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Stability and enhanced thermal conductivity of ethylene glycol-based SiC nanofluids
International Journal of Heat and Mass Transfer 89 (2015) 613–619
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Stability and enhanced thermal conductivity of ethylene glycol-based SiC nanofluids Xiaoke Li a, Changjun Zou a,⇑, Xinyu Lei b, Wenliang Li a a b
Department of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China Organic Chemicals Co., Ltd., Chinese Academy of Sciences, Chengdu 610041, PR China
a r t i c l e
i n f o
Article history: Received 25 March 2015 Received in revised form 22 May 2015 Accepted 24 May 2015
Keywords: Nanofluids SiC nanoparticles Stability Thermal conductivity
a b s t r a c t Nanofluid is one of the most popular fluids in recent scientific community, which exhibits more attractive heat transfer properties than the traditional thermal fluids. Nanofluids are fabricated by dispersing nanostructured solid particles in a selected base liquid. We recently reported on the rheological behavior of ethylene glycol (EG)-based nanofluids containing spherical-shaped silicon carbide (SiC) nanoparticles. In this study, we focused on the stability and thermo-physical property of EG/SiC nanofluids to investigate the enhanced thermal conductivity with respect to the effect of volume fraction and temperature. A series of SiC nanofluids with volume fraction up to 1.0 vol.% were made with this purpose. The results of sedimentation experiment and zeta potential analysis confirmed that the nanofluids exhibited good stability. In addition, enhancement up to 16.21% in thermal conductivity of nanofluids with volume fractions was found compared to base fluids and theoretical model. The thermal conductivity also increased with the measured temperature ranged from 20 to 50 °C. We do hope that our current work could be useful for future research of SiC nanofluids as sharing a desirable direction of nanofluids research. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nanofluids, which have been firstly proposed by Dr. Choi [1], are the colloidal suspensions of nano-sized particles (1–100 nm) dispersed in base fluids with the purpose to enhance their thermal performances [2]. Then, the rapid development of nanotechnology in the 21st century has led to considerable demands for nanofluids in different fields. This is because nanofluids could enhance the thermal conductivity (TC) significantly and eventually increase heat transfer coefficient of the fluid [3], which is required in various fields such as nuclear systems, electronic devices, heat exchanger systems, chemical industries and automobiles and so on [4]. Therefore, the advanced concepts of nanofluids have attracted attentions from the researchers worldwide. And there are numerous researches on the superior heat transfer properties of nanofluids. Choi et al. [5], Mahjoub et al. [6], Patrice Estellé et al. [7] and Ding et al. [8] observed great enhancement of TC of carbon nanotube nanofluids. The improved transient heat transfer performance of ZnO–propylene glycol nanofluids was reported by Rajan and his co-workers [9]. Besides, the enhanced thermophysical properties of nanofluids containing Al2O3, SiO2, and CuO etc. ⇑ Corresponding author at: No. 8 Xindu Avenue, Xindu District, Chengdu 610500, PR China. Tel.: +86 02883037327; fax: +86 02883037305. E-mail address: [email protected] (C. Zou). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.096 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
have attracted some more attention in recent years [10–15]. In addition, many review articles also have been published till date [16–21]. However, due to the diversities of nanoparticles and base fluids, the research gap in the literatures is still existed. For example, to the best of authors’ knowledge, limited studies have been reported about the enhanced thermal conductivity of SiC nanofluids [22–26]. Bang et al. [22] investigated the TC of deionized water based SiC nanofluids, and approximately 7.2% of enhancement was found compared with deionized water. The study of Paul et al. [23] showed that the TC of water-based SiC nanofluids can be enhanced by 12% with only 0.1 vol.% nanoparticles. The mixture of EG and H2O based SiC nanofluids were studied by Timofeeva et al. [24,25] and Toprak et al. [26]. The reported nanofluids displayed 1.5–20% TC enhancement with different nanoparticles concentrations and sizes. On the other hand, only one research was found to focus on the enhanced thermophysical properties of EG-based SiC nanofluids [27]. However, the stability of nanofluids was not mentioned in that study and the test temperature was very limited. The summary of the literatures on SiC nanofluids for heat transfer applications is shown in Table 1. We believe that the potential applications of SiC in nanofluids are far more than that. Because SiC is one of the most promising non-oxide materials and has excellent properties such as high thermal conductivity (270 W/(m K)) [28], low thermal expansion
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Nomenclature Symbols k thermal conductivity [W/(m K)] uv volume fraction of nanofluids [%] m mass [g] q density [g/cm2] kr relative thermal conductivity
Subscripts p nanoparticle b base liquid n nanofluids
coefficient and chemical inertness [29,30]. Furthermore, the production technology of SiC nanoparticles is mature, which makes the SiC nanoparticles commercially available and relatively inexpensive. Therefore, the objective of present work is to explore the thermophysical properties of EG–SiC nanofluids in order to fill the research gap in the literatures. The selected base liquid, EG, is one of the most conventional heat transfer fluids. It is widely used as the base liquid of nanofluids due to its wide applicable temperature range and popular price [31,32]. In this paper, homogeneous and stable EG-based SiC nanofluids with different volume fractions were fabricated. Then, the enhanced TC of the nanofluids at different volume concentrations and temperatures were examined, analyzed, and discussed.
2. Experimental 2.1. Preparation of nanofluid It is widely known that the nanofluid is not simply a solid–liquid mixture. It is more reasonable to treat nanofluid as a special kind of functionalized colloid suspensions [32]. Therefore, the well-known two-step method was used in this paper to prepare EG-based SiC nanofluids. The SiC nanoparticles used in this paper were purchased from C.W. Nanotechnology Company (Shanghai, China) and the average particle diameter is 30 nm. The base liquid EG, dispersant polyvinyl pyrrolidone (PVP) and pH regulator sodium hydroxide (NaOH) were obtained from Kelong chemical reagent factory (Chengdu, China). The transmission electron microscope (TEM) was used to identify the morphology of isolated nano-SiC particles. As shown in Fig. 1, SiC nanoparticles have a spherical shape and they are in the form of large agglomerates. A series of nanofluids were fabricated via two-step method. The process of two-step method was conducted as follows. (I) SiC nanoparticles were suspended in the weighed base fluid at different volume fraction. (II) The magnetic stirring was used for 1 h to mix the EG/SiC suspension. During this process, a specific amount of PVP as dispersant was added into ensure the stable dispersion. In addition, the pH value of samples was adjusted to 11 by NaOH and it was confirmed by a precise F-51 pH meter (HORIBA, China, Beijing). (III) An ultra-sonication homogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) was continuously used for
12 h to obtain uniform nanofluids. All the processes were conducted at the room temperature (22.5 °C). The volume fractions of nanofluids in this study were 0.1–1.0 vol.% and they were calculated using Eq. (1).
uv ¼
mn =qn ðmn =qn Þ þ ðmb =qb Þ
where uv means the volume fraction of nanofluids (%), m stands for quality and q determines the density. The subscripts n and b represent nanofluids and base liquid, respectively. 2.2. Stability of nanofluids and morphology The stability of nanofluids is a critical factor that must be taken into account because the sedimentation or stratification of nanoparticles would decrease the TC of nanofluids significantly [33,34]. Initially, the microstructure of SiC nanofluids (0.2 vol.%) was characterized by the scanning electron microscope (SEM) Quanta 450 (FEI, USA, Hillsboro) 24 h and 30 days after preparation for stability inspection, respectively. At the same time, the sedimentation experiment was conducted. In addition, zeta potential (f) analysis is one of the most common used method to evaluate the stability of nanofluids [35]. Therefore, a Zeta PALS 190 Plus instrument (Brookhaven Instruments Corporation, USA, Austin) was used to check the zeta-potential of each sample. 2.3. Measurement of thermal conductivity of nanofluids The TCs of nanofluids and base fluid were measured by using a KD2-Pro thermal analyzer (Decagon Devices Inc., USA) which was based on the transient hot wire. The KS-1 sensor was used for the TC measurement of liquid. In thermal conductivity measurement, a 30 mL test tube containing test sample was placed in an
Table 1 Summary of available literatures on thermo-physical properties of SiC nanofluids. Base fluid
pH
Size (nm)
Temperature (°C)
Water Water Water/EG Water Water/EG Water EG
11 Not reported 9.4 9–10 9.5 10 Not reported
100 27 16–90 170 30–115 26 26
22.5 3.7–7.2 Room temperature 12–26 22.5 7–12.5 22.5 4–28 20 20 4 9 4 10
Enhancement Refs. of TC (%) [20] [21] [22] [23] [24] [25] [25]
ð1Þ
Fig. 1. TEM micrograph of SiC nanoparticles.
X. Li et al. / International Journal of Heat and Mass Transfer 89 (2015) 613–619
isothermal water bath vertically and the probe was completely immersed in the nanofluids. The temperature of water bath could be controlled by a temperature controller and the complete set of equipment was placed on a horizontal place with poor vibration. In order to obtain accurate results, the experimental apparatus was initially calibrated by glycerin, which was provided by Decagon Devices Inc. for verification use only, with an estimated accuracy less than 2%. On the other hand, the vessel and probe were maintained at a constant temperature for 15 min to reach equilibration before each measurement. Once the equilibration is achieved, ten measurements are taken at a given temperature and the average value was reported. In the present work, the TCs of SiC nanofluids at different volume fractions (0.2–1.0%) were measured under the temperature of 20 °C. In addition, the effect of temperature on the enhancement of the thermal conductivity of the SiC nanofluids was also studied and the tested temperature were 20 °C, 30 °C, 40 °C and 50 °C, respectively. It should be pointed out that the maximum measurable temperature of KS-1 sensor is 50 °C. So far, many efforts have been made to predict the enhancement of thermal conductivity of nanofluids. And the Maxwell model [36] is one of the most classical and representative models, which was proposed to predict the thermal conductivity of suspensions with spherical particles. The form of the Maxwell model is as follows:
kn 3 ða 1Þ uv ¼1þ kb ða þ 2Þ ða 1Þuv
ð2Þ
where kn is the thermal conductivity of nanofluids, kb is the thermal conductivity of base fluid, u is the particle volume fraction, a equals kp/kb and kp represents the thermal conductivity of selected nanoparticles. Some other classical or modified models for thermal conductivity considering different factors were developed based on Maxwell model [37–40]. However, in this study, we only considered the Maxwell model because SiC nanoparticles used in our study were spheres. 3. Results and discussion
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SiC concentration was also investigated. The results are shown in Fig. 4(a) and (b), respectively. Generally, a suspension with an absolute zeta-potential above 30 mV is considered to have moderate stability and those above 45 mV indicate good stability and possible settling [41]. In this study, the absolute zeta potential values of test samples were between 53 and 54 mV approximately. The lowest absolute zeta potential value was found to be 52.7 mV on the sample of 1.0 vol.% SiC nanofluids. The reason might be that the probability of the collision of particles related to the Brown motion was higher at high volume fraction, which led to more particle clusters. And these clusters might decrease the stability of nanofluids. Bang and his co-workers [22] prepared stable water-based SiC nanofluids and found the similar value of absolute zeta potential (51.6 mV). Besides, the results of Fig. 4(b) indicated that there were fluctuations of zeta potential value against the standing time, but they were insignificant. To be clear, the values of zeta potential as a function of standing time for the SiC nanofluids with the other volume fractions were also tested, and the data showed similar tendency with Fig. 4(b) (see Supplementary material). Therefore, the results of zeta potential matched with qualitative tests above, which indicated the suitable stability of the SiC nanofluids fabricated in this paper. 3.2. Thermal conductivity of EG-based SiC nanofluids 3.2.1. Effect of volume fraction The thermal conductivity of the SiC nanofluids as a function of SiC volume fractions have been measured at temperature of 20 °C. The result is shown in Fig. 5(a). And Fig. 5(b) shows variation of thermal conductivity ratio (with respect to EG) as a function of particle concentration. It has been observed that the thermal conductivity ratio increases nonlinearly with increase in volume fractions. In addition, Fig. 6 reveals the relative thermal conductivity (kr = 100(kn kb)/kb) as a function of volume fraction at temperature of 20 °C. As can be seen obviously, both the TC and kr of SiC nanofluids increased with the increasing of volume fractions until the maximum value (0.294 W/(m K) and 16.21%, respectively) was reached at 1.0 vol.%, and the increasing lines were nearly linear. Linear fit of the recorded experimental data with a R2 value of 0.91187 was expressed as follows:
3.1. Stability of nanofluids
kr ¼ 4:583 þ 12:71 uv
In the present, homogeneous and stable EG-based SiC nanofluids were made by the two-step method. During the sedimentation experiment, no visually observable sedimentation or stratification was found even after one month (see Fig. 2). It should be pointed out that in the sedimentation experiment, the nanofluids with different volume fractions were stored in open containers for 30 days. In addition, the SEM micrograph of nanofluids containing 0.2 vol.% SiC nanoparticles are shown in Fig. 3. As shown in Fig. 3(a), although few small agglomerates were existed in the nanofluids, the nanoparticles or agglomerates spread evenly. After the sedimentation experiment (30 days after preparation), another SEM image was acquired (see Fig. 3(b)). It should be pointed out that we used the same sample (0.2 vol.% nanofluids) to do the comparative experiment. More small agglomerates (highlighted by red circles) showed up in the micrograph, but these agglomerations of nanoparticles were not visually observable and their distribution in the base fluid was still well-proportioned. However, the sedimentation experiment and micrograph analysis are qualitative tests. In order to investigate the stability of nanofluids exactly, the zeta potential of SiC nanofluids at different volume fractions was measured. In addition, the zeta potential as a function of standing time for the nanofluid sample with 0.2 vol.%
Similar trend of thermal conductivity enhancement was reported by Paul et al. [23] and Xie et al. [27] on the water-based SiC nanofluids. It should be noted that the relative thermal conductivities recorded in their studies were higher than that in our present work. This is mainly because the thermal conductivity of pure water is higher than that of EG. Besides, the experimental result was much larger than that from the Maxwell model at the measured volume fractions. Similar results were reported by Bang and co-workers [22]. So far, various modified models were proposed to predict the enhanced thermal conductivity of different nanofluids considering many other possible factors, such as size and shape of nanoparticles, interface resistance between the nanoparticles, intensity of Brownian motion and aggregation of nanoparticles etc. However, controversy about prediction of thermal conductivity is still existed and there is no well-accepted conclusion on it. Therefore, more theoretical and experimental works are needed in the future. However, we do hope that our current work could be useful for future research of SiC nanofluids. On the other hand, the enhanced thermal conductivity of SiC nanofluids could be well understood according to effective medium theories [42]. The nanoparticles have clustered into tiny
ð3Þ
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Fig. 2. SiC–EG nanofluids prepared with (a) 0.2 vol.%, (b) 0.4 vol.%, (c) 0.6 vol.%, (d) 0.8 vol.%, (e) 1.0 vol.% 30 days after preparation.
Fig. 3. SEM micrograph of EG-based SiC nanofluids (0.2 vol.%): (a) 24 h after preparation; (b) 30 days after preparation.
aggregates due to the Brownian motion and the collisions of nanoparticles. Since the volume of clusters is much bigger than that of individual nanoparticles, the volume fraction of the aggregates is larger than the volume fraction of nanoparticles, which results to the increasing of the effective volume of a highly conductive region. According to the Hamilton–Crosser theory [43,44], the effective thermal conductivity could be expressed as follow:
kef ¼ kb
kp þ ðn 1Þkb þ ðn 1Þðkp kb Þ V p kp þ ðn 1Þkb V p ðkp kb Þ
ð4Þ
where kef is the effective thermal conductivity, Vp is the effective volume and n is three for spheres particles. For the SiC nanoparticles, kp kb. Thus, Eq. (4) could be expressed as follow:
kef ¼ kb
1 þ 2V p 1 Vp
ð5Þ
Therefore, the higher value of Vp may increase the thermal conductivity significantly.
X. Li et al. / International Journal of Heat and Mass Transfer 89 (2015) 613–619
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Fig. 4. Zeta potential of SiC nanofluids (a) at different volume fractions; (b) as a function of standing time.
Fig. 5. Thermal conductivity–volume fraction curve of EG/SiC nanofluids.
Fig. 6. Relative thermal conductivity–volume fraction curve of EG/SiC nanofluids.
Fig. 7. Temperature dependence of thermal conductivity of EG/SiC nanofluids.
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3.2.2. Effect of temperature Thermal conductivity of the nanofluids as a function of measurement temperature is exhibited in Fig. 7. As can be seen form Fig. 7, the values of TC of SiC nanofluids for all concentrations increased almost linearly with temperature up to 50 °C. However, the increasing of pure EG was not significant as SiC nanofluids. Farbod et al. [45] found similar results on water-based carbon nanotube nanofluids. The same phenomenon occurred on Ag/TiO2-water nanofluids [46] whose TC exhibited obvious increases with temperature. Therefore, the conclusion could be conducted that the main reason of observed temperature dependence was the addition of nanoparticles. In addition, the Brownian motion of nanoparticles became more intense with the increase of temperature, which allowed a much more rapid heat flow among particles, resulted to the enhancement of thermal conductivity. 4. Conclusions In this paper, experimental work was conducted and theoretical analyses were performed scientifically and objectively. Initially, homogeneous and highly stable EG-based SiC nanofluids containing 1.0 vol.% were made by using two-step method. The sedimentation experiment, micrograph analysis and zeta potential analysis were conducted to confirm the excellent stability of nanofluids. Then the thermophysical properties of SiC nanofluids were investigated. The results showed that nanoparticles volume fractions affect the thermal conductivity of nanofluids significantly. It increased with increasing of volume fraction, and the measured maximum relative thermal conductivity value was 16.21% on the 1.0 vol.% EG/SiC nanofluids. Interpretation of the results from effective medium theories help to understand the enhanced thermophysical properties of nanofluids. According to the experimental date, a simple fitting equation was obtained for computing purpose. In addition, the temperature also puts an obvious influence on the thermal conductivity of nanofluids. It increased with the temperature up to 50 °C. The current results would be useful for future research of SiC nanofluids as well as filling the research gap about EG-based SiC nanofluids in the literatures. Conflict of interest None declared. Acknowledgments The authors would like to acknowledge the financial support of The National Natural Science Foundation of China, China National Petroleum Corporation Petrochemical Unite Funded Project (U1262111). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.096. References [1] S.U.S. Choi, Developments and Applications of Non-Newtonian Flows, 231, ASME, New York, 1995. 99–102. [2] P. Keblinski, J.A. Eastman, D.G. Cahill, Nanofluids for thermal transport, Mater. Today 8 (2005) 36–44. [3] W. Yu, D.M. France, J.L. Routbort, S.U.S. Choi, Review and comparison of nanofluid thermal conductivity and heat transfer enhancements, Heat Transfer Eng. 29 (2008) 432–460.
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Stability and enhanced thermal conductivity of ethylene glycol-based SiC nanofluids Xiaoke Li a, Changjun Zou a,⇑, Xinyu Lei b, Wenliang Li a a b
Department of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China Organic Chemicals Co., Ltd., Chinese Academy of Sciences, Chengdu 610041, PR China
a r t i c l e
i n f o
Article history: Received 25 March 2015 Received in revised form 22 May 2015 Accepted 24 May 2015
Keywords: Nanofluids SiC nanoparticles Stability Thermal conductivity
a b s t r a c t Nanofluid is one of the most popular fluids in recent scientific community, which exhibits more attractive heat transfer properties than the traditional thermal fluids. Nanofluids are fabricated by dispersing nanostructured solid particles in a selected base liquid. We recently reported on the rheological behavior of ethylene glycol (EG)-based nanofluids containing spherical-shaped silicon carbide (SiC) nanoparticles. In this study, we focused on the stability and thermo-physical property of EG/SiC nanofluids to investigate the enhanced thermal conductivity with respect to the effect of volume fraction and temperature. A series of SiC nanofluids with volume fraction up to 1.0 vol.% were made with this purpose. The results of sedimentation experiment and zeta potential analysis confirmed that the nanofluids exhibited good stability. In addition, enhancement up to 16.21% in thermal conductivity of nanofluids with volume fractions was found compared to base fluids and theoretical model. The thermal conductivity also increased with the measured temperature ranged from 20 to 50 °C. We do hope that our current work could be useful for future research of SiC nanofluids as sharing a desirable direction of nanofluids research. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nanofluids, which have been firstly proposed by Dr. Choi [1], are the colloidal suspensions of nano-sized particles (1–100 nm) dispersed in base fluids with the purpose to enhance their thermal performances [2]. Then, the rapid development of nanotechnology in the 21st century has led to considerable demands for nanofluids in different fields. This is because nanofluids could enhance the thermal conductivity (TC) significantly and eventually increase heat transfer coefficient of the fluid [3], which is required in various fields such as nuclear systems, electronic devices, heat exchanger systems, chemical industries and automobiles and so on [4]. Therefore, the advanced concepts of nanofluids have attracted attentions from the researchers worldwide. And there are numerous researches on the superior heat transfer properties of nanofluids. Choi et al. [5], Mahjoub et al. [6], Patrice Estellé et al. [7] and Ding et al. [8] observed great enhancement of TC of carbon nanotube nanofluids. The improved transient heat transfer performance of ZnO–propylene glycol nanofluids was reported by Rajan and his co-workers [9]. Besides, the enhanced thermophysical properties of nanofluids containing Al2O3, SiO2, and CuO etc. ⇑ Corresponding author at: No. 8 Xindu Avenue, Xindu District, Chengdu 610500, PR China. Tel.: +86 02883037327; fax: +86 02883037305. E-mail address: [email protected] (C. Zou). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.096 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
have attracted some more attention in recent years [10–15]. In addition, many review articles also have been published till date [16–21]. However, due to the diversities of nanoparticles and base fluids, the research gap in the literatures is still existed. For example, to the best of authors’ knowledge, limited studies have been reported about the enhanced thermal conductivity of SiC nanofluids [22–26]. Bang et al. [22] investigated the TC of deionized water based SiC nanofluids, and approximately 7.2% of enhancement was found compared with deionized water. The study of Paul et al. [23] showed that the TC of water-based SiC nanofluids can be enhanced by 12% with only 0.1 vol.% nanoparticles. The mixture of EG and H2O based SiC nanofluids were studied by Timofeeva et al. [24,25] and Toprak et al. [26]. The reported nanofluids displayed 1.5–20% TC enhancement with different nanoparticles concentrations and sizes. On the other hand, only one research was found to focus on the enhanced thermophysical properties of EG-based SiC nanofluids [27]. However, the stability of nanofluids was not mentioned in that study and the test temperature was very limited. The summary of the literatures on SiC nanofluids for heat transfer applications is shown in Table 1. We believe that the potential applications of SiC in nanofluids are far more than that. Because SiC is one of the most promising non-oxide materials and has excellent properties such as high thermal conductivity (270 W/(m K)) [28], low thermal expansion
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Nomenclature Symbols k thermal conductivity [W/(m K)] uv volume fraction of nanofluids [%] m mass [g] q density [g/cm2] kr relative thermal conductivity
Subscripts p nanoparticle b base liquid n nanofluids
coefficient and chemical inertness [29,30]. Furthermore, the production technology of SiC nanoparticles is mature, which makes the SiC nanoparticles commercially available and relatively inexpensive. Therefore, the objective of present work is to explore the thermophysical properties of EG–SiC nanofluids in order to fill the research gap in the literatures. The selected base liquid, EG, is one of the most conventional heat transfer fluids. It is widely used as the base liquid of nanofluids due to its wide applicable temperature range and popular price [31,32]. In this paper, homogeneous and stable EG-based SiC nanofluids with different volume fractions were fabricated. Then, the enhanced TC of the nanofluids at different volume concentrations and temperatures were examined, analyzed, and discussed.
2. Experimental 2.1. Preparation of nanofluid It is widely known that the nanofluid is not simply a solid–liquid mixture. It is more reasonable to treat nanofluid as a special kind of functionalized colloid suspensions [32]. Therefore, the well-known two-step method was used in this paper to prepare EG-based SiC nanofluids. The SiC nanoparticles used in this paper were purchased from C.W. Nanotechnology Company (Shanghai, China) and the average particle diameter is 30 nm. The base liquid EG, dispersant polyvinyl pyrrolidone (PVP) and pH regulator sodium hydroxide (NaOH) were obtained from Kelong chemical reagent factory (Chengdu, China). The transmission electron microscope (TEM) was used to identify the morphology of isolated nano-SiC particles. As shown in Fig. 1, SiC nanoparticles have a spherical shape and they are in the form of large agglomerates. A series of nanofluids were fabricated via two-step method. The process of two-step method was conducted as follows. (I) SiC nanoparticles were suspended in the weighed base fluid at different volume fraction. (II) The magnetic stirring was used for 1 h to mix the EG/SiC suspension. During this process, a specific amount of PVP as dispersant was added into ensure the stable dispersion. In addition, the pH value of samples was adjusted to 11 by NaOH and it was confirmed by a precise F-51 pH meter (HORIBA, China, Beijing). (III) An ultra-sonication homogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) was continuously used for
12 h to obtain uniform nanofluids. All the processes were conducted at the room temperature (22.5 °C). The volume fractions of nanofluids in this study were 0.1–1.0 vol.% and they were calculated using Eq. (1).
uv ¼
mn =qn ðmn =qn Þ þ ðmb =qb Þ
where uv means the volume fraction of nanofluids (%), m stands for quality and q determines the density. The subscripts n and b represent nanofluids and base liquid, respectively. 2.2. Stability of nanofluids and morphology The stability of nanofluids is a critical factor that must be taken into account because the sedimentation or stratification of nanoparticles would decrease the TC of nanofluids significantly [33,34]. Initially, the microstructure of SiC nanofluids (0.2 vol.%) was characterized by the scanning electron microscope (SEM) Quanta 450 (FEI, USA, Hillsboro) 24 h and 30 days after preparation for stability inspection, respectively. At the same time, the sedimentation experiment was conducted. In addition, zeta potential (f) analysis is one of the most common used method to evaluate the stability of nanofluids [35]. Therefore, a Zeta PALS 190 Plus instrument (Brookhaven Instruments Corporation, USA, Austin) was used to check the zeta-potential of each sample. 2.3. Measurement of thermal conductivity of nanofluids The TCs of nanofluids and base fluid were measured by using a KD2-Pro thermal analyzer (Decagon Devices Inc., USA) which was based on the transient hot wire. The KS-1 sensor was used for the TC measurement of liquid. In thermal conductivity measurement, a 30 mL test tube containing test sample was placed in an
Table 1 Summary of available literatures on thermo-physical properties of SiC nanofluids. Base fluid
pH
Size (nm)
Temperature (°C)
Water Water Water/EG Water Water/EG Water EG
11 Not reported 9.4 9–10 9.5 10 Not reported
100 27 16–90 170 30–115 26 26
22.5 3.7–7.2 Room temperature 12–26 22.5 7–12.5 22.5 4–28 20 20 4 9 4 10
Enhancement Refs. of TC (%) [20] [21] [22] [23] [24] [25] [25]
ð1Þ
Fig. 1. TEM micrograph of SiC nanoparticles.
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isothermal water bath vertically and the probe was completely immersed in the nanofluids. The temperature of water bath could be controlled by a temperature controller and the complete set of equipment was placed on a horizontal place with poor vibration. In order to obtain accurate results, the experimental apparatus was initially calibrated by glycerin, which was provided by Decagon Devices Inc. for verification use only, with an estimated accuracy less than 2%. On the other hand, the vessel and probe were maintained at a constant temperature for 15 min to reach equilibration before each measurement. Once the equilibration is achieved, ten measurements are taken at a given temperature and the average value was reported. In the present work, the TCs of SiC nanofluids at different volume fractions (0.2–1.0%) were measured under the temperature of 20 °C. In addition, the effect of temperature on the enhancement of the thermal conductivity of the SiC nanofluids was also studied and the tested temperature were 20 °C, 30 °C, 40 °C and 50 °C, respectively. It should be pointed out that the maximum measurable temperature of KS-1 sensor is 50 °C. So far, many efforts have been made to predict the enhancement of thermal conductivity of nanofluids. And the Maxwell model [36] is one of the most classical and representative models, which was proposed to predict the thermal conductivity of suspensions with spherical particles. The form of the Maxwell model is as follows:
kn 3 ða 1Þ uv ¼1þ kb ða þ 2Þ ða 1Þuv
ð2Þ
where kn is the thermal conductivity of nanofluids, kb is the thermal conductivity of base fluid, u is the particle volume fraction, a equals kp/kb and kp represents the thermal conductivity of selected nanoparticles. Some other classical or modified models for thermal conductivity considering different factors were developed based on Maxwell model [37–40]. However, in this study, we only considered the Maxwell model because SiC nanoparticles used in our study were spheres. 3. Results and discussion
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SiC concentration was also investigated. The results are shown in Fig. 4(a) and (b), respectively. Generally, a suspension with an absolute zeta-potential above 30 mV is considered to have moderate stability and those above 45 mV indicate good stability and possible settling [41]. In this study, the absolute zeta potential values of test samples were between 53 and 54 mV approximately. The lowest absolute zeta potential value was found to be 52.7 mV on the sample of 1.0 vol.% SiC nanofluids. The reason might be that the probability of the collision of particles related to the Brown motion was higher at high volume fraction, which led to more particle clusters. And these clusters might decrease the stability of nanofluids. Bang and his co-workers [22] prepared stable water-based SiC nanofluids and found the similar value of absolute zeta potential (51.6 mV). Besides, the results of Fig. 4(b) indicated that there were fluctuations of zeta potential value against the standing time, but they were insignificant. To be clear, the values of zeta potential as a function of standing time for the SiC nanofluids with the other volume fractions were also tested, and the data showed similar tendency with Fig. 4(b) (see Supplementary material). Therefore, the results of zeta potential matched with qualitative tests above, which indicated the suitable stability of the SiC nanofluids fabricated in this paper. 3.2. Thermal conductivity of EG-based SiC nanofluids 3.2.1. Effect of volume fraction The thermal conductivity of the SiC nanofluids as a function of SiC volume fractions have been measured at temperature of 20 °C. The result is shown in Fig. 5(a). And Fig. 5(b) shows variation of thermal conductivity ratio (with respect to EG) as a function of particle concentration. It has been observed that the thermal conductivity ratio increases nonlinearly with increase in volume fractions. In addition, Fig. 6 reveals the relative thermal conductivity (kr = 100(kn kb)/kb) as a function of volume fraction at temperature of 20 °C. As can be seen obviously, both the TC and kr of SiC nanofluids increased with the increasing of volume fractions until the maximum value (0.294 W/(m K) and 16.21%, respectively) was reached at 1.0 vol.%, and the increasing lines were nearly linear. Linear fit of the recorded experimental data with a R2 value of 0.91187 was expressed as follows:
3.1. Stability of nanofluids
kr ¼ 4:583 þ 12:71 uv
In the present, homogeneous and stable EG-based SiC nanofluids were made by the two-step method. During the sedimentation experiment, no visually observable sedimentation or stratification was found even after one month (see Fig. 2). It should be pointed out that in the sedimentation experiment, the nanofluids with different volume fractions were stored in open containers for 30 days. In addition, the SEM micrograph of nanofluids containing 0.2 vol.% SiC nanoparticles are shown in Fig. 3. As shown in Fig. 3(a), although few small agglomerates were existed in the nanofluids, the nanoparticles or agglomerates spread evenly. After the sedimentation experiment (30 days after preparation), another SEM image was acquired (see Fig. 3(b)). It should be pointed out that we used the same sample (0.2 vol.% nanofluids) to do the comparative experiment. More small agglomerates (highlighted by red circles) showed up in the micrograph, but these agglomerations of nanoparticles were not visually observable and their distribution in the base fluid was still well-proportioned. However, the sedimentation experiment and micrograph analysis are qualitative tests. In order to investigate the stability of nanofluids exactly, the zeta potential of SiC nanofluids at different volume fractions was measured. In addition, the zeta potential as a function of standing time for the nanofluid sample with 0.2 vol.%
Similar trend of thermal conductivity enhancement was reported by Paul et al. [23] and Xie et al. [27] on the water-based SiC nanofluids. It should be noted that the relative thermal conductivities recorded in their studies were higher than that in our present work. This is mainly because the thermal conductivity of pure water is higher than that of EG. Besides, the experimental result was much larger than that from the Maxwell model at the measured volume fractions. Similar results were reported by Bang and co-workers [22]. So far, various modified models were proposed to predict the enhanced thermal conductivity of different nanofluids considering many other possible factors, such as size and shape of nanoparticles, interface resistance between the nanoparticles, intensity of Brownian motion and aggregation of nanoparticles etc. However, controversy about prediction of thermal conductivity is still existed and there is no well-accepted conclusion on it. Therefore, more theoretical and experimental works are needed in the future. However, we do hope that our current work could be useful for future research of SiC nanofluids. On the other hand, the enhanced thermal conductivity of SiC nanofluids could be well understood according to effective medium theories [42]. The nanoparticles have clustered into tiny
ð3Þ
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Fig. 2. SiC–EG nanofluids prepared with (a) 0.2 vol.%, (b) 0.4 vol.%, (c) 0.6 vol.%, (d) 0.8 vol.%, (e) 1.0 vol.% 30 days after preparation.
Fig. 3. SEM micrograph of EG-based SiC nanofluids (0.2 vol.%): (a) 24 h after preparation; (b) 30 days after preparation.
aggregates due to the Brownian motion and the collisions of nanoparticles. Since the volume of clusters is much bigger than that of individual nanoparticles, the volume fraction of the aggregates is larger than the volume fraction of nanoparticles, which results to the increasing of the effective volume of a highly conductive region. According to the Hamilton–Crosser theory [43,44], the effective thermal conductivity could be expressed as follow:
kef ¼ kb
kp þ ðn 1Þkb þ ðn 1Þðkp kb Þ V p kp þ ðn 1Þkb V p ðkp kb Þ
ð4Þ
where kef is the effective thermal conductivity, Vp is the effective volume and n is three for spheres particles. For the SiC nanoparticles, kp kb. Thus, Eq. (4) could be expressed as follow:
kef ¼ kb
1 þ 2V p 1 Vp
ð5Þ
Therefore, the higher value of Vp may increase the thermal conductivity significantly.
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Fig. 4. Zeta potential of SiC nanofluids (a) at different volume fractions; (b) as a function of standing time.
Fig. 5. Thermal conductivity–volume fraction curve of EG/SiC nanofluids.
Fig. 6. Relative thermal conductivity–volume fraction curve of EG/SiC nanofluids.
Fig. 7. Temperature dependence of thermal conductivity of EG/SiC nanofluids.
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3.2.2. Effect of temperature Thermal conductivity of the nanofluids as a function of measurement temperature is exhibited in Fig. 7. As can be seen form Fig. 7, the values of TC of SiC nanofluids for all concentrations increased almost linearly with temperature up to 50 °C. However, the increasing of pure EG was not significant as SiC nanofluids. Farbod et al. [45] found similar results on water-based carbon nanotube nanofluids. The same phenomenon occurred on Ag/TiO2-water nanofluids [46] whose TC exhibited obvious increases with temperature. Therefore, the conclusion could be conducted that the main reason of observed temperature dependence was the addition of nanoparticles. In addition, the Brownian motion of nanoparticles became more intense with the increase of temperature, which allowed a much more rapid heat flow among particles, resulted to the enhancement of thermal conductivity. 4. Conclusions In this paper, experimental work was conducted and theoretical analyses were performed scientifically and objectively. Initially, homogeneous and highly stable EG-based SiC nanofluids containing 1.0 vol.% were made by using two-step method. The sedimentation experiment, micrograph analysis and zeta potential analysis were conducted to confirm the excellent stability of nanofluids. Then the thermophysical properties of SiC nanofluids were investigated. The results showed that nanoparticles volume fractions affect the thermal conductivity of nanofluids significantly. It increased with increasing of volume fraction, and the measured maximum relative thermal conductivity value was 16.21% on the 1.0 vol.% EG/SiC nanofluids. Interpretation of the results from effective medium theories help to understand the enhanced thermophysical properties of nanofluids. According to the experimental date, a simple fitting equation was obtained for computing purpose. In addition, the temperature also puts an obvious influence on the thermal conductivity of nanofluids. It increased with the temperature up to 50 °C. The current results would be useful for future research of SiC nanofluids as well as filling the research gap about EG-based SiC nanofluids in the literatures. Conflict of interest None declared. Acknowledgments The authors would like to acknowledge the financial support of The National Natural Science Foundation of China, China National Petroleum Corporation Petrochemical Unite Funded Project (U1262111). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.096. References [1] S.U.S. Choi, Developments and Applications of Non-Newtonian Flows, 231, ASME, New York, 1995. 99–102. [2] P. Keblinski, J.A. Eastman, D.G. Cahill, Nanofluids for thermal transport, Mater. Today 8 (2005) 36–44. [3] W. Yu, D.M. France, J.L. Routbort, S.U.S. Choi, Review and comparison of nanofluid thermal conductivity and heat transfer enhancements, Heat Transfer Eng. 29 (2008) 432–460.
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