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Viscosity of hybrid nanofluids: A critical review

The advanced class of conventional heat transfer fluids called hybrid nanofluids is one of the astonishing outcome of the recent advancement in nanotechnology. The remarkable enhancement in heat transfer capabilities of conventional fluids with the addition of nanosized metallic and non-metallic particles appealed the attention of scientists and scholars towards the suspension of hybrid nanocomposites as a substitute of mono particles. The presence of nanocomposites enhances the heat transfer rate as a result of the thermal conductivity enhancement of utilized fluid. Although these fluids manifest captivating thermal characteristics, the drawback associated with their application includes high frictional effects and pumping power requirements. The major cause of aforementioned problems is the elevated viscosity. The current study summarizes the work of different investigators and discusses the critical factors affecting the viscosity of hybrid nanofluids such as temperature, particle concentration, pH value, particle size and morphology along with a concise discussion on the reasons reported in the literature for the viscosity augmentation. Furthermore, the models developed by different investigators have also been discoursed with specified limitations. A brief comparison between the viscosity of mono and hybrid nanofluid is also presented. It is revealed that most of the studies considered the effect of particle concentration and temperature that the effect of these factors is more significant than that of others. Water-based nanofluids delivered the better results in comparison of ethylene glycol-based nanofluids while the oil-based nanofluids preferred in the applications where the pumping power is not more significant. It has been observed that the fluids containing tube shaped nanoparticles comparatively showed enhanced viscosity than that of spherically shaped nanoparticles. It has also been noticed that the studies preferred to develop their own models for the prediction of viscosity of utilized hybrid nanofluid rather than to use the existing models and failed to provide a universal model. Moreover, the presented models are acceptable within the prescribed ranges for variables with the restriction of ensuring the same procedure and conditions for the preparation of hybrid nanofluids.
PAPER REVISED: 2018-12-19
PAPER ACCEPTED: 2018-12-20
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