Influence of particle properties on convective heat transfer of nanofluids

An experimental study is performed in order to examine how particle properties such as size and thermal conductivity affect the convection heat transfer of nanofluids. For this purpose, we prepare and study self-synthesized water-based nanofluids with different kinds of particles: polystyrene, SiO2, Al2O3 and micelles. Concentrations of the nanofluids vary in the range of 0.1-1.8 vol-% and particle sizes between 8 and 58 nm. Full-scale convective heat transfer experiments are carried out using an annular tube heat exchanger with the Reynolds numbers varying in the range of 1000-11000. The pressure losses are also taken into account in the analysis in order to assess the feasibility of the nanofluids for practical forced convection heat transfer applications. The fluids are thoroughly characterized: viscosities, thermal conductivities, densities, particle size distributions, shapes and zeta potentials are all determined experimentally. In many previous studies, anomalous enhancement in convective heat transfer is observed based on comparison of the Nusselt numbers with equal Reynolds numbers. Also in this work, the nanofluids exhibit Nusselt numbers higher than water when compared on this basis. However, this comparison neglects the impact of differences in the Prandtl numbers, and therefore the altered thermal properties of nanofluids are not properly taken into account. In this study, no difference in Nusselt numbers is observed when the Prandtl number is properly considered in the analysis. All nanofluids performed as the Gnielinski correlation predicts, and the widely reported anomalous convective heat transfer enhancement was not observed with any nanoparticle types. Instead, we show that the convection heat transfer behavior of nanofluids can be explained through the altered thermal properties alone. However, addition of any type of nanoparticles was observed to change the fluid properties in an unfavorable manner: the viscosity increases significantly, while only moderate enhancement in the thermal conductivity is obtained. The more viscous nanofluids reach lower Reynolds numbers than water with equal pumping powers resulting in lower heat transfer coefficients. However, the increase in viscosity, and therefore also the deterioration of the convective heat transfer, is less pronounced for the nanofluids with smaller particle size indicating that small particle size is preferable for convective heat transfer applications.