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Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids
Applied Thermal Engineering 88 (2015) 363e368
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids Haoran Li, Li Wang, Yurong He*, Yanwei Hu, Jiaqi Zhu, Baocheng Jiang Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2014 Received in revised form 8 October 2014 Accepted 20 October 2014 Available online 27 October 2014
In this work, well dispersed ethylene glycol (EG) based nanofluids containing ZnO nanoparticles with different mass fractions between 1.75% and 10.5% were prepared by a typical two-step method. Structural properties of the dry ZnO nanoparticles were measured with X-ray diffraction（XRD）and Transmission Electron Microscopy (TEM). Thermal transport properties including thermal conductivity and viscosity were experimentally measured for the nanofluids. The experimental results show that thermal conductivity increases slightly with increasing the temperature from 15 to 55 C. It depends strongly on particle concentration and increases nonlinearly with the concentration within the range studied. The enhanced value is higher than the value predicted by the Hamilton and Crosser (HeC) model. Moreover, viscosity increases with concentration as usual for ZnO nanoparticles and decreases with temperature. For an analysis of the rheological behaviors, it shows that ZnO-EG nanofluids with mass fraction wt.% 10.5 demonstrate Newtonian behaviors. © 2014 Elsevier Ltd. All rights reserved.
Keywords: ZnO-EG nanofluids Thermal conductivity Viscosity Experimental investigation
1. Introduction Nanofluids are suspension of nanoparticles into traditional fluids including water, engine oil and glycerol. Since the 1990s, researchers began to explore nano-materials technology to apply in the field of enhanced heat transfer, and were focusing on the research work relating to high efficient heat transfer of cooling technology. Thus, nanofluids with dispersed metal or nonmetal nanopowders into traditional heat transfer medium, and keeping uniform, stable and high thermal conductivity, are used in thermal engineering [1e3]. Meanwhile, nanofluids have huge potential application prospects in energy industry, chemical engineering industry, automobile, construction, microelectronics, information and other fields, and become the research hotspot in materials, physics, and chemistry field. Up to now, many kinds of metallic oxide nanopartices are used as the additives of nanofluids. Among the additives, CuO, Al2O3, TiO2 and Fe3O4 are most commonly investigated and the transport properties of those nanofluids are more likely to reach to the consensus. However, the analysis of ethylene glycol based ZnO nanofluids has not been well investigated.
* Corresponding author. Tel.: þ86 0451 86413233. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.applthermaleng.2014.10.071 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
Chung et al. [4] explored the effectiveness of ultrasonic dispersion, and they suggested that processing volumes should be kept small, or mixed well by mechanical stirring during the process so as to circulate all material through the cavitation field. Yu et al. [5] investigated the thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids and announced that the absolute thermal conductivity increases with increasing the temperature for different temperature ranging from 10 to 60 C, while the enhanced ratios are almost constant, and the thermal conductivities of the nanofluids track the thermal conductivities of the base liquid. Goharshadi et al. [6] researched the preparation, structural characterization, semiconductor and photo luminescent properties of ZnO nanoparticles in a phosphonium based ionic liquid. They concluded that the UVevis absorption spectrum of ZnO nanoparticles dispersed in ethylene glycol at room temperature revealed a blue-shifted onset of absorption. Raykar et al. [7] reported the thermal and rheological behavior of acetylacetone stabilized ZnO nanofluids. Yang et al. [8] had obtained ZnO hexagonal pyramids in hydrophilic media without any traditional stabilizers and found a nice model system that allows for both theory and experiment, and the work described the synthesis mechanism of ZnO nanoparticles in detail. Venerus et al. [9] examined the influence of particle concentration on the viscosity of eight kinds of nanofluids and compared the data with predictions from classical theories on suspension rheology. They made a consensus that nanofluids with lower concentrations (vol. ¼ 0.01) were Newtonian fluid, while
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those with higher concentrations (vol. ¼ 0.03) were nonNewtonian fluid. An article about experimental investigation and theoretical study of viscosity of Al2O3-water nanofluids was published by Ghanbarpour et al. [10]. In their work, viscosity enhancement is in the range of 18.1%e300% for mass concentration and temperature in the range of 3%e50% and 293e313 K, respectively. Santra et al. [11] calculated the shear stresses of copperwater nanofluids using Ostwald-de Waele model for an incompressible non-Newtonian fluid. Effect of solid volume on flow characteristics were discussed in detail in their work. It is worth mentioning that most of the research results show that the viscosity of ZnO-EG nanofluids decreased with increasing the temperature, but Esfe and Saedodin [12] have reported the correlation of viscosity of ZnO-EG nanofluids at various temperature (from room condition up to 50 C) and found that the dynamic viscosity did not significantly change (decrease) with temperature. Kole and Dey [13], previously, explored the correlation of viscosity with different volume fractions between 0.5% and 3.75% at two laboratories, in surprise, they observed no worthwhile enhancement in the viscosity of nanofluids. All the above research focuses on the basic propertiesdespecially the thermal conductivity and viscosity of nanofluids. That is to say, the thermal conductivity and viscosity of nanofluids are two important transport properties for applications of nanofluids as a new class of heat transfer fluid in thermal devices or systems such as heat exchangers. As mentioned above, although plenty research has been done on thermal properties of nanofluids, the analysis of thermal properties of ZnO-EG nanofulid has not been well investigated. In this work, ZnO-EG nanofulid was prepared and the structural properties of the dry ZnO nanoparticles were measured using XRD and TEM. The effects of concentration and temperature on the thermal properties of nanofluids were analyzed. Thermal conductivity was measured by hot-wire measuring method and the viscosity was measured by a high-precision viscometer. 2. Experiment
agent in ZnO-EG nanofluids. Photograph of prepared ZnO-EG nanofluids (Fig. 1) showed no sedimentation after 48 h. The structural properties of the dry ZnO nanoparticles were measured using X-ray diffraction (D8 ADVANCE, BRUKER AXS GMBH, Germany) and Transmission Electron Microscopy (JEM2100, JEOL, Japan). A transient hot-wire apparatus (TC 3020L, Xi'an Xiatech Electronic Technology Co., Ltd, China) with an accuracy of ±2e3% and measuring range of 0.001e20 W/m K was applied to measure the thermal conductivities of the nanofluids. The instrument can endure an extensive temperature, which is between 160 and 150 C. Besides, a temperature-controlled bath was used to maintain constant temperature of nanofluids during the measurement process. What's more, the viscosity of ZnO nanofluids was measured by a Kinexus pro þ Super Rotation Rheometer (Malvern Instruments Ltd, Britain) with a Silent Air Compressor (Shanghai Dynamic Industry Co., Ltd, China). The angular velocity of the rheometer is range from 10 nrad/s to 500 rad/s and measuring temperature of specimen is between 40 and 200 C with a temperature resolution of 0.01 C. To ensure the uniformity of temperatures between thermal conductivity and viscosity, all measurements were started at a temperature of 15 C and up to 55 C in a 10 C interval. 3. Results and discussion Fig. 2 shows the powder XRD pattern for ZnO nanoparticles used in this work. It has intense peaks (100, 002, 101, 102, 110, 103, 200, 112, 201, 004 and 202) which can be indexed as a monoclinic structure of ZnO (JCPDS card No. 36-1451). A TEM image of the ZnO nanoparticles is shown in Fig. 3. It is observed that the nanoparticles are spherical and rectangular. 3.1. Thermal conductivities investigation A transient hot-wire apparatus was applied with a temperaturecontrolled bath to measure the thermal conductivities of ZnO-EG nanofluids. Nagasaka and Nagashima [14] proposed the following equation to calculate the thermal conductivity of electrically conducting liquids by the transient hot-wire method.
ZnO nanoparticles (Beijing Dk Nano technology Co. LTD, China) with an average diameter of 30 nm, the density 5.6 g/cm3 and the purity 99.9% were used in this work. The mass fraction of nanofulids was calculated from the weight of dry ZnO powder and the total weight of the suspension. By using a sensitive electronic balance with an accuracy of 1 mg, nanoparticle sample preparation was carried out. Nanoparticles were dispersed into a constant volume (100 mL) of ethylene glycol with different mass concentrations (1.75%, 3.5%, 5.25%, 7%, 8.75% and 10.5%, respectively). With magnetic stirring for 12 h before ultrasonic oscillation continuously for 4 h, the nanofluids mixture was well blended. In order to keep the stability of the suspension, polyvinylpyrrolidone (PVP) was used as dispersing
where k is the thermal conductivity of liquid, q is the input power per meter length of heating wire, T and T0 are temperatures at two different times t and t0 , respectively.
Fig. 1. Photography of ZnO nanofluids.
Fig. 2. Powder XRD pattern for ZnO nanoparticles.
k¼
q lnðt 0 =tÞ , 4p T 0 T
(1)
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Fig. 3. Powder TEM pattern for ZnO nanoparticles.
To ensure the accuracy of the devices to measure the thermal conductivity, pure water was measured as standard sample. 20 min were taken to promote the balance of the sample temperature and bath temperature before the sample temperature was monitored at least 15 min. After that, 5 min were taken for each data. Results were shown in Table 1. It can be seen that it has a maximum deviation about ±2.35%. Fig. 4(a) and (b) show the distribution of thermal conductivities of ZnO-EG nanofluids with different concentrations ranging from 1.75% to 10.5% and temperature ranging from 15 to 55 C respectively. It's confirmed that the absolute thermal conductivity gradually increases with mass fraction and temperature, but the growth rate is obviously decreased at 15 C in the range of wt.% 8.75e10.5. Ethylene glycol is a low thermal conductivity liquid with higher viscosity, the thermal conductivity of ZnO-EG nanofluids does not always increase with increasing concentration possibly due to the nanoparticles in the suspension aggregate into clusters of virtually shaped nano-structure because of strongly intermolecular attractive force which is caused by the increase of the concentration. Fig. 5 shows the enhanced ratio of thermal conductivities at 25 C and 30 C. In this figure， (kk0)/k0 is the relative percentage of thermal conductivities with k and k0 being the thermal conductivities of the ZnO-EG nanofluids and ethylene glycol (base fluid), respectively. Some researchers also studied the properties of ZnO nanofluids using water and ethylene glycol [15e18] as base fluids, and they had got the similar variations. Lee et al. [19] had shown that the relative percentage of thermal conductivities increased linearly with the mass fraction of nanoparticles, and it had also been proven by Kim et al. [20]. Although there is also a disagreement whether the enhancement ratio of ZnO nanofluids is linear, almost come to believe that it is unlinearly. Many publishing
Fig. 4. Thermal conductivity distribution of ZnO-EG nanofluids as a function of (a) mass fraction and (b) temperature.
Table 1 Test results of thermal conductivity of pure water. Temperature
Measurement value
Reference value
Deviation
304.33 304.43 304.41 363.78 363.77 363.78
0.6029 0.6113 0.6105 0.6615 0.6607 0.6622
0.6174 0.6176 0.6175 0.6755 0.6755 0.6755
2.35% 1.02% 1.13% 2.07% 2.19% 1.97%
K K K K K K
W/(m W/(m W/(m W/(m W/(m W/(m
K) K) K) K) K) K)
W/(m W/(m W/(m W/(m W/(m W/(m
K) K) K) K) K) K)
Fig. 5. Relative thermal conductivity distribution as a function of concentration.
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Viscosity is one of the most important properties of nanofluids, which indicates the resistance of fluid. Considerable engineering problems arise while dealing with high viscous liquids that include higher energy requirement for pumping as well as for mixing. Turbulence, an essential means by which high rates of heat and mass transfer can be achieved, is delayed to higher velocities for liquids with higher viscosity [30].
To ensure the accuracy of the devices to measure the viscosity, pure water was measured as standard sample within the condition of this study. Results show that the measurement value is 1.1248 103 Pa s while reference value is 1.1404 103 Pa s at 15 C and proportional error is ±1.37%. At a higher temperature the measurement value is 0.7065 103 Pa,s while reference value is 0.7225 103 Pa,s at 35 C and proportional error is ±2.21%. Shear stress of ZnO-EG nanofluids as a function of shear rate for different concentrations at 25 C is shown in Fig. 6. The rheological behaviors of the nanofluids show that ZnO-EG nanofluids under the condition of wt.% 10.5% demonstrate Newtonian behaviors (Fig. 7). As viscosity increases with concentration, the nanofluids may turn to non-Newtonian behaviors at a peak point. Therefore, further study will focus on the rheological properties of higher concentration nanoparticles. Newtonian or non-Newtonian behaviors of nanofluids depend strongly on many factors [31e34], such as the kind and the shape of nanoparticles, their volume fraction and temperature. To investigate the temperature-independence viscosity of ZnOEG nanofluids, the viscosity was measured against the temperature. Fig. 8 exhibits the experimental viscosity distribution against temperature in the range of 15e55 C and concentration in the range of 1.75e10.5 wt.%, respectively. The viscosity decreases with increasing temperature and increases with increasing the mass fraction, (Fig. 8(a)) showing highly agreement with most of the previous work [35e37]. The relative viscosity of ZnO-EG nanofluids (Fig. 8(b)) shows the same temperature-dependency since the viscosity of the base fluid (EG) and that of the nanofluids nearly decrease by a similar degree as increasing the temperature. On the other hand, the enhanced ratio of the viscosity decreases with increasing the temperature between 15 and 55 C at a constant concentration. The relative viscosity is higher than some of the predecessors' work, which is most possibly caused by the use of PVP (dispersing agent). Models of viscosity have been well used to estimate the effective viscosity of nanofluids as a function of solid volume fraction. In this study, Einstein [38] and Wang [39] models, which can be used to calculate the viscosity of nanofluids with spherical particles in volume concentrations less than 5.0% vol., were applied to compare the predicted viscosity with the measured data. The comparison of the effective viscosity with respect to particle mass fraction for experimental data at 25 C and theoretical models is shown in Fig. 9. As can be seen, both of the theoretical correlations are unable
Fig. 6. Shear stress of ZnO-EG nanofluids as a function of shear rate for different mass concentrations.
Fig. 7. Rheological behaviors of ZnO-EG nanofluids.
literature provided the evidence that metal oxide nanofluids based on EG Refs. [21e28] could improve the thermal conductivity effectively, even to this day there is no reliable theory to predict the thermal conductivity of nanofluids, but the existing classical models for solideliquid mixtures are often used to calculate the thermal conductivity. In order to analyze the thermal conductivity enhancement mechanism and explore suitable thermal conductivity models for ethylene glycol based ZnO nanofluids, the following analysis is presented based on Hamilton and Crosser (HeC) model. Hamilton and Crosser [29] had put forward the theoretical model to calculate the thermal conductivity of solideliquid mixture. Under the assumption of ZnO nanoparticles were suspended in the ethylene glycol stably and uniformly, the thermal conductivity can be calculated as Equation (2).
kp þ n 1 k0 n 1 4 k0 kp k ¼ k0 kp þ n 1 k0 þ 4 k0 kp
(2)
where k0 and kp are the thermal conductivity of base fluid and nanoparticles, respectively, 4 is the volume fraction of nanoparticles, which was calculated by the mass fraction and density of ZnO nanoparticles provided by the supplier. In this work, the relative percentage of thermal conductivities increases nonlinearly with the mass fraction of nanoparticles. Experimental thermal conductivities values at 25 and 30 C were compared with previously reported values, for instance, the enhanced value is 9.13% for 7 wt.% ZnO-EG nanofluids, which is higher than the prediction value of 3.78% by calculating with the HeC model. The results of Yu et al. give a good agreement with present data and that of Lee et al., Moosavi et al., and Kim et al. under measured the experimental enhancement of ZnO-EG nanofluids at 30 C. 3.2. Viscosity investigation
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for the higher viscosity for experimental data. As high degree of accuracy for viscometer, residual errors for experimental data, for example, it is 0.25 mPa s for 1.75 wt.% nanofluids, are mostly caused by the analysis of measuring result. Judging from the above results, it should be noted that the thermal conductivity increased with increasing the temperature and the viscosity shows opposite tendency. In order to enhance heat transport in engineering applications, heat-transfer medium with a higher thermal conductivity should be the prime choice. What's more, the conveyer pipe may be blocked if the viscosity of medium is too large, and a pump with larger power is also needed. That is to say, it is vital that the ethylene glycol based ZnO nanofluids should be used in a higher temperature. 4. Conclusion
Fig. 8. (a) Absolute value of the viscosity and (b) the relative viscosity distribution for various concentrations of ZnO-EG nanofluids against the temperature.
to predict the viscosity of ZnO-EG accurately and the predicted viscosity is lower than experimental values. The aggregation of nanosize particles and quite a lot of PVP in the nanofluids elevate the resistance of suspension, so it may be the best reason to account
In this work, the thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids have been experimentally investigated. For this purpose, well dispersed suspensions were obtained by dispersing different mass fractions of ZnO nanoparticles with an average diameter of 50 nm into a fixed volume of ethylene glycol. The thermal transport properties including thermal conductivity and viscosity were measured. The following conclusions are obtained. The absolute thermal conductivity increases with increasing the temperature ranging from 15 to 55 C. The thermal conductivity depends on particle concentration, and it increases nonlinearly with the mass fraction of nanoparticles. Experimental thermal conductivities values were compared with previously reported values, the enhanced value is higher than the prediction value of HeC model. The results of Yu et al. give a good agreement with present data and that of Lee et al., Moosavi et al., and Kim et al. under measured the experimental enhancement of ZnO-EG nanofluids at 30 C. The rheological behaviors of the nanofluids show that ZnO-EG nanofluids with mass fraction wt.% 10.5 demonstrate Newtonian behaviors and the viscosity decreases with increasing the temperature and that increases with increasing the mass concentration. Acknowledgements This work is financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004), the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant No. 2014RFYXJ004) and the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETIV.201315). References
Fig. 9. Comparison between theoretical models and measured data at 25 C.
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids Haoran Li, Li Wang, Yurong He*, Yanwei Hu, Jiaqi Zhu, Baocheng Jiang Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2014 Received in revised form 8 October 2014 Accepted 20 October 2014 Available online 27 October 2014
In this work, well dispersed ethylene glycol (EG) based nanofluids containing ZnO nanoparticles with different mass fractions between 1.75% and 10.5% were prepared by a typical two-step method. Structural properties of the dry ZnO nanoparticles were measured with X-ray diffraction（XRD）and Transmission Electron Microscopy (TEM). Thermal transport properties including thermal conductivity and viscosity were experimentally measured for the nanofluids. The experimental results show that thermal conductivity increases slightly with increasing the temperature from 15 to 55 C. It depends strongly on particle concentration and increases nonlinearly with the concentration within the range studied. The enhanced value is higher than the value predicted by the Hamilton and Crosser (HeC) model. Moreover, viscosity increases with concentration as usual for ZnO nanoparticles and decreases with temperature. For an analysis of the rheological behaviors, it shows that ZnO-EG nanofluids with mass fraction wt.% 10.5 demonstrate Newtonian behaviors. © 2014 Elsevier Ltd. All rights reserved.
Keywords: ZnO-EG nanofluids Thermal conductivity Viscosity Experimental investigation
1. Introduction Nanofluids are suspension of nanoparticles into traditional fluids including water, engine oil and glycerol. Since the 1990s, researchers began to explore nano-materials technology to apply in the field of enhanced heat transfer, and were focusing on the research work relating to high efficient heat transfer of cooling technology. Thus, nanofluids with dispersed metal or nonmetal nanopowders into traditional heat transfer medium, and keeping uniform, stable and high thermal conductivity, are used in thermal engineering [1e3]. Meanwhile, nanofluids have huge potential application prospects in energy industry, chemical engineering industry, automobile, construction, microelectronics, information and other fields, and become the research hotspot in materials, physics, and chemistry field. Up to now, many kinds of metallic oxide nanopartices are used as the additives of nanofluids. Among the additives, CuO, Al2O3, TiO2 and Fe3O4 are most commonly investigated and the transport properties of those nanofluids are more likely to reach to the consensus. However, the analysis of ethylene glycol based ZnO nanofluids has not been well investigated.
* Corresponding author. Tel.: þ86 0451 86413233. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.applthermaleng.2014.10.071 1359-4311/© 2014 Elsevier Ltd. All rights reserved.
Chung et al. [4] explored the effectiveness of ultrasonic dispersion, and they suggested that processing volumes should be kept small, or mixed well by mechanical stirring during the process so as to circulate all material through the cavitation field. Yu et al. [5] investigated the thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids and announced that the absolute thermal conductivity increases with increasing the temperature for different temperature ranging from 10 to 60 C, while the enhanced ratios are almost constant, and the thermal conductivities of the nanofluids track the thermal conductivities of the base liquid. Goharshadi et al. [6] researched the preparation, structural characterization, semiconductor and photo luminescent properties of ZnO nanoparticles in a phosphonium based ionic liquid. They concluded that the UVevis absorption spectrum of ZnO nanoparticles dispersed in ethylene glycol at room temperature revealed a blue-shifted onset of absorption. Raykar et al. [7] reported the thermal and rheological behavior of acetylacetone stabilized ZnO nanofluids. Yang et al. [8] had obtained ZnO hexagonal pyramids in hydrophilic media without any traditional stabilizers and found a nice model system that allows for both theory and experiment, and the work described the synthesis mechanism of ZnO nanoparticles in detail. Venerus et al. [9] examined the influence of particle concentration on the viscosity of eight kinds of nanofluids and compared the data with predictions from classical theories on suspension rheology. They made a consensus that nanofluids with lower concentrations (vol. ¼ 0.01) were Newtonian fluid, while
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those with higher concentrations (vol. ¼ 0.03) were nonNewtonian fluid. An article about experimental investigation and theoretical study of viscosity of Al2O3-water nanofluids was published by Ghanbarpour et al. [10]. In their work, viscosity enhancement is in the range of 18.1%e300% for mass concentration and temperature in the range of 3%e50% and 293e313 K, respectively. Santra et al. [11] calculated the shear stresses of copperwater nanofluids using Ostwald-de Waele model for an incompressible non-Newtonian fluid. Effect of solid volume on flow characteristics were discussed in detail in their work. It is worth mentioning that most of the research results show that the viscosity of ZnO-EG nanofluids decreased with increasing the temperature, but Esfe and Saedodin [12] have reported the correlation of viscosity of ZnO-EG nanofluids at various temperature (from room condition up to 50 C) and found that the dynamic viscosity did not significantly change (decrease) with temperature. Kole and Dey [13], previously, explored the correlation of viscosity with different volume fractions between 0.5% and 3.75% at two laboratories, in surprise, they observed no worthwhile enhancement in the viscosity of nanofluids. All the above research focuses on the basic propertiesdespecially the thermal conductivity and viscosity of nanofluids. That is to say, the thermal conductivity and viscosity of nanofluids are two important transport properties for applications of nanofluids as a new class of heat transfer fluid in thermal devices or systems such as heat exchangers. As mentioned above, although plenty research has been done on thermal properties of nanofluids, the analysis of thermal properties of ZnO-EG nanofulid has not been well investigated. In this work, ZnO-EG nanofulid was prepared and the structural properties of the dry ZnO nanoparticles were measured using XRD and TEM. The effects of concentration and temperature on the thermal properties of nanofluids were analyzed. Thermal conductivity was measured by hot-wire measuring method and the viscosity was measured by a high-precision viscometer. 2. Experiment
agent in ZnO-EG nanofluids. Photograph of prepared ZnO-EG nanofluids (Fig. 1) showed no sedimentation after 48 h. The structural properties of the dry ZnO nanoparticles were measured using X-ray diffraction (D8 ADVANCE, BRUKER AXS GMBH, Germany) and Transmission Electron Microscopy (JEM2100, JEOL, Japan). A transient hot-wire apparatus (TC 3020L, Xi'an Xiatech Electronic Technology Co., Ltd, China) with an accuracy of ±2e3% and measuring range of 0.001e20 W/m K was applied to measure the thermal conductivities of the nanofluids. The instrument can endure an extensive temperature, which is between 160 and 150 C. Besides, a temperature-controlled bath was used to maintain constant temperature of nanofluids during the measurement process. What's more, the viscosity of ZnO nanofluids was measured by a Kinexus pro þ Super Rotation Rheometer (Malvern Instruments Ltd, Britain) with a Silent Air Compressor (Shanghai Dynamic Industry Co., Ltd, China). The angular velocity of the rheometer is range from 10 nrad/s to 500 rad/s and measuring temperature of specimen is between 40 and 200 C with a temperature resolution of 0.01 C. To ensure the uniformity of temperatures between thermal conductivity and viscosity, all measurements were started at a temperature of 15 C and up to 55 C in a 10 C interval. 3. Results and discussion Fig. 2 shows the powder XRD pattern for ZnO nanoparticles used in this work. It has intense peaks (100, 002, 101, 102, 110, 103, 200, 112, 201, 004 and 202) which can be indexed as a monoclinic structure of ZnO (JCPDS card No. 36-1451). A TEM image of the ZnO nanoparticles is shown in Fig. 3. It is observed that the nanoparticles are spherical and rectangular. 3.1. Thermal conductivities investigation A transient hot-wire apparatus was applied with a temperaturecontrolled bath to measure the thermal conductivities of ZnO-EG nanofluids. Nagasaka and Nagashima [14] proposed the following equation to calculate the thermal conductivity of electrically conducting liquids by the transient hot-wire method.
ZnO nanoparticles (Beijing Dk Nano technology Co. LTD, China) with an average diameter of 30 nm, the density 5.6 g/cm3 and the purity 99.9% were used in this work. The mass fraction of nanofulids was calculated from the weight of dry ZnO powder and the total weight of the suspension. By using a sensitive electronic balance with an accuracy of 1 mg, nanoparticle sample preparation was carried out. Nanoparticles were dispersed into a constant volume (100 mL) of ethylene glycol with different mass concentrations (1.75%, 3.5%, 5.25%, 7%, 8.75% and 10.5%, respectively). With magnetic stirring for 12 h before ultrasonic oscillation continuously for 4 h, the nanofluids mixture was well blended. In order to keep the stability of the suspension, polyvinylpyrrolidone (PVP) was used as dispersing
where k is the thermal conductivity of liquid, q is the input power per meter length of heating wire, T and T0 are temperatures at two different times t and t0 , respectively.
Fig. 1. Photography of ZnO nanofluids.
Fig. 2. Powder XRD pattern for ZnO nanoparticles.
k¼
q lnðt 0 =tÞ , 4p T 0 T
(1)
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Fig. 3. Powder TEM pattern for ZnO nanoparticles.
To ensure the accuracy of the devices to measure the thermal conductivity, pure water was measured as standard sample. 20 min were taken to promote the balance of the sample temperature and bath temperature before the sample temperature was monitored at least 15 min. After that, 5 min were taken for each data. Results were shown in Table 1. It can be seen that it has a maximum deviation about ±2.35%. Fig. 4(a) and (b) show the distribution of thermal conductivities of ZnO-EG nanofluids with different concentrations ranging from 1.75% to 10.5% and temperature ranging from 15 to 55 C respectively. It's confirmed that the absolute thermal conductivity gradually increases with mass fraction and temperature, but the growth rate is obviously decreased at 15 C in the range of wt.% 8.75e10.5. Ethylene glycol is a low thermal conductivity liquid with higher viscosity, the thermal conductivity of ZnO-EG nanofluids does not always increase with increasing concentration possibly due to the nanoparticles in the suspension aggregate into clusters of virtually shaped nano-structure because of strongly intermolecular attractive force which is caused by the increase of the concentration. Fig. 5 shows the enhanced ratio of thermal conductivities at 25 C and 30 C. In this figure， (kk0)/k0 is the relative percentage of thermal conductivities with k and k0 being the thermal conductivities of the ZnO-EG nanofluids and ethylene glycol (base fluid), respectively. Some researchers also studied the properties of ZnO nanofluids using water and ethylene glycol [15e18] as base fluids, and they had got the similar variations. Lee et al. [19] had shown that the relative percentage of thermal conductivities increased linearly with the mass fraction of nanoparticles, and it had also been proven by Kim et al. [20]. Although there is also a disagreement whether the enhancement ratio of ZnO nanofluids is linear, almost come to believe that it is unlinearly. Many publishing
Fig. 4. Thermal conductivity distribution of ZnO-EG nanofluids as a function of (a) mass fraction and (b) temperature.
Table 1 Test results of thermal conductivity of pure water. Temperature
Measurement value
Reference value
Deviation
304.33 304.43 304.41 363.78 363.77 363.78
0.6029 0.6113 0.6105 0.6615 0.6607 0.6622
0.6174 0.6176 0.6175 0.6755 0.6755 0.6755
2.35% 1.02% 1.13% 2.07% 2.19% 1.97%
K K K K K K
W/(m W/(m W/(m W/(m W/(m W/(m
K) K) K) K) K) K)
W/(m W/(m W/(m W/(m W/(m W/(m
K) K) K) K) K) K)
Fig. 5. Relative thermal conductivity distribution as a function of concentration.
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Viscosity is one of the most important properties of nanofluids, which indicates the resistance of fluid. Considerable engineering problems arise while dealing with high viscous liquids that include higher energy requirement for pumping as well as for mixing. Turbulence, an essential means by which high rates of heat and mass transfer can be achieved, is delayed to higher velocities for liquids with higher viscosity [30].
To ensure the accuracy of the devices to measure the viscosity, pure water was measured as standard sample within the condition of this study. Results show that the measurement value is 1.1248 103 Pa s while reference value is 1.1404 103 Pa s at 15 C and proportional error is ±1.37%. At a higher temperature the measurement value is 0.7065 103 Pa,s while reference value is 0.7225 103 Pa,s at 35 C and proportional error is ±2.21%. Shear stress of ZnO-EG nanofluids as a function of shear rate for different concentrations at 25 C is shown in Fig. 6. The rheological behaviors of the nanofluids show that ZnO-EG nanofluids under the condition of wt.% 10.5% demonstrate Newtonian behaviors (Fig. 7). As viscosity increases with concentration, the nanofluids may turn to non-Newtonian behaviors at a peak point. Therefore, further study will focus on the rheological properties of higher concentration nanoparticles. Newtonian or non-Newtonian behaviors of nanofluids depend strongly on many factors [31e34], such as the kind and the shape of nanoparticles, their volume fraction and temperature. To investigate the temperature-independence viscosity of ZnOEG nanofluids, the viscosity was measured against the temperature. Fig. 8 exhibits the experimental viscosity distribution against temperature in the range of 15e55 C and concentration in the range of 1.75e10.5 wt.%, respectively. The viscosity decreases with increasing temperature and increases with increasing the mass fraction, (Fig. 8(a)) showing highly agreement with most of the previous work [35e37]. The relative viscosity of ZnO-EG nanofluids (Fig. 8(b)) shows the same temperature-dependency since the viscosity of the base fluid (EG) and that of the nanofluids nearly decrease by a similar degree as increasing the temperature. On the other hand, the enhanced ratio of the viscosity decreases with increasing the temperature between 15 and 55 C at a constant concentration. The relative viscosity is higher than some of the predecessors' work, which is most possibly caused by the use of PVP (dispersing agent). Models of viscosity have been well used to estimate the effective viscosity of nanofluids as a function of solid volume fraction. In this study, Einstein [38] and Wang [39] models, which can be used to calculate the viscosity of nanofluids with spherical particles in volume concentrations less than 5.0% vol., were applied to compare the predicted viscosity with the measured data. The comparison of the effective viscosity with respect to particle mass fraction for experimental data at 25 C and theoretical models is shown in Fig. 9. As can be seen, both of the theoretical correlations are unable
Fig. 6. Shear stress of ZnO-EG nanofluids as a function of shear rate for different mass concentrations.
Fig. 7. Rheological behaviors of ZnO-EG nanofluids.
literature provided the evidence that metal oxide nanofluids based on EG Refs. [21e28] could improve the thermal conductivity effectively, even to this day there is no reliable theory to predict the thermal conductivity of nanofluids, but the existing classical models for solideliquid mixtures are often used to calculate the thermal conductivity. In order to analyze the thermal conductivity enhancement mechanism and explore suitable thermal conductivity models for ethylene glycol based ZnO nanofluids, the following analysis is presented based on Hamilton and Crosser (HeC) model. Hamilton and Crosser [29] had put forward the theoretical model to calculate the thermal conductivity of solideliquid mixture. Under the assumption of ZnO nanoparticles were suspended in the ethylene glycol stably and uniformly, the thermal conductivity can be calculated as Equation (2).
kp þ n 1 k0 n 1 4 k0 kp k ¼ k0 kp þ n 1 k0 þ 4 k0 kp
(2)
where k0 and kp are the thermal conductivity of base fluid and nanoparticles, respectively, 4 is the volume fraction of nanoparticles, which was calculated by the mass fraction and density of ZnO nanoparticles provided by the supplier. In this work, the relative percentage of thermal conductivities increases nonlinearly with the mass fraction of nanoparticles. Experimental thermal conductivities values at 25 and 30 C were compared with previously reported values, for instance, the enhanced value is 9.13% for 7 wt.% ZnO-EG nanofluids, which is higher than the prediction value of 3.78% by calculating with the HeC model. The results of Yu et al. give a good agreement with present data and that of Lee et al., Moosavi et al., and Kim et al. under measured the experimental enhancement of ZnO-EG nanofluids at 30 C. 3.2. Viscosity investigation
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for the higher viscosity for experimental data. As high degree of accuracy for viscometer, residual errors for experimental data, for example, it is 0.25 mPa s for 1.75 wt.% nanofluids, are mostly caused by the analysis of measuring result. Judging from the above results, it should be noted that the thermal conductivity increased with increasing the temperature and the viscosity shows opposite tendency. In order to enhance heat transport in engineering applications, heat-transfer medium with a higher thermal conductivity should be the prime choice. What's more, the conveyer pipe may be blocked if the viscosity of medium is too large, and a pump with larger power is also needed. That is to say, it is vital that the ethylene glycol based ZnO nanofluids should be used in a higher temperature. 4. Conclusion
Fig. 8. (a) Absolute value of the viscosity and (b) the relative viscosity distribution for various concentrations of ZnO-EG nanofluids against the temperature.
to predict the viscosity of ZnO-EG accurately and the predicted viscosity is lower than experimental values. The aggregation of nanosize particles and quite a lot of PVP in the nanofluids elevate the resistance of suspension, so it may be the best reason to account
In this work, the thermal conductivity and viscosity of ethylene glycol based ZnO nanofluids have been experimentally investigated. For this purpose, well dispersed suspensions were obtained by dispersing different mass fractions of ZnO nanoparticles with an average diameter of 50 nm into a fixed volume of ethylene glycol. The thermal transport properties including thermal conductivity and viscosity were measured. The following conclusions are obtained. The absolute thermal conductivity increases with increasing the temperature ranging from 15 to 55 C. The thermal conductivity depends on particle concentration, and it increases nonlinearly with the mass fraction of nanoparticles. Experimental thermal conductivities values were compared with previously reported values, the enhanced value is higher than the prediction value of HeC model. The results of Yu et al. give a good agreement with present data and that of Lee et al., Moosavi et al., and Kim et al. under measured the experimental enhancement of ZnO-EG nanofluids at 30 C. The rheological behaviors of the nanofluids show that ZnO-EG nanofluids with mass fraction wt.% 10.5 demonstrate Newtonian behaviors and the viscosity decreases with increasing the temperature and that increases with increasing the mass concentration. Acknowledgements This work is financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004), the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant No. 2014RFYXJ004) and the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETIV.201315). References
Fig. 9. Comparison between theoretical models and measured data at 25 C.
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