Laminar convective heat transfer of alumina-polyalphaolefin nanofluids containing spherical and non-spherical nanoparticles

As a potential candidate for advanced heat transfer fluid, nanofluids have been studied extensively in the literature. Past investigations were largely limited to thermophysical property measurements of aqueous nanofluids synthesized with spherical nanoparticles. No comprehensive information is yet available for convective thermal transport of nanofluids containing non-spherical particles, especially of the non-aqueous nanofluids. In this work, an experimental study was conducted to investigate the thermophysical properties and convective heat transfer of Al2O3-polyalphaolefin (PAO) nanofluids containing both spherical and rod-like nanoparticles. The effective viscosity and effective thermal conductivity of the nanofluids were measured and compared to predictions from several existing theories in the literature. It was found that, in addition to the particle volume fraction, other parameters, including the aspect ratio, the dispersion state and the aggregation of nanoparticles as well as the shear field, have significant impact on the effective properties of the nanofluids, especially of those containing non-spherical particles. The pressure drop and convection heat transfer coefficient were also measured for the nanofluids in the laminar flow regime. Although established theoretical correlations provide satisfactory prediction of the friction factor and Nusselt number for nanofluids containing spherical nanoparticles, they fail for nanofluids containing rod-like nanoparticles. The results indicate that in a convective flow, the shear-induced alignment and orientational motion of the particles must be considered in order to correctly interpret the experimental data of the nanofluids containing non-spherical nanoparticles.