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Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications
Applied Energy 97 (2012) 828–833
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications Gianpiero Colangelo ⇑, Ernani Favale, Arturo de Risi, Domenico Laforgia Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Via per Arnesano, 73100 Lecce, Italy
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 5 November 2011 Accepted 13 November 2011 Available online 7 December 2011 Keywords: Nanofluid Thermal conductivity Diathermic oil Energy
a b s t r a c t The work reported in this paper shows the experimental results from a study on diathermic oil based nanofluids. Diathermic oil finds application in renewable energy, cogeneration and cooling systems. For example, it is used in solar thermodynamic or biomass plants, where high efficiency, compact volumes and high energy fluxes are required. Besides diathermic oil is very important in those applications where high temperatures are reached or where the use of water or vapor is not suitable. Therefore an improvement of diathermic oil thermo-physical properties, by using of nanoparticles, can increase the performance of the systems. In literature there are not many experimental data on diathermic oil based nanofluids because many experimental campaigns are focused on water nanofluids. Samples of nanofluids, with nanoparticles of CuO, Al2O3, ZnO and Cu, having different shapes and concentrations varying from 0.0% up to 3.0%, have been produced and their thermal conductivity has been measured by means of hot-wire technique, according to the standard ASTM D 2717-95. Measurements were carried out to investigate the effects of volume fraction, particle size of nanoparticles on the thermal conductivity of the nanofluid. The effect of temperature has been also investigated in the range 20–60 °C. A dependence was observed on the measured parameters and the results showed that the heat transfer performance of diathermic oil enhances more than water with the same nanoparticles. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Heat transfer fluids are used in many fields of applications and often play a key role to define the performance of an energy system [1]. Their low thermal conductivity is one of the limits of the traditional heat transfer fluids, however, using nanostructured materials, a new type of fluids have been proposed in the last decade, to improve heat transfer performance, with the enhancement of thermal conductivity. They are traditional fluids, containing suspended solid nanoparticles (1–100 nm) of metal or metal oxide, that can improve thermal conductivity. These multicomponent liquids, containing a suspension of nano-sized solid particles, are generally better known as nanofluids (the name generally used [2]). Early studies, carried out by using millimeter or micrometer particles dispersed in traditional heat transfer fluids, on one hand showed enhancement in their thermal conductivity, but, on the other hand, the size of the particles was such to yield quick sedimentation, abrasion and clogging, that strongly reduced any potential for practical application [3,4]. Therefore the interest for these suspensions reduced.
⇑ Corresponding author. Tel.: +39 0832297752; fax: +39 297777. E-mail address: [email protected] (G. Colangelo). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.11.026
Nanoparticles play a key role in this scenario, because they can stay suspended much longer than larger particles and their surface area, referred to the volume unit, is significantly larger than that of microparticles. Different studies revealed an increase in thermal conductivity of 20% for 4 vol.% CuO nanoparticles, with average diameter of 35 nm, dispersed in ethylene glycol [5] and similar behavior was observed also for Al2O3 nanoparticles. Eastman et al. [6] measured thermal conductivity of ethylene glycol–Cu nanofluids with average diameter of nanoparticles of less than 10 nm. In order to improve the stability of the suspension a small quantity of dispersant was added in the nanofluid sample. It was recorded thermal conductivity enhancement as large as 40% through the dispersion of 0.3 vol.% of Cu nanoparticles. Hwang et al. [7] measured thermal conductivity of oil-Multi Walled Carbon Nanotube (MWCNT) and the thermal conductivity enhancement was up to 8.7% with 0.5 vol.% of MWCNT. Yu et al. [8] obtained an increase of thermal conductivity of 26.5% for 5.0 vol.% of ZnO dispersed in ethylene glycol, while Murshed et al. [9] measured ethylene glycol–TiO2 nanofluids and ethylene glycol–Al nanofluids thermal conductivity. They obtained enhancements of 18% with 5.0 vol.% of TiO2 and 45% with 5.0 vol.% of Al respectively. Even if many studies and measurements have been carried out on water based nanofluids, diathermic oil based nanofluids have not been studied deeply so far.
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Nomenclature keff effective thermal conductivity of suspension (W/m K) kBF thermal conductivity of base fluid (W/m K) thermal conductivity of solid phase (W/m K) ksolid a = ksolid/kBF ratio between the thermal conductivities of the base fluid and solid phase t volume fraction of particles w sphericity n = 3/w shape factor In order to predict the thermal conductivity of solid–fluid mixtures, many theories and models have been developed. The first model was proposed by Maxwell [10] and it is reported as follows:
keff 3ða 1Þt ¼1þ kBF ða þ 2Þ ða 1Þt
ð1Þ
This model was derived under the hypothesis of spherical particles, with the radius much smaller compared to inter-particle distance. In 1962 Hamilton and Crosser [11] proposed a model to consider the particles shape effect on the thermal conductivity of the particle–fluid mixture:
keff a þ ðn 1Þ ðn 1Þð1 aÞt ¼ km a þ ðn 1Þ þ ð1 aÞt
ð2Þ
n ¼ 3=w
ð3Þ
T T0 DT
a q t
fluid temperature (K) reference temperature (K) difference between fluid temperature and a reference temperature (K) fluid thermal diffusivity (m2/s) heat flux (W/m) time (s)
temperature is less than 0 °C. Therefore the improvement of the thermophysical properties of diathermic oil (by using nanoparticles) can be useful to increase the performance of applications as solar thermodynamic systems, cogeneration, cooling systems [19,20] where the use of water is not suitable. The purpose of this work is to study the behavior of diathermic oil based nanofluids (made by diathermic oil and different types of nanoparticles) in order to investigate their performance and dependence from parameters such as thermal conductivity of the base fluid, particle size and temperature. This study covers a lack of information in this scientific field. 2. Experimental setup 2.1. Thermal conductivity measurements
where w is the sphericity, defined as the ratio between the surface area of a sphere particle with a volume equal to that of the particle and the surface area of the particle. Keblinski et al. [12] assert that to calculate the thermal properties of nanofluids it has to be investigated the ballistic nature of the heat transport phenomenon in nanoparticles and the clustering effects, while shape factor and Brownian motion give negligible contributions to the thermal conductivity enhancement. Besides within a nanofluid, each solid particle is surrounded by a thin film fluid layer, having better conductive characteristics than the base fluid [12]. The atomic structure of the liquid layer is more ordered than that of the bulk liquid, therefore it is similar to crystalline solids that have better thermal transport properties than liquids. Hong and Yang [13] showed that not always better results can be obtained by using nanoparticles with high thermal conductivity. Enhancement for Fe nanofluids was higher than that of Cu nanofluids, even if Fe thermal conductivity is smaller than that of Cu. Some authors, as Jang and Choi [14], assert that there is a nanoconvection motion phenomenon near the nanoparticle, due to Brownian motion. They include three modes of heat transfer: heat transfer due to collision of base fluid molecules; thermal diffusion in nanoparticles; nanoconvection due to the Brownian motion. However thermal conductivity of nanofluids, stability and viscosity of the suspensions depend on storage time and surfactant or dispersant used during the sample preparation [15]. In addition, the heat transfer coefficient of nanofluids, h [W/m2 K], is higher than that of the base fluid (it also depends on thermal conductivity enhancement) and therefore it is possible to use nanofluids either to reduce heat transfer surface area or mass flow rate [16]. Alternatively an increase of heat flux can be obtained, at the same conditions [17]. Finally nanofluids are suitable for energy storage systems [18]. Diathermic oil is a heat transfer fluid used in many applications, especially when low pressure and high temperatures are reached, in order to reduce the construction costs of the systems and to guarantee a high safety level and an easy management. Besides diathermic oil is necessary in applications where working
Nanofluids thermal conductivity was measured through a device based on the hot-wire technique, according to the standard ASTM D 2717-95 [21]. Hot-wire method is based on electric resistance variation with temperature. The ideal apparatus should have to be made by an infinitely long vertical wire, with null heat capacity and negligible diameter, immersed in the sample fluid. The wire, because of electric current, undergoes to a temperature rise and to an electric resistance variation. Temperature rise depends on the thermal conductivity of the fluid around the wire. The hot wire (the local heat source) develops temperature gradients that yield convective motions, depending on intensity and time duration of the heat flux. Because of this phenomenon, the time of the measurements must be as short as possible and, thus, the transient hot wire method is used in this investigation. It is based on the following heat equation solution, for two coaxial cylinders system in transient mode, with a heat flux, q:
DTðr; tÞ ¼ Tðr; tÞ T 0 ¼
4a ln t þ ln 2 0:5772 4pk r q
ð4Þ
The constant 0.5772 is the Eulero–Mascheroni constant [22]. Eq. (4) is a straight line on the [ln t, DT] plane and its slope is:
dDT q ¼ d ln t 4pk
ð5Þ
In Eq. (5), the thermal conductivity, k, can be obtained without requiring the actual temperature and geometry of the system. The real apparatus (Fig. 1) is a system made by a short platinum wire (with a diameter of 0.1 mm and length of 35 mm) welded on a support and immersed in a cylindrical cell, where the nanofluid sample is placed. Besides, a thermocouple measures the average temperature in the measuring cell. Border effects on the wire are negligible and the thermal conductivity value is not dependent on eccentricity between wire and internal surface cell, according to Eq. (5). The thermocouple and the platinum wire are placed on the measuring head, screwed on the cylindrical cell. The experimental apparatus was calibrated to measure fluid having a temperature between 20 °C and 150 °C and a thermal
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Fig. 1. Experimental apparatus for thermal conductivity measurements.
Table 2 Measured thermal conductivity of the base fluids.
Table 1 Particles properties. Material
Effective density (kg/m3)
Particle size (nm)
Shape
Thermal conductivity (W/m K)
CuO A12O3 ZnO Cu Cu Cu
6.50 3.97 5.60 8.94 8.94 8.94
30 45 60 100 50 25
Spherical Spherical Elongated Spherical Spherical Spherical
20.0 25.1 29.0 390 390 390
Base fluid
Temperature (°C)
k (W/m K)
Diathermic oil
20 30 40 50 60
0.119 0.117 0.115 0.113 0.112
Demineralized water
20
0.606
conductivity in the range between 10 and 1000 mW/mK, with an experimental error less than ±1%. The average temperature of the samples was assured by a thermostatic bath. 2.2. Nanoparticles and base fluids properties In this investigation metal oxide and metal nanoparticles were used, in particular: CuO and Al2O3 nanoparticles with spherical shape and particle size with average diameter 30 nm and 45 nm respectively; ZnO with elongated particle shape and average dimension of 60 nm. Finally, three types of Cu nanoparticles with spherical shape and particle size with average diameter of 100 nm, 50 nm and 25 nm respectively were used. Table 1 reports their properties. A diathermic oil and demineralized water with non-ionic dispersant, to stabilize the suspension, were used as base fluids. The concentration of the dispersant was such to reach the Critical Micelle Concentration (CMC), defined as the concentration of a surfactant above which micelles are spontaneously formed. The temperature operation range of the selected diathermic oil is between 10 °C and 345 °C. The vapor pressure at the maximum value of temperature is 70.68 kPa [23]. Demineralized water was also used in order to compare oil nanofluids with water nanofluids. Table 2 summarized the measured thermal conductivity of diathermic oil as function of temperature and of demineralized water at a reference temperature of 20 °C.
Fig. 2. Flowchart of the nanofluid samples preparation and measurement of the thermal conductivity.
Fig. 3 shows the sonication effect on diathermic oil–Al2O3 0.7 vol.%; it is possible to see a better stability of the solid phase in a sample with sonication. The nanofluids, after a further mixing with a magnetic homogenizator, were then submitted to a stability analysis. The stability test consisted in sampling part of the nanofluid in a glass tube and let it rest for 90 min. If any significant precipitation of the solid phase was observed then the thermal conductivity was measured, otherwise the nanofluid sample was rejected.
2.3. Sample preparation 3. Experimental results and discussion Fig. 2 shows the flowchart of the procedure used to prepare the nanofluid samples and to measure the thermal conductivity. For each nanofluid sample, first of all the solid and the liquid phases were weighed by means of a precision balance with a resolution of 0.01 g and were placed in a beaker. The second step, after weighting both the liquid and the solid phases, is the mixing for 60 min, with a magnetic stirrer at 700 rpm. The suspension is therefore vibrated in an ultrasonic homogenizer at 20 kHz and 70 W to break the clusters of the nanoparticles and to improve the stability of the suspension.
Table 3 shows the enhancement of the thermal conductivity of the base fluid of nanofluids. In each row the base fluid (DW and DO are demineralized water and diathermic oil respectively), the measurement temperature, the nanoparticles type (Cu100, Cu050 and Cu025 are copper with 100 nm, 50 nm and 25 nm particle size respectively) and the thermal conductivity enhancement are indicated with the variation of the volume fraction of the nanoparticles. The temperature range is between 20 °C and 60 °C, while the volume fraction is between 0.1% and 3.0%. For both demineralized water
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Fig. 3. Sonication effect on diathermic oil based nanofluids (a – no sonicated, b – sonicated).
and diathermic oil, added with metal oxides, better thermal conductivity enhancements have been obtained with ZnO nanoparticles. Instead with metal nanoparticles in diathermic oil, better results have been obtained with 25 nm Cu nanoparticles. The thermal conductivity enhancement is directly proportional to the volume fraction (see Table 3). Fig. 4 shows the thermal conductivity enhancement, knf/k0 (where knf is the thermal conductivity of the nanofluid and k0 is the thermal conductivity of the base fluids respectively), for nanofluid samples with metal oxide nanoparticles (Al2O3, CuO and ZnO), at 30 °C and predicted values with Hamilton–Crosser model (HC). For ZnO nanoparticles the needed constants are: w = 0.7786 and n = 3.853, obtained from TEM images of the nanoparticles. Hamilton–Crosser model does not match experimental data (see Fig. 3), therefore the thermal conductivity behavior depends on other mechanisms, that this model does not take into account, such as the effect of clustering of the nanoparticles, ballistic phonon transport [11] or Brownian motion [13]. However there is not yet a common theory in order to explain thermal conductivity enhancement of nanofluids. As it is possible to see in Fig. 5, the particle size is determinant in order to improve the nanofluids thermal conductivity. This figure shows the enhancement of the thermal conductivity of DO– Cu100, DO–Cu050 and DO–Cu025 nanofluids, where particle sizes of solid phases are 100 nm, 50 nm and 25 nm respectively. The slope difference between the trend line of DO–Cu100 and the trend
Fig. 4. Comparison of the experimental data with predicted values from Hamilton– Crosser model for nanofluids samples with oil and metal oxide nanoparticles at 30 °C.
Fig. 5. Experimental data for nanofluids samples with oil and Cu nanoparticles with different particle sizes.
line of DO–Cu050 (with 50 nm particle size difference) is comparable to the slope difference between the trend line of Cu050 and that of Cu025 (with 25 nm particle size difference). Therefore not only the thermal conductivity is inversely proportional to the particle size, but the gradient enhancement rises, while the particle size decreases. Many authors propose various mechanisms in order
Table 3 Experimental results. Base fluid
Temperature (°C)
Nanoparticles
Volume fraction of nanoparticles (%) 0.1
Enhancement (%) DW 20 DW 20 DW 20 DO 20 DO 30 DO 40 DO 50 DO 60 DO 20 DO 30 DO 40 DO 50 DO 60 DO 20 DO 30 DO 40 DO 50 DO 60 DO 30 DO 30 DO 30
A12O3 CuO ZnO A12O3 A12O3 A12O3 A12O3 A12O3 CuO CuO CuO CuO CuO ZnO ZnO ZnO ZnO ZnO Cu100 Cu050 Cu025
0.3
0.40 0.33 0.48 0.46
0.76 0.92 1.23 1.12
0.67 1.11 0.66 0.50
1.67 2.04 2.05 1.82
0.58 1.01 1.11 0.66
2.49 2.29 1.98 1.70
0.5
2.33 2.16 2.18 2.42 2.99 2.59 2.98 2.42 3.50 3.54 3.52 3.60 3.59 4.45 5.75
0.7
2.85 3.49 3.49 3.03 3.57 3.38 3.08 2.96 4.55 4.22 4.14 3.96
1.0 2.51 3.18 3.58 3.92 4.02 4.02 3.98 3.97 5.45 5.27 5.23 5.47 4.97 6.48 6.51 6.40 6.11 6.50 7.04 8.38 10.11
1.5
2.0 4.74 5.95 8.45
5.50 5.89 5.87 5.81
8.29 8.04 7.88 7.72
7.84 8.02 8.04 7.83
10.71 10.35 10.44 9.96
9.77 9.49 9.67 9.11 10.85 13.55 16.85
12.85 12.90 12.59 12.70 14.56 18.09 21.76
3.0 6.70 8.31 11.40
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Fig. 9. Enhancement of thermal conductivity for diathermic oil and ZnO nanofluids samples. Fig. 6. Comparison among the enhancements for nanofluids samples with metal oxide nanoparticles.
Fig. 7. Enhancement of thermal conductivity for diathermic oil and Al2O3 nanofluids samples.
The enhancements of the thermal conductivity at a temperature of 20 °C were measured for diathermic oil and demineralized water, with dispersant, nanofluids samples, with 1.0% Al2O3, CuO and ZnO respectively and are showed in Fig. 6. The enhancements of diathermic oil samples are higher than those of demineralized water, with dispersant, samples with the same volume fraction (1.0%). This result is very encouraging, because the thermal conductivity enhancement is inversely proportional to the base fluid thermal conductivity. Finally, Figs. 7–9 show the thermal conductivity enhancements for samples of diathermic oil nanofluids, with metal oxide nanoparticles. For all the investigated samples, the thermal conductivity behavior was not influenced by the measurement temperature. However, using the diathermic oil nanofluids, the performance of the energy systems can be improved, but it needs to take into account that the nanofluids are liquid–solid suspensions and, therefore, an appropriate design of the equipment to avoid sedimentation is necessary, such as the shape and the slope of the piping and stirring system in tanks. The nanofluids sedimentation yields a decrease in the thermal conductivity and in the convective heat transfer. For example, in thermal solar systems it is possible to use innovative thermal solar collectors with the bottom pipe and the top pipe with a variable cross section along the tubes, to maintain a constant velocity and to avoid sedimentation [24].
4. Conclusions
Fig. 8. Enhancement of thermal conductivity for diathermic oil and CuO nanofluids samples.
to explain this behavior. For example, Keblinky et al. [12] assert that one of the reasons is the ballistic phonon transport when particles distance is very small: if the ballistic phonons initiated in one particle, they can persist in the liquid and reach another particle; because the phonon mean free path is shorter in the liquid than in the solid, this phenomenon can only happen if the separation between particles is in the order of the thickness of the liquid layer film around the particle. The effect of this phenomenon is inversely proportional to particle size. For other authors Brownian motion plays a key role to enhance the thermal conductivity of the nanofluids, such as Jang and Choi [14], including a nanoconvection mode, induced by Brownian motion, that contributes to the heat transfer and it is inversely proportional to the particle size of the solid phase.
An experimental study on thermal conductivity of diathermic oil based nanofluids was carried out. The diathermic oil is a heat transfer fluid used in high temperature applications. The following results were obtained by experimental data: for all the investigated samples and temperature ranges, the thermal conductivity enhancement is directly proportional to the volume fraction; Hamilton–Crosser model undervalues the experimental data, therefore the nanofluids thermal conductivity depends on other heat transfer mechanisms, not considered in this model; the nanofluids thermal conductivity depends on the particle size, that could influence the heat transfer mechanisms; the thermal conductivity enhancement of the nanofluids with diathermic oil is higher than that with demineralized water, with the same nanoparticles and at the same conditions; temperature does not influence the thermal conductivity enhancement of the nanofluids; better results were obtained with ZnO, for nanofluids with metal oxide nanoparticles.
G. Colangelo et al. / Applied Energy 97 (2012) 828–833
References [1] Kulkarni Devdatta P, Das Debendra K, Vajjha Ravikanth S. Application of nanofluids in heating buildings and reducing pollution. Appl Energy 2009;86:2566–73. [2] Xuan Yimin, Li Qiang. Heat transfer enhancement of nanofluids. Int J Heat Transfer Fluid Flow 2000;21:58–64. [3] He Yurong, Jin Yi, Chen Haisheng, Ding Yulong, Cang Daqiang, Lu Huilin. Heat transfer and flow behaviour of aqueous of TiO2 nanoparticles (nanofluids) flowing through a vertical pipe. Int J Heat Mass Transfer 2007;50:2272–81. [4] Agwu Nnanna AG. Experimental model of temperature-driven nanofluids. J Heat Transfer 2007;129(June):697–704. [5] Xue Qing-Zhong. Model for effective thermal conductivity of nanofluids. Phys Lett A 2003;307:313–7. [6] Eastman JA, Choi SUS, Li S, Yu W, Thompson LKJ. Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper particles. Appl Phys Lett 2001;78(6). [7] Hwang Y, Lee JK, Lee CH, Jung YM, Cheong SI, Lee CG, et al. Stability and thermal conductivity characteristics of nanofluids. Thermochim Acta 2007;544:70–4. [8] Yu Wei, Xie Huaqing, Chen Lifei, Li Yang. Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid. Thermochim Acta 2009;544:92–6. [9] Murshed SMS, Leong KC, Yang C. Investigation of thermal conductivity and viscosity of nanofluids. Int J Therm Sci 2008;47:560–8. [10] Maxwell JC. A treatise on electricity and magnetism, 2nd ed., vol. 1. Cambridge: Oxford University Press; 1881. p. 435. [11] Hamilton RL, Crosser OK. Thermal conductivity of heterogeneous twocomponent system. IEC Fundam 1962;2:187. [12] Keblinski P, Phillpot SR, Choi SUS, Eastmam LA. Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids). Int J Heat Mass Transfer 2000;45:855–63.
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[13] Hong Tae-Keun, Yang Ho-Soon. Nanoparticle dispersion dependent thermal conductivity in nanofluids. J Korean Phys Soc 2005;47(September):S321–4. [14] Jang SP, Choi SUS. Effects of various parameters on nanofluids thermal conductivity. J Heat Transfer 2007;129(May):617–23. [15] Lin Cherng-Yuan, Wang Jung-Chang, Chen Teng-Chieh. Analysis of suspension and heat transfer characteristics of Al2O3 nanofluids prepared through ultrasonic vibration. Appl Energy 2011;88:4527–33. [16] Kulkarni Devdatta P, Das Debendra K, Vajjha Ravikanth S. Application of nanofluids in heating buildings and reduction pollution. Appl Energy 2009;86:2566–73. [17] Teng Tun-Ping, Hung Yi-Hsuan, Teng Tun-Chien, Chen Jyun-Hong. Performance evaluation on an air-cooled heat exchanger for alumina nanofluids under laminar flow. Nanoscale Res Lett 2011;6:488. [18] Songping Mo, Ying Chen, Lisi Jia, Xianglong Luo. Investigation on crystallization of TiO2–water nanofluids and deionized water. Appl Energy 2012;93:65–70. [19] Calise F. Design of a hybrid polygeneration system with solar collectors and a solid oxide fuel cell: dynamic simulation and economic assessment. Int J Hydrogen Energy 2011;36:6128–50. [20] Moya M, Bruno JC, Eguia P, Torres E, Zamora I, Coronas A, et al. Performance analysis of trigeneration system based on a micro gas turbine and an aircooled, indirect fired, ammonia–water absorption chiller. Appl Energy 2011;88:4424–40. [21] ASTM D 2717-95. Standard test method for thermal conductivity of liquids. [22] Franco A. An apparatus for the routine measurements of thermal conductivity of materials for building application based on a transient hot-wire method. Appl Therm Eng 2007;27:2495–504. [23] Diathermic oil data sheet. [24] Colangelo G, Favale E, de Risi A, Starace G, Laforgia D. Un nuovo pannello solare termico a nanofluidi. 66th ATI National Congress Session ‘‘Solar Energy and other Renewable Source’’, 5–9 September 2011, Università della Calabria – Rende (Cosenza), Italy.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications Gianpiero Colangelo ⇑, Ernani Favale, Arturo de Risi, Domenico Laforgia Dipartimento di Ingegneria dell’Innovazione, Università del Salento, Via per Arnesano, 73100 Lecce, Italy
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 5 November 2011 Accepted 13 November 2011 Available online 7 December 2011 Keywords: Nanofluid Thermal conductivity Diathermic oil Energy
a b s t r a c t The work reported in this paper shows the experimental results from a study on diathermic oil based nanofluids. Diathermic oil finds application in renewable energy, cogeneration and cooling systems. For example, it is used in solar thermodynamic or biomass plants, where high efficiency, compact volumes and high energy fluxes are required. Besides diathermic oil is very important in those applications where high temperatures are reached or where the use of water or vapor is not suitable. Therefore an improvement of diathermic oil thermo-physical properties, by using of nanoparticles, can increase the performance of the systems. In literature there are not many experimental data on diathermic oil based nanofluids because many experimental campaigns are focused on water nanofluids. Samples of nanofluids, with nanoparticles of CuO, Al2O3, ZnO and Cu, having different shapes and concentrations varying from 0.0% up to 3.0%, have been produced and their thermal conductivity has been measured by means of hot-wire technique, according to the standard ASTM D 2717-95. Measurements were carried out to investigate the effects of volume fraction, particle size of nanoparticles on the thermal conductivity of the nanofluid. The effect of temperature has been also investigated in the range 20–60 °C. A dependence was observed on the measured parameters and the results showed that the heat transfer performance of diathermic oil enhances more than water with the same nanoparticles. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Heat transfer fluids are used in many fields of applications and often play a key role to define the performance of an energy system [1]. Their low thermal conductivity is one of the limits of the traditional heat transfer fluids, however, using nanostructured materials, a new type of fluids have been proposed in the last decade, to improve heat transfer performance, with the enhancement of thermal conductivity. They are traditional fluids, containing suspended solid nanoparticles (1–100 nm) of metal or metal oxide, that can improve thermal conductivity. These multicomponent liquids, containing a suspension of nano-sized solid particles, are generally better known as nanofluids (the name generally used [2]). Early studies, carried out by using millimeter or micrometer particles dispersed in traditional heat transfer fluids, on one hand showed enhancement in their thermal conductivity, but, on the other hand, the size of the particles was such to yield quick sedimentation, abrasion and clogging, that strongly reduced any potential for practical application [3,4]. Therefore the interest for these suspensions reduced.
⇑ Corresponding author. Tel.: +39 0832297752; fax: +39 297777. E-mail address: [email protected] (G. Colangelo). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.11.026
Nanoparticles play a key role in this scenario, because they can stay suspended much longer than larger particles and their surface area, referred to the volume unit, is significantly larger than that of microparticles. Different studies revealed an increase in thermal conductivity of 20% for 4 vol.% CuO nanoparticles, with average diameter of 35 nm, dispersed in ethylene glycol [5] and similar behavior was observed also for Al2O3 nanoparticles. Eastman et al. [6] measured thermal conductivity of ethylene glycol–Cu nanofluids with average diameter of nanoparticles of less than 10 nm. In order to improve the stability of the suspension a small quantity of dispersant was added in the nanofluid sample. It was recorded thermal conductivity enhancement as large as 40% through the dispersion of 0.3 vol.% of Cu nanoparticles. Hwang et al. [7] measured thermal conductivity of oil-Multi Walled Carbon Nanotube (MWCNT) and the thermal conductivity enhancement was up to 8.7% with 0.5 vol.% of MWCNT. Yu et al. [8] obtained an increase of thermal conductivity of 26.5% for 5.0 vol.% of ZnO dispersed in ethylene glycol, while Murshed et al. [9] measured ethylene glycol–TiO2 nanofluids and ethylene glycol–Al nanofluids thermal conductivity. They obtained enhancements of 18% with 5.0 vol.% of TiO2 and 45% with 5.0 vol.% of Al respectively. Even if many studies and measurements have been carried out on water based nanofluids, diathermic oil based nanofluids have not been studied deeply so far.
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Nomenclature keff effective thermal conductivity of suspension (W/m K) kBF thermal conductivity of base fluid (W/m K) thermal conductivity of solid phase (W/m K) ksolid a = ksolid/kBF ratio between the thermal conductivities of the base fluid and solid phase t volume fraction of particles w sphericity n = 3/w shape factor In order to predict the thermal conductivity of solid–fluid mixtures, many theories and models have been developed. The first model was proposed by Maxwell [10] and it is reported as follows:
keff 3ða 1Þt ¼1þ kBF ða þ 2Þ ða 1Þt
ð1Þ
This model was derived under the hypothesis of spherical particles, with the radius much smaller compared to inter-particle distance. In 1962 Hamilton and Crosser [11] proposed a model to consider the particles shape effect on the thermal conductivity of the particle–fluid mixture:
keff a þ ðn 1Þ ðn 1Þð1 aÞt ¼ km a þ ðn 1Þ þ ð1 aÞt
ð2Þ
n ¼ 3=w
ð3Þ
T T0 DT
a q t
fluid temperature (K) reference temperature (K) difference between fluid temperature and a reference temperature (K) fluid thermal diffusivity (m2/s) heat flux (W/m) time (s)
temperature is less than 0 °C. Therefore the improvement of the thermophysical properties of diathermic oil (by using nanoparticles) can be useful to increase the performance of applications as solar thermodynamic systems, cogeneration, cooling systems [19,20] where the use of water is not suitable. The purpose of this work is to study the behavior of diathermic oil based nanofluids (made by diathermic oil and different types of nanoparticles) in order to investigate their performance and dependence from parameters such as thermal conductivity of the base fluid, particle size and temperature. This study covers a lack of information in this scientific field. 2. Experimental setup 2.1. Thermal conductivity measurements
where w is the sphericity, defined as the ratio between the surface area of a sphere particle with a volume equal to that of the particle and the surface area of the particle. Keblinski et al. [12] assert that to calculate the thermal properties of nanofluids it has to be investigated the ballistic nature of the heat transport phenomenon in nanoparticles and the clustering effects, while shape factor and Brownian motion give negligible contributions to the thermal conductivity enhancement. Besides within a nanofluid, each solid particle is surrounded by a thin film fluid layer, having better conductive characteristics than the base fluid [12]. The atomic structure of the liquid layer is more ordered than that of the bulk liquid, therefore it is similar to crystalline solids that have better thermal transport properties than liquids. Hong and Yang [13] showed that not always better results can be obtained by using nanoparticles with high thermal conductivity. Enhancement for Fe nanofluids was higher than that of Cu nanofluids, even if Fe thermal conductivity is smaller than that of Cu. Some authors, as Jang and Choi [14], assert that there is a nanoconvection motion phenomenon near the nanoparticle, due to Brownian motion. They include three modes of heat transfer: heat transfer due to collision of base fluid molecules; thermal diffusion in nanoparticles; nanoconvection due to the Brownian motion. However thermal conductivity of nanofluids, stability and viscosity of the suspensions depend on storage time and surfactant or dispersant used during the sample preparation [15]. In addition, the heat transfer coefficient of nanofluids, h [W/m2 K], is higher than that of the base fluid (it also depends on thermal conductivity enhancement) and therefore it is possible to use nanofluids either to reduce heat transfer surface area or mass flow rate [16]. Alternatively an increase of heat flux can be obtained, at the same conditions [17]. Finally nanofluids are suitable for energy storage systems [18]. Diathermic oil is a heat transfer fluid used in many applications, especially when low pressure and high temperatures are reached, in order to reduce the construction costs of the systems and to guarantee a high safety level and an easy management. Besides diathermic oil is necessary in applications where working
Nanofluids thermal conductivity was measured through a device based on the hot-wire technique, according to the standard ASTM D 2717-95 [21]. Hot-wire method is based on electric resistance variation with temperature. The ideal apparatus should have to be made by an infinitely long vertical wire, with null heat capacity and negligible diameter, immersed in the sample fluid. The wire, because of electric current, undergoes to a temperature rise and to an electric resistance variation. Temperature rise depends on the thermal conductivity of the fluid around the wire. The hot wire (the local heat source) develops temperature gradients that yield convective motions, depending on intensity and time duration of the heat flux. Because of this phenomenon, the time of the measurements must be as short as possible and, thus, the transient hot wire method is used in this investigation. It is based on the following heat equation solution, for two coaxial cylinders system in transient mode, with a heat flux, q:
DTðr; tÞ ¼ Tðr; tÞ T 0 ¼
4a ln t þ ln 2 0:5772 4pk r q
ð4Þ
The constant 0.5772 is the Eulero–Mascheroni constant [22]. Eq. (4) is a straight line on the [ln t, DT] plane and its slope is:
dDT q ¼ d ln t 4pk
ð5Þ
In Eq. (5), the thermal conductivity, k, can be obtained without requiring the actual temperature and geometry of the system. The real apparatus (Fig. 1) is a system made by a short platinum wire (with a diameter of 0.1 mm and length of 35 mm) welded on a support and immersed in a cylindrical cell, where the nanofluid sample is placed. Besides, a thermocouple measures the average temperature in the measuring cell. Border effects on the wire are negligible and the thermal conductivity value is not dependent on eccentricity between wire and internal surface cell, according to Eq. (5). The thermocouple and the platinum wire are placed on the measuring head, screwed on the cylindrical cell. The experimental apparatus was calibrated to measure fluid having a temperature between 20 °C and 150 °C and a thermal
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Fig. 1. Experimental apparatus for thermal conductivity measurements.
Table 2 Measured thermal conductivity of the base fluids.
Table 1 Particles properties. Material
Effective density (kg/m3)
Particle size (nm)
Shape
Thermal conductivity (W/m K)
CuO A12O3 ZnO Cu Cu Cu
6.50 3.97 5.60 8.94 8.94 8.94
30 45 60 100 50 25
Spherical Spherical Elongated Spherical Spherical Spherical
20.0 25.1 29.0 390 390 390
Base fluid
Temperature (°C)
k (W/m K)
Diathermic oil
20 30 40 50 60
0.119 0.117 0.115 0.113 0.112
Demineralized water
20
0.606
conductivity in the range between 10 and 1000 mW/mK, with an experimental error less than ±1%. The average temperature of the samples was assured by a thermostatic bath. 2.2. Nanoparticles and base fluids properties In this investigation metal oxide and metal nanoparticles were used, in particular: CuO and Al2O3 nanoparticles with spherical shape and particle size with average diameter 30 nm and 45 nm respectively; ZnO with elongated particle shape and average dimension of 60 nm. Finally, three types of Cu nanoparticles with spherical shape and particle size with average diameter of 100 nm, 50 nm and 25 nm respectively were used. Table 1 reports their properties. A diathermic oil and demineralized water with non-ionic dispersant, to stabilize the suspension, were used as base fluids. The concentration of the dispersant was such to reach the Critical Micelle Concentration (CMC), defined as the concentration of a surfactant above which micelles are spontaneously formed. The temperature operation range of the selected diathermic oil is between 10 °C and 345 °C. The vapor pressure at the maximum value of temperature is 70.68 kPa [23]. Demineralized water was also used in order to compare oil nanofluids with water nanofluids. Table 2 summarized the measured thermal conductivity of diathermic oil as function of temperature and of demineralized water at a reference temperature of 20 °C.
Fig. 2. Flowchart of the nanofluid samples preparation and measurement of the thermal conductivity.
Fig. 3 shows the sonication effect on diathermic oil–Al2O3 0.7 vol.%; it is possible to see a better stability of the solid phase in a sample with sonication. The nanofluids, after a further mixing with a magnetic homogenizator, were then submitted to a stability analysis. The stability test consisted in sampling part of the nanofluid in a glass tube and let it rest for 90 min. If any significant precipitation of the solid phase was observed then the thermal conductivity was measured, otherwise the nanofluid sample was rejected.
2.3. Sample preparation 3. Experimental results and discussion Fig. 2 shows the flowchart of the procedure used to prepare the nanofluid samples and to measure the thermal conductivity. For each nanofluid sample, first of all the solid and the liquid phases were weighed by means of a precision balance with a resolution of 0.01 g and were placed in a beaker. The second step, after weighting both the liquid and the solid phases, is the mixing for 60 min, with a magnetic stirrer at 700 rpm. The suspension is therefore vibrated in an ultrasonic homogenizer at 20 kHz and 70 W to break the clusters of the nanoparticles and to improve the stability of the suspension.
Table 3 shows the enhancement of the thermal conductivity of the base fluid of nanofluids. In each row the base fluid (DW and DO are demineralized water and diathermic oil respectively), the measurement temperature, the nanoparticles type (Cu100, Cu050 and Cu025 are copper with 100 nm, 50 nm and 25 nm particle size respectively) and the thermal conductivity enhancement are indicated with the variation of the volume fraction of the nanoparticles. The temperature range is between 20 °C and 60 °C, while the volume fraction is between 0.1% and 3.0%. For both demineralized water
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Fig. 3. Sonication effect on diathermic oil based nanofluids (a – no sonicated, b – sonicated).
and diathermic oil, added with metal oxides, better thermal conductivity enhancements have been obtained with ZnO nanoparticles. Instead with metal nanoparticles in diathermic oil, better results have been obtained with 25 nm Cu nanoparticles. The thermal conductivity enhancement is directly proportional to the volume fraction (see Table 3). Fig. 4 shows the thermal conductivity enhancement, knf/k0 (where knf is the thermal conductivity of the nanofluid and k0 is the thermal conductivity of the base fluids respectively), for nanofluid samples with metal oxide nanoparticles (Al2O3, CuO and ZnO), at 30 °C and predicted values with Hamilton–Crosser model (HC). For ZnO nanoparticles the needed constants are: w = 0.7786 and n = 3.853, obtained from TEM images of the nanoparticles. Hamilton–Crosser model does not match experimental data (see Fig. 3), therefore the thermal conductivity behavior depends on other mechanisms, that this model does not take into account, such as the effect of clustering of the nanoparticles, ballistic phonon transport [11] or Brownian motion [13]. However there is not yet a common theory in order to explain thermal conductivity enhancement of nanofluids. As it is possible to see in Fig. 5, the particle size is determinant in order to improve the nanofluids thermal conductivity. This figure shows the enhancement of the thermal conductivity of DO– Cu100, DO–Cu050 and DO–Cu025 nanofluids, where particle sizes of solid phases are 100 nm, 50 nm and 25 nm respectively. The slope difference between the trend line of DO–Cu100 and the trend
Fig. 4. Comparison of the experimental data with predicted values from Hamilton– Crosser model for nanofluids samples with oil and metal oxide nanoparticles at 30 °C.
Fig. 5. Experimental data for nanofluids samples with oil and Cu nanoparticles with different particle sizes.
line of DO–Cu050 (with 50 nm particle size difference) is comparable to the slope difference between the trend line of Cu050 and that of Cu025 (with 25 nm particle size difference). Therefore not only the thermal conductivity is inversely proportional to the particle size, but the gradient enhancement rises, while the particle size decreases. Many authors propose various mechanisms in order
Table 3 Experimental results. Base fluid
Temperature (°C)
Nanoparticles
Volume fraction of nanoparticles (%) 0.1
Enhancement (%) DW 20 DW 20 DW 20 DO 20 DO 30 DO 40 DO 50 DO 60 DO 20 DO 30 DO 40 DO 50 DO 60 DO 20 DO 30 DO 40 DO 50 DO 60 DO 30 DO 30 DO 30
A12O3 CuO ZnO A12O3 A12O3 A12O3 A12O3 A12O3 CuO CuO CuO CuO CuO ZnO ZnO ZnO ZnO ZnO Cu100 Cu050 Cu025
0.3
0.40 0.33 0.48 0.46
0.76 0.92 1.23 1.12
0.67 1.11 0.66 0.50
1.67 2.04 2.05 1.82
0.58 1.01 1.11 0.66
2.49 2.29 1.98 1.70
0.5
2.33 2.16 2.18 2.42 2.99 2.59 2.98 2.42 3.50 3.54 3.52 3.60 3.59 4.45 5.75
0.7
2.85 3.49 3.49 3.03 3.57 3.38 3.08 2.96 4.55 4.22 4.14 3.96
1.0 2.51 3.18 3.58 3.92 4.02 4.02 3.98 3.97 5.45 5.27 5.23 5.47 4.97 6.48 6.51 6.40 6.11 6.50 7.04 8.38 10.11
1.5
2.0 4.74 5.95 8.45
5.50 5.89 5.87 5.81
8.29 8.04 7.88 7.72
7.84 8.02 8.04 7.83
10.71 10.35 10.44 9.96
9.77 9.49 9.67 9.11 10.85 13.55 16.85
12.85 12.90 12.59 12.70 14.56 18.09 21.76
3.0 6.70 8.31 11.40
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Fig. 9. Enhancement of thermal conductivity for diathermic oil and ZnO nanofluids samples. Fig. 6. Comparison among the enhancements for nanofluids samples with metal oxide nanoparticles.
Fig. 7. Enhancement of thermal conductivity for diathermic oil and Al2O3 nanofluids samples.
The enhancements of the thermal conductivity at a temperature of 20 °C were measured for diathermic oil and demineralized water, with dispersant, nanofluids samples, with 1.0% Al2O3, CuO and ZnO respectively and are showed in Fig. 6. The enhancements of diathermic oil samples are higher than those of demineralized water, with dispersant, samples with the same volume fraction (1.0%). This result is very encouraging, because the thermal conductivity enhancement is inversely proportional to the base fluid thermal conductivity. Finally, Figs. 7–9 show the thermal conductivity enhancements for samples of diathermic oil nanofluids, with metal oxide nanoparticles. For all the investigated samples, the thermal conductivity behavior was not influenced by the measurement temperature. However, using the diathermic oil nanofluids, the performance of the energy systems can be improved, but it needs to take into account that the nanofluids are liquid–solid suspensions and, therefore, an appropriate design of the equipment to avoid sedimentation is necessary, such as the shape and the slope of the piping and stirring system in tanks. The nanofluids sedimentation yields a decrease in the thermal conductivity and in the convective heat transfer. For example, in thermal solar systems it is possible to use innovative thermal solar collectors with the bottom pipe and the top pipe with a variable cross section along the tubes, to maintain a constant velocity and to avoid sedimentation [24].
4. Conclusions
Fig. 8. Enhancement of thermal conductivity for diathermic oil and CuO nanofluids samples.
to explain this behavior. For example, Keblinky et al. [12] assert that one of the reasons is the ballistic phonon transport when particles distance is very small: if the ballistic phonons initiated in one particle, they can persist in the liquid and reach another particle; because the phonon mean free path is shorter in the liquid than in the solid, this phenomenon can only happen if the separation between particles is in the order of the thickness of the liquid layer film around the particle. The effect of this phenomenon is inversely proportional to particle size. For other authors Brownian motion plays a key role to enhance the thermal conductivity of the nanofluids, such as Jang and Choi [14], including a nanoconvection mode, induced by Brownian motion, that contributes to the heat transfer and it is inversely proportional to the particle size of the solid phase.
An experimental study on thermal conductivity of diathermic oil based nanofluids was carried out. The diathermic oil is a heat transfer fluid used in high temperature applications. The following results were obtained by experimental data: for all the investigated samples and temperature ranges, the thermal conductivity enhancement is directly proportional to the volume fraction; Hamilton–Crosser model undervalues the experimental data, therefore the nanofluids thermal conductivity depends on other heat transfer mechanisms, not considered in this model; the nanofluids thermal conductivity depends on the particle size, that could influence the heat transfer mechanisms; the thermal conductivity enhancement of the nanofluids with diathermic oil is higher than that with demineralized water, with the same nanoparticles and at the same conditions; temperature does not influence the thermal conductivity enhancement of the nanofluids; better results were obtained with ZnO, for nanofluids with metal oxide nanoparticles.
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