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Experimental investigation on the thermo-physical properties of Al2O3 nanoparticles suspended in car radiator coolant
International Communications in Heat and Mass Transfer 54 (2014) 48–53
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Experimental investigation on the thermo-physical properties of Al2O3 nanoparticles suspended in car radiator coolant☆ M.M. Elias ⁎, I.M. Mahbubul, R. Saidur ⁎, M.R. Sohel, I.M. Shahrul, S.S. Khaleduzzaman, S. Sadeghipour Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 26 March 2014 Keywords: Nanofluid Volume concentration Temperature Thermal conductivity Viscosity Density Specific heat
a b s t r a c t Nanofluid is a new type of heat transfer fluid with superior thermal performance characteristics, which is very promising for thermal engineering applications. This paper presents new findings on the thermal conductivity, viscosity, density, and specific heat of Al2O3 nanoparticles dispersed into water and ethylene glycol based coolant used in car radiator. The nanofluids were prepared by the two-step method by using an ultrasonic homogenizer with no surfactants. Thermal conductivity, viscosity, density, and specific heat have been measured at different volume concentrations (i.e. 0 to 1 vol.%) of nanoparticles and various temperature ranges (i.e. from 10 °C to 50 °C). It was found that thermal conductivity, viscosity, and density of the nanofluid increased with the increase of volume concentrations. However, specific heat of nanofluid was found to be decreased with the increase of nanoparticle volume concentrations. Moreover, by increasing the temperature, thermal conductivity and specific heat were observed to be intensified, while the viscosity and density were decreased. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Heat transfer enhancement is a major concern in the field of thermal engineering. Therefore, efforts need to be put to improve the heat transfer performance of thermal devices used in many engineering applications. Heat transfer improvement can be made by increasing (i) heat transfer area, (ii) temperature, and (iii) heat transfer co-efficient [1]. However, technologies have already reached their limit for the cases (i) and (ii). Recently many researchers found that dispersing nanosized particles into the liquids result in higher heat transfer co-efficient of these newly developed fluids called nanofluids compared to the traditional liquids [2]. For this reason, nanometer size particles with the diameter of 1–100 nm are suspended in a liquid called nanofluid which was invented by Choi [2] and the author found considerable enhancement of the thermal conductivity of the suspension. Since then, many investigations were carried out by a number of researchers on the thermal conductivity of various nanofluids [1]. Lee et al. [3] experimentally studied the mixture of ethylene glycol and CuO nanoparticles of 35 nm size at the concentration of 4.0 vol.% and found a 20% increase in thermal conductivity. Yu et al. [4] experimentally investigated that, the thermal conductivity of nanofluid strongly depends on nanoparticle volume concentrations and it increases nonlinearly with the increase of volume concentration ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding authors. E-mail addresses: [email protected] (M.M. Elias), [email protected], [email protected] (R. Saidur).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.03.005 0735-1933/© 2014 Elsevier Ltd. All rights reserved.
and the enhanced thermal conductivity was found to be 26.5% at 5.0 vol.% concentration. Duangthongsuk and Wongwises [5] experimentally investigated that the TiO2–water nanofluid offered 3–7% higher thermal conductivity than base fluid for 0.2 to 2.0 vol.% of TiO2 nanoparticles. Viscosity of nanofluids is a parameter as crucial as thermal conductivity for the thermal performance investigation [6,7]. Pumping power is directly related to the pressure drop, which is in turn related to the viscosity of the fluid. Therefore, minimization of viscosity is also a critical factor in addition to the augmentation of thermal conductivity [8]. Nguyen et al. [9] experimentally investigated the effect of volume concentration and temperature on the dynamic viscosity of Al2O3–water nanofluid and found that viscosity of the nanofluid considerably increases with the increase of particle volume concentrations, but it decreases with the increase of temperature. Wang et al. [10] investigated the viscosity of Al2O3–water nanofluid prepared by mechanical blending with particle size of 28 nm at 5 vol.% concentration and viscosity increased by 86% compared to the base fluid. They also investigated Al2O3/ethylene glycol nanofluid and found a 40% increase in viscosity. Das et al. [11] also observed that with the increase of particle volume concentration, viscosity of the nanofluid increases. Density of a fluid is another important thermophysical property. Like viscosity, density of any fluid has direct impact on the pressure drop and pumping power. Density is strongly dependent on the nanoparticle material used, whereas the other parameters such as nanoparticles size, shape, zeta potential and additives do not affect the density of nanofluids [12]. Solids have a greater density compared to liquids; therefore, the density of nanofluids is found to be increased with the
M.M. Elias et al. / International Communications in Heat and Mass Transfer 54 (2014) 48–53
Nomenclature Cp mf n vol. k mn RC
specific heat mass of base fluid shape factor volume thermal conductivity, W/m.K Mass of nanoparticle radiator coolant
Greek symbols ρ Density, kg/m3 ϕ Volume fraction μ Dynamic viscosity
Subscripts f Base fluid eff Effective p Particle nf Nanofluid
increase in concentration of nanoparticles in the fluid. Literatures about the density of nanofluids are still scarce. Pak and Cho [13] for the first time, measured the density of γAl2O3 and TiO2 with distilled water and found that density of nanofluid increases with the increase of nanofluid. Specific heat is also a very important characteristic of nanofluids. To study the energy performance, specific heat of nanofluid must be determined. From the previous literatures it is found that, the specific heat of nanofluids depends on different parameters such as type, size and volume concentration of nanoparticles and base fluids at different temperatures. Pak and Cho [13] first reported that the specific heat of Al2O3– water and TiO2–water nanofluids decreases with the increase of particle volume concentration. Vajjha and Das [14] measured the specific heat of three nanofluids containing Al2O3, ZnO and SiO2 nanoparticles. They also discovered that the specific heat value decreases as the volumetric concentration of nanoparticles increases but the specific heat of those nanofluids increases with the increase of temperature. Preparation of nanofluid is an emergent step to enhance the thermal conductivity and better stability of nanofluids. Thermal conductivity varies according to the particle size, shape, and material of the nanoparticle. For example, the thermal conductivity of nanofluid with metallic/ oxide particles was found higher compared to the nanofluids with other particles. Moreover, oxide based nanofluids have better stability in comparison with other nanofluids. Therefore, in this research Al2O3 nanoparticles were used. Radiators are important accessories for automobiles. It is a kind of heat exchanger system that is used to cool the engines. Thermal management of the engine is a great problem like other thermal devices. If the heat transfer performance of the radiators can be increased, the engine performance can be improved as well. Traditionally, water is used as the coolant in car radiator. However, recently, the 50:50 mixtures of water and ethylene glycol are commercially introduced for better performance of the car radiators. Nanofluid is pronounced for improving the heat transfer performance of the heat exchanger systems [15]. However, before investigating the heat transfer performance, thermal conductivity, viscosity, density and specific heat capacity need to be examined as the performance of the car radiator is dependent on these thermo physical properties. There are limited literatures available on the fundamental properties of radiator coolant based nanofluid. To the best of the authors' knowledge, there is only one literature available on the viscosity of a radiator coolant based nanofluid [16], where the
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authors dispersed Al2O3 nanoparticles in a car radiator coolant and measured the viscosity of that nanofluid for different concentrations at different temperatures. Therefore, in the present study, thermal conductivity, viscosity, density, and specific heat of the radiator coolant based nanofluid containing Al2O3 nanoparticles have been investigated for 0 to 1 vol.% of nanoparticles with temperature ranging from 10 °C to 50 °C. The obtained results have been compared with empirical equations as well. It is expected that, this research will fill the gaps on the fundamental thermophysical properties of the radiator coolant based nanofluids. 2. Experimental procedure The commercial radiator coolant (50:50 mixtures of ethylene glycol and water), which is widely used in automobiles, has been used in this research. Al2O3 nanoparticles were used throughout this experiment; nanoparticles were purchased from Sigma-Aldrich, Malaysia with diameters of 13 nm. The properties were taken without using any surfactant. An ultrasonic homogenizer (Fisher Scientific Model 505 Sonic Dismembrator) was used for preparing the nanofluid for different volume fractions ranging from 0 to 1 vol.%. The ultra-sonication time was 30 min for all of the dispersion. No sedimentations were found within few hours after preparation. Thermal conductivity, viscosity, density, and specific heat were measured in this experiment with 0 to 1 volume concentrations and in 10 °C to 50 °C temperature range. Eq. (1) was used to calculate the volume fractions of nanofluids:
ϕ¼
mn ρn
mn m f þ ρn ρ f
ð1Þ
2.1. Thermal conductivity A KD2-Pro thermal properties analyzer (made by Decagon, USA) was used to measure the thermal conductivity of nanofluids. The accuracy of this equipment was found to be ±0.1 of readings. A refrigerating circulating bath was used to control the temperature of the nanofluid. For the measurement of thermal conductivity, the vessel with tested sample was placed inside the temperature controlled bath and sample temperature was checked by a thermocouple inserted into the vessel. All of the data were taken 3 times and the average of 3 values was used for analysis. Some of the data (about 5%) was eliminated since they were found to be outliers. As there is no literature available on the experimental value of thermal conductivity of the radiator coolant based nanofluid, the following two models were used to compare thermal conductivity of the present work. Maxwell model [17] was used to determine thermal conductivity of the nanofluid:
keff
kp þ2k f þ 2 kp ‐k f ϕ ¼ kp þ2k f − kp ‐k f ϕ
ð2Þ
Hamilton–Crosser model [18] as expressed below was also used to measure thermal conductivity of the nanofluid
keff
kp þ ðn−1Þk f −2ðn−1Þϕ k f −kp kf ¼ kp þ ðn−1Þk f þ ϕ k f ‐kp
ð3Þ
2.2. Viscosity Viscosity of the nanofluids for different volume concentrations of nanoparticles along with the different temperatures was measured by a Brookfield programmable viscometer (model: LVDV-III ultra) which
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M.M. Elias et al. / International Communications in Heat and Mass Transfer 54 (2014) 48–53
was interfaced with a personal computer for data collection and storage. The spindle connected with the viscometer was submerged into the nanofluids. The viscous is developed against the spindle due to deflection of the calibrated spring with the range of viscosity between 1 to 6,000,000 MPa.s with the aid of Ultra low adapter (ULA) attached with the main equipment [19]. In this experiment, the Brookfield ULA with spindle model ULA-49EAY (spindle code 01) has been used. The adapter has the provision for the circulation of temperature of bath fluid. The measurement of viscosity for different volume concentrations ranging from 0 to 1 vol.% were started from 10 °C, and gradually increased to 50 °C at an interval of 10 °C. All of the data have been taken 3 times and the average value has been considered for the analysis. 2.3. Density A portable density meter, KEM-DA 130N (Kyoto, Japan) was used to measure the density of nanofluid ranging from 0 to 1 vol.% concentration of nanoparticles. This device can measure the density within the range of 0 to 2000 kg/m3 with the precision of ±0.001 kg/m3 [20]. All the data has been taken 3 times to get better accuracy and the average value has been taken for the analysis. The density was taken within the temperature range of 15 °C to 50 °C. As no literature exists about the density of a radiator coolant based nanofluid, the model of Pak and Cho [13] was used to be compared with the obtained experimental densities of the nanofluids using Eq. (4). ρnf ¼ ð1−φÞρ f þ φρρ
ð4Þ
2.4. Specific heat
concentrations (%) of Al2O3/radiator coolant nanofluid. Images of nanofluid were taken just after preparation until after 7 days of preparation with a time interval of 1 day (24 h). Fig. 1 shows that, sedimentation starts after 3 days of preparation. The sedimentation of mixtures has been seen at the bottom of the specimen and leveled as well. It is easily understandable from the picture that sedimentation slowly increased each day after 3 days until 7 days. However, until 7 days the sedimentation level was not so high. 3.1. Thermal conductivity
A Differential Scanning Calorimeter (model: DSC 4000, Perkin Elmer, USA) measured the specific heat of the nanofluid for 0 to 1 vol.% concentrations of nanoparticles along with different temperatures. The measurement of specific heat for different temperatures was started from 10 °C, and gradually increased to 50 °C at an interval of 10 °C. All of the data have been taken 3 times and the average value has been considered for the analysis. Because there are no literature available about the specific heat of radiator coolant based nanofluid therefore, the following equation of Xuan and Roetzel [21] has been used to compare the experimental values of this study. C p; nf ¼
Fig. 2. Thermal conductivity of the nanofluid (Al2O3–RC) at different temperatures.
ð1−φÞρ f C p; f þ φρρ C p;p ρnf
ð5Þ
Where, Cp is the specific heat of nanofluids, ρ is the density of nanofluid, and Ø is the volume fraction of nanoparticles. Also nf, f, and s refer to nanofluid, fluid, and solid, respectively. The density of nanofluid for this Eq. (5) has been determined by using Eq. (4) of Pak and Cho [13]. 3. Results and discussion The stability of the nanofluid has been checked with Photograph Capturing Method. Fig. 1 shows the picture of 0.2 volume
The thermal conductivity of Al2O3-radiator coolant nanofluid for different volume concentrations at different temperatures is depicted in Fig. 2. From the figure, it is seen that, thermal conductivity of the nanofluid increases with the increase of volume concentration and temperature. For both base fluid and nanofluid, thermal conductivity increases with the increase of temperature, and the nanofluids show higher thermal conductivity than the base fluid for every volume concentration. This is because of particle movement during the temperature rise. With the intensification of fluid temperature, the movement among the particles increases thus raising the thermal conductivity. At 10 °C, the base fluid thermal conductivity was 0.250 W/m.K and at the same temperature, nanofluid thermal conductivity was 0.270 W/m.K for 1.0 vol.% concentration which is 8% higher than the base fluid. This means that with the increase of volume concentration, thermal conductivity of nanofluid increases. At 50 °C, the thermal conductivities of the base fluid and 1 vol.% of nanofluid were 0.259 W/m.K and 0.280 W/m.K, respectively. The thermal conductivity of the nanofluid at 1 vol.% concentration shows 8.30% increment compared to the base fluid for the same temperature. For each and every case, nanofluid showed increased thermal conductivity than the base fluid. An almost similar increasing trend of thermal conductivity of nanofluid has been observed for the various volume concentrations and temperatures. However, the graph lines are not parallel. The possible reason is that,
Sedimentation level Just after After preparation1 day
After 2 days
After 3 days
After 4 days
After 5 days
After 6 days
After 7 days
Fig. 1. Image of 0.2 volume concentration (%) of Al2O3/radiator coolant nanofluid.
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Fig. 5. Relative viscosity of the nanofluids versus different volume concentrations. Fig. 3. Comparison of the experimental thermal conductivity with two proposed models.
thermal conductivity of the nanofluids not only depends on particle concentration and temperature but also controlled by the other parameters like size and shape. However, in this case it is expected that the negligible variation of the trends could be due to the particle alignment and clustering [22]. There is no experimental data of thermal conductivity of radiator coolant based nanofluids. Therefore, the experimental values of the thermal conductivity of Al2O3–radiator coolant nanofluid at 30 °C have been plotted in Fig. 3 to compare this experimental value with different empirical equations under different volume concentrations ranging from 0 to 1 vol.%. The experimental results of the nanofluid indicated higher thermal conductivities than the models developed by Maxwell [17] and Hamilton–Crosser [18] for all the volume concentrations. It has been observed that in both experimental data and developed models, thermal conductivity increases with the increase of volume concentration. However, the thermal conductivity calculated by Hamilton and Crosser model [18] for Al2O3 based radiator coolant is 1.92% greater than that of Maxwell model [17]. Thermal conductivities obtained from the experiments for 1 vol.% concentration of Al2O3 based nanofluid were 4.26% higher than the calculated thermal conductivities using the proposed model by Maxwell [17] and 2.2% greater than the other proposed model by Hamilton–Crosser [18]. 3.2. Viscosity Viscosities of Al2O3-radiator coolant based nanofluid were measured within the temperature range of 10 °C to 50 °C with different volume
Fig. 4. Viscosity of Al2O3–RC nanofluid as a function of temperature with different volume concentrations.
concentrations ranging from 0 to 1 vol.% of nanoparticles and plotted in Fig. 4. It is shown in Fig. 4 that, the viscosity of nanofluid exponentially decreases with the increase of temperature. The viscosity value of the nanofluid is higher than the base fluid for every volume concentration. It is also observed that, with the increase of nanoparticle concentration in the base fluid, viscosity of the nanofluid intensifies and this amount decreases with the increase of temperature. At 1 vol.% concentration, viscosity of the nanofluid is found to be decreased by 57.5% when the temperature increases from 10 °C to 50 °C and for the base fluid, viscosity is reduced by 73.21% when temperature is elevated from 10 °C to 50 °C. This is a natural phenomenon that in most cases, viscosity of the liquids decreases with the increase of temperature. When temperature of any substance increases the movement among the molecules also increased. For the higher movement of the molecules, the resistance to flow a material (which in terms called viscosity) decreases. The relative viscosities of the Al2O3–RC nanofluids at 30 °C have been determined to be compared with the experimental values available in the literature. Results of the relative viscosity of Al2O3/radiator coolant by Kole and Dey [16] have been selected for the comparison, which have been plotted in Fig. 5. The viscosity value of Al2O3–RC nanofluid, at 0.2 vol.% is the same as the value of Kole and Dey [16]. However, the measured viscosity of this experiment apparently showed higher values compared to those of Kole and Dey [16]. Though, in both cases the nanoparticles and the base fluid are alike, the sources were different. Therefore, there are possibilities that there may be some differences between the purities and ingredients of the two sources, resulting in this slight difference between the viscosity values. Such deviations have been also seen in other experimental values among other researchers for the same type of nanofluids. For example, Pak and Cho [13] found lower viscosities compared to the results of Masuda et al. [23] for the same TiO2–water nanofluid.
Fig. 6. Density of Al2O3–RC nanofluid as a function of temperatures.
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M.M. Elias et al. / International Communications in Heat and Mass Transfer 54 (2014) 48–53
Fig. 9. Comparison of experimental specific heat capacity of the nanofluids with Xuan and Roetzel model [21] model under different volume concentrations at 30 °C. Fig. 7. Comparison of experimental density of the nanofluids with Pak and Cho [13] model under different volume concentrations at 30 °C.
3.3. Density The densities of Al2O3–RC nanofluid and pure radiator coolant with different temperatures for the volume concentrations of 0 to 1.0 vol.% are portrayed in Fig. 6. The figure shows that the density of Al2O3–RC nanofluid and pure radiator coolant decreases with the increase of temperature, which is related to the increase in the fluid volume by increasing the temperature. For example, the density of the base fluid decreases by 1.8%, when the temperature increases from 15 °C to 50 °C. However, density increases with the increase of volume concentration and in all cases, nanofluid gave higher density than the base fluid. The higher density of the dispersed nanoparticles in the fluid evidently causes this. As there is no literature available about density of the radiator coolant based nanofluids, therefore to validate the experimental results, the measured densities have been compared with the model of Pak and Cho [13]. Experimental values of the density at 30 °C temperature and the densities obtained by the model are depicted in Fig. 7. Here, it is shown that, nanofluid densities measured by Pak and Cho model [13] present almost similar values for Al2O3 based nanofluid. However, the experimental values of Al2O3–RC showed little deviation from the model results of density of the nanofluid especially at 0.6 vol.%. 3.4. Specific heat Fig. 8 shows the effect of temperatures on specific heat for different volume concentrations of nanofluid. Here, specific heat increased with the increase of temperature. The increasing trend was constant and
Fig. 8. Specific heat of Al2O3–RC nanofluid as a function of temperature.
linear. However, specific heat decreased with the increase of volume concentration of nanofluid. This decrease is due to the lower specific heat of the added particles compared to that of the base fluid. Moreover, higher specific heat of the base fluid is the reason why the specific heat value of the mixture becomes higher than that of the nanoparticles. Therefore, it becomes lower than the specific heat value of the base fluid. In Fig. 8, though the gap between two concentrations was constant however, the gaps between decreasing line for two consecutive concentrations were not same. This may have happened because a very small amount of sample was used to measure the specific heat. Some milligrams only (a fraction of a drop of suspension) was used in DSC machine. In this small amount of nanofluids the ratio between two concentrations may have varied. Therefore, the specific heat values slightly changed. Fig. 9 shows the comparison among the experimental specific heat capacity of nanofluid with the computed values obtained from Xuan and Roetzel model [21]. All of the data here were considered at 30 °C. From this figure, it is obvious that specific heat capacity of nanofluid decreases with the increase of particle volume concentrations. This can be attributed to the addition of the nanoparticles with lower specific heats, which is equivalent to the decrease in the proportion of the fluid of a higher specific heat. However, the decreasing trend was found to be higher in this experiment compared to the theoretical values. It means that the effect of nanoparticles on the specific heat of suspension was more than the expectation. The average deviation of the experimental values from theoretical values was found to be 4.5% only. 4. Conclusions Based on the obtained experimental results, the following conclusions can be drawn: (a) It was observed that for the Al2O3–radiator coolant nanofluid as well as the base fluid, thermal conductivity increases with the increase of temperature from 10 °C to 50 °C and higher thermal conductivities were found for higher volume concentrations of the nanoparticles. The average thermal conductivity for Al2O3– RC nanofluid increase 3.26% when temperature increases from 10 °C to 50 °C. Moreover, nanofluid's thermal conductivity with any volume concentration of nanoparticles was higher than the base fluid. The highest thermal conductivity enhancement was found to be 8.30% for 1 vol.% of Al2O3–RC nanofluid. (b) The obtained nanofluid viscosities were higher than the base fluid and greater viscosities were found for higher volume concentrations. The highest viscosity enhancement was found to be 150% for 1 vol.% of Al2O3–RC at 10 °C whereas the lowest viscosity enhancement was found to be 4% for 0.2 vol.% of Al2O3–RC at 50 °C. The average viscosities of Al2O3–RC nanofluid
M.M. Elias et al. / International Communications in Heat and Mass Transfer 54 (2014) 48–53
(c)
(d)
(e)
(f)
(g)
decrease about 195% when temperature increases from 10 °C to 50 °C. Density of the Al2O3 radiator coolant nanofluid was increased with the increase of volume concentration. The highest density enhancement was found at 2.91% for 1 vol.% of Al2O3–RC at 15 °C whereas the lowest density enhancement was found to be 0.36% for 0.2 vol.% of Al2O3–RC at 50 °C. However, nanofluid densities slightly deviated from the densities obtained by the proposed model of Pak and Cho [13]. The average density for Al2O3–RC nanofluid decreases 1.71% when temperature increases from 15 °C to 50 °C. Contrary to the thermal conductivity, viscosity and density, the specific heat capacity of nanofluid decreased with the increase of particle concentrations. However, it increased with the increase of temperature. Unlike thermal conductivity; viscosity and density of the nanofluids decreased with the increase of temperature. Moreover, in the case of viscosity, when temperature increases from 10 °C to 50 °C, viscosity of the nanofluid at 1 vol.% concentration and the base fluid, decreases by 50.4%. That means, the decreasing trend is about exponential, whereas this trend for density is almost linear. By increasing the temperature, thermal conductivity increases, while viscosity and density decrease. Therefore, the nanofluid can show better performances at higher temperatures. It would be better if the effect of particle size and shape could be considered with the variation of temperature and particle concentrations for the investigation of the thermo-physical properties.
Acknowledgment The authors would like to acknowledge the financial support from the Ministry of Energy, Green Technology and Water (KeTTHA) under the University of Malaya ERGS project no: 53-02-03-1101 to carry out this research. References [1] R. Saidur, K.Y. Leong, H.A. Mohammad, A review on applications and challenges of nanofluids, Renew. Sustain. Energy Rev. 15 (3) (2011) 1646–1668.
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[2] S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, in: D.A. Siginer, H.P. Wang (Eds.), Developments Applications of Non-Newtonian Flows, FED-vol 231/MD-vol 66, ASME, New York, 1995, pp. 99–105. [3] S. Lee, S.U. Choi, S. Li, J. Eastman, Measuring thermal conductivity of fluids containing oxide nanoparticles, Journal of Heat Transfer 121 (2) (1999) 280–289. [4] W. Yu, H. Xie, L. Chen, Y. Li, Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid, Thermochimica Acta 491 (1–2) (2009) 92–96. [5] W. Duangthongsuk, S. Wongwises, Measurement of temperature-dependent thermal conductivity and viscosity of TiO2–water nanofluids, Experimental Thermal and Fluid Science 33 (4) (2009) 706–714. [6] I.M. Mahbubul, R. Saidur, M.A. Amalina, Latest developments on the viscosity of nanofluids, International Journal of Heat and Mass Transfer 55 (4) (2012) 877–888. [7] I.M. Mahbubul, R. Saidur, M.A. Amalina, Investigation of viscosity of R123–TiO2 nanorefrigerant, Int. J. Mech. Mater. Eng. 7 (2) (2012) 146–151. [8] M. Kole, T.K. Dey, Effect of prolonged ultrasonication on the thermal conductivity of ZnO–ethylene glycol nanofluids, Thermochimica Acta 535 (2012) 58–65. [9] C.T. Nguyen, F. Desgranges, G. Roy, N. Galanis, T. Maré, S. Boucher, H. Angue Mintsa, Temperature and particle-size dependent viscosity data for water-based nanofluids — Hysteresis phenomenon, International Journal of Heat and Fluid Flow 28 (6) (2007) 1492–1506. [10] X. Wang, X. Xu, S.U.S. Choi, Thermal conductivity of nanoparticle–fluid mixture, Journal of Thermophysics and Heat Transfer 13 (4) (1999) 474–480. [11] S.K. Das, N. Putra, W. Roetzel, Pool boiling characteristics of nano-fluids, International Journal of Heat and Mass Transfer 46 (5) (2003) 851–862. [12] E.V. Timofeeva, W. Yu, D.M. France, D. Singh, J.L. Routbort, Nanofluids for heat transfer: an engineering approach, Nanoscale Research Letters 6 (1) (2011) 182. [13] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Exp. Heat Transf. 11 (2) (1998) 151–170. [14] R.S. Vajjha, D.K. Das, Specific heat measurement of three nanofluids and development of new correlations, ASME J. Heat Transf. 131 (7) (2009) 071601–071607. [15] M.M. Elias, M. Miqdad, I.M. Mahbubul, R. Saidur, M. Kamalisarvestani, M.R. Sohel, A. Hepbasli, N.A. Rahim, M.A. Amalina, Effect of nanoparticle shape on the heat transfer and thermodynamic performance of a shell and tube heat exchanger, Int. Commun. Heat and Mass Transfer 44 (2013) 93–99. [16] M. Kole, T.K. Dey, Viscosity of alumina nanoparticles dispersed in car engine coolant, Experimental Thermal and Fluid Science 34 (6) (2010) 677–683. [17] J. Maxwell, 3rd ed., A Treatise on Electricity and Magnetisms, Chap 9, Art 310–314, Vol. 1, Clarendon Press, Oxford, 1891. [18] R.L. Hamilton, O.K. Crosser, Thermal conductivity of heterogeneous two-component systems, Ind. Eng. Chem. Fundam. 1 (3) (1962) 187–191. [19] I.M. Mahbubul, R. Saidur, M.A. Amalina, Influence of particle concentration and temperature on thermal conductivity and viscosity of Al2O3/R141b nanorefrigerant, Int. Commun. Heat and Mass Transfer 43 (2013) 100–104. [20] I.M. Mahbubul, R. Saidur, M.A. Amalina, Thermal conductivity, viscosity and density of R141b refrigerant based nanofluid, Procedia Eng. 56 (0) (2013) 310–315. [21] Y. Xuan, W. Roetzel, Conceptions for heat transfer correlation of nanofluids, International Journal of Heat and Mass Transfer 43 (19) (2000) 3701–3707. [22] H. Zhu, C. Zhang, S. Liu, Y. Tang, Y. Yin, Effects of nanoparticle clustering and alignment on thermal conductivities of Fe3O4 aqueous nanofluids, Applied Physics Letters 89 (2) (2006) 023123. [23] H. Masuda, A. Ebata, K. Teramae, N. Hishinuma, Alteration of thermal conductivity and viscosity of liquid by dispersing ultra-fine particles, Netsu Bussei 7 (2) (1993) 227–233.
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Experimental investigation on the thermo-physical properties of Al2O3 nanoparticles suspended in car radiator coolant☆ M.M. Elias ⁎, I.M. Mahbubul, R. Saidur ⁎, M.R. Sohel, I.M. Shahrul, S.S. Khaleduzzaman, S. Sadeghipour Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 26 March 2014 Keywords: Nanofluid Volume concentration Temperature Thermal conductivity Viscosity Density Specific heat
a b s t r a c t Nanofluid is a new type of heat transfer fluid with superior thermal performance characteristics, which is very promising for thermal engineering applications. This paper presents new findings on the thermal conductivity, viscosity, density, and specific heat of Al2O3 nanoparticles dispersed into water and ethylene glycol based coolant used in car radiator. The nanofluids were prepared by the two-step method by using an ultrasonic homogenizer with no surfactants. Thermal conductivity, viscosity, density, and specific heat have been measured at different volume concentrations (i.e. 0 to 1 vol.%) of nanoparticles and various temperature ranges (i.e. from 10 °C to 50 °C). It was found that thermal conductivity, viscosity, and density of the nanofluid increased with the increase of volume concentrations. However, specific heat of nanofluid was found to be decreased with the increase of nanoparticle volume concentrations. Moreover, by increasing the temperature, thermal conductivity and specific heat were observed to be intensified, while the viscosity and density were decreased. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Heat transfer enhancement is a major concern in the field of thermal engineering. Therefore, efforts need to be put to improve the heat transfer performance of thermal devices used in many engineering applications. Heat transfer improvement can be made by increasing (i) heat transfer area, (ii) temperature, and (iii) heat transfer co-efficient [1]. However, technologies have already reached their limit for the cases (i) and (ii). Recently many researchers found that dispersing nanosized particles into the liquids result in higher heat transfer co-efficient of these newly developed fluids called nanofluids compared to the traditional liquids [2]. For this reason, nanometer size particles with the diameter of 1–100 nm are suspended in a liquid called nanofluid which was invented by Choi [2] and the author found considerable enhancement of the thermal conductivity of the suspension. Since then, many investigations were carried out by a number of researchers on the thermal conductivity of various nanofluids [1]. Lee et al. [3] experimentally studied the mixture of ethylene glycol and CuO nanoparticles of 35 nm size at the concentration of 4.0 vol.% and found a 20% increase in thermal conductivity. Yu et al. [4] experimentally investigated that, the thermal conductivity of nanofluid strongly depends on nanoparticle volume concentrations and it increases nonlinearly with the increase of volume concentration ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding authors. E-mail addresses: [email protected] (M.M. Elias), [email protected], [email protected] (R. Saidur).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.03.005 0735-1933/© 2014 Elsevier Ltd. All rights reserved.
and the enhanced thermal conductivity was found to be 26.5% at 5.0 vol.% concentration. Duangthongsuk and Wongwises [5] experimentally investigated that the TiO2–water nanofluid offered 3–7% higher thermal conductivity than base fluid for 0.2 to 2.0 vol.% of TiO2 nanoparticles. Viscosity of nanofluids is a parameter as crucial as thermal conductivity for the thermal performance investigation [6,7]. Pumping power is directly related to the pressure drop, which is in turn related to the viscosity of the fluid. Therefore, minimization of viscosity is also a critical factor in addition to the augmentation of thermal conductivity [8]. Nguyen et al. [9] experimentally investigated the effect of volume concentration and temperature on the dynamic viscosity of Al2O3–water nanofluid and found that viscosity of the nanofluid considerably increases with the increase of particle volume concentrations, but it decreases with the increase of temperature. Wang et al. [10] investigated the viscosity of Al2O3–water nanofluid prepared by mechanical blending with particle size of 28 nm at 5 vol.% concentration and viscosity increased by 86% compared to the base fluid. They also investigated Al2O3/ethylene glycol nanofluid and found a 40% increase in viscosity. Das et al. [11] also observed that with the increase of particle volume concentration, viscosity of the nanofluid increases. Density of a fluid is another important thermophysical property. Like viscosity, density of any fluid has direct impact on the pressure drop and pumping power. Density is strongly dependent on the nanoparticle material used, whereas the other parameters such as nanoparticles size, shape, zeta potential and additives do not affect the density of nanofluids [12]. Solids have a greater density compared to liquids; therefore, the density of nanofluids is found to be increased with the
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Nomenclature Cp mf n vol. k mn RC
specific heat mass of base fluid shape factor volume thermal conductivity, W/m.K Mass of nanoparticle radiator coolant
Greek symbols ρ Density, kg/m3 ϕ Volume fraction μ Dynamic viscosity
Subscripts f Base fluid eff Effective p Particle nf Nanofluid
increase in concentration of nanoparticles in the fluid. Literatures about the density of nanofluids are still scarce. Pak and Cho [13] for the first time, measured the density of γAl2O3 and TiO2 with distilled water and found that density of nanofluid increases with the increase of nanofluid. Specific heat is also a very important characteristic of nanofluids. To study the energy performance, specific heat of nanofluid must be determined. From the previous literatures it is found that, the specific heat of nanofluids depends on different parameters such as type, size and volume concentration of nanoparticles and base fluids at different temperatures. Pak and Cho [13] first reported that the specific heat of Al2O3– water and TiO2–water nanofluids decreases with the increase of particle volume concentration. Vajjha and Das [14] measured the specific heat of three nanofluids containing Al2O3, ZnO and SiO2 nanoparticles. They also discovered that the specific heat value decreases as the volumetric concentration of nanoparticles increases but the specific heat of those nanofluids increases with the increase of temperature. Preparation of nanofluid is an emergent step to enhance the thermal conductivity and better stability of nanofluids. Thermal conductivity varies according to the particle size, shape, and material of the nanoparticle. For example, the thermal conductivity of nanofluid with metallic/ oxide particles was found higher compared to the nanofluids with other particles. Moreover, oxide based nanofluids have better stability in comparison with other nanofluids. Therefore, in this research Al2O3 nanoparticles were used. Radiators are important accessories for automobiles. It is a kind of heat exchanger system that is used to cool the engines. Thermal management of the engine is a great problem like other thermal devices. If the heat transfer performance of the radiators can be increased, the engine performance can be improved as well. Traditionally, water is used as the coolant in car radiator. However, recently, the 50:50 mixtures of water and ethylene glycol are commercially introduced for better performance of the car radiators. Nanofluid is pronounced for improving the heat transfer performance of the heat exchanger systems [15]. However, before investigating the heat transfer performance, thermal conductivity, viscosity, density and specific heat capacity need to be examined as the performance of the car radiator is dependent on these thermo physical properties. There are limited literatures available on the fundamental properties of radiator coolant based nanofluid. To the best of the authors' knowledge, there is only one literature available on the viscosity of a radiator coolant based nanofluid [16], where the
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authors dispersed Al2O3 nanoparticles in a car radiator coolant and measured the viscosity of that nanofluid for different concentrations at different temperatures. Therefore, in the present study, thermal conductivity, viscosity, density, and specific heat of the radiator coolant based nanofluid containing Al2O3 nanoparticles have been investigated for 0 to 1 vol.% of nanoparticles with temperature ranging from 10 °C to 50 °C. The obtained results have been compared with empirical equations as well. It is expected that, this research will fill the gaps on the fundamental thermophysical properties of the radiator coolant based nanofluids. 2. Experimental procedure The commercial radiator coolant (50:50 mixtures of ethylene glycol and water), which is widely used in automobiles, has been used in this research. Al2O3 nanoparticles were used throughout this experiment; nanoparticles were purchased from Sigma-Aldrich, Malaysia with diameters of 13 nm. The properties were taken without using any surfactant. An ultrasonic homogenizer (Fisher Scientific Model 505 Sonic Dismembrator) was used for preparing the nanofluid for different volume fractions ranging from 0 to 1 vol.%. The ultra-sonication time was 30 min for all of the dispersion. No sedimentations were found within few hours after preparation. Thermal conductivity, viscosity, density, and specific heat were measured in this experiment with 0 to 1 volume concentrations and in 10 °C to 50 °C temperature range. Eq. (1) was used to calculate the volume fractions of nanofluids:
ϕ¼
mn ρn
mn m f þ ρn ρ f
ð1Þ
2.1. Thermal conductivity A KD2-Pro thermal properties analyzer (made by Decagon, USA) was used to measure the thermal conductivity of nanofluids. The accuracy of this equipment was found to be ±0.1 of readings. A refrigerating circulating bath was used to control the temperature of the nanofluid. For the measurement of thermal conductivity, the vessel with tested sample was placed inside the temperature controlled bath and sample temperature was checked by a thermocouple inserted into the vessel. All of the data were taken 3 times and the average of 3 values was used for analysis. Some of the data (about 5%) was eliminated since they were found to be outliers. As there is no literature available on the experimental value of thermal conductivity of the radiator coolant based nanofluid, the following two models were used to compare thermal conductivity of the present work. Maxwell model [17] was used to determine thermal conductivity of the nanofluid:
keff
kp þ2k f þ 2 kp ‐k f ϕ ¼ kp þ2k f − kp ‐k f ϕ
ð2Þ
Hamilton–Crosser model [18] as expressed below was also used to measure thermal conductivity of the nanofluid
keff
kp þ ðn−1Þk f −2ðn−1Þϕ k f −kp kf ¼ kp þ ðn−1Þk f þ ϕ k f ‐kp
ð3Þ
2.2. Viscosity Viscosity of the nanofluids for different volume concentrations of nanoparticles along with the different temperatures was measured by a Brookfield programmable viscometer (model: LVDV-III ultra) which
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was interfaced with a personal computer for data collection and storage. The spindle connected with the viscometer was submerged into the nanofluids. The viscous is developed against the spindle due to deflection of the calibrated spring with the range of viscosity between 1 to 6,000,000 MPa.s with the aid of Ultra low adapter (ULA) attached with the main equipment [19]. In this experiment, the Brookfield ULA with spindle model ULA-49EAY (spindle code 01) has been used. The adapter has the provision for the circulation of temperature of bath fluid. The measurement of viscosity for different volume concentrations ranging from 0 to 1 vol.% were started from 10 °C, and gradually increased to 50 °C at an interval of 10 °C. All of the data have been taken 3 times and the average value has been considered for the analysis. 2.3. Density A portable density meter, KEM-DA 130N (Kyoto, Japan) was used to measure the density of nanofluid ranging from 0 to 1 vol.% concentration of nanoparticles. This device can measure the density within the range of 0 to 2000 kg/m3 with the precision of ±0.001 kg/m3 [20]. All the data has been taken 3 times to get better accuracy and the average value has been taken for the analysis. The density was taken within the temperature range of 15 °C to 50 °C. As no literature exists about the density of a radiator coolant based nanofluid, the model of Pak and Cho [13] was used to be compared with the obtained experimental densities of the nanofluids using Eq. (4). ρnf ¼ ð1−φÞρ f þ φρρ
ð4Þ
2.4. Specific heat
concentrations (%) of Al2O3/radiator coolant nanofluid. Images of nanofluid were taken just after preparation until after 7 days of preparation with a time interval of 1 day (24 h). Fig. 1 shows that, sedimentation starts after 3 days of preparation. The sedimentation of mixtures has been seen at the bottom of the specimen and leveled as well. It is easily understandable from the picture that sedimentation slowly increased each day after 3 days until 7 days. However, until 7 days the sedimentation level was not so high. 3.1. Thermal conductivity
A Differential Scanning Calorimeter (model: DSC 4000, Perkin Elmer, USA) measured the specific heat of the nanofluid for 0 to 1 vol.% concentrations of nanoparticles along with different temperatures. The measurement of specific heat for different temperatures was started from 10 °C, and gradually increased to 50 °C at an interval of 10 °C. All of the data have been taken 3 times and the average value has been considered for the analysis. Because there are no literature available about the specific heat of radiator coolant based nanofluid therefore, the following equation of Xuan and Roetzel [21] has been used to compare the experimental values of this study. C p; nf ¼
Fig. 2. Thermal conductivity of the nanofluid (Al2O3–RC) at different temperatures.
ð1−φÞρ f C p; f þ φρρ C p;p ρnf
ð5Þ
Where, Cp is the specific heat of nanofluids, ρ is the density of nanofluid, and Ø is the volume fraction of nanoparticles. Also nf, f, and s refer to nanofluid, fluid, and solid, respectively. The density of nanofluid for this Eq. (5) has been determined by using Eq. (4) of Pak and Cho [13]. 3. Results and discussion The stability of the nanofluid has been checked with Photograph Capturing Method. Fig. 1 shows the picture of 0.2 volume
The thermal conductivity of Al2O3-radiator coolant nanofluid for different volume concentrations at different temperatures is depicted in Fig. 2. From the figure, it is seen that, thermal conductivity of the nanofluid increases with the increase of volume concentration and temperature. For both base fluid and nanofluid, thermal conductivity increases with the increase of temperature, and the nanofluids show higher thermal conductivity than the base fluid for every volume concentration. This is because of particle movement during the temperature rise. With the intensification of fluid temperature, the movement among the particles increases thus raising the thermal conductivity. At 10 °C, the base fluid thermal conductivity was 0.250 W/m.K and at the same temperature, nanofluid thermal conductivity was 0.270 W/m.K for 1.0 vol.% concentration which is 8% higher than the base fluid. This means that with the increase of volume concentration, thermal conductivity of nanofluid increases. At 50 °C, the thermal conductivities of the base fluid and 1 vol.% of nanofluid were 0.259 W/m.K and 0.280 W/m.K, respectively. The thermal conductivity of the nanofluid at 1 vol.% concentration shows 8.30% increment compared to the base fluid for the same temperature. For each and every case, nanofluid showed increased thermal conductivity than the base fluid. An almost similar increasing trend of thermal conductivity of nanofluid has been observed for the various volume concentrations and temperatures. However, the graph lines are not parallel. The possible reason is that,
Sedimentation level Just after After preparation1 day
After 2 days
After 3 days
After 4 days
After 5 days
After 6 days
After 7 days
Fig. 1. Image of 0.2 volume concentration (%) of Al2O3/radiator coolant nanofluid.
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Fig. 5. Relative viscosity of the nanofluids versus different volume concentrations. Fig. 3. Comparison of the experimental thermal conductivity with two proposed models.
thermal conductivity of the nanofluids not only depends on particle concentration and temperature but also controlled by the other parameters like size and shape. However, in this case it is expected that the negligible variation of the trends could be due to the particle alignment and clustering [22]. There is no experimental data of thermal conductivity of radiator coolant based nanofluids. Therefore, the experimental values of the thermal conductivity of Al2O3–radiator coolant nanofluid at 30 °C have been plotted in Fig. 3 to compare this experimental value with different empirical equations under different volume concentrations ranging from 0 to 1 vol.%. The experimental results of the nanofluid indicated higher thermal conductivities than the models developed by Maxwell [17] and Hamilton–Crosser [18] for all the volume concentrations. It has been observed that in both experimental data and developed models, thermal conductivity increases with the increase of volume concentration. However, the thermal conductivity calculated by Hamilton and Crosser model [18] for Al2O3 based radiator coolant is 1.92% greater than that of Maxwell model [17]. Thermal conductivities obtained from the experiments for 1 vol.% concentration of Al2O3 based nanofluid were 4.26% higher than the calculated thermal conductivities using the proposed model by Maxwell [17] and 2.2% greater than the other proposed model by Hamilton–Crosser [18]. 3.2. Viscosity Viscosities of Al2O3-radiator coolant based nanofluid were measured within the temperature range of 10 °C to 50 °C with different volume
Fig. 4. Viscosity of Al2O3–RC nanofluid as a function of temperature with different volume concentrations.
concentrations ranging from 0 to 1 vol.% of nanoparticles and plotted in Fig. 4. It is shown in Fig. 4 that, the viscosity of nanofluid exponentially decreases with the increase of temperature. The viscosity value of the nanofluid is higher than the base fluid for every volume concentration. It is also observed that, with the increase of nanoparticle concentration in the base fluid, viscosity of the nanofluid intensifies and this amount decreases with the increase of temperature. At 1 vol.% concentration, viscosity of the nanofluid is found to be decreased by 57.5% when the temperature increases from 10 °C to 50 °C and for the base fluid, viscosity is reduced by 73.21% when temperature is elevated from 10 °C to 50 °C. This is a natural phenomenon that in most cases, viscosity of the liquids decreases with the increase of temperature. When temperature of any substance increases the movement among the molecules also increased. For the higher movement of the molecules, the resistance to flow a material (which in terms called viscosity) decreases. The relative viscosities of the Al2O3–RC nanofluids at 30 °C have been determined to be compared with the experimental values available in the literature. Results of the relative viscosity of Al2O3/radiator coolant by Kole and Dey [16] have been selected for the comparison, which have been plotted in Fig. 5. The viscosity value of Al2O3–RC nanofluid, at 0.2 vol.% is the same as the value of Kole and Dey [16]. However, the measured viscosity of this experiment apparently showed higher values compared to those of Kole and Dey [16]. Though, in both cases the nanoparticles and the base fluid are alike, the sources were different. Therefore, there are possibilities that there may be some differences between the purities and ingredients of the two sources, resulting in this slight difference between the viscosity values. Such deviations have been also seen in other experimental values among other researchers for the same type of nanofluids. For example, Pak and Cho [13] found lower viscosities compared to the results of Masuda et al. [23] for the same TiO2–water nanofluid.
Fig. 6. Density of Al2O3–RC nanofluid as a function of temperatures.
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Fig. 9. Comparison of experimental specific heat capacity of the nanofluids with Xuan and Roetzel model [21] model under different volume concentrations at 30 °C. Fig. 7. Comparison of experimental density of the nanofluids with Pak and Cho [13] model under different volume concentrations at 30 °C.
3.3. Density The densities of Al2O3–RC nanofluid and pure radiator coolant with different temperatures for the volume concentrations of 0 to 1.0 vol.% are portrayed in Fig. 6. The figure shows that the density of Al2O3–RC nanofluid and pure radiator coolant decreases with the increase of temperature, which is related to the increase in the fluid volume by increasing the temperature. For example, the density of the base fluid decreases by 1.8%, when the temperature increases from 15 °C to 50 °C. However, density increases with the increase of volume concentration and in all cases, nanofluid gave higher density than the base fluid. The higher density of the dispersed nanoparticles in the fluid evidently causes this. As there is no literature available about density of the radiator coolant based nanofluids, therefore to validate the experimental results, the measured densities have been compared with the model of Pak and Cho [13]. Experimental values of the density at 30 °C temperature and the densities obtained by the model are depicted in Fig. 7. Here, it is shown that, nanofluid densities measured by Pak and Cho model [13] present almost similar values for Al2O3 based nanofluid. However, the experimental values of Al2O3–RC showed little deviation from the model results of density of the nanofluid especially at 0.6 vol.%. 3.4. Specific heat Fig. 8 shows the effect of temperatures on specific heat for different volume concentrations of nanofluid. Here, specific heat increased with the increase of temperature. The increasing trend was constant and
Fig. 8. Specific heat of Al2O3–RC nanofluid as a function of temperature.
linear. However, specific heat decreased with the increase of volume concentration of nanofluid. This decrease is due to the lower specific heat of the added particles compared to that of the base fluid. Moreover, higher specific heat of the base fluid is the reason why the specific heat value of the mixture becomes higher than that of the nanoparticles. Therefore, it becomes lower than the specific heat value of the base fluid. In Fig. 8, though the gap between two concentrations was constant however, the gaps between decreasing line for two consecutive concentrations were not same. This may have happened because a very small amount of sample was used to measure the specific heat. Some milligrams only (a fraction of a drop of suspension) was used in DSC machine. In this small amount of nanofluids the ratio between two concentrations may have varied. Therefore, the specific heat values slightly changed. Fig. 9 shows the comparison among the experimental specific heat capacity of nanofluid with the computed values obtained from Xuan and Roetzel model [21]. All of the data here were considered at 30 °C. From this figure, it is obvious that specific heat capacity of nanofluid decreases with the increase of particle volume concentrations. This can be attributed to the addition of the nanoparticles with lower specific heats, which is equivalent to the decrease in the proportion of the fluid of a higher specific heat. However, the decreasing trend was found to be higher in this experiment compared to the theoretical values. It means that the effect of nanoparticles on the specific heat of suspension was more than the expectation. The average deviation of the experimental values from theoretical values was found to be 4.5% only. 4. Conclusions Based on the obtained experimental results, the following conclusions can be drawn: (a) It was observed that for the Al2O3–radiator coolant nanofluid as well as the base fluid, thermal conductivity increases with the increase of temperature from 10 °C to 50 °C and higher thermal conductivities were found for higher volume concentrations of the nanoparticles. The average thermal conductivity for Al2O3– RC nanofluid increase 3.26% when temperature increases from 10 °C to 50 °C. Moreover, nanofluid's thermal conductivity with any volume concentration of nanoparticles was higher than the base fluid. The highest thermal conductivity enhancement was found to be 8.30% for 1 vol.% of Al2O3–RC nanofluid. (b) The obtained nanofluid viscosities were higher than the base fluid and greater viscosities were found for higher volume concentrations. The highest viscosity enhancement was found to be 150% for 1 vol.% of Al2O3–RC at 10 °C whereas the lowest viscosity enhancement was found to be 4% for 0.2 vol.% of Al2O3–RC at 50 °C. The average viscosities of Al2O3–RC nanofluid
M.M. Elias et al. / International Communications in Heat and Mass Transfer 54 (2014) 48–53
(c)
(d)
(e)
(f)
(g)
decrease about 195% when temperature increases from 10 °C to 50 °C. Density of the Al2O3 radiator coolant nanofluid was increased with the increase of volume concentration. The highest density enhancement was found at 2.91% for 1 vol.% of Al2O3–RC at 15 °C whereas the lowest density enhancement was found to be 0.36% for 0.2 vol.% of Al2O3–RC at 50 °C. However, nanofluid densities slightly deviated from the densities obtained by the proposed model of Pak and Cho [13]. The average density for Al2O3–RC nanofluid decreases 1.71% when temperature increases from 15 °C to 50 °C. Contrary to the thermal conductivity, viscosity and density, the specific heat capacity of nanofluid decreased with the increase of particle concentrations. However, it increased with the increase of temperature. Unlike thermal conductivity; viscosity and density of the nanofluids decreased with the increase of temperature. Moreover, in the case of viscosity, when temperature increases from 10 °C to 50 °C, viscosity of the nanofluid at 1 vol.% concentration and the base fluid, decreases by 50.4%. That means, the decreasing trend is about exponential, whereas this trend for density is almost linear. By increasing the temperature, thermal conductivity increases, while viscosity and density decrease. Therefore, the nanofluid can show better performances at higher temperatures. It would be better if the effect of particle size and shape could be considered with the variation of temperature and particle concentrations for the investigation of the thermo-physical properties.
Acknowledgment The authors would like to acknowledge the financial support from the Ministry of Energy, Green Technology and Water (KeTTHA) under the University of Malaya ERGS project no: 53-02-03-1101 to carry out this research. References [1] R. Saidur, K.Y. Leong, H.A. Mohammad, A review on applications and challenges of nanofluids, Renew. Sustain. Energy Rev. 15 (3) (2011) 1646–1668.
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