Thermophysical profile of ethylene glycol-based ZnO nanofluids

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Highlights

Abstract

This work presents values of the experimental thermal conductivity, viscosity and density of homogeneous and stable nanofluids consisting of both synthesized and commercial ZnO nanoparticles dispersed in ethylene glycol (EG). The influence of variables such as particle size, temperature and volume fraction on their thermophysical properties were studied at concentrations up to 6.2%. The experimental results provide the evidence that thermal conductivity increases non-linearly with concentration and temperature within the range studied. Viscosity increases with concentration as usual for this type of dispersion and decreases with temperature. Density as a function of pressure has been also examined. The mixing volumes show little dependence on temperature and pressure over the range studied. Nevertheless, these results show a contractive behaviour on mixing and a departure from ideal behaviour, and this effect increases with concentration. The influence of particle size is significant for the measured properties, especially on viscosity. Finally, experimental values of thermal conductivity and viscosity have been compared to estimations provided by several simple theoretical models.

Introduction

In many applications in the fields of thermal science and engineering, a better understanding of processes involved in micro-scale liquid flow has become a major concern [1]. It is agreed that the thermal conductivity of the working fluids employed plays a relevant role in the heat transfer efficiency in processes of thermal convection. Concerning this, a new family of materials, termed as nanofluids, and consisting of nano-additives dispersed on a base fluid [2], represents a promising possibility because their thermal transport capacities are significantly higher than the usual values exhibited by conventional liquids.

Thermal conductivity of nanocolloids containing suspended metallic nanoparticles has been reported to be higher than the average values of the most widely used heat transfer fluids [1], [2], [3], [4], [5], [6], [7]. There are some discrepancies between different thermal conductivity data sets reported in literature for this type of systems, and the reason for these differences is due to the combination of several factors, as for instance the diversity of preparation processes, particle size dispersion, stability, non uniformity of the particle shape, clustering, sedimentation, etc. The recent work by Buongiorno et al. [8], where thermal conductivity of the same sample of a given nanofluid was measured simultaneously by several research laboratories involved in nanofluid research, each of them applying their expertise and their particular experimental approaches, represents a remarkable effort towards standardization and search of reproducibility for nanofluids characterisation. Although in the few last years key contributions intended to clarify concerns on thermophysical properties of nanofluids have been presented, there is still an important need of reliable experimental data, including thermal conductivity because it was the first property that brought nanofluids to scene, but considering as well other properties that may include volumetric or rheological behaviour [1], [2], [9], [10], [11], [12], [13], [14], [15], [16], because it is the nature and mechanisms of these nanometric scale systems that are still to be unveiled.

Some studies concerning ZnO nanofluids, using ethylene glycol (EG) as base fluid have been lately published [17], [18], [19], [20], [21]. However, further research is clearly needed to resolve controversies and to help to describe the underlying molecular level interactions and mesoscale processes governing their particular behaviour, including the enhanced thermal conductivity, viscosity and volumetric behaviour for these ZnO/EG nanofluids. Regarding thermal conductivity research on ZnO/EG nanofluids containing no surfactants, Lee et al. [18] proposed a comparative study conducted by five Laboratories that evidences large enhancements in thermal conductivity, that represent a remarkable departure from the classical Maxwell model. The authors [18] point out the conclusion that further research is needed. Previously, Yu et al. [22] determined that the thermal conductivity of ZnO/EG nanofluids increases with temperature and also but with a non linear dependence with concentration. They observed also that thermal conductivity enhancement ratios are temperature independent. Moreover, the authors reported Newtonian behaviour at low volume concentrations (up to 2 vol.%) while shear-thinning was noticed for volume fractions higher than 3%. However, the torque values corresponding to non-Newtonian behaviour shear rates were not provided, raising the issue about potential problems with the device inertia. Yu et al. [20] studied samples using ZnO nanoparticles with a declared diameter in the range (10 to 20) nm, and they stated the strong dependence of the sample particle size in the measured thermal conductivity. Recently, Lee et al. [18] reviewed the studies published on thermal conductivity of ZnO nanofluids in water, EG or mixtures. Moosavi et al. [19] studied the thermal conductivity, viscosity and interfacial tension of ZnO nanoparticles of 67 nm dispersed in EG, but used ammonium citrate as dispersant agent, a factor that is to be carefully considered as it may alter the fluid thermophysical properties. They found that thermal conductivity ratio increases nonlinearly with temperature but is lower when using base fluids of higher thermal conductivity, while viscosity increases with nanoparticle concentration and as usual decreases with temperature. Previously, Kim et al. [17] analysed the effect of the dispersed nanoparticle size, finding that thermal conductivity is inversely proportional to the mean diameter of nanoparticles (30 and 60 nm), and this dependence appears to be linear.

Following our previous research on nanofluids [5], [23], [24] in the present study we have synthesized ZnO nanoparticles, which were used to disperse ZnO/EG nanofluids in a range of volume fractions, and then we have studied the concentration, particle size and temperature influence on the experimental thermal conductivity, viscosity and density. The influence of pressure in density data was also measured. Finally, data for experimental thermal conductivity and viscosity were compared with the estimations of some simplified empirical models.

Section snippets

Synthesis of ZnO nanoparticles

ZnO nanoparticles were synthetized using Zn acetate as reagent, in an alcohol solution with controlled pH conditions. This route is a version of the original method first proposed by Haase et al. [25]. Following this technique, 1.37 g of Zinc acetate and 1.4 g of NaOH were dissolved in Ethanol (250 mL) under ultra-sonication water bath at room temperature during two hours. Then, the precipitate was washed three times with deionized water, centrifuged at 9000 rpm and the remaining solid was dried in

Results and discussion

Thermal conductivities of five different concentrations of S1 nanofluids have been measured at temperatures from (283.15 to 343.15) K. The volume fraction, ϕ, varied between (1 and 6.2)%, estimated from EG [5] and the bulk solid oxide densities. Experimental results, knf, are presented in table 2. Thermal conductivity increases with concentration, see figure 5a, in agreement with literature trends, [17], [18], [19], [20], corresponding to an enhancement of the thermal conductivity from (4 to

Conclusions

ZnO nanoparticles were synthesized using a precipitation method and different ZnO/EG nanofluids were prepared by dispersing these synthesized and also commercial ZnO nanoparticles in EG. The behaviour of the density as a function of temperature and pressure has been experimentally determined up to 1% volume fraction. As concentration increases, the interactions between nanoparticles do show a non-negligible effect on the volumetric trend.

Within the shear stress range used for viscosity

Acknowledgements

The authors acknowledge CACTI (Universidade de Vigo) for assistance in microscopy and chemical analysis techniques, Xunta de Galicia (Spain) for both Grant Ref. PGIDIT-07-PXIB-314181PR, and Postdoctoral Grant for MJPG cofinanced with FSE funds. L.L. also acknowledges financial support from Ramón y Cajal Grant Program (Ministerio de Ciencia e Innovación, Spain).

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