Laminar Convective Heat Transfer of Alumina-Polyalphaolefin Nanofluids Containing Spherical and Non-Spherical Nanoparticles

As a promising candidate for advanced heat transfer fluids, nanofluids have been studied extensively during the past decade. In contrast to the early reports of dramatic heat transfer enhancement even at extremely low particle concentrations, the most recent studies suggest the laminar convective heat transfer of nanofluids is only mildly augmented and can be predicted by the conventional Navier-Stokes equations. The majority of the past studies were limited to water-based nanofluids synthesized from spherical nanoparticles. No systematic information is yet available for the convective heat transfer of nanofluids containing non-spherical particles, especially those formulated with the base fluid other than water. An experimental study was conducted in this work to investigate the thermophysical properties and convective heat transfer characteristics of Al₂O₃-Polyalphaolefin (PAO) nanofluids containing both spherical and rod-like nanoparticles. The effective viscosity and thermal conductivity were measured and compared to predictions from the effective medium theory. The friction factor and local Nusselt number were also measured for the laminar flow regime. It was found that established theoretical correlations can satisfactorily predict the experimental data for nanofluids containing spherical nanoparticles; however, they are less successful for nanofluids with nanorods. The possible reasons may be attributed to the shear-induced alignment of non-spherical nanoparticles and its subsequent influence on the development of the thermal boundary layer. The results suggest that the hydrodynamic interactions between the non-spherical nanoparticles and the surrounding fluid medium have a significant impact on the thermophysical properties as well as on the thermal transport characteristics of nanofluids.