Combined effect of physical properties and convective heat transfer coefficient of nanofluids on their cooling efficiency

• The Maxwell and Modified Krieger-Dougherty models can predict the thermal conductivity and viscosity of nanofluids (+/- 10%).
• However, for the later the ratio of aggregated to primary particles and the fractional index must be known.
• Comparing base fluid with nanofluid at the same Reynolds number is not appropriate, but at the same pumping power or velocity.
• Investigating the hottest wall temperature at the outlet of a heat sink is a good method to look into cooling performance.
• Nanofluids did not show any benefit in turbulent flow; however, very small advantage in laminar flow.

The advantages of using Al2O3, TiO2, SiO2 and CeO2 nanofluids as coolants have been investigated by analysing the combined effect of nanoparticles on thermophysical properties and heat transfer coefficient. The thermal conductivity and viscosity of these nanofluids were measured at two leading European universities to ensure the accuracy of the results. The relative thermal conductivity of nanofluids agreed with the prediction of the Maxwell model within +/− 10% even at elevated temperature of 50 °C indicating that the Brownian motion of nanoparticles does not affect thermal conductivity of nanofluids. The viscosity of nanofluids is well correlated by the modified Krieger–Dougherty model providing that the effect of nanoparticle aggregation is taken into account. It was found that at the same Reynolds number the advantage of using a nanofluid increases with increasing nanofluid viscosity which is counterintuitive. At the same pumping power nanofluids do not offer any advantage in terms of cooling efficiency over base fluids since the increase in viscosity outweighs the enhancement of thermal conductivity.