or vehicle thermal management), microelectronics, solar energy technologies, micro-electromechanical systems (MEMS), electronics and instrumentations, heat exchangers, heating-ventilating and air-conditioning (HVAC), cooling electronics, microfluidics, defense, medical and many other applications [1,2,6,13,17–22]. Many MEMS and other micro- or mini-electronic systems produce very high heat flux and cooling for desirable performance and durability of such devices is a key technical challenge, which cannot be met by conventional fluids or solid (even micro-sized particles)-fluid mixtures. While inherently poor thermal properties of the conventional fluids greatly limit the cooling performance, other particle (micro-sized)-fluid mixture are not suitable for those devices and cooling systems as they can clog and damage the flow channels besides rapid settling of these particles. With high thermal conductivity nanofluids can meet the cooling demands of such small devices and those high-tech industries [22]. Thus, the impact of nanofluids is great given the heat transfer performance of heat exchangers or cooling devices is vital in many industries. Nanofluids and the technologies using these fluids can also save energy cost of billions of dollars. According to a study [23] if nanofluids improve chiller efficiency by 1%, a saving of 320 billion kWh of electricity or equivalent to 5.5 million barrels of oil per year would be realized in USA alone. An estimation of the potential worldwide market for nanofluids only in heat transfer applications was made to be over 2 billion dollars per year and it is expected to grow further in the future [18]. Despite above-mentioned impacts, potential applications, and extensive research been performed in the last decade, except few areas progresses on nanofluids towards developing them as real value-added materials are rather slow. Most of the research works have been conducted on thermal conductivity of nanofluids and they were found to have considerably higher values compared to their base fluids [5–10,12–16]. However, results obtained from various laboratories are not consistent and there are also controversies on the heat transfer mechanisms of nanofluids [2,6,12,24]. Research on the other key areas such as convective heat transfer of nanofluids are also receiving growing interest in recent years and results are even more promising as nanofluids showed substantially higher heat transfer features (e.g., convective heat transfer coefficient) compared to those of their base fluids. Also, researchers mostly used nanofluids in minichannel-based flow and heat exchangers systems. However, compared to aqueous nanofluids [23,25–27], only a handful convective heat transfer works have been done on EG or EG/W mixture-based nanofluids.
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Having desirable thermal characteristics such as low freezing point, high boiling point, better stability over a wide range of temperatures, and good thermophysical properties ethylene glycol is a very popular antifreeze and heat transfer fluid in numerous commercial and industrial applications. Aqueous mixtures of EG are also commonly used as cooling media due to their higher thermal properties and lower viscosity as well as cost than those of pure EG. Furthermore, nanoparticles are generally better dispersed in EG than in water. Thus researchers widely employed EG and its aqueous mixtures as base fluids for nanofluids and research showed that the heat transfer performances of these nanofluids are significantly higher than those of base EG and EG/W [9,27,28]. Since coining the concept of nanofluids significant research works have been performed and good progress has been made, it is timely and of great importance to provide a state of the art review on the key thermal areas of these nanofluids. A comprehensive review on the research and development of conduction and convection heat transfer of EG and EG/W based nanofluids is provided in this paper. Preparation of nanofluids and their stabilization methods have been discussed and summarized. Literature studies of the influences of nanoparticles type, size and concentration as well as fluids temperature on the thermal conductivity of available EG and EG/W mixture-based nanofluids have been critically reviewed. Convective heat transfer studies of these nanofluids have been analyzed and results from various research groups are also compared. In addition, research works on potential application of nanofluids in energy technologies particularly in solar energy systems have been briefly discussed. 2. Nanofluids preparation One of the major challenges of nanofluids research is to achieve long-term stability of nanofluids, which mostly depends on the preparation of nanofluids. The preparation of nanofluids is widely performed through two ways- one-step and two-step methods. The one-step method is directly synthesizing nanoparticles in base fluids by applying various chemical or physical methods such as chemical vapour deposition. The two-step method is to synthesize or purchase nanoparticles first and then disperse them in a base fluid. Nanoparticles are synthesized by two general approachesbottom-up and top-down approaches. In recent years, nanoparticles production techniques have been improved considerably and wide varieties of nanoparticles (types and shapes) are commercially available and some of them are reasonably cheap as well. Thus, researchers working or dealing with nanofluids mostly used two-step method to prepare their nanofluids. Some other methods are also used to prepare nanofluids and most of them are chemical solution, reaction based-route or other methods [29]. For example, while phase transfer method was used for preparing nanofluids by some researchers [30,31], microfluidic flow system was also employed to synthesis nanofluids (e.g., [32,33]). Besides shortterm stability of prepared nanofluids, the other limitations of these new one-step methods are that they cannot be used to produce large quantity of nanofluids. It is also difficult to control the size distribution of nanoparticles and the purity of the nanofluids by these new techniques. Research advancements in the areas of nanofluids preparation methods, stability mechanisms and other related issues have been well-discussed in the literature [29,34].
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
Regardless of preparation methods used, proper dispersion and stabilization of the nanoparticles in base fluids are crucial for nanofluids’ optimum properties and applications. Homogenization of nanoparticles in base fluid is commonly performed using ultrasonication, which is very effective in breaking the larger clusters of nanoparticles into smaller clusters. Thus almost every study employed sonication and/or magnetic stirrer to disperse or breaking agglomerated nanoparticles. However, sonication time can be varied from tens of minutes to several hours depending on nanoparticles concentration, types and size, base fluids as well as power or frequency setting of the sonicator. Studies showed that through the state of dispersion sonication time can influence the thermal properties of nanofluids. Thus special attention should be given in sonication and its duration. In many cases, even sonication was found insufficient to obtain stable and well-dispersed nanofluids and thus various types of dispersant agents or surfactants such as sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS), polyvinyl pyrrolidone (PVP), chitosan, and gum arabic (GA) as well as functionalization or surface-treatment of nanoparticles (e.g., CNT) were also employed in nanofluids [7,8,13,35–44]. Among various conventional fluids, ethylene glycol is the second most popular base fluids for nanofluids after water. Here only EG and its water mixture-based nanofluids and their preparation as well as stability are presented. A summary of various EGbased and surfactant added-nanofluids and their stabilization techniques used in the literature is provided in Table 1. Since all studies employed sonication, this information is not given in Table 1. As mentioned before, Table 1 further confirms that researchers mostly adopted the two-step methods and a wide variety of dispersants were also used for better stability of nanofluids. It is also of great importance to study the morphology of nanoparticles in nanofluids as most of the cases particularly for purchased nanoparticles or two-step preparation the size and shape of the dispersed nanoparticles are very different (mostly larger size and non-uniform shape) from those of the data provided by the company/supplier. There are numerous reasons such as nanoparticle synthesis route, purity, degree of agglomeration, reliability of data provided by the supplier behind such anomalies among actual and company/supplier’s provided morphological information of the nanoparticles. A detailed description and protocol of selection of nanoparticles and nanofluids preparation
Table 1 Various EG-based nanofluids and their stabilization techniques employed in the literature studies. Nanofluids
Preparation Surfactant/surface technique treatment
Researcher
Cu/EG MWCNT/EG TiO2/EG ZnO/EG MWCNT/EG + W TiO2/EG; Al/EG Al2O3/EG + W
One step Two-step Two-step One step Two-step Two-step Two-step
Eastman et al. [35] Assael et al. [36] Ding et al. [28] Kim et al. [37] Chen et al. [38] Murshed et al. [7,8] Timofeeva et al. [39]
ZnO/EG MWCNT/EG
One step Two-step
MWCNT/EG + W Al2O3/EG MWCNT/EG + W TiO2/EG + W Fe3O4/EG Al2O3/EG + W ZnO/EG
Two-step Two step Two-step Two step One step Two-step Two-step
Thioglycolic acid SDS surfactant SDBS surfactant SDS surfactant Surface treatment CTAB surfactant Monovalent acids (nitric, acetic or formic) Ammonium citrate 12-3-12, 2 Br 1 surfactant Functionalization CTAB surfactant Chitosan dispersant Oleic acid and CTAB Citric acid surfactant SDBS surfactant CTAB surfactant
Moosavi et al. [40] Xie and Chen [41] Aravind et al. [42] Murshed [13] Teng and Yu [43] Reddy and Rao [44] Altan et al. [45] Mojarrad et al. [46] Esfe and Saedodin [47]
3
method can be found in a study by Lourenço and Vieira [48] and this will not be elaborated here further.
3. Literature survey on thermal conductivity As mentioned before, extensive experimental and theoretical research works have been conducted on the thermal conductivity of nanofluids. However, results from different researchers are not consistent and sometimes contradictory as well [9,24,49]. For example, while some researchers showed significant influence of temperature in increasing relative thermal conductivity of nanofluids, others found no considerable effect of temperature on the enhancement of this property. Also, for the same nanofluids and at the same concentration of nanoparticle, the enhancements of thermal conductivity reported by different researchers vary widely. The effect of nanoparticle size on the thermal conductivity of nanofluids is also not conclusive. Nevertheless, it is obvious that nanofluids exhibit higher thermal conductivity compared to their base fluids. These can be evidenced from Table 2, which summarizes literature results on thermal conductivity of various types of EG-based nanofluids at room temperature. In addition to reported scattered data of all these nanofluids, Table 2 reveals that researchers mostly used transient hot-wire technique to measure the thermal conductivity. Furthermore, despite large number of theoretical efforts devoted to the development of models for the thermal conductivity of nanofluids no widely accepted model is yet available and the underlying heat transfer mechanisms are not well understood [12]. Here only experimental studies and findings of the thermal conductivity of EG-based nanofluids have been summarized first followed by thermal conductivity of individual nanofluid containing each type of nanoparticle in EG. Available studies on thermal conductivity of EG/W mixture based nanofluids have also been reviewed. 3.1. Overview of EG-based nanofluids The enhancement of thermal conductivity of nanofluids is solely due to the dispersed nanoparticle, which in addition to other properties influences the thermal conductivity of nanofluids. Significant increase in thermal conductivity of EG-based nanofluids is demonstrated in Table 2. As anticipated nanoparticles with higher thermal conductivity such as CNT, diamond, and Cu showed very large enhancement of thermal conductivity even at very small concentration. For instance, Eastman et al. [35] reported about 41% increase in thermal conductivity of EG for loading only 0.28 vol.% of Cu (<10 nm) nanoparticles, Kang et al. [58] found 75% enhancement at 1.32 vol.% concentration of diamond (30–40 nm) nanoparticles, and Aravind et al. [42] observed about 40% enhancement at almost vanishingly small concentration of 0.03 vol.% of MWCNT. Nonetheless, the enhanced thermal conductivity of nanofluids can lead to higher energy efficiency, better performance of heat exchangers as well as any thermal management and energy conversion systems using this new class of fluids. Besides presenting and comparing results of thermal conductivity of available EG-based nanofluids some representative studies and key findings on each type of nanofluids have been briefly summarized in the following sections. 3.2. Al2O3/EG nanofluids Among early studies, Lee et al. [50] measured the effective thermal conductivity of EG-based Al2O3 (38 nm) nanofluids and showed a moderate enhancement of thermal conductivity of this nanofluid. A maximum 18% increase in thermal conductivity of
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Table 2 Summary of representative studies on thermal conductivity of EG-based nanofluids. Nanoparticle (size in nm)
Measurement technique
Thermal conductivity increase (%) over EG at nanoparticle concentration
Researchers
Al2O3 (38) CuO (24) Al2O3 (28) Cu (<10) CuO (35) Al2O3 (60.4) Al2O3 (29) TiO2 (40) TMWCNT (15 nm 30 lm) Fe (10) CuO (12) MWCNT (120) CuO (29) Diamond (30–40) TiO2 (15) Al (80) TiO2 (10 40) Al2O3 (80) SWCNT (1 4) Fe (10) WO3 (38) Al (80) Cu (80) CuO (31) Graphene (-) MWCNT (-) SWCNT (0.8–1.6 500) ZnO (<50) Al2O3 (105) ZnO (48) Fe3O4 (6) CeO2 (30–50) Co3O4 (<50)
Transient hot-wire Method (THWM) Steady-state method (SSM) THWM
18% at 5 vol.% of Al2O3 20% at 4 vol.% of CuO 26% at 5 vol.% 41% at 0.28 vol.% of Cu 22% at 4 vol.% of CuO 30% at 5 vol.% 18% at 4 vol.% of Al2O3 13% at 5 vol.% of TiO2 12.7% at 1 vol.% 18% at 0.55 vol.% 6% at 1 vol.% 21% at 0.6 vol.% 23% at 5 vol.% 75% at 1.32 vol.% 17.5% at 5 vol.% of TiO2 45% at 5 vol.% of Al 20% at 5 vol.% of TiO2 18% at 5 vol.% of Al2O3 20% at 2.5 vol.% 16.5% at 0.3 vol.% of Fe 13.8% at 0.3 vol.% of WO3 18% at 3 vol.% of Al 26% at 3 vol.% of Cu 13% at 3 vol.% of CuO 3–4% at 0.005 vol.% 40% at 0.03 vol.% 14.8% at 0.2 vol.% 40% at 3.7 vol.% 27% at 3 vol.% 33% at 6.1 vol.% Slight decrease at 1.73 wt.% 13% at 1 vol.% 25% at 5.67 vol.% at 30 °C
Lee et al. [50]
THWM SSM THWM THWM
THWM- KD2 Pro Lambda Instruments THWM THWM Transient hot-disk THWM- KD2 Pro Lambda Instrument THWM- KD2 Pro THWM- KD2 Pro
EG was reported for 5 vol.% loading of Al2O3 nanoparticles in their study. The lead author also previously investigated the thermal conductivity of Al2O3 (80 nm)/EG and found about 18% increase in thermal conductivity over base EG at nanoparticle concentration of 5 vol.% [8]. Wang et al. [53] also observed similar thermal conductivity for their Al2O3 (29 nm)/EG-based nanofluids. However, another early study by Wang et al. [51] reported slight higher increase (26%) in thermal conductivity of EG at the same 5 vol.% of Al2O3 (28 nm). Whereas Xie et al. [52] observed even higher (30%) enhancement of thermal conductivity of EG at the same 5 vol.% concentration of larger size (60.4 nm) Al2O3 nanoparticles. Recently, Esfe et al. [69] investigated the effect of concentration of this nanoparticle and temperature on the thermal conductivity of Al2O3 (5 nm)/EG nanofluids. Their results showed significant enhancement of thermal conductivity of nanofluids with increasing Al2O3 concentration and temperature as well. At room temperature a maximum 29% increase in thermal conductivity was reported at 5 vol.% of Al2O3 (5 nm). Results of these studies [8,51–53,69] demonstrated no conclusive effect of nanoparticle size on the thermal conductivity enhancement of this Al2O3/EG nanofluid. Representative results on the enhancement of thermal conductivity of Al2O3/EG nanofluids as a function of volumetric concentration of this nanoparticle at room temperature are presented in Fig. 1 [8,50–52,61,65,67,69–72]. The thermal conductivity enhancement has been denoted with the ratio (or relative) of thermal conductivity of nanofluids (knf) over thermal conductivity of base fluid (kf). The size and type of this nanoparticle used by researchers are also provided in Fig. 1. All these results demonstrate that the enhanced thermal conductivity of this nanofluid increases considerably with increasing concentration of this nanoparticle. It can also be noticed from Fig. 1 that the data from
1.4
Thermal conductivity ratio (knf/kf)
THWM Parallel plateSSM THWM THWM THWM THWM THWM THWM THWM
Wang et al. [51] Eastman et al. [35] Xie et al. [52] Wang et al. [53] Xie et al. [54] Hong et al. [55] Kwak and Kim [56] Assael et al. [36] Liu et al. [57] Kang et al. [58] Murshed et al. [8] Murshed et al. [7] Amrollahi et al. [59] Yoo et al. [60] Patel et al. [61]
Baby and Ramaprabhu [62] Aravind et al. [42] Harish et al. [63] Kole and Dey [64] Longo and Zilio [65] Gallego et al. [66] Altan et al. [45] Mary et al. [67] Mariano et al. [68]
38 nm [50] 28 nm [51] 80 nm [8] α type,15 nm [52]
α type,60.4 nm [52] 11 nm [61] 150 nm [61] 16 nm [70]
2
4
105 nm [65] 5 nm [69] 40-50 nm [71] 15-45 nm (α type) [72]
1.3
1.2
1.1
1.0 0
6
Al2O3 nanoparticle concentration (vol.%) Fig. 1. Enhancement of thermal conductivity of EG-based nanofluids and effect of concentration of Al2O3 nanoparticle.
various research groups are scattered. While some researchers (e.g., [65,72]) showed very high enhancement of thermal conductivity at low concentrations, others (e.g., [8,50,52]) showed moderate increase in thermal conductivity with volumetric loading of this popular nanoparticle. Even for nanoparticles of similar sizes, the thermal conductivity data from different researchers vary considerably (Fig. 1). Effect of temperature on thermal conductivity of this Al2O3/EG nanofluid obtained from literatures has also been presented in Fig. 2 [8,61,65,69,71,72]. It confirms that the temperature also has some positive influence on the relative thermal conductivity of nanofluids (knf/kf). However, the magnitude of
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
Al2O3/EG
Thermal conductivity ratio (knf /kf)
1.8
5 vol.%(5 nm) [69] 1.5 vol.%(40-50 nm) [71]
0.5 vol.%(80 nm) [8] 1 vol.%(80 nm) [8] 0.5 vol.%(11 nm) [61] 1 vol.%(11 nm) [61] 0.5 vol.%(150 nm) [61] 3 vol.%(150 nm) [61] 3 vol.%(105 nm) [65] 1 vol.%(5 nm) [69]
1.6
4.8 vol.%(40-50 nm) [71] 0.25 vol.%(15-45 nm) [72] 1 vol.%(15-45 nm) [72]
1.4
1.2
1.0 0
20
40
60
80
100
Temperature (°C) Fig. 2. Effect of temperature on the enhanced thermal conductivity of Al2O3/EG nanofluids.
influence of temperature is smaller than that of the nanoparticle concentration (Figs. 1 and 2). Nevertheless such increase in thermal conductivity with increasing temperature makes nanofluids even more interesting for applications at elevated temperature conditions such as high temperature heat exchangers. 3.3. TiO2/EG nanofluids Fig. 3 presents literature data on the enhancement of thermal conductivity of nanofluids as a function concentration (vol.%) of TiO2 nanoparticle at room temperature [7,53,65,73–75]. It is evidenced from Fig. 3 that this nanofluid exhibits mostly moderate and linear increase (within 20%) in thermal conductivity with increasing concentration of this nanoparticle. Given the smaller thermal conductivity value of TiO2 nanoparticle (about 8 W/m-K) as compared to most of the other types of nanoparticles such as CNT, Al, Cu, Al2O3 and CuO, such thermal conductivity enhancement is very fascinating. The scattered data could be due to different sizes of nanoparticles and nanofluids preparation as well as measurement techniques used by different researchers. Nonetheless, the trends of the reported results are more consistent as compared to Al2O3/EG nanofluids. The influence of nanoparticle volume fraction on the thermal conductivity, thermal diffusivity, as well as specific heat of several EG-based nanofluids including TiO2/EG [13] was previously
3.4. ZnO/EG nanofluids Kim et al. [37] was the first to study the thermal conductivity of ZnO/EG nanofluids that was synthesized directly by one-step
TiO2 (15 nm)/EG [7] TiO2 (10×40 nm)/EG [7]
Thermal conductivity ratio (knf /kf)
Thermal conductivity ratio (knf /kf)
1.25
studied by the lead author. Two different shapes TiO2 were used and the effective thermal conductivity of these nanofluids was found to increase substantially with increasing nanoparticle volume fraction. For a maximum 5 vol.% loading of TiO2 nanoparticles of 15 nm (spherical) and 10 40 nm (cylindrical) in EG, the maximum increases in thermal conductivity were 17.5% and 20%, respectively. Wang et al. [53] also reported similar results for their TiO2 (40 nm)/EG-based nanofluids particularly for 3% and 4 vol.% concentrations of nanoparticles. TiO2 nanofluids with cylindrical nanoparticles show a little higher enhancement in thermal conductivity compared to those with spherical nanoparticles. These indicate that along with the particle volume fraction, particle shape also influences the thermal conductivity of nanofluids. Thermophysical properties of TiO2 (150 nm)/EG-based nanofluids was measured by Longo and Zilio [65]. A transient hot-disk apparatus was used to measure the thermal conductivity of nanofluids as a function of concentration and temperature. The effective thermal conductivity of nanofluids was found to increase with increasing both the concentration of nanoparticle and temperature. Although their nanoparticle size was the largest among others, significant enhancement of thermal conductivity was observed. For example, at room temperature, about 16% increase in thermal conductivity was found at 3 vol.% concentration of this nanoparticle. Recently, Cabaleiro et al. [73] studied thermal conductivity of EG-based two different nanocrystalline (anatase and rutile) TiO2 nanofluids. A transient based direct thermal conductivity-meter (DTC-25) was employed to measure the thermal conductivity at different concentrations of nanoparticles and temperatures. Although the enhanced thermal conductivity of all their TiO2/EG nanofluids increased with increasing loading of nanoparticle, it was almost temperature independent. For 25 wt.% (8.85 vol.%) concentration of tetragonal anatase TiO2 (33 nm), thermal conductivity of EG was increased about 12%. Literature data of the effect of temperature on the thermal conductivity of TiO2/EG nanofluids are depicted in Fig. 4 [65,73–75]. Except a couple of studies, most of the researchers didn’t observed any noticeable influence of temperature on the relative thermal conductivity of this nanofluid (knf/kff). This suggests that the effect of temperature on thermal conductivities of both the nanofluid and base fluid are of similar magnitudes [73–75].
TiO2 (40 nm)/EG [53]
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TiO2 (33 nm, anatase) [73] TiO2 (<100 nm)/EG [74] TiO2 (150 nm)/EG [65]
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TiO2 (<40 nm)/EG [75]
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TiO2/EG:
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1 vol.% (<100nm) [74] 4 vol.% (<100nm) [74] 1 vol.% (150 nm) [65] 3 vol.% (150 nm) [65]
1.5 vol.% (33nm,anatase) [73] 4.9 vol.% (33nm,anatase) [73] 0.5 vol.% (<40 nm) [75]
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TiO2 nanoparticle concentration (vol.%) Fig. 3. Enhancement of thermal conductivity of TiO2/EG nanofluids as a function of concentration of nanoparticle.
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Temperature (°C) Fig. 4. Effect of temperature on the enhanced thermal conductivity of TiO2/EG nanofluids.
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method. The SDS surfactant was also added for better dispersion and stability of nanofluids. Effect of concentration and size of nanoparticle on the thermal conductivity was investigated. Their results demonstrated considerable increase in thermal conductivity of nanofluids with concentration of ZnO nanofluids. At 3 vol.% of 30 nm and 60 nm nanoparticles the thermal conductivity of EG was reported to increase about 21 and 10%, respectively. They found that the larger the particle size, the smaller the thermal conductivity of nanofluids [37]. Later other researchers also reported similar results. For example, Moosavi et al. [40] showed about 10.5% increase in thermal conductivity of EG at about 3 vol.% of ZnO nanoparticles of 67.2 nm. Suganthi et al. [76] also reported similar increase in thermal conductivity for their ZnO (25– 40 nm)/EG nanofluids as about 24% increase in thermal conductivity at about 3 vol.% of ZnO was observed in their study. Furthermore, with 3 vol.% concentration of larger ZnO nanoparticles (<100 nm) Lee et al. [77] observed about 13% enhancement of thermal conductivity of EG. Yu et al. [78] measured thermal conductivity of ZnO (10 20)/EG nanofluids and found a maximum 26.5% increase in thermal conductivity at about 5 vol.% concentration of this nanoparticle. Whereas, for 3.7 vol.% of ZnO (<50 nm) nanoparticle about 40% enhancement of thermal conductivity of ZnO (<50 nm)/EG was reported by Kole and Dey [64]. A summary of literature results on the thermal conductivity enhancement as a function of concentration of ZnO nanoparticle is presented in Fig. 5 [37,40,64,66,76–80]. All these results demonstrated that the enhanced thermal conductivity of ZnO/EG nanofluids further increased almost linearly with increasing concentration of ZnO at room temperature. Like other nanofluids, the results of this nanofluid are also scattered in some extend. Fig. 6 compares literature results of the effect of temperature on the thermal conductivity of nanofluids over pure EG [40,64,66,76– 78]. Most of the studies showed that the thermal conductivity enhancement of this nanofluid is temperature independent. Interestingly, one study [76] reported considerable negative effect of temperature on the thermal conductivity of their nanofluid at two different concentrations (Fig. 6). 3.5. CNT/EG nanofluids Literature results on the enhancement of thermal conductivity of EG-based CNT nanofluids as a function of CNT concentration are presented in Fig. 7, which clearly shows that the thermal conductivity of this nanofluid increases substantially with increasing the volumetric loading of CNT [36,38,54,59,81–84]. Interestingly,
ZnO(30nm)/EG [37] ZnO(60nm)/EG [37] ZnO(67.2nm)/EG [40] ZnO(<50nm)/EG [64] ZnO(48nm)/EG [66] ZnO(25-40nm)/EG [76] ZnO(<100nm)/EG [77]
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1 vol.% (25-40 nm) [76] 4 vol.% (25-40 nm) [76] 1 vol.% (<100 nm) [77] 5.5 vol.% (<100 nm) [77] 5 vol.% (10-20 nm) [78]
2 vol.% (67.2 nm) [40] 0.5 vol.% (<50 nm) [64] 3.75 vol.% (<50 nm) [64] 1 vol.% (48 nm) [66] 4.7 vol.% (48 nm) [66]
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Temperature (°C) Fig. 6. Effect of temperature on the enhanced thermal conductivity of ZnO/EG nanofluids.
the hybrid Ag-MWCNT/EG nanofluids showed very high enhancement of thermal conductivity at almost vanishingly small concentration [81]. Since the thermal conductivity CNT is about order (s) magnitude higher as compared to most of other nanoparticles, it is anticipated to have significantly high thermal conductivity enhancement for CNT nanofluids. It was also found that most of the researchers used multi-walled carbon nanotubes (MWCNT). On the other hand, effect of temperature on the relative thermal conductivity of this nanofluid is demonstrated in Fig. 8, which reveals that except results from one study [38], the thermal conductivity of this nanofluid increases almost linearly with increasing temperature [41,59,81]. It is noted that most of the studies used very small concentration of CNT. Xie et al. [54] reported thermal conductivity of treated CNT/EG nanofluids measured by transient hot-wire apparatus. In order to have stable and homogenous suspensions, their nanotubes went through surface treatment process. Results showed considerable increase in the enhanced thermal conductivity of nanofluids with increasing CNT loading. The thermal conductivity of several MWCNT (mean diameter of 120 nm) dispersed nanofluids in different base fluids including EG was experimentally studied by Assael et al. [36]. Different concentrations of SDS surfactant were also added in their EG-based MWCNT nanofluids. Thermal conductivity of these nanofluids
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ZnO(10-20nm)/EG [78] ZnO(50nm)/EG [79] ZnO(-)/EG [80]
Thermal conductivity ratio (knf/kf)
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ZnO/EG:
Thermal conductivity ratio (knf /kf)
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MWCNT/EG [36] MWCNT/EG [38] MWCNT/EG [54] SWCNT/EG [59] Au-MWCNT/EG [81] Ag-MWCNT/EG [81]
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MWCNT/EG [82] MWCNT/EG [83] MWCNT/EG [84]
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ZnO nanoparticle concentration (vol.%) Fig. 5. Enhancement of thermal conductivity of ZnO/EG nanofluids with concentration of nanoparticle.
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CNT volume fraction (%) Fig. 7. Enhancement of thermal conductivity of various CNT/EG nanofluids as a function of CNT concentration.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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MWCNT (0.6 vol.%)/EG [38] MWCNT (0.175 wt.%)/EG [41] SWCNT (0.5 vol.%)/EG [59] SWCNT (1 vol.%)/EG [59] Ag-MWCNT (0.012 vol.%)/EG [81]
Thermal conductivity ratio (knf /kf)
Thermal conductivity ratio (knf/kf)
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CuO(<50 nm)/EG [75] CuO(25-35 nm)/EG [85] CuO(23-37 nm)/EG [86] CuO(29 nm)/EG [87] CuO(31 nm)/EG [61] CuO(12 nm)/EG [56] CuO(35 nm)/EG [35] CuO(24 nm)/EG [50]
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increased significantly with concentration of MWCNT as a maximum 21% increase in thermal conductivity was observed at only 0.6 vol.% concentration of MWCNT. It was also concluded that the thermal conductivity enhancement was only marginally affected by the quantity of surfactant used. Using a short hot-wire method Chen et al. [38] measured thermal conductivity of treated multi-walled carbon nanotubes (TCNT) dispersed in EG and observed up to 17.5% enhancement of thermal conductivity of this nanofluid at 1 vol.% loading of TCNT. Their nanotubes were treated by using mechanochemical reaction method for better dispersion in EG. They however did not observe any obvious effects of temperature on the thermal conductivity enhancement of their nanofluids. Amrollahi et al. [59] investigated the effects of temperature, volume fraction, and vibration time on the thermo-physical properties of a SWCNT/EG nanofluid. For 2.5 vol.% concentration of SWCNT the thermal conductivity of EG increased up to 20%. They also reported a strong influence of temperature on the thermal conductivity of this nanofluid. In a different study, Jha and Ramaprabhu [81] investigated thermal conductivity of several hybrid nanofluids containing high conductive nanoparticles including Ag, Au and Pd decorated MWCNT in EG. Results showed that the enhanced thermal conductivity of these hybrid nanofluids increase considerably with increasing concentration of metal-MWCNT and temperature as well. At a very small concentration (0.03 vol.%) of Ag-MWCNT and Au-MWCNT, thermal conductivity of EG increased about 11% and 10%, respectively. The sequence of thermal conductivity enhancements of their three metal-MWCNT/EG nanofluids which followed the same sequence of the thermal conductivity value of each metal, was found to be Ag-MWCNT > Au-MWCNT > Pd-MWCNT. Aravind et al. [42] reported significantly higher thermal conductivity enhancement at much lower volumetric concentrations of their functionalized CNT (f-MWCNT). The MWCNT was functionalized for better dispersion and stability. Thermal conductivity of EG increased up to 40% at only 0.03 vol.% loading of f-MWCNT. Such enhancement was attributed to the homogeneous dispersion of CNT in base fluids and formation of more hydrophilic phase.
3.6. CuO/EG nanofluids Fig. 9 compares literature results on the enhancement of thermal conductivity of CuO/EG nanofluid as a function of nanoparticle concentration at room temperature [35,50,56,61,75,85–87]. Like
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Fig. 9. Enhancement of thermal conductivity of CuO/EG nanofluids as a function of nanoparticle concentration.
other nanofluids, the enhanced thermal conductivity of this nanofluid increases almost linearly with volume % of nanoparticle loading. Compared to other nanofluids, the thermal conductivity enhancement trends of this nanofluid from different researchers are more consistent. Literature results on the temperature dependence of the thermal conductivity of this CuO nanofluid are presented in Fig. 10 [61,85,86]. Although majority of these studies observed an increase in the enhanced thermal conductivity of nanofluid with increasing temperature, one study [85] reported very strange trend of influence of temperature on the thermal conductivity of their CuO nanofluid. The thermal conductivity of their nanofluids at two different concentrations was first decreased with increasing temperature until temperature of around 35 °C and then increased with further increasing the temperature (Fig. 10) [85]. The first study with CuO/EG nanofluids was conducted by Lee et al. [50] who measured the thermal conductivity of nanofluids using transient hot-wire system and a maximum 20% increase in thermal conductivity of EG was reported at 4 vol.% of CuO nanoparticle of about 24 nm size. Two years later the same group leading by Eastman et al. [35] reported about 22% increase in thermal conductivity of EG for the same 4 vol.% of CuO nanoparticles of 35 nm. The thermal conductivity in both studies was measured using a transient hot-wire method. These results indicate that nanofluids with larger size CuO nanoparticles possess higher enhancement of thermal conductivity than that of smaller nanoparticles. 0.5 vol.% CuO(31nm)/EG[61] 3 vol.% CuO(31nm)/EG[61] 1.5 vol.% CuO(23-37nm)/EG[86] 3 vol.% CuO(23-37nm)/EG[86] 0.5 vol.% CuO(25-35nm)/EG[85] 1 vol.% CuO(25-35nm)/EG[85]
1.35
Thermal conductivity ratio (knf /kf)
Fig. 8. Effect of temperature on the enhanced thermal conductivity of CNT/EG nanofluids.
1
CuO nanoparticle concentration (vol.%)
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Temperature (°C) Fig. 10. Effect of temperature on the enhanced thermal conductivity of CuO/EG nanofluids.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
However, some researchers like Kim et al. [37] reported exactly opposite effect of nanoparticle size where the smaller the nanoparticles the higher the enhancement of thermal conductivity of nanofluids. For 3 vol.% of CuO (31 nm), Patel et al. [61] showed about 13% increase in thermal conductivity of EG. Liu et al. [87] also found similar enhancement (13.6%) for the same 3 vol.% concentration of similar sized CuO (29 nm) nanoparticle. However, the maximum enhancement of thermal conductivity was about 23% at 5 vol.% of CuO [87]. Barbes et al. [86] measured thermal conductivity of EG based-CuO nanofluids and found 10% increase in thermal conductivity at 3 vol.% concentration of CuO (23–37 nm) nanoparticle. No considerable influence of temperature on the thermal conductivity was also observed in their study. Whereas Patel et al. [61] demonstrated an increase in thermal conductivity with temperature for their CuO/EG nanofluids. On the other hand, a recent study by Zennifer et al. [85] showed that the thermal conductivity of their CuO (25–35 nm)/EG nanofluids increases with loading of nanoparticles but decreases with increasing temperature. However, such decreasing nature of thermal conductivity with temperature is not common and any possible reason for such results was not explained.
3.7. Cu/EG nanofluids Despite high thermal conductivity of Cu nanoparticles, only a few studies have been reported on EG based Cu nanofluids. A summary of the thermal conductivity enhancements of this nanofluid as a function of Cu concentration at room temperature is shown in Fig. 11 [31,35,55,61,88]. At low concentration, this nanofluid showed substantially higher thermal conductivity compared to its base EG and it increases with loading of nanoparticle. An early study by Eastman et al. [35] showed about 41% increase in thermal conductivity of EG for loading only 0.28 vol.% of Cu (<10 nm) nanoparticles. They also used thioglycolic acid as dispersant agent in their nanofluids. However, the enhancement of thermal conductivity was much smaller for their old Cu/EG nanofluid compared to that of the acid added one. Thermal conductivity of larger Cu nanoparticles (200 nm) dispersed nanofluids was reported by Garg et al. [88] and results showed moderate increase in thermal conductivity with increasing the concentration of Cu nanoparticle. Patel et al. [61] measured the thermal conductivity of Cu (80 nm)/EG nanofluids at different concentrations and temperatures. Thermal conductivity was found to increase with increasing concentration and temperature as well. Yu et al. [31] also reported significant enhancement of thermal conductivity of their Cu (5–
10 nm)/EG nanofluids with loading of nanoparticle and temperature as well. A maximum 15% increase in thermal conductivity was observed at only 0.5 vol.% of Cu nanoparticle.
3.8. Other EG-based nanofluids Fig. 12 presents literature data on the enhancement of thermal conductivity of other (except above discussed nanofluids) EGbased nanofluids [8,55,61,67,68,89,90]. As usual, results in Fig. 12 confirm the positive effect of concentrations of these nanoparticles on the enhancement of the thermal conductivity of nanofluids. Since data of various types of nanofluids are presented, they look scattered but the thermal conductivities of all these nanofluids increase with increasing loading of nanoparticles. As anticipated nanofluids having high thermal conductive nanoparticles exhibit relatively higher thermal conductivity compared to nanofluids with low thermal conductive nanoparticles. Findings from some of the representative studies are briefly elaborated here. Literature results on thermal conductivity of various other EG-based nanofluids as a function of temperature are also depicted in Fig. 13, which shows that except CeO2 nanofluids of Mary et al. [67], the enhanced thermal conductivity of all other nanofluids increased with increasing temperature [31,61,89].
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Fig. 12. Enhancement of thermal conductivity of various other EG-based nanofluids as a function of concentration of nanoparticles.
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Nanoparticle concentration (vol.%)
1.5 Cu (<10 nm)/EG(+acid) [35] Cu (<10 nm)/EG(old) [35] Cu/EG [55] Cu (200 nm)/EG [88] Cu (5-10 nm)/EG [31] Cu (80 nm)/EG [61]
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CeO2 (30-50 nm)/EG [67] Co3O4 (<50 nm)/EG at 30°C [68] Fe3O4 (15 nm)/EG at 30°C [89] Fe2O3 (29 nm)/EG at 30°C [89] Al (80 nm)/EG [61] Al (80 nm)/EG [8] Fe (10 nm)/EG [55] SiC (26 nm)/EG [90] SiC (600 nm)/EG [90]
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Thermal conductivity ratio (knf/kf)
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0.5 vol.% CeO2 (30-50 nm)/EG[67] 1 vol.% CeO2 (30-50 nm)/EG [67] 5 vol.% Fe2O3 (29 nm)/EG [89] 5.3 vol.% Fe3O4 (15 nm)/EG [89] 0.5 vol.% Cu (5-10 nm)/EG [31] 0.5 vol.% Cu (80 nm)/EG [61] 3 vol.% Cu (80 nm)/EG [61] 0.5 vol.% Al (80 nm)/EG [61] 3 vol.% Al (80 nm)/EG [61]
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Cu nanoparticle concentration (vol.%) Fig. 11. Enhancement of thermal conductivity of Cu/EG nanofluids with concentration of nanoparticle.
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Temperature (°C) Fig. 13. Effect of temperature on the enhancement of thermal conductivity of various other EG-based nanofluids.
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Thermal conductivity ratio (knf/kf)
A relatively large increase in thermal conductivity of Fe (10 nm)/EG-based nanofluids was reported by Hong et al. [55]. At a small concentration of Fe nanoparticle (0.55 vol.%) the thermal conductivity of EG increased 18%. They also concluded that sonication has a significant effect on the thermal conductivity of nanofluids. Yoo et al. [60] measured thermal conductivity of several nanofluids including EG-based Fe and tungsten oxide (WO3) nanofluids. These nanofluids showed large enhancements of thermal conductivity as compared to the base fluid (EG). For instance, Fe (10 nm)/EG nanofluid showed 16.5% increase in thermal conductivity for loading 0.3 vol.% of this nanoparticle while WO3 (38 nm)/EG nanofluid showed 13.8% enhancement for the same 0.3 vol.% WO3 nanoparticles. Thermal conductivity study using WO3/EG nanofluids is of first kind and cannot be compared. The influence of nanoparticle volume fraction on the thermal conductivity of Al (80 nm)/EG nanofluids was previously investigated by the lead author [8]. The thermal conductivity of EG was found to increase up to 45% at 5 vol.% of Al nanoparticle. It was also demonstrated that nanofluids having highly thermal conductive nanoparticles (Al) exhibit much higher thermal conductivity than those of nanofluids with low thermal conductive nanoparticles (e.g., TiO2). For the first time, Baby and Ramaprabhu [62] investigated thermal and electrical conductivity of EG-based f-TEG (functionalized thermally exfoliated graphene) nanofluids. In their study, thermal conductivity was found to increase with increasing concentration of this graphene and temperature as well. A small enhancement (3–4%) in thermal conductivity was reported for adding very small concentration (0.05 vol.%) of f-TEG in EG at room temperature. Sharma et al. [91] synthesized EG-based nanofluids of different concentrations (1000–10,000 ppm) of silver (Ag) nanoparticles and measured their thermal conductivity using a transient hot-wire technique. The thermal conductivity of this nanofluids increased with concentration of Ag nanoparticle. A maximum 18% increase in thermal conductivity of EG was observed at 10,000 ppm concentration of Ag nanoparticle. Gallego et al. [89] prepared two sets of magnetic nanofluids containing Fe2O3 (20 nm) and Fe3O4 (15 nm) nanoparticles dispersed in EG and measured their thermal conductivities as a function of nanoparticles volume fractions and temperature as well. Their results showed that the enhanced thermal conductivities of these nanofluids increase further with volumetric loading of nanoparticles but were temperature independent. The maximum enhancement of thermal conductivity was only 15% for loading 6.6 vol.% of Fe2O3 nanoparticles in EG. Among very few studies, Altan et al. [45] observed no enhancement but slight deterioration of thermal conductivity of their EGbased magnetic (Fe3O4) nanofluids with nanoparticle concentration as well as temperature. At 35 °C, the relative thermal conductivity was about 0.99 (1% decrease) at 1.73 wt.% of Fe3O4 nanoparticle. They added citric acid as stabilization agent in the nanofluids and measured the thermal conductivity using a Flucon GmBH Lambda instrument. Recently, Mary et al. [67] measured thermal conductivity of EGbased ceria (CeO2) nanofluids using a thermal property meter (KD2 device). A maximum 13% increase in thermal conductivity was observed at 1 vol.% concentration of this CeO2 (30–50 nm) nanoparticle. Another very recent study conducted by Mariano et al. [68] showed that thermal conductivity of Co3O4 (<50)/EG nanofluids increased about 25% at 5.67 vol.% of this nanoparticle and at 30 °C. They also used KD2 device to measure the thermal conductivity. This is one of the rare types of nanofluids studied by researchers and it was found promising in heat transfer. However,
CuO(27 nm)/EG+W(50/50 wt%)[94] TiO2(21 nm)/EG+W(40/60 wt%) [44] ZnO(77 nm)/EG+W(60/40 wt.%)[93] Fe3O4(13 nm)/EG+W(60/40 wt%) [95]
Al2O3(20-30 nm)/EG+W(50/50 vol.%) [46] Al2O3(53 nm)/EG+W(60/40 wt.%) [93] Al2O3(36.5 nm)/EG+W(50/50 wt%) [94] Al2O3(45 nm)/EG+W [61] CuO(100 nm)/EG+W(60/40)[92] CuO(29 nm)/EG+W(60/40 wt.%)[93]
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Nanoparticle concentration (vol.%) Fig. 14. Enhancement of thermal conductivity of various EG/W mixture-based nanofluids as a function of concentration of nanoparticles.
the thermophysical properties and dispersion behavior of this nanofluid are not well known. 3.9. EG/W mixture-based nanofluids As mentioned before, in addition to pure EG, its mixture with water (EG/W) at various proportions are also commonly used as base fluid in nanofluids. This is mostly due to using nanofluids in flow and convectional heat exchange applications where relatively more quantity of nanofluids is needed and addition of water in EG also reduces the viscosity and increases the effective thermal conductivity of the mixture. Thus literature studies on thermal conductivity EG/W mixture-based various nanofluids have been reviewed and results are presented in Fig. 14 [44,46,61,92–95]. Similar to all other EG-based nanofluids, results of these EG/W based nanofluids also showed significant enhancement of thermal conductivity, which further increased with increasing the volumetric loading of nanoparticles. It can also be noticed that the enhancements of thermal conductivity are high at low concentrations of nanoparticles. While some researchers showed very linear increase in thermal conductivity of nanofluids up to a relatively higher concentration of nanoparticles (e.g., [92,93]), others used low concentrations of nanoparticles (up to 2 vol.%) and reported high enhancement of thermal conductivity (e.g., [94,95]). Hwang et al. [82] measured thermal conductivity of CuO (35.4 nm)/EG + W nanofluids using transient hot-wire technique and found about 9% increase in thermal conductivity at 1 vol.% of this nanoparticle. In an extensive experimental study, Vajjha and Das [93] measured thermal conductivity of three nanofluids containing Al2O3 (53 nm), CuO (29 nm), and ZnO (77 nm) nanoparticles in EG + W (EG/W = 60/40 by mass) mixture using a steady-state method. Besides concentration of these nanoparticles, effect of temperature on thermal conductivity was also investigated. At room temperature and 6 vol.% of Al2O3, CuO and ZnO nanoparticles the enhancements of thermal conductivities of these nanofluids were 23%, 32%, and 26%, respectively. It shows CuO dispersed nanofluid having superior thermal conductivity than those of other two nanofluids. They also found substantial increase in thermal conductivity of these nanofluids with temperature. Sundar et al. [94] also used the same steady-state apparatus (of P.A. Hilton, U.K.) to measure the thermal conductivity of their EG/W (50/50 by mass) based Al2O3 (36.5 nm) and CuO (27 nm) nanofluids at relatively low concentrations of these nanoparticles (up to 0.8 vol.%). Thermal conductivity of these nanofluids was found to increase with
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
increasing concentration of nanoparticles and temperature as well. At room temperature (25 °C) and 0.8 vol.% concentration, thermal conductivity of their Al2O3 and CuO nanofluids were increased by 13% and 17%, respectively. Similar to Vajjha and Das [93], CuO nanofluid was found to have higher enhancement of thermal conductivity as compared to Al2O3 nanofluid. Juneja and Gangacharyulu [96] investigated the effects of temperature and nanoparticle concentration on the thermo-physical properties (thermal conductivity, viscosity, and density) of Al2O3 (30–40 nm)/EG + W (25/75)-based nanofluids. The concentration of nanoparticles was between 0.1 and 1.0 vol.%. Results showed that thermal conductivity increases with increasing nanoparticles concentration and temperature as well. At room temperature, a maximum enhancement of 6% of thermal conductivity was observed for loading 1.0 vol.% of this nanoparticle. In a convective heat transfer study, Mojarrad et al. [46] measured the thermal conductivity of Al2O3 (20–30 nm)/EG + W (50/50 by volume) nanofluids. SDBS surfactant was added for better dispersion of nanoparticles and results showed small enhancement (4.5%) of thermal conductivity at 0.7 vol.% concentration of this nanoparticle.
4. Studies on convective heat transfer Despite limited numbers of studies been performed, the results of convective heat transfer of nanofluids from the literature are more consistent and conclusive as compared to thermal conductivity data. Nanofluids exhibit significantly higher heat transfer coefficient (HTC) compared to its base fluid and it further increases with increasing the concentration of nanoparticles as well as Reynolds number (Re) [9,23,25–28]. The enhancement of heat transfer coefficient is even more significant at turbulent regime. Thus, it is presumed that nanofluids can perform better heat transfer in any flow conditions as compared to their base fluids which are conventional heat transfer fluids. Nonetheless, a clear understanding of the convective heat transfer mechanisms of nanofluids is also not yet achieved. In an attempt to establish a strong explanation of the reported anomalously high convective heat transfer coefficient of nanofluids, Buongiorno [97] considered seven-slip mechanisms. He concluded that among those seven only Brownian diffusion and thermophoresis are the two most important mechanisms in nanofluids. He also proposed a new convection correlation and claimed that the enhanced laminar flow convective heat transfer
can be attributed to a reduction of viscosity within and consequent thinning of the laminar sublayer. Since not all the studies reported convective heat transfer results with the same convective parameters, literature results are thus presented and compared with different parameters such as Nusselt number (Nu) and convective heat transfer coefficient (h) as a function of concentration of nanoparticles and Reynolds number (Re). A summary of results on convective heat transfer coefficient (Nu) of Al2O3 nanofluids as a function of concentration of this nanoparticle in laminar flow regime is presented in Fig. 15 [46,98–100]. As anticipated, Nu increases with volumetric loading of this nanoparticle. Due to relatively low viscosity, most of the researchers used EG/W mixture instead of pure EG for the convectional heat transfer studies. Fig. 16 presents literature data showing the influence of Re on this heat transfer coefficient (Nu) of Al2O3 nanofluids at laminar regime [46,98–100]. It shows that at low Re (within 1000) the enhancement of Nu was comparatively low and it increased slowly with increasing Re [98,100] while at higher Re the enhancement of Nu is considerably high (Fig. 16). Nonetheless, Re has positive (increasing) impact on the convective heat transfer performance of nanofluids. On the other hand, numerical results of a research group [101] on the influence of Reynolds number on the Nusselt number of nanofluids at turbulent flow regime are presented in Fig. 17. It confirms that Nu of their two nanofluids increased lin100
5 vol.% Al 2O3/EG[98] 0.7 vol.% Al 2O3/EG+W(50/50 vol.)[46] 1 vol.% Al 2O3/EG (single phase)[99]
80
1 vol.% Al 2O3/EG (two phase)[99] 0.5 vol.% Al 2O3/EG+W(50/50)[100]
60
Nu
10
40
20
0 0
500
1000
1500
2000
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Re Fig. 16. Effect of Reynolds number on Nusselt number for Al2O3 nanofluids at laminar flow regime.
80 1600 6 vol.% SiO2/EG+W(60/40 wt.)[101]
70 1400
6 vol.% CuO/EG+W(60/40 wt.)[101]
60 1200
50
Nu
Nu
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40 30
800 600
Al2O3/EG+W(50/50 vol.) (Re =650)[46] Al2O3/EG+W(50/50 vol.) (Re =1700)[46] Al2O3/EG (Re~1670)[98] Al2O3/EG (Re =1630, single phase) [99] Al2O3/EG (Re =1630, two phase)[99] Al2O3/EG+W(50/50) (Re =120)[100]
20 10 0 0
1
2
3
4
5
400 200
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Nanoparticle concentration (vol.%) Fig. 15. Effect of concentration of nanoparticles on convective heat transfer coefficient (Nu) of Al2O3 nanofluids at laminar flow condition.
20x103
40x103
60x103
80x103
100x103
Re Fig. 17. Effect of Reynolds number on Nusselt number of nanofluids at turbulent flow regime.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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2.2 2.0
4 vol.% SiO2/EG+W(60/40 wt.)[102]
0.3 vol.% CuO/EG[106]
4 vol.% Al2O3/EG+W(60/40 wt.)[102] 8 vol.% Al2O3/EG+W(60/40 wt.)[102]
25000
4 vol.% CuO/EG+W(60/40 wt.)[102] 6 vol.% SiO2/EG+W(60/40 wt.)[103]
1.8
h (W/m2K)
Relative h (hnf /hf)
30000
Al2O3/EG+W(60/40) (Re =4000)[102] SiO2/EG+W(60/40 wt.) (Re =3200)[103] Al2O3 /EG (Re =250)[104] Al2O3 /EG (Re =50000)[104]
1.6 1.4
6 vol.% SiO2/EG+W(60/40 wt.)[103] 3.7 vol.% SiC/EG+W(50/50 vol.)[105]
20000
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10000
1.2 1.0
5000
0
2
4
6
8
10
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Nanoparticle concentration (vol.%) Fig. 18. Enhancement of convective heat transfer coefficient (h) of several nanofluids as a function of concentration of nanoparticles.
early with the Re or flow rate at turbulent flow conditions and CuO nanofluid was found to have higher Nu compared to SiO2 nanofluids. Fig. 18 demonstrates the effect of nanoparticle concentration on the enhanced convective heat transfer coefficient (h) of different nanofluids in laminar to turbulent regimes [102–104]. The relative h, which is h of nanofluids (hnf) over corresponding base fluid (hf), represents the enhancement (hnf/hf > 1) or deterioration (hnf/hf < 1) of the convection heat transfer. The convective heat transfer performance of all these nanofluids (hnf) increased considerably with increasing concentration of nanoparticles in all flow regimes (Fig. 18). Literature results on the heat transfer coefficient (h) of various nanofluids as a function of Re are presented in Fig. 19 [102,103,105,106]. It can be evidenced that h of these nanofluids increased slightly non-linearly (except data from [105]) with increasing Re (i.e., flow rate for fixed system) and the higher the concentration of nanoparticles the larger the hnf. Recently, Sivakumar et al. [106] studied heat transfer characteristics of microchannel heat sink using different nanofluids including CuO (15 nm)/EG in laminar flow condition. The heat transfer coefficient (h) of this nanofluid increased surprisingly high with increasing Re. For example, at 0.3 vol.% of CuO nanoparticle hnf increased from 4050 to 42,000 W/m2 K for increasing Re from 100 to 1300 (Fig. 19 depicts data up to Re of 900 of this nanofluid). The first numerical study on the forced convection heat transfer and friction factor of water and EG-based Al2O3 nanofluids at laminar and turbulent regimes was performed by Maiga et al. [104]. Their results showed that both the heat transfer coefficient and friction factor increased with increasing volume fraction of this nanoparticle. They also found that Al2O3/EG nanofluids exhibited larger enhancement in heat transfer coefficient than Al2O3/W nanofluids. The enhancement was more significant with increasing Re and eventually at the turbulent flow regime (Fig. 18). Forced convective heat transfer of several aqueous and EGbased TiO2 (20 nm) nanofluids flowing in vertically oriented straight copper tube was experimentally investigated by Ding et al. [28]. Although aqueous-based nanofluids showed significant enhancement of convective heat transfer coefficient, it surprisingly deteriorates for EG based nanofluids at low Re. They presumed that high viscosity of EG could be responsible for such deterioration of heat transfer coefficient. In a comprehensive study, Vajjha et al. [102] experimentally determined convective heat transfer and friction factor of three nanofluids containing Al2O3 (45 m), CuO (29 nm) and SiO2 (three sizes – 20 nm, 50 nm and 100 nm) in EG/W mixture (60/40 by mass)
0
2000
4000
6000
8000
10000
12000
Re Fig. 19. Effect of Reynolds number on convective heat transfer coefficient (h) of various EG/W mixture-based nanofluids.
in turbulent regime. Based on their experimental results they also developed correlations for convective heat transfer of these nanofluids. Results showed significantly high convective heat transfer performance of these nanofluids compared to base fluid (EG/W) and the heat transfer coefficient (HTC) increases further with increasing concentration as well as Reynolds number (Fig. 19). For example, the h of Al2O3 (45 m) nanofluid increased substantially and almost linearly with increasing loading of this nanoparticle and a maximum 120% increase in h was reported for 10 vol.% concentration of Al2O3 nanoparticle in EG/W mixture (Fig. 18). Timofeeva et al. [107] measured convective heat transfer coefficient of EG/W (50/50 by volume)-based SiC (four sizes between 16 and 90 nm) nanofluids at turbulent flow condition. With increasing flow velocity the Nu of these nanofluids increased considerably. When all other parameters are same, heat transfer coefficients of nanofluids increased with increasing nanoparticle sizes. The smaller particles (16 nm and 29 nm) gave lower heat transfer coefficients than those for the base EG/W. They also demonstrated that adding SiC nanoparticles significantly improve the cooling efficiency of EG/W mixture. Aravind et al. [42] conducted an experimental investigation on convective heat transfer with aqueous and EG-based functionalized CNT (f-MWCNT)-nanofluids and found almost the same trend of the heat transfer coefficient profiles for their f-MWCNT/EG and f-MWCNT/W nanofluids. For a CNT concentration of only 0.03 vol.% and flow rate of 56 mL/s, the maximum increase in convective heat transfer coefficient of MWCNT/EG-based nanofluids was as high as 180%. In a different study, Putra et al. [80] experimentally determined the thermal performance of screen mesh wick heat pipes with different nanofluids comprising of Al2O3, TiO2 and ZnO nanoparticles in EG and W. The concentration of the nanoparticles was varied from 1 to 5 vol.% and nanofluids were deposited as a coating on the screen mesh in the heat pipes. However, the coating was found to be advantageous since it produced good capillary structures and therefore higher thermal performance. The screen mesh wick heat pipe with the best performance was resulted with Al2O3/W nanofluid at 5 vol.% concentration. In an intertube falling-film flow system, Ruan and Jacobi [108] studied convective heat transfer characteristics of MWCNT/EG nanofluids. With an increased concentration of CNT, the heat transfer coefficients of nanofluids were decreased monotonously and the maximum deviation with those of pure EG was around 26%. However, at concentration of 0.5 wt.% and any Reynolds number the falling-film HTC of nanofluids were larger than the base EG and the enhancement of HTC was up to 20% at Re = 5.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Experiments on heat transfer characteristics of EG/W mixture (30/70 by volume)-based MWCNT nanofluid in a tubular heat exchanger were performed by Kumaresan et al. [109]. The convective heat transfer coefficient of this nanofluid was found to increase to a maximum of 160% at 0.45 vol.% of MWCNT. They ascribed several factors such as nanotube rearrangement and high aspect ratio as well as delay in boundary layer development for the observed heat transfer enhancement of their nanofluid. Hamid et al. [110] experimentally investigated convective heat transfer of TiO2/W/EG mixture at Re range of 2000 < Re < 10,000. Results showed that the Nu of the nanofluid increases with Reynolds number at 1.5 vol.% concentration of TiO2. The heat transfer coefficient (h) was increased by 2.1% for 1.5 vol.% of TiO2 nanoparticle. For 0.5% and 1.0% volumetric concentrations and at temperature of 30 °C, no enhancement of heat transfer was found for the fluid flow under transition regime. In the presence of electric field, a natural convection heat transfer study of Fe3O4/EG nanofluids was conducted by Asadzadeh et al. [111]. An increase in heat transfer up to 0.02 vol.% of nanoparticle was reported. The enhancement of heat transfer was also found to increase with electric field intensity while decreased with Rayleigh number. Huminic and Huminic [112] performed a numerical analysis of laminar flow heat transfer of CuO/EG nanofluids in a flattened tube. The convective heat transfer coefficient of nanofluid was found to be higher than that of pure EG. Also at any Reynolds number and concentration of nanoparticle the heat transfer coefficient for flattened tube was larger than those of the elliptic and circular tubes. In order to evaluate the cooling performance nanofluids have also been used in commercial automotive radiators in which circulating fluids is cooled mainly by forced convection. For instance, Peyghambarzadeh et al. [113] employed EG and EG/W mixturebased Al2O3 nanofluids in a car radiator and showed that these nanofluids substantially increase (up to 40% compared to base fluids) the cooling performance of the radiator. Later Vajjha et al. [114] compared the numerically obtained performance of EG/W mixture based Al2O3 and CuO nanoparticles in flat tubes of a radiator. They reported about 36.6% and 49.7%, enhancements of the average heat transfer coefficient of EG/W mixture for adding 3 vol.% concentration of Al2O3 and CuO nanofluids, respectively. Nieh et al. [115] also reported an improvement of the heat dissipation and efficiency of the radiator cooling systems using EG/W (50/50) TiO2 nanofluids. For 0.2% volumetric loading of TiO2 they found maximum enhanced ratios of heat dissipation capacity and efficiency factor to be 25.6% and 27.2%, respectively. Results of all these studies [113–115] demonstrate that EG and EG/W based nanofluids are better coolants for commercial radiator systems as compared to their base fluids. Very recently, an experimental study on the convective heat transfer of Al2O3/EG nanofluid in a corrugated tube fitted with twisted tapes and under turbulent flow conditions was reported by Mohammadiun et al. [116]. The utilization of both twisted tapes and nanofluids increased the heat transfer and friction factor of the tube. The maximum thermal performance factor was 4.2 for twist ratio of 2 and at 0.5 vol.% concentration of Al2O3 nanoparticle. However, the heat transfer performance was weakened at high concentration of nanoparticle. Although a couple of studies reported deterioration in convective heat transfer of EG or EG/W based nanofluids mostly at high concentration of nanoparticles (e.g., [28]), most of the above reported studies demonstrate that the performance of nanofluids in convection heat transfer is better than that of the base fluid. Since the practical applications of nanofluids as heat transfer fluids are mainly in flowing systems, having such high convective heat transfer coefficients nanofluids can be considered as the advanced coolants for flow-based cooling systems such as minichannels,
microchannels, and miniaturized heat exchangers. Several studies also revealed that the cooling performance of automobile radiators can be improved by using nanofluids. Also, most of the convective heat transfer studies used Al2O3 nanoparticles either in EG or EG/W mixture. Thus other types of nanoparticles need to be investigated in more systematic convective heat transfer studies. Furthermore, the mechanisms behind the observed convection heat transfer performance of nanofluids need to be elucidated. 5. Nanofluids in energy applications Because of the enhanced conduction and convection heat transfer characteristics and other thermophysical properties nanofluids offer great potential in energy related applications particularly in solar energy systems. Nanofluids also provide several key advantages in solar power plants including direct absorption of solar energy, high absorption in the solar range and low emittance in the infrared, enhanced convection and radiation heat transfer, and enhancing absorption efficiency by changing the size, type and concentration of nanoparticles [117]. Thus, in recent years, numbers of studies have been reported to evaluate the performance and applicability of nanofluids in solar energy storage and conversions by various means such as solar collectors [117–126]. Most of these studies demonstrated that nanofluids can considerably improve the efficiency of solar energy systems. For instances, Taylor et al. [117] revealed that the power tower solar collectors can be benefited from the potential efficiency improvements due to using a nanofluid as working fluid. They conducted experiments on a laboratory-scale nanofluid dish receiver and reported up to 10% increase in efficiency relative to a conventional fluid. It was also shown that such increase in efficiency can lead to generate huge amount of revenue from solar thermal power tower by incorporating a nanofluid. Shin and Banerjee [119] showed nanofluids with their higher thermal conductivity and heat capacity can remarkably enhance the capability and performance of solar thermal power plants. While Lenert and Wang [120] demonstrated that the efficiency of a concentrated solar power collector can be increased by >35% with nanofluids, Yousefi et al. [121] found about 28.3% increase in efficiency of flat-plate solar collectors using MWCNT nanofluids. Although most of the studies used aqueous nanofluids, only a handful of efforts have been reported on employing EG and EG/ W-based nanofluids in energy applications. These nanofluids were also found to enhance the thermal performance and efficiency of those energy technologies [127,128]. For example, Mercatelli et al. [127] investigated the potentiality of using single-wall carbon nanohorns (SWCNHs)/EG naofluid in solar collectors and demonstrated that this SWCNH-nanofluid is very promising for direct sunlight absorbers for thermal solar energy exploitation. Tang et al. [128] also showed that SiO2/MWCNT/PEG (poly (ethylene glycol)) composite nanofluid can effectively improve the efficiency of solar energy applications and this nanofluid can be applied in energy storage systems as well. Nevertheless, more studies are to be performed in order to evaluate the commercial applicability of nanofluids in energy technologies. 6. Conclusions Research progress and findings on both conduction (thermal conductivity) and convection heat transfer of EG and EG/W mixture-based nanofluids have extensively been reviewed in this paper. One of the major challenges of nanofluids research is to achieve long-term stability of nanofluids which mostly depends on the
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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preparation methodology of nanofluids. Survey on nanofluids preparation reveals that due to commercially availability of wide varieties of nanoparticles and easiest option, most of the researchers used two-step methods (i.e., suspensions of purchased nanoparticles in base fluids) for preparing their nanofluids. Almost every study employed sonication to disperse nanoparticles. However, sonication time in different studies varied from tens of minutes to several hours. Sonication and its duration must be carefully and precisely performed as long-time sonication can evaporate the base fluids and can also damage the nanoparticle structures particularly for non-spherical particles. Researchers also added various types of dispersants for better stability. The preparation and stability of nanofluids are of great importance for their optimum properties as well as for a wide range of applications. Majority of the literature studies focused on the thermal conductivity of nanofluids. Like other nanofluids, EG-based nanofluids also exhibit higher thermal conductivity compared to their base fluid and it further increases substantially with increasing loading of nanoparticles. However, most of the results from different researchers are scattered. While some researchers showed very high enhancement of thermal conductivity at low concentrations, others reported moderate increase in this thermal property. Even for nanoparticles of similar size and concentration, the thermal conductivity data from different researchers vary considerably. The effect of nanoparticle size on the thermal conductivity of nanofluids is also not conclusive as some studies found that nanofluids with larger size nanoparticles possess higher enhancement of thermal conductivity than that of smaller nanoparticles. Whereas, others reported that the smaller the nanoparticles the higher the enhancement of the thermal conductivity. EG/W-based nanofluids also showed similar enhancement of their thermal conductivity. Compared to volume of studies on thermal conductivity, limited research efforts have been made on convective heat transfer of these nanofluids. However, most of the researchers used EG/W mixture as base fluids instead of pure EG. This is mainly due to keeping the viscosity of nanofluids relatively low and to have higher conduction as well. Review demonstrates that these nanofluids exhibit enhanced convectional heat transfer performance (coefficient) which further increases significantly with increasing the concentration of nanoparticles as well as Reynolds number or flow rate. The convective heat transfer is even more pronounced at turbulent flow regime. Despite the fact that couple of studies reported deterioration of the convective heat transfer, nanofluids can perform better heat transfer as compared to their base fluids in any flow conditions. Nonetheless, a clear understanding of the convective heat transfer mechanisms of nanofluids is also not yet achieved. Thus more systematic experimental and analytical studies are to be performed on characterization of these thermal properties and features of these nanofluids. Studies also revealed that nanofluids exhibit enhanced efficiency and performance in energy storage and conversion systems as compared to conventional fluids. In addition to properties characterization it is also of great importance to conduct more rigorous investigations on the use of nanofluids in various energy- based applications. Acknowledgment This work was supported by the Fundação para a Ciência e Tecnologia (FCT), Portugal through grants SFRH/BPD/102518/2014 and PEst-OE/QUI/UI0100/2013. References [1] Das SK, Choi SUS, Yu W, Pradeep T. Nanofluids: science and technology. New Jersey: Wiley; 2008.
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[2] Murshed SMS, Nieto de Castro CA. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [3] Murshed SMS, Leong KC, Yang C. Enhanced thermal conductivity of TiO2water based nanofluids. Int J Therm Sci 2005;44:367–73. [4] Murshed SMS, Leong KC, Yang C. Determination of the effective thermal diffusivity of nanofluids by the double hot-wire technique. J Phys D: Appl Phys 2006;39:5316–22. [5] Yu W, France DM, Routbort JL, Choi SUS. Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat Transf Eng 2008;29:432–60. [6] Murshed SMS, Leong KC, Yang C. Thermophysical and electrokinetic properties of nanofluids – a critical review. Appl Therm Eng 2008;28:2109–25. [7] Murshed SMS, Leong KC, Yang C. Heat transfer of nanoparticle suspensions (nanofluids). In: Lombardi SL, editor. Nanoparticles: new research. New York: Nova Science Publishers; 2008. [8] Murshed SMS, Leong KC, Yang C. Investigations of thermal conductivity and viscosity of nanofluids. Int J Therm Sci 2008;47:560–8. [9] Özerinç S, Kakaç S, Yazıcıog˘lu AG. Enhanced thermal conductivity of nanofluids: a state-of-the-art review. Microfluid Nanofluid 2010;8:145–70. [10] Murshed SMS, Leong KC, Yang C. Thermophysical properties of nanofluids. In: Sattler KD, editor. Handbook of nanophysics: nanoparticles and quantum dots. Boca Raton: Taylor & Francis; 2010. [11] Murshed SMS. Determination of effective specific heat of nanofluids. J Exp Nanosci 2011;6:539–46. [12] Kleinstreuer C, Feng Y. Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review. Nanoscale Res Lett 2011;6:229. [13] Murshed SMS. Simultaneous measurement of thermal conductivity, thermal diffusivity, and specific heat of nanofluids. Heat Transf Eng 2012;33:722–31. [14] Murshed SMS, Nieto de Castro CA. Superior thermal features of carbon nanotubes based nanofluids – a review. Renew Sustain Energy Rev 2014;37:155–67. [15] Murshed SMS, Nieto de Castro CA. Thermophysical properties and heat transfer characteristics of carbon nanotubes dispersed nanofluids. In: Murshed SMS, Nieto de Castro CA, editors. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [16] Younes H, Christensen G, Li D, Hong H, Ghaferi AA. Thermal conductivity of nanofluids: review. J Nanofluid 2015;4:107–32. [17] Choi SUS, Zhang ZG, Keblinski P. Nanofluids. In: Nalwa HS, editor. Encyclopedia of nanoscience and nanotechnology. Los Angeles: American Scientific Publishers; 2004. [18] Wen D, Lin G, Vafaei S, Zhang K. Review of nanofluids for heat transfer applications. Particuology 2009;7:141–50. [19] Murshed SMS, Tan SH, Nguyen NT, Wong TN, Yobas L. Microdroplet formation of water and nanofluids in heat-induced microfluidic T-junction. Microfluid Nanofluid 2009;6:253–9. [20] Wong KV, De Leon O. Applications of nanofluids: current and future. Adv Mech Eng 2010;2010:519659. [21] Saidur R, Leong KY, Mohammad HA. A review on applications and challenges of nanofluids. Renew Sustain Energy Rev 2011;15:1646–68. [22] Murshed SMS, Nieto de Castro CA. Nanofluids as advanced coolants. In: Mohammad A, Inamuddin, editors. Green solvents I: properties and applications in chemistry. London: Springer; 2012. [23] Mohammed HA, Al-aswadi AA, Shuaib NH, Saidur R. Convective heat transfer and fluid flow study over a step using nanofluids: a review. Renew Sustain Energy Rev 2011;15:2921–39. [24] Murshed SMS. Correction and comment on ‘‘Thermal conductance of nanofluids: is the controversy over?”. J Nanopart Res 2009;11:511–2. [25] Murshed SMS, Nieto de Castro CA, Lourenço MJV, Lopes MLM, Santos FJV. A review of boiling and convective heat transfer with nanofluids. Renew Sustain Energy Rev 2011;15:2342–54. [26] Murshed SMS, Nieto de Castro CA. Forced convective heat transfer of nanofluids in minichannel. In: Ahsan A, editor. Two phase flow, phase change and numerical modeling. Rijeka: INTECH; 2011. [27] Sundar LS, Singh MK. Convective heat transfer and friction factor correlations of nanofluids in a tube and with inserts: a review. Renew Sustain Energy Rev 2013;20:23–35. [28] Ding Y, Chen H, He Y, Lapkin A, Yeganeh M, Siller L, et al. Forced convective heat transfer of nanofluids. Adv Powder Technol 2007;18:813–24. [29] Wang L, Fan J. Nanofluids research: key issues. Nanoscale Res Lett 2010;5:1241–52. [30] Feng X, Ma H, Huang S, Pan W, Zhang X, Tian F, et al. Aqueous-organic phasetransfer of highly stable gold, silver, and platinum nanoparticles and new route for fabrication of gold nanofilms at the oil/water interface and on solid supports. J Phys Chem B 2006;110:12311–7. [31] Yu W, Xie H, Chen L, Li Y. Investigation on the thermal transport properties of ethylene glycol-based nanofluids containing copper nanoparticles. Powder Technol 2010;197:218–21. [32] Boleininger J, Kurz A, Reuss V, Sönnichsen C. Microfluidic continuous flow synthesis of rod-shaped gold and silver nanocrystals. Phys Chem Chem Phys 2006;8:3824–7. [33] Wei X, Wang L. Synthesis and thermal conductivity of microfluidic copper nanofluids. Particuology 2010;8:262–71. [34] Yu W, Xie H. A review on nanofluids: preparation, stability mechanisms, and applications. J Nanomater 2012;2012:435873.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
14
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
[35] Eastman JA, Choi SUS, Li S, Yu W, Thompson LJ. Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles. Appl Phys Lett 2001;78:718–20. [36] Assael MJ, Metaxa IN, Kakosimos K, Constantinou D. Thermal conductivity of nanofluidsexperimental and theoretical. Int J Thermophys 2006;27:999–1016. [37] Kim SH, Choi SR Kim D. Thermal conductivity of metal-oxide nanofluids: particle size dependence and effect of laser irradiation. J Heat Transf 2007;129:298–307. [38] Chen L, Xie H, Li Y, Yu W. Nanofluids containing carbon nanotubes treated by mechanochemical reaction. Thermochim Acta 2008;477:21–4. [39] Timofeeva EV, Routbort JL, Singh D. Particle shape effects on thermophysical properties of alumina nanofluids. J Appl Phys 2009;106:014304. [40] Moosavi M, Goharshadi EK, Youssefi A. Fabrication, characterization, and measurement of some physicochemical properties of ZnO nanofluids. Int J Heat Fluid Flow 2010;31:599–605. [41] Xie H, Chen L. Review on the preparation and thermal performances of carbon nanotube contained nanofluids. J Chem Eng Data 2011;56:1030–41. [42] Aravind SSJ, Baskar P, Baby TT, Sabareesh RK, Das S, Ramaprabhu S. Investigation of structural stability, dispersion, viscosity, and conductive heat transfer properties of functionalized carbon nanotube based nanofluids. J Phys Chem C 2011;115:16737–44. [43] Teng T-P, Yu C-C. Heat dissipation performance of MWCNTs nano-coolant for vehicle. Exp Therm Fluid Sci 2013;49:22–30. [44] Reddy MCS, Rao VV. Experimental studies on thermal conductivity of blends of ethylene glycol-water-based TiO2 nanofluids. Int Commun Heat Mass Transf 2013;46:31–6. [45] Altan CL, Gurten B, Sommerdijk NAJM, Bucak S. Deterioration in effective thermal conductivity of aqueous magnetic nanofluids. J Appl Phys 2014;116:224904. [46] Mojarrad MS, Keshavarz A, Ziabasharhagh M, Raznahan MM. Experimental investigation on heat transfer enhancement of alumina/water and alumina/ water–ethylene glycol nanofluids in thermally developing laminar flow. Exp Therm Fluid Sci 2014;53:111–8. [47] Esfe MH, Saedodin S. Experimental investigation and proposed correlations for temperature-dependent thermal conductivity enhancement of ethylene glycol based nanofluid containing ZnO nanoparticles. J Heat Mass Transf Res 2014;1:47–54. [48] Lourenço MJV, Vieira SI. Nanofluids preparation methodology. In: Murshed SMS, Nieto de Castro CA, editors. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [49] Thomas S, Sobhan CBP. A review of experimental investigations on thermal phenomena in nanofluids. Nanoscale Res Lett 2011;6:377. [50] Lee S, Choi SUS, Li S, Eastman JA. Measuring thermal conductivity of fluids containing oxide nanoparticles. J Heat Transf 1999;121:280–9. [51] Wang X, Xu X, Choi SUS. Thermal conductivity of nanoparticle–fluid mixture. J Thermophys Heat Transf 1999;13:474–80. [52] Xie H, Wang J, Xi T, Liu Y, Ai F, Wu Q. Thermal conductivity enhancement of suspensions containing nanosized alumina particles. J Appl Phys 2002;9:4568–70. [53] Wang Y, Fisher TS, Davidson JL, Jiang L. Thermal conductivity of nanoparticle suspensions. In: 8th AIAA/ASME joint thermophys heat transfer conf, June 24–26, Missouri, USA; 2002. [54] Xie H, Lee H, Youn W, Choi M. Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities. J Appl Phys 2003;94:4967–71. [55] Hong KS, Hong TK, Yang HS. Thermal conductivity of Fe nanofluids depending on the cluster size of nanoparticles. Appl Phys Lett 2006;88:031901. [56] Kwak K, Kim C. Viscosity and thermal conductivity of copper oxide nanofluid dispersed in ethylene glycol. Korea-Austr Rheol J 2005;17:35–40. [57] Liu MS, Lin MC-C, Huang IT, Wang C-C. Enhanced thermal conductivity with CuO for nanofluids. Chem Eng Technol 2006;29:72–7. [58] Kang HU, Kim SH, Oh JM. Estimation of thermal conductivity of nanofluid using experimental effective particle volume. Exp Heat Transf 2006;19:181–91. [59] Amrollahi A, Hamidi AA, Rashidi AM. The effects of temperature, volume fraction and vibration time on the thermo-physical properties of a carbon nanotube suspension (carbon nanofluid). Nanotechnology 2008;19:315701. [60] Yoo D, Hong K, Yang H. Study of thermal conductivity of nanofluids for the application of heat transfer fluids. Thermochim Acta 2007;455:66–9. [61] Patel HE, Sundararajan T, Das SK. An experimental investigation into the thermal conductivity enhancement in oxide and metallic nanofluids. J Nanopart Res 2010;12:1015–31. [62] Baby TT, Ramaprabhu S. Investigation of thermal and electrical conductivity of graphene based nanofluids. J Appl Phys 2010;108:124308. [63] Harish S, Ishikawa K, Einarsson E, Aikawa S, Chiashi S, Shiomi J, et al. Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions. Int J Heat Mass Transf 2012;55:3885–90. [64] Kole M, Dey TK. Effect of prolonged ultrasonication on the thermal conductivity of ZnO– ethylene glycol nanofluids. Thermochim Acta 2012;535:58–65. [65] Longo GA, Zilio C. Experimental measurements of thermophysical properties of Al2O3– and TiO2–ethylene glycol nanofluids. Int J Thermophys 2013;34:1288–307. [66] Gallego MJP, Lugo L, Cabaleiro D, Legido JLL, Pineiro MMM. Thermophysical profile of ethylene glycol-based ZnO nanofluids. J Chem Thermodyn 2014;73:23–30.
[67] Mary EEJ, Suganthi KS, Manikandan S, Anusha N, Rajan KS. Cerium oxide ethylene glycol nanofluids with improved transport properties: preparation and elucidation of mechanism. J Taiwan Inst Chem Eng 2015;49:183–91. [68] Mariano A, Gallego MJP, Lugo L, Mussari L, Piñeiro MM. Co3O4 ethylene glycol-based nanofluids: thermal conductivity, viscosity and high pressure density. Int J Heat Mass Transf 2015;85:54–60. [69] Esfe MH, Karimipour A, Yan WM, Akbari M, Safaei MR, Dahari M. Experimental study on thermal conductivity of ethylene glycol based nanofluids containing Al2O3 nanoparticles. Int J Heat Mass Transf 2015;88:728–34. [70] Beck MP, Yuan Y, Warrier P, Teja AS. The effect of particle size on the thermal conductivity of alumina nanofluids. J Nanopart Res 2009;11:1129–36. [71] Gallego MJP, Lugo L, Legido JL, Pineiro MM. Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids. Nanoscale Res Lett 2011;6:221. [72] Gangwar J, Srivastava AK, Tripathi SK, Wan M, Yadav RR. Strong enhancement in thermal conductivity of ethylene glycol-based nanofluids by amorphous and crystalline Al2O3 nanoparticles. Appl Phys Lett 2014;105:063108. [73] Cabaleiro D, Nimo J, Gallego MJP, Piñeiro MM, Legido JL, Lugo L. Thermal conductivity of dry anatase and rutile nano-powders and ethylene and propylene glycol-based TiO2 nanofluids. J Chem Thermodyn 2015;83:67–76. [74] Lee GJ, Kim CK, Lee MK, Rhee CK. Characterization of ethylene glycol based TiO2 nanofluid prepared by pulsed wire evaporation (PWE) method. Rev Adv Mater Sci 2011;28:126–9. [75] John T, Krishnakumar TS. Experimental studies of thermal conductivity, viscosity and stability of ethylene glycol nanofluids. Int J Innov Res Sci Eng Technol 2013;2:611–7. [76] Suganthi KS, Vinodhan VL, Rajan KS. Heat transfer performance and transport properties of ZnO–ethylene glycol and ZnO–ethylene glycol–water nanofluid coolants. Appl Energy 2014;135:548–59. [77] Lee GJ, Kim CK, Lee MK, Rhee CK, Kim S, Kim C. Thermal conductivity enhancement of ZnO nanofluids using a one-step physical method. Thermochim Acta 2012;542:24–7. [78] Yu W, Xie H, Chen L, Li Y. Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid. Thermochim Acta 2009;491:92–6. [79] Li H, Wang L, He Y, Hu Y, Zhu J, Jiang B. Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids. Appl Therm Eng 2015;88:363–8. [80] Putra N, Septiadi WN, Rahman H, Irwansyah R. Thermal performance of screen mesh wick heat pipes with nanofluids. Exp Therm Fluid Sci 2012;40:10–7. [81] Jha N, Ramaprabhu S. Thermal conductivity studies of metal dispersed multiwalled carbon nanotubes in water and ethylene glycol based nanofluid. J Appl Phys 2009;106:084317. [82] Hwang YJ, Park HS, Lee JK, Jun WH. Thermal conductivity and lubrication characteristics of nanofluids. Curr Appl Phys 2006;6S1:e67–71. [83] Xie H, Chen L. Adjustable thermal conductivity in carbon nanotube nanofluids. Phys Lett A 2009;373:1861–4. [84] Liu M-S, Lin MC-C, Huang I-T, Wang C-C. Enhancement of thermal conductivity with carbon nanotube for nanofluids. Int Commun Heat Mass Transf 2005;32:1202–10. [85] Zennifer MA, Anikandan S, Sughanti KS, Vinodhan VL, Rajan KS. Development of CuO-ethylene glycol nanofluids for efficient energy management: assessment of potential for energy recovery. Energy Convers Manage 2015;105:685–96. [86] Barbes B, Paramo R, Blanco E, Casanova C. Thermal conductivity and specific heat capacity measurements of CuO nanofluids. J Therm Anal Calorim 2014;115:1883–91. [87] Liu M-S, Lin MC-C, Wang C-C. Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale Res Lett 2011;6:297. [88] Garg J, Poudel B, Chiesa M, Gordon JB, Ma JJ, Wang JB, et al. Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid. J Appl Phys 2008;103:074301. [89] Gallego MJP, Lugo L, Legido JL, Pineiro MM. Enhancement of thermal conductivity and volumetric behavior of FexOy nanofluids. J Appl Phys 2011;110:014309. [90] Xie H, Wang J, Xi T, Liu Y. Thermal conductivity of suspensions containing nanosized SiC particles. Int J Thermophys 2002;23:571–80. [91] Sharma P, Baek I-H, Cho T, Park S, Lee KB. Enhancement of thermal conductivity of ethylene glycol based silver nanofluids. Powder Technol 2011;208:7–19. [92] Kulkarni DP, Namburu PK, Das DK. Comparison of heat transfer rates of different nanofluids on the basis of the Mouromtseff number. Electro Cooling 2007;13:28–32. [93] Vajjha RS, Das DK. Experimental determination of thermal conductivity of three nanofluids and development of new correlations. Int J Heat Mass Transf 2009;52:4675–82. [94] Sundar LS, Farooky MH, Sarada SN, Singh MK. Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids. Int Commun Heat Mass Transf 2013;41:41–6. [95] Sundar LS, Singh MK, Sousa ACM. Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid. Int Commun Heat Mass Transf 2013;49:17–24.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx [96] Juneja M, Gangacharyulu D. Experimental analysis on influence of temperature and volume fraction of nanofluids on thermophysical properties. Int J Emerg Technol Comput Appl Sci 2013;5:233–8. [97] Buongiorno J. Convective transport in nanofluids. J Heat Transf 2006;128:240–50. [98] Hajimeyoshino Torii S. Heat transfer enhancement of Al2O3 nanofluid dispersed in ethylene glycol. Int J Earth Sci Eng 2012;5:532–4. [99] Delavari V, Hashemabadi SH. CFD simulation of heat transfer enhancement of Al2O3/water and Al2O3/ethylene glycol nanofluids in a car radiator. Appl Therm Eng 2014;73:380–90. [100] Zakaria I, Mohamed WANW, Mamat AMIB, Saidur R, Azmi WH, Mamat R, et al. Experimental investigation of Al2O3 - water ethylene glycol mixture nanofluid thermal behaviour in a single cooling plate for PEM fuel cell application. Energy Procedia 2015;79:252–8. [101] Namburu PK, Das DK, Tanguturi KM, Vajjha RS. Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties. Int J Therm Sci 2009;48:290–302. [102] Vajjha RS, Das DK, Kulkarni DP. Development of new correlations for convective heat transfer and friction factor in turbulent regime for nanofluids. Int J Heat Mass Transf 2010;53:4607–18. [103] Kulkarni DP, Namburu PK, Bargar HE, Das DK. Convective heat transfer and fluid dynamic characteristics of SiO2 ethylene glycol/water nanofluid. Heat Transf Eng 2008;29:1027–35. [104] Maiga SEB, Nguyen CT, Galanis N, Roy G. Heat transfer behaviours of nanofluids in a uniformly heated tube. Superlat Microstruct 2004;35:543–57. [105] Yu W, France D, Smith D, Routbort J, Timofeeva E, Singh D, et al. Nanofluids for thermal conditions– underhood heat transfer. In: DOE hydrogen program and vehicle technologies program annual merit review and peer evaluation meeting, May 18–22, Washington, DC, USA; 2009. [106] Sivakumar A, Alagumurthi N, Senthilvelan T. Effect of serpentine grooves on heat transfer characteristics of microchannel heat sink with different nanofluids. Heat Trans Asian Res 2015. doi: http://dx.doi.org/10.1002/ htj.21206. [107] Timofeeva EV, Yu W, France DM, Singh D, Routbort JL. Base fluid and temperature effects on the heat transfer characteristics of SiC in ethylene glycol/H2O and H2O nanofluids. J Appl Phys 2011;109:014914. [108] Ruan B, Jacobi AM. Heat transfer characteristics of multiwall carbon nanotube suspensions (MWCNT nanofluids) in intertube falling-film flow. Int J Heat Mass Transf 2012;55:3186–95. [109] Kumaresan V, Velraj R, Das SK. Convective heat transfer characteristics of secondary refrigerant based CNT nanofluids in a tubular heat exchanger. Int J Refrig 2012;35:2287–96. [110] Hamid KA, Azmi WH, Mamat R, Usri NA. Heat transfer performance of titanium oxide in ethylene glycol based nanofluids under transition flow. Appl Mech Mater 2014;660:684–8. [111] Asadzadeh F, Esfahany MN, Etesami N. Natural convective heat transfer of Fe3O4/ethylene glycol nanofluid in electric field. Int J Therm Sci 2012;62:114–9.
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[112] Huminic G, Huminic A. Numerical analysis of laminar flow heat transfer of nanofluids in a flattened tube. Int Commun Heat Mass Transf 2013;44:52–7. [113] Peyghambarzadeh SM, Hashemabadi SH, Hoseini SM, Jamnani MS. Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators. Int Commun Heat Mass Transf 2011;38:1283–90. [114] Vajjha RS, Das DK, Ray DR. Development of new correlations for the Nusselt number and the friction factor under turbulent flow of nanofluids in flat tubes. Int J Heat Mass Transf 2014;80:353–67. [115] Nieh H-M, Teng T-P, Yu C-C. Enhanced heat dissipation of a radiator using oxide nano-coolant. Int J Therm Sci 2014;77:252–61. [116] Mohammadiun H, Mohammadiun M, Hazbehian M, Maddah H. Experimental study of ethylene glycol-based Al2O3 nanofluid turbulent heat transfer enhancement in the corrugated tube with twisted tapes. Heat Mass Transfer 2016;52:141–51. [117] Taylor RA, Phelan PE, Otanicar TP, Walker CA, Nguyen M, Trimble S, et al. Applicability of nanofluids in high flux solar collectors. J Renew Sustain Energy 2011;3:023104. [118] Taylor RA, Phelan PE, Otanicar TP, Adrian R, Prasher RS. Nanofluid optical property characterization: towards efficient direct absorption solar collectors. Nanoscale Res Lett 2011;6:225. [119] Shin D, Banerjee D. Enhancement of specific heat capacity of hightemperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications. Int J Heat Mass Transf 2011;54:1064–70. [120] Lenert A, Wang EN. Optimization of nanofluid volumetric receivers for solar thermal energy conversion. Sol Energy 2012;86:253–65. [121] Yousefi T, Veysi F, Shojaeizadeh E, Zinadini S. An experimental investigation on the effect of Al2O3-H2O nanofluid on the efficiency of flat plate solar collector. Renew Energy 2012;39:293–8. [122] Javadi FS, Saidur R, Kamalisarvestani M. Investigating performance improvement of solar collectors by using nanofluids. Renew Sustain Energy Rev 2013;28:232–45. [123] Colangelo G, Favale E, de Risi A, Laforgia D. A new solution for reduced sedimentation flat panel solar thermal collector using nanofluids. Appl Energy 2013;111:80–93. [124] Mahian O, Kianifar A, Kalogirou SA, Pop I, Wongwises S. A review of the applications of nanofluids in solar energy. Int J Heat Mass Transf 2013;57:582–94. [125] Bozorgan N, Shafahi M. Performance evaluation of nanofluids in solar energy: a review of the recent literature. Micro Nano Syst Lett 2015;3:5. [126] Kasaeian A, Eshghi AT, Sameti M. A review on the applications of nanofluids in solar energy systems. Renew Sustain Energy Rev 2015;43:584–98. [127] Mercatelli L, Sani E, Zaccanti G, Martelli F, Ninni PD, Barison S, et al. Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers. Nanoscale Res Lett 2011;6:282. [128] Tang B, Wang Y, Qiu M, Zhang S. A full-band sunlight-driven carbon nanotube/PEG/SiO2 composites for solar energy storage. Sol Energy Mater Sol Cells 2014;123:7–12.
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review S.M. Sohel Murshed ⇑, C.A. Nieto de Castro Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal
h i g h l i g h t s A review on key heat transfer features of ethylene glycol-nanofluids is presented. Nanofluids preparation and stabilization methods are discussed and summarized. Effects of various key factors on their thermal conductivity have been analyzed. Nanofluids exhibit superior heat transfer compared to base conventional fluids. Energy applications and research needs of nanofluids are identified and addressed.
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Article history: Received 26 April 2016 Received in revised form 5 November 2016 Accepted 6 November 2016 Available online xxxx Keywords: Nanofluids Thermal conductivity Convection heat transfer Ethylene glycol Nanoparticles
a b s t r a c t This paper presents a state of the art review on the research and development of conduction (thermal conductivity) and convection heat transfer characteristics of ethylene glycol-based nanofluids. Nanofluids preparation and stabilization methods are summarized and discussed. The effects of nanoparticles type, size and concentration as well as temperature on the thermal conductivity of available ethylene glycol and ethylene glycol/water mixture-based nanofluids have been critically and individually analyzed and discussed. Studies on convective heat transfer of these nanofluids have also been reviewed and results from different studies are compared. The review clearly demonstrates that these nanofluids possess considerably higher thermal conductivity and convective heat transfer characteristics compared to their base fluids i.e., ethylene glycol and its aqueous mixture. These thermal features of nanofluids are key factors for their performance in thermal management and energy applications. With the enhanced thermal conductivity and convective heat transfer coefficient nanofluids offer immense potential in energy harvesting and storage as well as to advanced cooling applications. Ó 2016 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofluids preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature survey on thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Overview of EG-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Al2O3/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. TiO2/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. ZnO/EG nanofluids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. CNT/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. CuO/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Cu/EG nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Other EG-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. EG/W mixture-based nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Studies on convective heat transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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⇑ Corresponding author at: Centro de Química Estrutural, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail address: [email protected] (S.M.S. Murshed). http://dx.doi.org/10.1016/j.apenergy.2016.11.017 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Nanofluids in energy applications . Conclusions. . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . .
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1. Introduction Due to an overwhelming amount of attention from the researchers worldwide, nanofluids became one of the most popular research fronts in recent years. Nanofluids, which are the suspensions of nanometer-sized particles (typically <100 nm) in traditional heat transfer fluids such as water (W), ethylene glycol (EG), and engine oil possess superior thermophysical properties than those of their base fluids [1–16]. Besides highly desirable enhanced thermal properties, this new class of fluids can offer tremendous benefits and potential applications in industrial, electronics, and energy fields. Nanofluids can be used for better heat transfer and for other performances of systems or technologies in many engineering areas including transportations (engine cooling or vehicle thermal management), microelectronics, solar energy technologies, micro-electromechanical systems (MEMS), electronics and instrumentations, heat exchangers, heating-ventilating and air-conditioning (HVAC), cooling electronics, microfluidics, defense, medical and many other applications [1,2,6,13,17–22]. Many MEMS and other micro- or mini-electronic systems produce very high heat flux and cooling for desirable performance and durability of such devices is a key technical challenge, which cannot be met by conventional fluids or solid (even micro-sized particles)-fluid mixtures. While inherently poor thermal properties of the conventional fluids greatly limit the cooling performance, other particle (micro-sized)-fluid mixture are not suitable for those devices and cooling systems as they can clog and damage the flow channels besides rapid settling of these particles. With high thermal conductivity nanofluids can meet the cooling demands of such small devices and those high-tech industries [22]. Thus, the impact of nanofluids is great given the heat transfer performance of heat exchangers or cooling devices is vital in many industries. Nanofluids and the technologies using these fluids can also save energy cost of billions of dollars. According to a study [23] if nanofluids improve chiller efficiency by 1%, a saving of 320 billion kWh of electricity or equivalent to 5.5 million barrels of oil per year would be realized in USA alone. An estimation of the potential worldwide market for nanofluids only in heat transfer applications was made to be over 2 billion dollars per year and it is expected to grow further in the future [18]. Despite above-mentioned impacts, potential applications, and extensive research been performed in the last decade, except few areas progresses on nanofluids towards developing them as real value-added materials are rather slow. Most of the research works have been conducted on thermal conductivity of nanofluids and they were found to have considerably higher values compared to their base fluids [5–10,12–16]. However, results obtained from various laboratories are not consistent and there are also controversies on the heat transfer mechanisms of nanofluids [2,6,12,24]. Research on the other key areas such as convective heat transfer of nanofluids are also receiving growing interest in recent years and results are even more promising as nanofluids showed substantially higher heat transfer features (e.g., convective heat transfer coefficient) compared to those of their base fluids. Also, researchers mostly used nanofluids in minichannel-based flow and heat exchangers systems. However, compared to aqueous nanofluids [23,25–27], only a handful convective heat transfer works have been done on EG or EG/W mixture-based nanofluids.
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Having desirable thermal characteristics such as low freezing point, high boiling point, better stability over a wide range of temperatures, and good thermophysical properties ethylene glycol is a very popular antifreeze and heat transfer fluid in numerous commercial and industrial applications. Aqueous mixtures of EG are also commonly used as cooling media due to their higher thermal properties and lower viscosity as well as cost than those of pure EG. Furthermore, nanoparticles are generally better dispersed in EG than in water. Thus researchers widely employed EG and its aqueous mixtures as base fluids for nanofluids and research showed that the heat transfer performances of these nanofluids are significantly higher than those of base EG and EG/W [9,27,28]. Since coining the concept of nanofluids significant research works have been performed and good progress has been made, it is timely and of great importance to provide a state of the art review on the key thermal areas of these nanofluids. A comprehensive review on the research and development of conduction and convection heat transfer of EG and EG/W based nanofluids is provided in this paper. Preparation of nanofluids and their stabilization methods have been discussed and summarized. Literature studies of the influences of nanoparticles type, size and concentration as well as fluids temperature on the thermal conductivity of available EG and EG/W mixture-based nanofluids have been critically reviewed. Convective heat transfer studies of these nanofluids have been analyzed and results from various research groups are also compared. In addition, research works on potential application of nanofluids in energy technologies particularly in solar energy systems have been briefly discussed. 2. Nanofluids preparation One of the major challenges of nanofluids research is to achieve long-term stability of nanofluids, which mostly depends on the preparation of nanofluids. The preparation of nanofluids is widely performed through two ways- one-step and two-step methods. The one-step method is directly synthesizing nanoparticles in base fluids by applying various chemical or physical methods such as chemical vapour deposition. The two-step method is to synthesize or purchase nanoparticles first and then disperse them in a base fluid. Nanoparticles are synthesized by two general approachesbottom-up and top-down approaches. In recent years, nanoparticles production techniques have been improved considerably and wide varieties of nanoparticles (types and shapes) are commercially available and some of them are reasonably cheap as well. Thus, researchers working or dealing with nanofluids mostly used two-step method to prepare their nanofluids. Some other methods are also used to prepare nanofluids and most of them are chemical solution, reaction based-route or other methods [29]. For example, while phase transfer method was used for preparing nanofluids by some researchers [30,31], microfluidic flow system was also employed to synthesis nanofluids (e.g., [32,33]). Besides shortterm stability of prepared nanofluids, the other limitations of these new one-step methods are that they cannot be used to produce large quantity of nanofluids. It is also difficult to control the size distribution of nanoparticles and the purity of the nanofluids by these new techniques. Research advancements in the areas of nanofluids preparation methods, stability mechanisms and other related issues have been well-discussed in the literature [29,34].
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Regardless of preparation methods used, proper dispersion and stabilization of the nanoparticles in base fluids are crucial for nanofluids’ optimum properties and applications. Homogenization of nanoparticles in base fluid is commonly performed using ultrasonication, which is very effective in breaking the larger clusters of nanoparticles into smaller clusters. Thus almost every study employed sonication and/or magnetic stirrer to disperse or breaking agglomerated nanoparticles. However, sonication time can be varied from tens of minutes to several hours depending on nanoparticles concentration, types and size, base fluids as well as power or frequency setting of the sonicator. Studies showed that through the state of dispersion sonication time can influence the thermal properties of nanofluids. Thus special attention should be given in sonication and its duration. In many cases, even sonication was found insufficient to obtain stable and well-dispersed nanofluids and thus various types of dispersant agents or surfactants such as sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS), polyvinyl pyrrolidone (PVP), chitosan, and gum arabic (GA) as well as functionalization or surface-treatment of nanoparticles (e.g., CNT) were also employed in nanofluids [7,8,13,35–44]. Among various conventional fluids, ethylene glycol is the second most popular base fluids for nanofluids after water. Here only EG and its water mixture-based nanofluids and their preparation as well as stability are presented. A summary of various EGbased and surfactant added-nanofluids and their stabilization techniques used in the literature is provided in Table 1. Since all studies employed sonication, this information is not given in Table 1. As mentioned before, Table 1 further confirms that researchers mostly adopted the two-step methods and a wide variety of dispersants were also used for better stability of nanofluids. It is also of great importance to study the morphology of nanoparticles in nanofluids as most of the cases particularly for purchased nanoparticles or two-step preparation the size and shape of the dispersed nanoparticles are very different (mostly larger size and non-uniform shape) from those of the data provided by the company/supplier. There are numerous reasons such as nanoparticle synthesis route, purity, degree of agglomeration, reliability of data provided by the supplier behind such anomalies among actual and company/supplier’s provided morphological information of the nanoparticles. A detailed description and protocol of selection of nanoparticles and nanofluids preparation
Table 1 Various EG-based nanofluids and their stabilization techniques employed in the literature studies. Nanofluids
Preparation Surfactant/surface technique treatment
Researcher
Cu/EG MWCNT/EG TiO2/EG ZnO/EG MWCNT/EG + W TiO2/EG; Al/EG Al2O3/EG + W
One step Two-step Two-step One step Two-step Two-step Two-step
Eastman et al. [35] Assael et al. [36] Ding et al. [28] Kim et al. [37] Chen et al. [38] Murshed et al. [7,8] Timofeeva et al. [39]
ZnO/EG MWCNT/EG
One step Two-step
MWCNT/EG + W Al2O3/EG MWCNT/EG + W TiO2/EG + W Fe3O4/EG Al2O3/EG + W ZnO/EG
Two-step Two step Two-step Two step One step Two-step Two-step
Thioglycolic acid SDS surfactant SDBS surfactant SDS surfactant Surface treatment CTAB surfactant Monovalent acids (nitric, acetic or formic) Ammonium citrate 12-3-12, 2 Br 1 surfactant Functionalization CTAB surfactant Chitosan dispersant Oleic acid and CTAB Citric acid surfactant SDBS surfactant CTAB surfactant
Moosavi et al. [40] Xie and Chen [41] Aravind et al. [42] Murshed [13] Teng and Yu [43] Reddy and Rao [44] Altan et al. [45] Mojarrad et al. [46] Esfe and Saedodin [47]
3
method can be found in a study by Lourenço and Vieira [48] and this will not be elaborated here further.
3. Literature survey on thermal conductivity As mentioned before, extensive experimental and theoretical research works have been conducted on the thermal conductivity of nanofluids. However, results from different researchers are not consistent and sometimes contradictory as well [9,24,49]. For example, while some researchers showed significant influence of temperature in increasing relative thermal conductivity of nanofluids, others found no considerable effect of temperature on the enhancement of this property. Also, for the same nanofluids and at the same concentration of nanoparticle, the enhancements of thermal conductivity reported by different researchers vary widely. The effect of nanoparticle size on the thermal conductivity of nanofluids is also not conclusive. Nevertheless, it is obvious that nanofluids exhibit higher thermal conductivity compared to their base fluids. These can be evidenced from Table 2, which summarizes literature results on thermal conductivity of various types of EG-based nanofluids at room temperature. In addition to reported scattered data of all these nanofluids, Table 2 reveals that researchers mostly used transient hot-wire technique to measure the thermal conductivity. Furthermore, despite large number of theoretical efforts devoted to the development of models for the thermal conductivity of nanofluids no widely accepted model is yet available and the underlying heat transfer mechanisms are not well understood [12]. Here only experimental studies and findings of the thermal conductivity of EG-based nanofluids have been summarized first followed by thermal conductivity of individual nanofluid containing each type of nanoparticle in EG. Available studies on thermal conductivity of EG/W mixture based nanofluids have also been reviewed. 3.1. Overview of EG-based nanofluids The enhancement of thermal conductivity of nanofluids is solely due to the dispersed nanoparticle, which in addition to other properties influences the thermal conductivity of nanofluids. Significant increase in thermal conductivity of EG-based nanofluids is demonstrated in Table 2. As anticipated nanoparticles with higher thermal conductivity such as CNT, diamond, and Cu showed very large enhancement of thermal conductivity even at very small concentration. For instance, Eastman et al. [35] reported about 41% increase in thermal conductivity of EG for loading only 0.28 vol.% of Cu (<10 nm) nanoparticles, Kang et al. [58] found 75% enhancement at 1.32 vol.% concentration of diamond (30–40 nm) nanoparticles, and Aravind et al. [42] observed about 40% enhancement at almost vanishingly small concentration of 0.03 vol.% of MWCNT. Nonetheless, the enhanced thermal conductivity of nanofluids can lead to higher energy efficiency, better performance of heat exchangers as well as any thermal management and energy conversion systems using this new class of fluids. Besides presenting and comparing results of thermal conductivity of available EG-based nanofluids some representative studies and key findings on each type of nanofluids have been briefly summarized in the following sections. 3.2. Al2O3/EG nanofluids Among early studies, Lee et al. [50] measured the effective thermal conductivity of EG-based Al2O3 (38 nm) nanofluids and showed a moderate enhancement of thermal conductivity of this nanofluid. A maximum 18% increase in thermal conductivity of
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Table 2 Summary of representative studies on thermal conductivity of EG-based nanofluids. Nanoparticle (size in nm)
Measurement technique
Thermal conductivity increase (%) over EG at nanoparticle concentration
Researchers
Al2O3 (38) CuO (24) Al2O3 (28) Cu (<10) CuO (35) Al2O3 (60.4) Al2O3 (29) TiO2 (40) TMWCNT (15 nm 30 lm) Fe (10) CuO (12) MWCNT (120) CuO (29) Diamond (30–40) TiO2 (15) Al (80) TiO2 (10 40) Al2O3 (80) SWCNT (1 4) Fe (10) WO3 (38) Al (80) Cu (80) CuO (31) Graphene (-) MWCNT (-) SWCNT (0.8–1.6 500) ZnO (<50) Al2O3 (105) ZnO (48) Fe3O4 (6) CeO2 (30–50) Co3O4 (<50)
Transient hot-wire Method (THWM) Steady-state method (SSM) THWM
18% at 5 vol.% of Al2O3 20% at 4 vol.% of CuO 26% at 5 vol.% 41% at 0.28 vol.% of Cu 22% at 4 vol.% of CuO 30% at 5 vol.% 18% at 4 vol.% of Al2O3 13% at 5 vol.% of TiO2 12.7% at 1 vol.% 18% at 0.55 vol.% 6% at 1 vol.% 21% at 0.6 vol.% 23% at 5 vol.% 75% at 1.32 vol.% 17.5% at 5 vol.% of TiO2 45% at 5 vol.% of Al 20% at 5 vol.% of TiO2 18% at 5 vol.% of Al2O3 20% at 2.5 vol.% 16.5% at 0.3 vol.% of Fe 13.8% at 0.3 vol.% of WO3 18% at 3 vol.% of Al 26% at 3 vol.% of Cu 13% at 3 vol.% of CuO 3–4% at 0.005 vol.% 40% at 0.03 vol.% 14.8% at 0.2 vol.% 40% at 3.7 vol.% 27% at 3 vol.% 33% at 6.1 vol.% Slight decrease at 1.73 wt.% 13% at 1 vol.% 25% at 5.67 vol.% at 30 °C
Lee et al. [50]
THWM SSM THWM THWM
THWM- KD2 Pro Lambda Instruments THWM THWM Transient hot-disk THWM- KD2 Pro Lambda Instrument THWM- KD2 Pro THWM- KD2 Pro
EG was reported for 5 vol.% loading of Al2O3 nanoparticles in their study. The lead author also previously investigated the thermal conductivity of Al2O3 (80 nm)/EG and found about 18% increase in thermal conductivity over base EG at nanoparticle concentration of 5 vol.% [8]. Wang et al. [53] also observed similar thermal conductivity for their Al2O3 (29 nm)/EG-based nanofluids. However, another early study by Wang et al. [51] reported slight higher increase (26%) in thermal conductivity of EG at the same 5 vol.% of Al2O3 (28 nm). Whereas Xie et al. [52] observed even higher (30%) enhancement of thermal conductivity of EG at the same 5 vol.% concentration of larger size (60.4 nm) Al2O3 nanoparticles. Recently, Esfe et al. [69] investigated the effect of concentration of this nanoparticle and temperature on the thermal conductivity of Al2O3 (5 nm)/EG nanofluids. Their results showed significant enhancement of thermal conductivity of nanofluids with increasing Al2O3 concentration and temperature as well. At room temperature a maximum 29% increase in thermal conductivity was reported at 5 vol.% of Al2O3 (5 nm). Results of these studies [8,51–53,69] demonstrated no conclusive effect of nanoparticle size on the thermal conductivity enhancement of this Al2O3/EG nanofluid. Representative results on the enhancement of thermal conductivity of Al2O3/EG nanofluids as a function of volumetric concentration of this nanoparticle at room temperature are presented in Fig. 1 [8,50–52,61,65,67,69–72]. The thermal conductivity enhancement has been denoted with the ratio (or relative) of thermal conductivity of nanofluids (knf) over thermal conductivity of base fluid (kf). The size and type of this nanoparticle used by researchers are also provided in Fig. 1. All these results demonstrate that the enhanced thermal conductivity of this nanofluid increases considerably with increasing concentration of this nanoparticle. It can also be noticed from Fig. 1 that the data from
1.4
Thermal conductivity ratio (knf/kf)
THWM Parallel plateSSM THWM THWM THWM THWM THWM THWM THWM
Wang et al. [51] Eastman et al. [35] Xie et al. [52] Wang et al. [53] Xie et al. [54] Hong et al. [55] Kwak and Kim [56] Assael et al. [36] Liu et al. [57] Kang et al. [58] Murshed et al. [8] Murshed et al. [7] Amrollahi et al. [59] Yoo et al. [60] Patel et al. [61]
Baby and Ramaprabhu [62] Aravind et al. [42] Harish et al. [63] Kole and Dey [64] Longo and Zilio [65] Gallego et al. [66] Altan et al. [45] Mary et al. [67] Mariano et al. [68]
38 nm [50] 28 nm [51] 80 nm [8] α type,15 nm [52]
α type,60.4 nm [52] 11 nm [61] 150 nm [61] 16 nm [70]
2
4
105 nm [65] 5 nm [69] 40-50 nm [71] 15-45 nm (α type) [72]
1.3
1.2
1.1
1.0 0
6
Al2O3 nanoparticle concentration (vol.%) Fig. 1. Enhancement of thermal conductivity of EG-based nanofluids and effect of concentration of Al2O3 nanoparticle.
various research groups are scattered. While some researchers (e.g., [65,72]) showed very high enhancement of thermal conductivity at low concentrations, others (e.g., [8,50,52]) showed moderate increase in thermal conductivity with volumetric loading of this popular nanoparticle. Even for nanoparticles of similar sizes, the thermal conductivity data from different researchers vary considerably (Fig. 1). Effect of temperature on thermal conductivity of this Al2O3/EG nanofluid obtained from literatures has also been presented in Fig. 2 [8,61,65,69,71,72]. It confirms that the temperature also has some positive influence on the relative thermal conductivity of nanofluids (knf/kf). However, the magnitude of
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Al2O3/EG
Thermal conductivity ratio (knf /kf)
1.8
5 vol.%(5 nm) [69] 1.5 vol.%(40-50 nm) [71]
0.5 vol.%(80 nm) [8] 1 vol.%(80 nm) [8] 0.5 vol.%(11 nm) [61] 1 vol.%(11 nm) [61] 0.5 vol.%(150 nm) [61] 3 vol.%(150 nm) [61] 3 vol.%(105 nm) [65] 1 vol.%(5 nm) [69]
1.6
4.8 vol.%(40-50 nm) [71] 0.25 vol.%(15-45 nm) [72] 1 vol.%(15-45 nm) [72]
1.4
1.2
1.0 0
20
40
60
80
100
Temperature (°C) Fig. 2. Effect of temperature on the enhanced thermal conductivity of Al2O3/EG nanofluids.
influence of temperature is smaller than that of the nanoparticle concentration (Figs. 1 and 2). Nevertheless such increase in thermal conductivity with increasing temperature makes nanofluids even more interesting for applications at elevated temperature conditions such as high temperature heat exchangers. 3.3. TiO2/EG nanofluids Fig. 3 presents literature data on the enhancement of thermal conductivity of nanofluids as a function concentration (vol.%) of TiO2 nanoparticle at room temperature [7,53,65,73–75]. It is evidenced from Fig. 3 that this nanofluid exhibits mostly moderate and linear increase (within 20%) in thermal conductivity with increasing concentration of this nanoparticle. Given the smaller thermal conductivity value of TiO2 nanoparticle (about 8 W/m-K) as compared to most of the other types of nanoparticles such as CNT, Al, Cu, Al2O3 and CuO, such thermal conductivity enhancement is very fascinating. The scattered data could be due to different sizes of nanoparticles and nanofluids preparation as well as measurement techniques used by different researchers. Nonetheless, the trends of the reported results are more consistent as compared to Al2O3/EG nanofluids. The influence of nanoparticle volume fraction on the thermal conductivity, thermal diffusivity, as well as specific heat of several EG-based nanofluids including TiO2/EG [13] was previously
3.4. ZnO/EG nanofluids Kim et al. [37] was the first to study the thermal conductivity of ZnO/EG nanofluids that was synthesized directly by one-step
TiO2 (15 nm)/EG [7] TiO2 (10×40 nm)/EG [7]
Thermal conductivity ratio (knf /kf)
Thermal conductivity ratio (knf /kf)
1.25
studied by the lead author. Two different shapes TiO2 were used and the effective thermal conductivity of these nanofluids was found to increase substantially with increasing nanoparticle volume fraction. For a maximum 5 vol.% loading of TiO2 nanoparticles of 15 nm (spherical) and 10 40 nm (cylindrical) in EG, the maximum increases in thermal conductivity were 17.5% and 20%, respectively. Wang et al. [53] also reported similar results for their TiO2 (40 nm)/EG-based nanofluids particularly for 3% and 4 vol.% concentrations of nanoparticles. TiO2 nanofluids with cylindrical nanoparticles show a little higher enhancement in thermal conductivity compared to those with spherical nanoparticles. These indicate that along with the particle volume fraction, particle shape also influences the thermal conductivity of nanofluids. Thermophysical properties of TiO2 (150 nm)/EG-based nanofluids was measured by Longo and Zilio [65]. A transient hot-disk apparatus was used to measure the thermal conductivity of nanofluids as a function of concentration and temperature. The effective thermal conductivity of nanofluids was found to increase with increasing both the concentration of nanoparticle and temperature. Although their nanoparticle size was the largest among others, significant enhancement of thermal conductivity was observed. For example, at room temperature, about 16% increase in thermal conductivity was found at 3 vol.% concentration of this nanoparticle. Recently, Cabaleiro et al. [73] studied thermal conductivity of EG-based two different nanocrystalline (anatase and rutile) TiO2 nanofluids. A transient based direct thermal conductivity-meter (DTC-25) was employed to measure the thermal conductivity at different concentrations of nanoparticles and temperatures. Although the enhanced thermal conductivity of all their TiO2/EG nanofluids increased with increasing loading of nanoparticle, it was almost temperature independent. For 25 wt.% (8.85 vol.%) concentration of tetragonal anatase TiO2 (33 nm), thermal conductivity of EG was increased about 12%. Literature data of the effect of temperature on the thermal conductivity of TiO2/EG nanofluids are depicted in Fig. 4 [65,73–75]. Except a couple of studies, most of the researchers didn’t observed any noticeable influence of temperature on the relative thermal conductivity of this nanofluid (knf/kff). This suggests that the effect of temperature on thermal conductivities of both the nanofluid and base fluid are of similar magnitudes [73–75].
TiO2 (40 nm)/EG [53]
1.20
TiO2 (33 nm, anatase) [73] TiO2 (<100 nm)/EG [74] TiO2 (150 nm)/EG [65]
1.15
TiO2 (<40 nm)/EG [75]
1.10
1.05
1.00
TiO2/EG:
1.25
1 vol.% (<100nm) [74] 4 vol.% (<100nm) [74] 1 vol.% (150 nm) [65] 3 vol.% (150 nm) [65]
1.5 vol.% (33nm,anatase) [73] 4.9 vol.% (33nm,anatase) [73] 0.5 vol.% (<40 nm) [75]
1.20
1.15
1.10
1.05
1.00 0
2
4
6
TiO2 nanoparticle concentration (vol.%) Fig. 3. Enhancement of thermal conductivity of TiO2/EG nanofluids as a function of concentration of nanoparticle.
0
20
40
60
80
100
Temperature (°C) Fig. 4. Effect of temperature on the enhanced thermal conductivity of TiO2/EG nanofluids.
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method. The SDS surfactant was also added for better dispersion and stability of nanofluids. Effect of concentration and size of nanoparticle on the thermal conductivity was investigated. Their results demonstrated considerable increase in thermal conductivity of nanofluids with concentration of ZnO nanofluids. At 3 vol.% of 30 nm and 60 nm nanoparticles the thermal conductivity of EG was reported to increase about 21 and 10%, respectively. They found that the larger the particle size, the smaller the thermal conductivity of nanofluids [37]. Later other researchers also reported similar results. For example, Moosavi et al. [40] showed about 10.5% increase in thermal conductivity of EG at about 3 vol.% of ZnO nanoparticles of 67.2 nm. Suganthi et al. [76] also reported similar increase in thermal conductivity for their ZnO (25– 40 nm)/EG nanofluids as about 24% increase in thermal conductivity at about 3 vol.% of ZnO was observed in their study. Furthermore, with 3 vol.% concentration of larger ZnO nanoparticles (<100 nm) Lee et al. [77] observed about 13% enhancement of thermal conductivity of EG. Yu et al. [78] measured thermal conductivity of ZnO (10 20)/EG nanofluids and found a maximum 26.5% increase in thermal conductivity at about 5 vol.% concentration of this nanoparticle. Whereas, for 3.7 vol.% of ZnO (<50 nm) nanoparticle about 40% enhancement of thermal conductivity of ZnO (<50 nm)/EG was reported by Kole and Dey [64]. A summary of literature results on the thermal conductivity enhancement as a function of concentration of ZnO nanoparticle is presented in Fig. 5 [37,40,64,66,76–80]. All these results demonstrated that the enhanced thermal conductivity of ZnO/EG nanofluids further increased almost linearly with increasing concentration of ZnO at room temperature. Like other nanofluids, the results of this nanofluid are also scattered in some extend. Fig. 6 compares literature results of the effect of temperature on the thermal conductivity of nanofluids over pure EG [40,64,66,76– 78]. Most of the studies showed that the thermal conductivity enhancement of this nanofluid is temperature independent. Interestingly, one study [76] reported considerable negative effect of temperature on the thermal conductivity of their nanofluid at two different concentrations (Fig. 6). 3.5. CNT/EG nanofluids Literature results on the enhancement of thermal conductivity of EG-based CNT nanofluids as a function of CNT concentration are presented in Fig. 7, which clearly shows that the thermal conductivity of this nanofluid increases substantially with increasing the volumetric loading of CNT [36,38,54,59,81–84]. Interestingly,
ZnO(30nm)/EG [37] ZnO(60nm)/EG [37] ZnO(67.2nm)/EG [40] ZnO(<50nm)/EG [64] ZnO(48nm)/EG [66] ZnO(25-40nm)/EG [76] ZnO(<100nm)/EG [77]
1.4
1.6
1 vol.% (25-40 nm) [76] 4 vol.% (25-40 nm) [76] 1 vol.% (<100 nm) [77] 5.5 vol.% (<100 nm) [77] 5 vol.% (10-20 nm) [78]
2 vol.% (67.2 nm) [40] 0.5 vol.% (<50 nm) [64] 3.75 vol.% (<50 nm) [64] 1 vol.% (48 nm) [66] 4.7 vol.% (48 nm) [66]
1.4
1.2
1.0 20
40
60
80
100
Temperature (°C) Fig. 6. Effect of temperature on the enhanced thermal conductivity of ZnO/EG nanofluids.
the hybrid Ag-MWCNT/EG nanofluids showed very high enhancement of thermal conductivity at almost vanishingly small concentration [81]. Since the thermal conductivity CNT is about order (s) magnitude higher as compared to most of other nanoparticles, it is anticipated to have significantly high thermal conductivity enhancement for CNT nanofluids. It was also found that most of the researchers used multi-walled carbon nanotubes (MWCNT). On the other hand, effect of temperature on the relative thermal conductivity of this nanofluid is demonstrated in Fig. 8, which reveals that except results from one study [38], the thermal conductivity of this nanofluid increases almost linearly with increasing temperature [41,59,81]. It is noted that most of the studies used very small concentration of CNT. Xie et al. [54] reported thermal conductivity of treated CNT/EG nanofluids measured by transient hot-wire apparatus. In order to have stable and homogenous suspensions, their nanotubes went through surface treatment process. Results showed considerable increase in the enhanced thermal conductivity of nanofluids with increasing CNT loading. The thermal conductivity of several MWCNT (mean diameter of 120 nm) dispersed nanofluids in different base fluids including EG was experimentally studied by Assael et al. [36]. Different concentrations of SDS surfactant were also added in their EG-based MWCNT nanofluids. Thermal conductivity of these nanofluids
1.30
ZnO(10-20nm)/EG [78] ZnO(50nm)/EG [79] ZnO(-)/EG [80]
Thermal conductivity ratio (knf/kf)
Thermal conductivity ratio (knf /kf)
1.5
ZnO/EG:
Thermal conductivity ratio (knf /kf)
6
1.3
1.2
1.1
1.0
MWCNT/EG [36] MWCNT/EG [38] MWCNT/EG [54] SWCNT/EG [59] Au-MWCNT/EG [81] Ag-MWCNT/EG [81]
1.25
MWCNT/EG [82] MWCNT/EG [83] MWCNT/EG [84]
1.20
1.15
1.10
1.05
1.00
0
1
2
3
4
5
6
ZnO nanoparticle concentration (vol.%) Fig. 5. Enhancement of thermal conductivity of ZnO/EG nanofluids with concentration of nanoparticle.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
CNT volume fraction (%) Fig. 7. Enhancement of thermal conductivity of various CNT/EG nanofluids as a function of CNT concentration.
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1.25
1.25
MWCNT (0.6 vol.%)/EG [38] MWCNT (0.175 wt.%)/EG [41] SWCNT (0.5 vol.%)/EG [59] SWCNT (1 vol.%)/EG [59] Ag-MWCNT (0.012 vol.%)/EG [81]
Thermal conductivity ratio (knf /kf)
Thermal conductivity ratio (knf/kf)
1.30
1.20 1.15 1.10 1.05
CuO(<50 nm)/EG [75] CuO(25-35 nm)/EG [85] CuO(23-37 nm)/EG [86] CuO(29 nm)/EG [87] CuO(31 nm)/EG [61] CuO(12 nm)/EG [56] CuO(35 nm)/EG [35] CuO(24 nm)/EG [50]
1.20
1.15
1.10
1.05
1.00
1.00 20
30
40
50
60
0
70
Temperature (°C)
increased significantly with concentration of MWCNT as a maximum 21% increase in thermal conductivity was observed at only 0.6 vol.% concentration of MWCNT. It was also concluded that the thermal conductivity enhancement was only marginally affected by the quantity of surfactant used. Using a short hot-wire method Chen et al. [38] measured thermal conductivity of treated multi-walled carbon nanotubes (TCNT) dispersed in EG and observed up to 17.5% enhancement of thermal conductivity of this nanofluid at 1 vol.% loading of TCNT. Their nanotubes were treated by using mechanochemical reaction method for better dispersion in EG. They however did not observe any obvious effects of temperature on the thermal conductivity enhancement of their nanofluids. Amrollahi et al. [59] investigated the effects of temperature, volume fraction, and vibration time on the thermo-physical properties of a SWCNT/EG nanofluid. For 2.5 vol.% concentration of SWCNT the thermal conductivity of EG increased up to 20%. They also reported a strong influence of temperature on the thermal conductivity of this nanofluid. In a different study, Jha and Ramaprabhu [81] investigated thermal conductivity of several hybrid nanofluids containing high conductive nanoparticles including Ag, Au and Pd decorated MWCNT in EG. Results showed that the enhanced thermal conductivity of these hybrid nanofluids increase considerably with increasing concentration of metal-MWCNT and temperature as well. At a very small concentration (0.03 vol.%) of Ag-MWCNT and Au-MWCNT, thermal conductivity of EG increased about 11% and 10%, respectively. The sequence of thermal conductivity enhancements of their three metal-MWCNT/EG nanofluids which followed the same sequence of the thermal conductivity value of each metal, was found to be Ag-MWCNT > Au-MWCNT > Pd-MWCNT. Aravind et al. [42] reported significantly higher thermal conductivity enhancement at much lower volumetric concentrations of their functionalized CNT (f-MWCNT). The MWCNT was functionalized for better dispersion and stability. Thermal conductivity of EG increased up to 40% at only 0.03 vol.% loading of f-MWCNT. Such enhancement was attributed to the homogeneous dispersion of CNT in base fluids and formation of more hydrophilic phase.
3.6. CuO/EG nanofluids Fig. 9 compares literature results on the enhancement of thermal conductivity of CuO/EG nanofluid as a function of nanoparticle concentration at room temperature [35,50,56,61,75,85–87]. Like
2
3
4
5
Fig. 9. Enhancement of thermal conductivity of CuO/EG nanofluids as a function of nanoparticle concentration.
other nanofluids, the enhanced thermal conductivity of this nanofluid increases almost linearly with volume % of nanoparticle loading. Compared to other nanofluids, the thermal conductivity enhancement trends of this nanofluid from different researchers are more consistent. Literature results on the temperature dependence of the thermal conductivity of this CuO nanofluid are presented in Fig. 10 [61,85,86]. Although majority of these studies observed an increase in the enhanced thermal conductivity of nanofluid with increasing temperature, one study [85] reported very strange trend of influence of temperature on the thermal conductivity of their CuO nanofluid. The thermal conductivity of their nanofluids at two different concentrations was first decreased with increasing temperature until temperature of around 35 °C and then increased with further increasing the temperature (Fig. 10) [85]. The first study with CuO/EG nanofluids was conducted by Lee et al. [50] who measured the thermal conductivity of nanofluids using transient hot-wire system and a maximum 20% increase in thermal conductivity of EG was reported at 4 vol.% of CuO nanoparticle of about 24 nm size. Two years later the same group leading by Eastman et al. [35] reported about 22% increase in thermal conductivity of EG for the same 4 vol.% of CuO nanoparticles of 35 nm. The thermal conductivity in both studies was measured using a transient hot-wire method. These results indicate that nanofluids with larger size CuO nanoparticles possess higher enhancement of thermal conductivity than that of smaller nanoparticles. 0.5 vol.% CuO(31nm)/EG[61] 3 vol.% CuO(31nm)/EG[61] 1.5 vol.% CuO(23-37nm)/EG[86] 3 vol.% CuO(23-37nm)/EG[86] 0.5 vol.% CuO(25-35nm)/EG[85] 1 vol.% CuO(25-35nm)/EG[85]
1.35
Thermal conductivity ratio (knf /kf)
Fig. 8. Effect of temperature on the enhanced thermal conductivity of CNT/EG nanofluids.
1
CuO nanoparticle concentration (vol.%)
1.30 1.25 1.20 1.15 1.10 1.05 1.00 10
20
30
40
50
60
70
Temperature (°C) Fig. 10. Effect of temperature on the enhanced thermal conductivity of CuO/EG nanofluids.
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However, some researchers like Kim et al. [37] reported exactly opposite effect of nanoparticle size where the smaller the nanoparticles the higher the enhancement of thermal conductivity of nanofluids. For 3 vol.% of CuO (31 nm), Patel et al. [61] showed about 13% increase in thermal conductivity of EG. Liu et al. [87] also found similar enhancement (13.6%) for the same 3 vol.% concentration of similar sized CuO (29 nm) nanoparticle. However, the maximum enhancement of thermal conductivity was about 23% at 5 vol.% of CuO [87]. Barbes et al. [86] measured thermal conductivity of EG based-CuO nanofluids and found 10% increase in thermal conductivity at 3 vol.% concentration of CuO (23–37 nm) nanoparticle. No considerable influence of temperature on the thermal conductivity was also observed in their study. Whereas Patel et al. [61] demonstrated an increase in thermal conductivity with temperature for their CuO/EG nanofluids. On the other hand, a recent study by Zennifer et al. [85] showed that the thermal conductivity of their CuO (25–35 nm)/EG nanofluids increases with loading of nanoparticles but decreases with increasing temperature. However, such decreasing nature of thermal conductivity with temperature is not common and any possible reason for such results was not explained.
3.7. Cu/EG nanofluids Despite high thermal conductivity of Cu nanoparticles, only a few studies have been reported on EG based Cu nanofluids. A summary of the thermal conductivity enhancements of this nanofluid as a function of Cu concentration at room temperature is shown in Fig. 11 [31,35,55,61,88]. At low concentration, this nanofluid showed substantially higher thermal conductivity compared to its base EG and it increases with loading of nanoparticle. An early study by Eastman et al. [35] showed about 41% increase in thermal conductivity of EG for loading only 0.28 vol.% of Cu (<10 nm) nanoparticles. They also used thioglycolic acid as dispersant agent in their nanofluids. However, the enhancement of thermal conductivity was much smaller for their old Cu/EG nanofluid compared to that of the acid added one. Thermal conductivity of larger Cu nanoparticles (200 nm) dispersed nanofluids was reported by Garg et al. [88] and results showed moderate increase in thermal conductivity with increasing the concentration of Cu nanoparticle. Patel et al. [61] measured the thermal conductivity of Cu (80 nm)/EG nanofluids at different concentrations and temperatures. Thermal conductivity was found to increase with increasing concentration and temperature as well. Yu et al. [31] also reported significant enhancement of thermal conductivity of their Cu (5–
10 nm)/EG nanofluids with loading of nanoparticle and temperature as well. A maximum 15% increase in thermal conductivity was observed at only 0.5 vol.% of Cu nanoparticle.
3.8. Other EG-based nanofluids Fig. 12 presents literature data on the enhancement of thermal conductivity of other (except above discussed nanofluids) EGbased nanofluids [8,55,61,67,68,89,90]. As usual, results in Fig. 12 confirm the positive effect of concentrations of these nanoparticles on the enhancement of the thermal conductivity of nanofluids. Since data of various types of nanofluids are presented, they look scattered but the thermal conductivities of all these nanofluids increase with increasing loading of nanoparticles. As anticipated nanofluids having high thermal conductive nanoparticles exhibit relatively higher thermal conductivity compared to nanofluids with low thermal conductive nanoparticles. Findings from some of the representative studies are briefly elaborated here. Literature results on thermal conductivity of various other EG-based nanofluids as a function of temperature are also depicted in Fig. 13, which shows that except CeO2 nanofluids of Mary et al. [67], the enhanced thermal conductivity of all other nanofluids increased with increasing temperature [31,61,89].
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Fig. 12. Enhancement of thermal conductivity of various other EG-based nanofluids as a function of concentration of nanoparticles.
Thermal conductivity ratio (knf/kf)
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CeO2 (30-50 nm)/EG [67] Co3O4 (<50 nm)/EG at 30°C [68] Fe3O4 (15 nm)/EG at 30°C [89] Fe2O3 (29 nm)/EG at 30°C [89] Al (80 nm)/EG [61] Al (80 nm)/EG [8] Fe (10 nm)/EG [55] SiC (26 nm)/EG [90] SiC (600 nm)/EG [90]
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Thermal conductivity ratio (knf/kf)
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0.5 vol.% CeO2 (30-50 nm)/EG[67] 1 vol.% CeO2 (30-50 nm)/EG [67] 5 vol.% Fe2O3 (29 nm)/EG [89] 5.3 vol.% Fe3O4 (15 nm)/EG [89] 0.5 vol.% Cu (5-10 nm)/EG [31] 0.5 vol.% Cu (80 nm)/EG [61] 3 vol.% Cu (80 nm)/EG [61] 0.5 vol.% Al (80 nm)/EG [61] 3 vol.% Al (80 nm)/EG [61]
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Temperature (°C) Fig. 13. Effect of temperature on the enhancement of thermal conductivity of various other EG-based nanofluids.
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1.5
Thermal conductivity ratio (knf/kf)
A relatively large increase in thermal conductivity of Fe (10 nm)/EG-based nanofluids was reported by Hong et al. [55]. At a small concentration of Fe nanoparticle (0.55 vol.%) the thermal conductivity of EG increased 18%. They also concluded that sonication has a significant effect on the thermal conductivity of nanofluids. Yoo et al. [60] measured thermal conductivity of several nanofluids including EG-based Fe and tungsten oxide (WO3) nanofluids. These nanofluids showed large enhancements of thermal conductivity as compared to the base fluid (EG). For instance, Fe (10 nm)/EG nanofluid showed 16.5% increase in thermal conductivity for loading 0.3 vol.% of this nanoparticle while WO3 (38 nm)/EG nanofluid showed 13.8% enhancement for the same 0.3 vol.% WO3 nanoparticles. Thermal conductivity study using WO3/EG nanofluids is of first kind and cannot be compared. The influence of nanoparticle volume fraction on the thermal conductivity of Al (80 nm)/EG nanofluids was previously investigated by the lead author [8]. The thermal conductivity of EG was found to increase up to 45% at 5 vol.% of Al nanoparticle. It was also demonstrated that nanofluids having highly thermal conductive nanoparticles (Al) exhibit much higher thermal conductivity than those of nanofluids with low thermal conductive nanoparticles (e.g., TiO2). For the first time, Baby and Ramaprabhu [62] investigated thermal and electrical conductivity of EG-based f-TEG (functionalized thermally exfoliated graphene) nanofluids. In their study, thermal conductivity was found to increase with increasing concentration of this graphene and temperature as well. A small enhancement (3–4%) in thermal conductivity was reported for adding very small concentration (0.05 vol.%) of f-TEG in EG at room temperature. Sharma et al. [91] synthesized EG-based nanofluids of different concentrations (1000–10,000 ppm) of silver (Ag) nanoparticles and measured their thermal conductivity using a transient hot-wire technique. The thermal conductivity of this nanofluids increased with concentration of Ag nanoparticle. A maximum 18% increase in thermal conductivity of EG was observed at 10,000 ppm concentration of Ag nanoparticle. Gallego et al. [89] prepared two sets of magnetic nanofluids containing Fe2O3 (20 nm) and Fe3O4 (15 nm) nanoparticles dispersed in EG and measured their thermal conductivities as a function of nanoparticles volume fractions and temperature as well. Their results showed that the enhanced thermal conductivities of these nanofluids increase further with volumetric loading of nanoparticles but were temperature independent. The maximum enhancement of thermal conductivity was only 15% for loading 6.6 vol.% of Fe2O3 nanoparticles in EG. Among very few studies, Altan et al. [45] observed no enhancement but slight deterioration of thermal conductivity of their EGbased magnetic (Fe3O4) nanofluids with nanoparticle concentration as well as temperature. At 35 °C, the relative thermal conductivity was about 0.99 (1% decrease) at 1.73 wt.% of Fe3O4 nanoparticle. They added citric acid as stabilization agent in the nanofluids and measured the thermal conductivity using a Flucon GmBH Lambda instrument. Recently, Mary et al. [67] measured thermal conductivity of EGbased ceria (CeO2) nanofluids using a thermal property meter (KD2 device). A maximum 13% increase in thermal conductivity was observed at 1 vol.% concentration of this CeO2 (30–50 nm) nanoparticle. Another very recent study conducted by Mariano et al. [68] showed that thermal conductivity of Co3O4 (<50)/EG nanofluids increased about 25% at 5.67 vol.% of this nanoparticle and at 30 °C. They also used KD2 device to measure the thermal conductivity. This is one of the rare types of nanofluids studied by researchers and it was found promising in heat transfer. However,
CuO(27 nm)/EG+W(50/50 wt%)[94] TiO2(21 nm)/EG+W(40/60 wt%) [44] ZnO(77 nm)/EG+W(60/40 wt.%)[93] Fe3O4(13 nm)/EG+W(60/40 wt%) [95]
Al2O3(20-30 nm)/EG+W(50/50 vol.%) [46] Al2O3(53 nm)/EG+W(60/40 wt.%) [93] Al2O3(36.5 nm)/EG+W(50/50 wt%) [94] Al2O3(45 nm)/EG+W [61] CuO(100 nm)/EG+W(60/40)[92] CuO(29 nm)/EG+W(60/40 wt.%)[93]
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Nanoparticle concentration (vol.%) Fig. 14. Enhancement of thermal conductivity of various EG/W mixture-based nanofluids as a function of concentration of nanoparticles.
the thermophysical properties and dispersion behavior of this nanofluid are not well known. 3.9. EG/W mixture-based nanofluids As mentioned before, in addition to pure EG, its mixture with water (EG/W) at various proportions are also commonly used as base fluid in nanofluids. This is mostly due to using nanofluids in flow and convectional heat exchange applications where relatively more quantity of nanofluids is needed and addition of water in EG also reduces the viscosity and increases the effective thermal conductivity of the mixture. Thus literature studies on thermal conductivity EG/W mixture-based various nanofluids have been reviewed and results are presented in Fig. 14 [44,46,61,92–95]. Similar to all other EG-based nanofluids, results of these EG/W based nanofluids also showed significant enhancement of thermal conductivity, which further increased with increasing the volumetric loading of nanoparticles. It can also be noticed that the enhancements of thermal conductivity are high at low concentrations of nanoparticles. While some researchers showed very linear increase in thermal conductivity of nanofluids up to a relatively higher concentration of nanoparticles (e.g., [92,93]), others used low concentrations of nanoparticles (up to 2 vol.%) and reported high enhancement of thermal conductivity (e.g., [94,95]). Hwang et al. [82] measured thermal conductivity of CuO (35.4 nm)/EG + W nanofluids using transient hot-wire technique and found about 9% increase in thermal conductivity at 1 vol.% of this nanoparticle. In an extensive experimental study, Vajjha and Das [93] measured thermal conductivity of three nanofluids containing Al2O3 (53 nm), CuO (29 nm), and ZnO (77 nm) nanoparticles in EG + W (EG/W = 60/40 by mass) mixture using a steady-state method. Besides concentration of these nanoparticles, effect of temperature on thermal conductivity was also investigated. At room temperature and 6 vol.% of Al2O3, CuO and ZnO nanoparticles the enhancements of thermal conductivities of these nanofluids were 23%, 32%, and 26%, respectively. It shows CuO dispersed nanofluid having superior thermal conductivity than those of other two nanofluids. They also found substantial increase in thermal conductivity of these nanofluids with temperature. Sundar et al. [94] also used the same steady-state apparatus (of P.A. Hilton, U.K.) to measure the thermal conductivity of their EG/W (50/50 by mass) based Al2O3 (36.5 nm) and CuO (27 nm) nanofluids at relatively low concentrations of these nanoparticles (up to 0.8 vol.%). Thermal conductivity of these nanofluids was found to increase with
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
increasing concentration of nanoparticles and temperature as well. At room temperature (25 °C) and 0.8 vol.% concentration, thermal conductivity of their Al2O3 and CuO nanofluids were increased by 13% and 17%, respectively. Similar to Vajjha and Das [93], CuO nanofluid was found to have higher enhancement of thermal conductivity as compared to Al2O3 nanofluid. Juneja and Gangacharyulu [96] investigated the effects of temperature and nanoparticle concentration on the thermo-physical properties (thermal conductivity, viscosity, and density) of Al2O3 (30–40 nm)/EG + W (25/75)-based nanofluids. The concentration of nanoparticles was between 0.1 and 1.0 vol.%. Results showed that thermal conductivity increases with increasing nanoparticles concentration and temperature as well. At room temperature, a maximum enhancement of 6% of thermal conductivity was observed for loading 1.0 vol.% of this nanoparticle. In a convective heat transfer study, Mojarrad et al. [46] measured the thermal conductivity of Al2O3 (20–30 nm)/EG + W (50/50 by volume) nanofluids. SDBS surfactant was added for better dispersion of nanoparticles and results showed small enhancement (4.5%) of thermal conductivity at 0.7 vol.% concentration of this nanoparticle.
4. Studies on convective heat transfer Despite limited numbers of studies been performed, the results of convective heat transfer of nanofluids from the literature are more consistent and conclusive as compared to thermal conductivity data. Nanofluids exhibit significantly higher heat transfer coefficient (HTC) compared to its base fluid and it further increases with increasing the concentration of nanoparticles as well as Reynolds number (Re) [9,23,25–28]. The enhancement of heat transfer coefficient is even more significant at turbulent regime. Thus, it is presumed that nanofluids can perform better heat transfer in any flow conditions as compared to their base fluids which are conventional heat transfer fluids. Nonetheless, a clear understanding of the convective heat transfer mechanisms of nanofluids is also not yet achieved. In an attempt to establish a strong explanation of the reported anomalously high convective heat transfer coefficient of nanofluids, Buongiorno [97] considered seven-slip mechanisms. He concluded that among those seven only Brownian diffusion and thermophoresis are the two most important mechanisms in nanofluids. He also proposed a new convection correlation and claimed that the enhanced laminar flow convective heat transfer
can be attributed to a reduction of viscosity within and consequent thinning of the laminar sublayer. Since not all the studies reported convective heat transfer results with the same convective parameters, literature results are thus presented and compared with different parameters such as Nusselt number (Nu) and convective heat transfer coefficient (h) as a function of concentration of nanoparticles and Reynolds number (Re). A summary of results on convective heat transfer coefficient (Nu) of Al2O3 nanofluids as a function of concentration of this nanoparticle in laminar flow regime is presented in Fig. 15 [46,98–100]. As anticipated, Nu increases with volumetric loading of this nanoparticle. Due to relatively low viscosity, most of the researchers used EG/W mixture instead of pure EG for the convectional heat transfer studies. Fig. 16 presents literature data showing the influence of Re on this heat transfer coefficient (Nu) of Al2O3 nanofluids at laminar regime [46,98–100]. It shows that at low Re (within 1000) the enhancement of Nu was comparatively low and it increased slowly with increasing Re [98,100] while at higher Re the enhancement of Nu is considerably high (Fig. 16). Nonetheless, Re has positive (increasing) impact on the convective heat transfer performance of nanofluids. On the other hand, numerical results of a research group [101] on the influence of Reynolds number on the Nusselt number of nanofluids at turbulent flow regime are presented in Fig. 17. It confirms that Nu of their two nanofluids increased lin100
5 vol.% Al 2O3/EG[98] 0.7 vol.% Al 2O3/EG+W(50/50 vol.)[46] 1 vol.% Al 2O3/EG (single phase)[99]
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20x103
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Re Fig. 17. Effect of Reynolds number on Nusselt number of nanofluids at turbulent flow regime.
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4 vol.% SiO2/EG+W(60/40 wt.)[102]
0.3 vol.% CuO/EG[106]
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early with the Re or flow rate at turbulent flow conditions and CuO nanofluid was found to have higher Nu compared to SiO2 nanofluids. Fig. 18 demonstrates the effect of nanoparticle concentration on the enhanced convective heat transfer coefficient (h) of different nanofluids in laminar to turbulent regimes [102–104]. The relative h, which is h of nanofluids (hnf) over corresponding base fluid (hf), represents the enhancement (hnf/hf > 1) or deterioration (hnf/hf < 1) of the convection heat transfer. The convective heat transfer performance of all these nanofluids (hnf) increased considerably with increasing concentration of nanoparticles in all flow regimes (Fig. 18). Literature results on the heat transfer coefficient (h) of various nanofluids as a function of Re are presented in Fig. 19 [102,103,105,106]. It can be evidenced that h of these nanofluids increased slightly non-linearly (except data from [105]) with increasing Re (i.e., flow rate for fixed system) and the higher the concentration of nanoparticles the larger the hnf. Recently, Sivakumar et al. [106] studied heat transfer characteristics of microchannel heat sink using different nanofluids including CuO (15 nm)/EG in laminar flow condition. The heat transfer coefficient (h) of this nanofluid increased surprisingly high with increasing Re. For example, at 0.3 vol.% of CuO nanoparticle hnf increased from 4050 to 42,000 W/m2 K for increasing Re from 100 to 1300 (Fig. 19 depicts data up to Re of 900 of this nanofluid). The first numerical study on the forced convection heat transfer and friction factor of water and EG-based Al2O3 nanofluids at laminar and turbulent regimes was performed by Maiga et al. [104]. Their results showed that both the heat transfer coefficient and friction factor increased with increasing volume fraction of this nanoparticle. They also found that Al2O3/EG nanofluids exhibited larger enhancement in heat transfer coefficient than Al2O3/W nanofluids. The enhancement was more significant with increasing Re and eventually at the turbulent flow regime (Fig. 18). Forced convective heat transfer of several aqueous and EGbased TiO2 (20 nm) nanofluids flowing in vertically oriented straight copper tube was experimentally investigated by Ding et al. [28]. Although aqueous-based nanofluids showed significant enhancement of convective heat transfer coefficient, it surprisingly deteriorates for EG based nanofluids at low Re. They presumed that high viscosity of EG could be responsible for such deterioration of heat transfer coefficient. In a comprehensive study, Vajjha et al. [102] experimentally determined convective heat transfer and friction factor of three nanofluids containing Al2O3 (45 m), CuO (29 nm) and SiO2 (three sizes – 20 nm, 50 nm and 100 nm) in EG/W mixture (60/40 by mass)
0
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Re Fig. 19. Effect of Reynolds number on convective heat transfer coefficient (h) of various EG/W mixture-based nanofluids.
in turbulent regime. Based on their experimental results they also developed correlations for convective heat transfer of these nanofluids. Results showed significantly high convective heat transfer performance of these nanofluids compared to base fluid (EG/W) and the heat transfer coefficient (HTC) increases further with increasing concentration as well as Reynolds number (Fig. 19). For example, the h of Al2O3 (45 m) nanofluid increased substantially and almost linearly with increasing loading of this nanoparticle and a maximum 120% increase in h was reported for 10 vol.% concentration of Al2O3 nanoparticle in EG/W mixture (Fig. 18). Timofeeva et al. [107] measured convective heat transfer coefficient of EG/W (50/50 by volume)-based SiC (four sizes between 16 and 90 nm) nanofluids at turbulent flow condition. With increasing flow velocity the Nu of these nanofluids increased considerably. When all other parameters are same, heat transfer coefficients of nanofluids increased with increasing nanoparticle sizes. The smaller particles (16 nm and 29 nm) gave lower heat transfer coefficients than those for the base EG/W. They also demonstrated that adding SiC nanoparticles significantly improve the cooling efficiency of EG/W mixture. Aravind et al. [42] conducted an experimental investigation on convective heat transfer with aqueous and EG-based functionalized CNT (f-MWCNT)-nanofluids and found almost the same trend of the heat transfer coefficient profiles for their f-MWCNT/EG and f-MWCNT/W nanofluids. For a CNT concentration of only 0.03 vol.% and flow rate of 56 mL/s, the maximum increase in convective heat transfer coefficient of MWCNT/EG-based nanofluids was as high as 180%. In a different study, Putra et al. [80] experimentally determined the thermal performance of screen mesh wick heat pipes with different nanofluids comprising of Al2O3, TiO2 and ZnO nanoparticles in EG and W. The concentration of the nanoparticles was varied from 1 to 5 vol.% and nanofluids were deposited as a coating on the screen mesh in the heat pipes. However, the coating was found to be advantageous since it produced good capillary structures and therefore higher thermal performance. The screen mesh wick heat pipe with the best performance was resulted with Al2O3/W nanofluid at 5 vol.% concentration. In an intertube falling-film flow system, Ruan and Jacobi [108] studied convective heat transfer characteristics of MWCNT/EG nanofluids. With an increased concentration of CNT, the heat transfer coefficients of nanofluids were decreased monotonously and the maximum deviation with those of pure EG was around 26%. However, at concentration of 0.5 wt.% and any Reynolds number the falling-film HTC of nanofluids were larger than the base EG and the enhancement of HTC was up to 20% at Re = 5.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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Experiments on heat transfer characteristics of EG/W mixture (30/70 by volume)-based MWCNT nanofluid in a tubular heat exchanger were performed by Kumaresan et al. [109]. The convective heat transfer coefficient of this nanofluid was found to increase to a maximum of 160% at 0.45 vol.% of MWCNT. They ascribed several factors such as nanotube rearrangement and high aspect ratio as well as delay in boundary layer development for the observed heat transfer enhancement of their nanofluid. Hamid et al. [110] experimentally investigated convective heat transfer of TiO2/W/EG mixture at Re range of 2000 < Re < 10,000. Results showed that the Nu of the nanofluid increases with Reynolds number at 1.5 vol.% concentration of TiO2. The heat transfer coefficient (h) was increased by 2.1% for 1.5 vol.% of TiO2 nanoparticle. For 0.5% and 1.0% volumetric concentrations and at temperature of 30 °C, no enhancement of heat transfer was found for the fluid flow under transition regime. In the presence of electric field, a natural convection heat transfer study of Fe3O4/EG nanofluids was conducted by Asadzadeh et al. [111]. An increase in heat transfer up to 0.02 vol.% of nanoparticle was reported. The enhancement of heat transfer was also found to increase with electric field intensity while decreased with Rayleigh number. Huminic and Huminic [112] performed a numerical analysis of laminar flow heat transfer of CuO/EG nanofluids in a flattened tube. The convective heat transfer coefficient of nanofluid was found to be higher than that of pure EG. Also at any Reynolds number and concentration of nanoparticle the heat transfer coefficient for flattened tube was larger than those of the elliptic and circular tubes. In order to evaluate the cooling performance nanofluids have also been used in commercial automotive radiators in which circulating fluids is cooled mainly by forced convection. For instance, Peyghambarzadeh et al. [113] employed EG and EG/W mixturebased Al2O3 nanofluids in a car radiator and showed that these nanofluids substantially increase (up to 40% compared to base fluids) the cooling performance of the radiator. Later Vajjha et al. [114] compared the numerically obtained performance of EG/W mixture based Al2O3 and CuO nanoparticles in flat tubes of a radiator. They reported about 36.6% and 49.7%, enhancements of the average heat transfer coefficient of EG/W mixture for adding 3 vol.% concentration of Al2O3 and CuO nanofluids, respectively. Nieh et al. [115] also reported an improvement of the heat dissipation and efficiency of the radiator cooling systems using EG/W (50/50) TiO2 nanofluids. For 0.2% volumetric loading of TiO2 they found maximum enhanced ratios of heat dissipation capacity and efficiency factor to be 25.6% and 27.2%, respectively. Results of all these studies [113–115] demonstrate that EG and EG/W based nanofluids are better coolants for commercial radiator systems as compared to their base fluids. Very recently, an experimental study on the convective heat transfer of Al2O3/EG nanofluid in a corrugated tube fitted with twisted tapes and under turbulent flow conditions was reported by Mohammadiun et al. [116]. The utilization of both twisted tapes and nanofluids increased the heat transfer and friction factor of the tube. The maximum thermal performance factor was 4.2 for twist ratio of 2 and at 0.5 vol.% concentration of Al2O3 nanoparticle. However, the heat transfer performance was weakened at high concentration of nanoparticle. Although a couple of studies reported deterioration in convective heat transfer of EG or EG/W based nanofluids mostly at high concentration of nanoparticles (e.g., [28]), most of the above reported studies demonstrate that the performance of nanofluids in convection heat transfer is better than that of the base fluid. Since the practical applications of nanofluids as heat transfer fluids are mainly in flowing systems, having such high convective heat transfer coefficients nanofluids can be considered as the advanced coolants for flow-based cooling systems such as minichannels,
microchannels, and miniaturized heat exchangers. Several studies also revealed that the cooling performance of automobile radiators can be improved by using nanofluids. Also, most of the convective heat transfer studies used Al2O3 nanoparticles either in EG or EG/W mixture. Thus other types of nanoparticles need to be investigated in more systematic convective heat transfer studies. Furthermore, the mechanisms behind the observed convection heat transfer performance of nanofluids need to be elucidated. 5. Nanofluids in energy applications Because of the enhanced conduction and convection heat transfer characteristics and other thermophysical properties nanofluids offer great potential in energy related applications particularly in solar energy systems. Nanofluids also provide several key advantages in solar power plants including direct absorption of solar energy, high absorption in the solar range and low emittance in the infrared, enhanced convection and radiation heat transfer, and enhancing absorption efficiency by changing the size, type and concentration of nanoparticles [117]. Thus, in recent years, numbers of studies have been reported to evaluate the performance and applicability of nanofluids in solar energy storage and conversions by various means such as solar collectors [117–126]. Most of these studies demonstrated that nanofluids can considerably improve the efficiency of solar energy systems. For instances, Taylor et al. [117] revealed that the power tower solar collectors can be benefited from the potential efficiency improvements due to using a nanofluid as working fluid. They conducted experiments on a laboratory-scale nanofluid dish receiver and reported up to 10% increase in efficiency relative to a conventional fluid. It was also shown that such increase in efficiency can lead to generate huge amount of revenue from solar thermal power tower by incorporating a nanofluid. Shin and Banerjee [119] showed nanofluids with their higher thermal conductivity and heat capacity can remarkably enhance the capability and performance of solar thermal power plants. While Lenert and Wang [120] demonstrated that the efficiency of a concentrated solar power collector can be increased by >35% with nanofluids, Yousefi et al. [121] found about 28.3% increase in efficiency of flat-plate solar collectors using MWCNT nanofluids. Although most of the studies used aqueous nanofluids, only a handful of efforts have been reported on employing EG and EG/ W-based nanofluids in energy applications. These nanofluids were also found to enhance the thermal performance and efficiency of those energy technologies [127,128]. For example, Mercatelli et al. [127] investigated the potentiality of using single-wall carbon nanohorns (SWCNHs)/EG naofluid in solar collectors and demonstrated that this SWCNH-nanofluid is very promising for direct sunlight absorbers for thermal solar energy exploitation. Tang et al. [128] also showed that SiO2/MWCNT/PEG (poly (ethylene glycol)) composite nanofluid can effectively improve the efficiency of solar energy applications and this nanofluid can be applied in energy storage systems as well. Nevertheless, more studies are to be performed in order to evaluate the commercial applicability of nanofluids in energy technologies. 6. Conclusions Research progress and findings on both conduction (thermal conductivity) and convection heat transfer of EG and EG/W mixture-based nanofluids have extensively been reviewed in this paper. One of the major challenges of nanofluids research is to achieve long-term stability of nanofluids which mostly depends on the
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
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preparation methodology of nanofluids. Survey on nanofluids preparation reveals that due to commercially availability of wide varieties of nanoparticles and easiest option, most of the researchers used two-step methods (i.e., suspensions of purchased nanoparticles in base fluids) for preparing their nanofluids. Almost every study employed sonication to disperse nanoparticles. However, sonication time in different studies varied from tens of minutes to several hours. Sonication and its duration must be carefully and precisely performed as long-time sonication can evaporate the base fluids and can also damage the nanoparticle structures particularly for non-spherical particles. Researchers also added various types of dispersants for better stability. The preparation and stability of nanofluids are of great importance for their optimum properties as well as for a wide range of applications. Majority of the literature studies focused on the thermal conductivity of nanofluids. Like other nanofluids, EG-based nanofluids also exhibit higher thermal conductivity compared to their base fluid and it further increases substantially with increasing loading of nanoparticles. However, most of the results from different researchers are scattered. While some researchers showed very high enhancement of thermal conductivity at low concentrations, others reported moderate increase in this thermal property. Even for nanoparticles of similar size and concentration, the thermal conductivity data from different researchers vary considerably. The effect of nanoparticle size on the thermal conductivity of nanofluids is also not conclusive as some studies found that nanofluids with larger size nanoparticles possess higher enhancement of thermal conductivity than that of smaller nanoparticles. Whereas, others reported that the smaller the nanoparticles the higher the enhancement of the thermal conductivity. EG/W-based nanofluids also showed similar enhancement of their thermal conductivity. Compared to volume of studies on thermal conductivity, limited research efforts have been made on convective heat transfer of these nanofluids. However, most of the researchers used EG/W mixture as base fluids instead of pure EG. This is mainly due to keeping the viscosity of nanofluids relatively low and to have higher conduction as well. Review demonstrates that these nanofluids exhibit enhanced convectional heat transfer performance (coefficient) which further increases significantly with increasing the concentration of nanoparticles as well as Reynolds number or flow rate. The convective heat transfer is even more pronounced at turbulent flow regime. Despite the fact that couple of studies reported deterioration of the convective heat transfer, nanofluids can perform better heat transfer as compared to their base fluids in any flow conditions. Nonetheless, a clear understanding of the convective heat transfer mechanisms of nanofluids is also not yet achieved. Thus more systematic experimental and analytical studies are to be performed on characterization of these thermal properties and features of these nanofluids. Studies also revealed that nanofluids exhibit enhanced efficiency and performance in energy storage and conversion systems as compared to conventional fluids. In addition to properties characterization it is also of great importance to conduct more rigorous investigations on the use of nanofluids in various energy- based applications. Acknowledgment This work was supported by the Fundação para a Ciência e Tecnologia (FCT), Portugal through grants SFRH/BPD/102518/2014 and PEst-OE/QUI/UI0100/2013. References [1] Das SK, Choi SUS, Yu W, Pradeep T. Nanofluids: science and technology. New Jersey: Wiley; 2008.
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[2] Murshed SMS, Nieto de Castro CA. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [3] Murshed SMS, Leong KC, Yang C. Enhanced thermal conductivity of TiO2water based nanofluids. Int J Therm Sci 2005;44:367–73. [4] Murshed SMS, Leong KC, Yang C. Determination of the effective thermal diffusivity of nanofluids by the double hot-wire technique. J Phys D: Appl Phys 2006;39:5316–22. [5] Yu W, France DM, Routbort JL, Choi SUS. Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat Transf Eng 2008;29:432–60. [6] Murshed SMS, Leong KC, Yang C. Thermophysical and electrokinetic properties of nanofluids – a critical review. Appl Therm Eng 2008;28:2109–25. [7] Murshed SMS, Leong KC, Yang C. Heat transfer of nanoparticle suspensions (nanofluids). In: Lombardi SL, editor. Nanoparticles: new research. New York: Nova Science Publishers; 2008. [8] Murshed SMS, Leong KC, Yang C. Investigations of thermal conductivity and viscosity of nanofluids. Int J Therm Sci 2008;47:560–8. [9] Özerinç S, Kakaç S, Yazıcıog˘lu AG. Enhanced thermal conductivity of nanofluids: a state-of-the-art review. Microfluid Nanofluid 2010;8:145–70. [10] Murshed SMS, Leong KC, Yang C. Thermophysical properties of nanofluids. In: Sattler KD, editor. Handbook of nanophysics: nanoparticles and quantum dots. Boca Raton: Taylor & Francis; 2010. [11] Murshed SMS. Determination of effective specific heat of nanofluids. J Exp Nanosci 2011;6:539–46. [12] Kleinstreuer C, Feng Y. Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review. Nanoscale Res Lett 2011;6:229. [13] Murshed SMS. Simultaneous measurement of thermal conductivity, thermal diffusivity, and specific heat of nanofluids. Heat Transf Eng 2012;33:722–31. [14] Murshed SMS, Nieto de Castro CA. Superior thermal features of carbon nanotubes based nanofluids – a review. Renew Sustain Energy Rev 2014;37:155–67. [15] Murshed SMS, Nieto de Castro CA. Thermophysical properties and heat transfer characteristics of carbon nanotubes dispersed nanofluids. In: Murshed SMS, Nieto de Castro CA, editors. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [16] Younes H, Christensen G, Li D, Hong H, Ghaferi AA. Thermal conductivity of nanofluids: review. J Nanofluid 2015;4:107–32. [17] Choi SUS, Zhang ZG, Keblinski P. Nanofluids. In: Nalwa HS, editor. Encyclopedia of nanoscience and nanotechnology. Los Angeles: American Scientific Publishers; 2004. [18] Wen D, Lin G, Vafaei S, Zhang K. Review of nanofluids for heat transfer applications. Particuology 2009;7:141–50. [19] Murshed SMS, Tan SH, Nguyen NT, Wong TN, Yobas L. Microdroplet formation of water and nanofluids in heat-induced microfluidic T-junction. Microfluid Nanofluid 2009;6:253–9. [20] Wong KV, De Leon O. Applications of nanofluids: current and future. Adv Mech Eng 2010;2010:519659. [21] Saidur R, Leong KY, Mohammad HA. A review on applications and challenges of nanofluids. Renew Sustain Energy Rev 2011;15:1646–68. [22] Murshed SMS, Nieto de Castro CA. Nanofluids as advanced coolants. In: Mohammad A, Inamuddin, editors. Green solvents I: properties and applications in chemistry. London: Springer; 2012. [23] Mohammed HA, Al-aswadi AA, Shuaib NH, Saidur R. Convective heat transfer and fluid flow study over a step using nanofluids: a review. Renew Sustain Energy Rev 2011;15:2921–39. [24] Murshed SMS. Correction and comment on ‘‘Thermal conductance of nanofluids: is the controversy over?”. J Nanopart Res 2009;11:511–2. [25] Murshed SMS, Nieto de Castro CA, Lourenço MJV, Lopes MLM, Santos FJV. A review of boiling and convective heat transfer with nanofluids. Renew Sustain Energy Rev 2011;15:2342–54. [26] Murshed SMS, Nieto de Castro CA. Forced convective heat transfer of nanofluids in minichannel. In: Ahsan A, editor. Two phase flow, phase change and numerical modeling. Rijeka: INTECH; 2011. [27] Sundar LS, Singh MK. Convective heat transfer and friction factor correlations of nanofluids in a tube and with inserts: a review. Renew Sustain Energy Rev 2013;20:23–35. [28] Ding Y, Chen H, He Y, Lapkin A, Yeganeh M, Siller L, et al. Forced convective heat transfer of nanofluids. Adv Powder Technol 2007;18:813–24. [29] Wang L, Fan J. Nanofluids research: key issues. Nanoscale Res Lett 2010;5:1241–52. [30] Feng X, Ma H, Huang S, Pan W, Zhang X, Tian F, et al. Aqueous-organic phasetransfer of highly stable gold, silver, and platinum nanoparticles and new route for fabrication of gold nanofilms at the oil/water interface and on solid supports. J Phys Chem B 2006;110:12311–7. [31] Yu W, Xie H, Chen L, Li Y. Investigation on the thermal transport properties of ethylene glycol-based nanofluids containing copper nanoparticles. Powder Technol 2010;197:218–21. [32] Boleininger J, Kurz A, Reuss V, Sönnichsen C. Microfluidic continuous flow synthesis of rod-shaped gold and silver nanocrystals. Phys Chem Chem Phys 2006;8:3824–7. [33] Wei X, Wang L. Synthesis and thermal conductivity of microfluidic copper nanofluids. Particuology 2010;8:262–71. [34] Yu W, Xie H. A review on nanofluids: preparation, stability mechanisms, and applications. J Nanomater 2012;2012:435873.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
14
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx
[35] Eastman JA, Choi SUS, Li S, Yu W, Thompson LJ. Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles. Appl Phys Lett 2001;78:718–20. [36] Assael MJ, Metaxa IN, Kakosimos K, Constantinou D. Thermal conductivity of nanofluidsexperimental and theoretical. Int J Thermophys 2006;27:999–1016. [37] Kim SH, Choi SR Kim D. Thermal conductivity of metal-oxide nanofluids: particle size dependence and effect of laser irradiation. J Heat Transf 2007;129:298–307. [38] Chen L, Xie H, Li Y, Yu W. Nanofluids containing carbon nanotubes treated by mechanochemical reaction. Thermochim Acta 2008;477:21–4. [39] Timofeeva EV, Routbort JL, Singh D. Particle shape effects on thermophysical properties of alumina nanofluids. J Appl Phys 2009;106:014304. [40] Moosavi M, Goharshadi EK, Youssefi A. Fabrication, characterization, and measurement of some physicochemical properties of ZnO nanofluids. Int J Heat Fluid Flow 2010;31:599–605. [41] Xie H, Chen L. Review on the preparation and thermal performances of carbon nanotube contained nanofluids. J Chem Eng Data 2011;56:1030–41. [42] Aravind SSJ, Baskar P, Baby TT, Sabareesh RK, Das S, Ramaprabhu S. Investigation of structural stability, dispersion, viscosity, and conductive heat transfer properties of functionalized carbon nanotube based nanofluids. J Phys Chem C 2011;115:16737–44. [43] Teng T-P, Yu C-C. Heat dissipation performance of MWCNTs nano-coolant for vehicle. Exp Therm Fluid Sci 2013;49:22–30. [44] Reddy MCS, Rao VV. Experimental studies on thermal conductivity of blends of ethylene glycol-water-based TiO2 nanofluids. Int Commun Heat Mass Transf 2013;46:31–6. [45] Altan CL, Gurten B, Sommerdijk NAJM, Bucak S. Deterioration in effective thermal conductivity of aqueous magnetic nanofluids. J Appl Phys 2014;116:224904. [46] Mojarrad MS, Keshavarz A, Ziabasharhagh M, Raznahan MM. Experimental investigation on heat transfer enhancement of alumina/water and alumina/ water–ethylene glycol nanofluids in thermally developing laminar flow. Exp Therm Fluid Sci 2014;53:111–8. [47] Esfe MH, Saedodin S. Experimental investigation and proposed correlations for temperature-dependent thermal conductivity enhancement of ethylene glycol based nanofluid containing ZnO nanoparticles. J Heat Mass Transf Res 2014;1:47–54. [48] Lourenço MJV, Vieira SI. Nanofluids preparation methodology. In: Murshed SMS, Nieto de Castro CA, editors. Nanofluids: synthesis, properties and applications. New York: Nova Science Publishers; 2014. [49] Thomas S, Sobhan CBP. A review of experimental investigations on thermal phenomena in nanofluids. Nanoscale Res Lett 2011;6:377. [50] Lee S, Choi SUS, Li S, Eastman JA. Measuring thermal conductivity of fluids containing oxide nanoparticles. J Heat Transf 1999;121:280–9. [51] Wang X, Xu X, Choi SUS. Thermal conductivity of nanoparticle–fluid mixture. J Thermophys Heat Transf 1999;13:474–80. [52] Xie H, Wang J, Xi T, Liu Y, Ai F, Wu Q. Thermal conductivity enhancement of suspensions containing nanosized alumina particles. J Appl Phys 2002;9:4568–70. [53] Wang Y, Fisher TS, Davidson JL, Jiang L. Thermal conductivity of nanoparticle suspensions. In: 8th AIAA/ASME joint thermophys heat transfer conf, June 24–26, Missouri, USA; 2002. [54] Xie H, Lee H, Youn W, Choi M. Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities. J Appl Phys 2003;94:4967–71. [55] Hong KS, Hong TK, Yang HS. Thermal conductivity of Fe nanofluids depending on the cluster size of nanoparticles. Appl Phys Lett 2006;88:031901. [56] Kwak K, Kim C. Viscosity and thermal conductivity of copper oxide nanofluid dispersed in ethylene glycol. Korea-Austr Rheol J 2005;17:35–40. [57] Liu MS, Lin MC-C, Huang IT, Wang C-C. Enhanced thermal conductivity with CuO for nanofluids. Chem Eng Technol 2006;29:72–7. [58] Kang HU, Kim SH, Oh JM. Estimation of thermal conductivity of nanofluid using experimental effective particle volume. Exp Heat Transf 2006;19:181–91. [59] Amrollahi A, Hamidi AA, Rashidi AM. The effects of temperature, volume fraction and vibration time on the thermo-physical properties of a carbon nanotube suspension (carbon nanofluid). Nanotechnology 2008;19:315701. [60] Yoo D, Hong K, Yang H. Study of thermal conductivity of nanofluids for the application of heat transfer fluids. Thermochim Acta 2007;455:66–9. [61] Patel HE, Sundararajan T, Das SK. An experimental investigation into the thermal conductivity enhancement in oxide and metallic nanofluids. J Nanopart Res 2010;12:1015–31. [62] Baby TT, Ramaprabhu S. Investigation of thermal and electrical conductivity of graphene based nanofluids. J Appl Phys 2010;108:124308. [63] Harish S, Ishikawa K, Einarsson E, Aikawa S, Chiashi S, Shiomi J, et al. Enhanced thermal conductivity of ethylene glycol with single-walled carbon nanotube inclusions. Int J Heat Mass Transf 2012;55:3885–90. [64] Kole M, Dey TK. Effect of prolonged ultrasonication on the thermal conductivity of ZnO– ethylene glycol nanofluids. Thermochim Acta 2012;535:58–65. [65] Longo GA, Zilio C. Experimental measurements of thermophysical properties of Al2O3– and TiO2–ethylene glycol nanofluids. Int J Thermophys 2013;34:1288–307. [66] Gallego MJP, Lugo L, Cabaleiro D, Legido JLL, Pineiro MMM. Thermophysical profile of ethylene glycol-based ZnO nanofluids. J Chem Thermodyn 2014;73:23–30.
[67] Mary EEJ, Suganthi KS, Manikandan S, Anusha N, Rajan KS. Cerium oxide ethylene glycol nanofluids with improved transport properties: preparation and elucidation of mechanism. J Taiwan Inst Chem Eng 2015;49:183–91. [68] Mariano A, Gallego MJP, Lugo L, Mussari L, Piñeiro MM. Co3O4 ethylene glycol-based nanofluids: thermal conductivity, viscosity and high pressure density. Int J Heat Mass Transf 2015;85:54–60. [69] Esfe MH, Karimipour A, Yan WM, Akbari M, Safaei MR, Dahari M. Experimental study on thermal conductivity of ethylene glycol based nanofluids containing Al2O3 nanoparticles. Int J Heat Mass Transf 2015;88:728–34. [70] Beck MP, Yuan Y, Warrier P, Teja AS. The effect of particle size on the thermal conductivity of alumina nanofluids. J Nanopart Res 2009;11:1129–36. [71] Gallego MJP, Lugo L, Legido JL, Pineiro MM. Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids. Nanoscale Res Lett 2011;6:221. [72] Gangwar J, Srivastava AK, Tripathi SK, Wan M, Yadav RR. Strong enhancement in thermal conductivity of ethylene glycol-based nanofluids by amorphous and crystalline Al2O3 nanoparticles. Appl Phys Lett 2014;105:063108. [73] Cabaleiro D, Nimo J, Gallego MJP, Piñeiro MM, Legido JL, Lugo L. Thermal conductivity of dry anatase and rutile nano-powders and ethylene and propylene glycol-based TiO2 nanofluids. J Chem Thermodyn 2015;83:67–76. [74] Lee GJ, Kim CK, Lee MK, Rhee CK. Characterization of ethylene glycol based TiO2 nanofluid prepared by pulsed wire evaporation (PWE) method. Rev Adv Mater Sci 2011;28:126–9. [75] John T, Krishnakumar TS. Experimental studies of thermal conductivity, viscosity and stability of ethylene glycol nanofluids. Int J Innov Res Sci Eng Technol 2013;2:611–7. [76] Suganthi KS, Vinodhan VL, Rajan KS. Heat transfer performance and transport properties of ZnO–ethylene glycol and ZnO–ethylene glycol–water nanofluid coolants. Appl Energy 2014;135:548–59. [77] Lee GJ, Kim CK, Lee MK, Rhee CK, Kim S, Kim C. Thermal conductivity enhancement of ZnO nanofluids using a one-step physical method. Thermochim Acta 2012;542:24–7. [78] Yu W, Xie H, Chen L, Li Y. Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid. Thermochim Acta 2009;491:92–6. [79] Li H, Wang L, He Y, Hu Y, Zhu J, Jiang B. Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids. Appl Therm Eng 2015;88:363–8. [80] Putra N, Septiadi WN, Rahman H, Irwansyah R. Thermal performance of screen mesh wick heat pipes with nanofluids. Exp Therm Fluid Sci 2012;40:10–7. [81] Jha N, Ramaprabhu S. Thermal conductivity studies of metal dispersed multiwalled carbon nanotubes in water and ethylene glycol based nanofluid. J Appl Phys 2009;106:084317. [82] Hwang YJ, Park HS, Lee JK, Jun WH. Thermal conductivity and lubrication characteristics of nanofluids. Curr Appl Phys 2006;6S1:e67–71. [83] Xie H, Chen L. Adjustable thermal conductivity in carbon nanotube nanofluids. Phys Lett A 2009;373:1861–4. [84] Liu M-S, Lin MC-C, Huang I-T, Wang C-C. Enhancement of thermal conductivity with carbon nanotube for nanofluids. Int Commun Heat Mass Transf 2005;32:1202–10. [85] Zennifer MA, Anikandan S, Sughanti KS, Vinodhan VL, Rajan KS. Development of CuO-ethylene glycol nanofluids for efficient energy management: assessment of potential for energy recovery. Energy Convers Manage 2015;105:685–96. [86] Barbes B, Paramo R, Blanco E, Casanova C. Thermal conductivity and specific heat capacity measurements of CuO nanofluids. J Therm Anal Calorim 2014;115:1883–91. [87] Liu M-S, Lin MC-C, Wang C-C. Enhancements of thermal conductivities with Cu, CuO, and carbon nanotube nanofluids and application of MWNT/water nanofluid on a water chiller system. Nanoscale Res Lett 2011;6:297. [88] Garg J, Poudel B, Chiesa M, Gordon JB, Ma JJ, Wang JB, et al. Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid. J Appl Phys 2008;103:074301. [89] Gallego MJP, Lugo L, Legido JL, Pineiro MM. Enhancement of thermal conductivity and volumetric behavior of FexOy nanofluids. J Appl Phys 2011;110:014309. [90] Xie H, Wang J, Xi T, Liu Y. Thermal conductivity of suspensions containing nanosized SiC particles. Int J Thermophys 2002;23:571–80. [91] Sharma P, Baek I-H, Cho T, Park S, Lee KB. Enhancement of thermal conductivity of ethylene glycol based silver nanofluids. Powder Technol 2011;208:7–19. [92] Kulkarni DP, Namburu PK, Das DK. Comparison of heat transfer rates of different nanofluids on the basis of the Mouromtseff number. Electro Cooling 2007;13:28–32. [93] Vajjha RS, Das DK. Experimental determination of thermal conductivity of three nanofluids and development of new correlations. Int J Heat Mass Transf 2009;52:4675–82. [94] Sundar LS, Farooky MH, Sarada SN, Singh MK. Experimental thermal conductivity of ethylene glycol and water mixture based low volume concentration of Al2O3 and CuO nanofluids. Int Commun Heat Mass Transf 2013;41:41–6. [95] Sundar LS, Singh MK, Sousa ACM. Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid. Int Commun Heat Mass Transf 2013;49:17–24.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017
S.M.S. Murshed, C.A. Nieto de Castro / Applied Energy xxx (2016) xxx–xxx [96] Juneja M, Gangacharyulu D. Experimental analysis on influence of temperature and volume fraction of nanofluids on thermophysical properties. Int J Emerg Technol Comput Appl Sci 2013;5:233–8. [97] Buongiorno J. Convective transport in nanofluids. J Heat Transf 2006;128:240–50. [98] Hajimeyoshino Torii S. Heat transfer enhancement of Al2O3 nanofluid dispersed in ethylene glycol. Int J Earth Sci Eng 2012;5:532–4. [99] Delavari V, Hashemabadi SH. CFD simulation of heat transfer enhancement of Al2O3/water and Al2O3/ethylene glycol nanofluids in a car radiator. Appl Therm Eng 2014;73:380–90. [100] Zakaria I, Mohamed WANW, Mamat AMIB, Saidur R, Azmi WH, Mamat R, et al. Experimental investigation of Al2O3 - water ethylene glycol mixture nanofluid thermal behaviour in a single cooling plate for PEM fuel cell application. Energy Procedia 2015;79:252–8. [101] Namburu PK, Das DK, Tanguturi KM, Vajjha RS. Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties. Int J Therm Sci 2009;48:290–302. [102] Vajjha RS, Das DK, Kulkarni DP. Development of new correlations for convective heat transfer and friction factor in turbulent regime for nanofluids. Int J Heat Mass Transf 2010;53:4607–18. [103] Kulkarni DP, Namburu PK, Bargar HE, Das DK. Convective heat transfer and fluid dynamic characteristics of SiO2 ethylene glycol/water nanofluid. Heat Transf Eng 2008;29:1027–35. [104] Maiga SEB, Nguyen CT, Galanis N, Roy G. Heat transfer behaviours of nanofluids in a uniformly heated tube. Superlat Microstruct 2004;35:543–57. [105] Yu W, France D, Smith D, Routbort J, Timofeeva E, Singh D, et al. Nanofluids for thermal conditions– underhood heat transfer. In: DOE hydrogen program and vehicle technologies program annual merit review and peer evaluation meeting, May 18–22, Washington, DC, USA; 2009. [106] Sivakumar A, Alagumurthi N, Senthilvelan T. Effect of serpentine grooves on heat transfer characteristics of microchannel heat sink with different nanofluids. Heat Trans Asian Res 2015. doi: http://dx.doi.org/10.1002/ htj.21206. [107] Timofeeva EV, Yu W, France DM, Singh D, Routbort JL. Base fluid and temperature effects on the heat transfer characteristics of SiC in ethylene glycol/H2O and H2O nanofluids. J Appl Phys 2011;109:014914. [108] Ruan B, Jacobi AM. Heat transfer characteristics of multiwall carbon nanotube suspensions (MWCNT nanofluids) in intertube falling-film flow. Int J Heat Mass Transf 2012;55:3186–95. [109] Kumaresan V, Velraj R, Das SK. Convective heat transfer characteristics of secondary refrigerant based CNT nanofluids in a tubular heat exchanger. Int J Refrig 2012;35:2287–96. [110] Hamid KA, Azmi WH, Mamat R, Usri NA. Heat transfer performance of titanium oxide in ethylene glycol based nanofluids under transition flow. Appl Mech Mater 2014;660:684–8. [111] Asadzadeh F, Esfahany MN, Etesami N. Natural convective heat transfer of Fe3O4/ethylene glycol nanofluid in electric field. Int J Therm Sci 2012;62:114–9.
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[112] Huminic G, Huminic A. Numerical analysis of laminar flow heat transfer of nanofluids in a flattened tube. Int Commun Heat Mass Transf 2013;44:52–7. [113] Peyghambarzadeh SM, Hashemabadi SH, Hoseini SM, Jamnani MS. Experimental study of heat transfer enhancement using water/ethylene glycol based nanofluids as a new coolant for car radiators. Int Commun Heat Mass Transf 2011;38:1283–90. [114] Vajjha RS, Das DK, Ray DR. Development of new correlations for the Nusselt number and the friction factor under turbulent flow of nanofluids in flat tubes. Int J Heat Mass Transf 2014;80:353–67. [115] Nieh H-M, Teng T-P, Yu C-C. Enhanced heat dissipation of a radiator using oxide nano-coolant. Int J Therm Sci 2014;77:252–61. [116] Mohammadiun H, Mohammadiun M, Hazbehian M, Maddah H. Experimental study of ethylene glycol-based Al2O3 nanofluid turbulent heat transfer enhancement in the corrugated tube with twisted tapes. Heat Mass Transfer 2016;52:141–51. [117] Taylor RA, Phelan PE, Otanicar TP, Walker CA, Nguyen M, Trimble S, et al. Applicability of nanofluids in high flux solar collectors. J Renew Sustain Energy 2011;3:023104. [118] Taylor RA, Phelan PE, Otanicar TP, Adrian R, Prasher RS. Nanofluid optical property characterization: towards efficient direct absorption solar collectors. Nanoscale Res Lett 2011;6:225. [119] Shin D, Banerjee D. Enhancement of specific heat capacity of hightemperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications. Int J Heat Mass Transf 2011;54:1064–70. [120] Lenert A, Wang EN. Optimization of nanofluid volumetric receivers for solar thermal energy conversion. Sol Energy 2012;86:253–65. [121] Yousefi T, Veysi F, Shojaeizadeh E, Zinadini S. An experimental investigation on the effect of Al2O3-H2O nanofluid on the efficiency of flat plate solar collector. Renew Energy 2012;39:293–8. [122] Javadi FS, Saidur R, Kamalisarvestani M. Investigating performance improvement of solar collectors by using nanofluids. Renew Sustain Energy Rev 2013;28:232–45. [123] Colangelo G, Favale E, de Risi A, Laforgia D. A new solution for reduced sedimentation flat panel solar thermal collector using nanofluids. Appl Energy 2013;111:80–93. [124] Mahian O, Kianifar A, Kalogirou SA, Pop I, Wongwises S. A review of the applications of nanofluids in solar energy. Int J Heat Mass Transf 2013;57:582–94. [125] Bozorgan N, Shafahi M. Performance evaluation of nanofluids in solar energy: a review of the recent literature. Micro Nano Syst Lett 2015;3:5. [126] Kasaeian A, Eshghi AT, Sameti M. A review on the applications of nanofluids in solar energy systems. Renew Sustain Energy Rev 2015;43:584–98. [127] Mercatelli L, Sani E, Zaccanti G, Martelli F, Ninni PD, Barison S, et al. Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers. Nanoscale Res Lett 2011;6:282. [128] Tang B, Wang Y, Qiu M, Zhang S. A full-band sunlight-driven carbon nanotube/PEG/SiO2 composites for solar energy storage. Sol Energy Mater Sol Cells 2014;123:7–12.
Please cite this article in press as: Murshed SMS, Nieto de Castro CA. Conduction and convection heat transfer characteristics of ethylene glycol based nanofluids – A review. Appl Energy (2016), http://dx.doi.org/10.1016/j.apenergy.2016.11.017