Heat Transfer and Rheological Behaviour of Nanofluids – A Review

Abstract

Nanofluids refer to dilute liquid suspensions of nanoparticles. Over the past decade, such materials generated lots of excitement mainly because a number of researchers reported drastic thermal conductivity enhancement with very small particle loadings. This also sparked hot debates on the underlying physics governing the experimentally observed phenomena. This paper gives an updated review on the topic. It is not intended to be exhaustive but meant to cover the main aspects associated with nanofluids with a specific focus on heat transfer applications. The review covers transport properties of nanofluids in particular thermal conductivity and shear viscosity, and heat transfer of nanofluids under convective and boiling conditions. No new physics appears to be behind the experimentally observed thermal conductivity enhancement as the vast majority of the experimental data fall within the range predicted by the conventional effective medium theory in combination with information of nanoparticle structuring. There seems to be no new physics either in terms of the experimentally observed increase in the shear viscosity of nanofluids as almost all the experimental data can be quantitatively interpreted by the conventional rheological and colloidal theories. There is no sufficient quantitative information, however, to infer the dominant mechanisms for heat transfer enhancement under convective and boiling conditions, where many controversies remain and require further research.