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

The advantages of using Al<inf>2</inf>O<inf>3</inf>, TiO<inf>2</inf>, SiO<inf>2</inf> and CeO<inf>2</inf> 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.