Enhanced Heat And Mass Transfer In Nanofluids

Nanofluids are a new class of liquids whose properties are controllable by the addition of nanoparticles. A great deal of attention has been drawn to their enhanced heat transfer characteristics relative to that of a pure fluid. Numerous experiments have demonstrated that the thermal conductivity of a nanofluid is generally greater than what can be predicted by classical mean-field theories, such as the Maxwell-Garnett model. Various mechanisms for this enhancement—still the subject of considerable debate—have been proposed, including energy transfer via interparticle potentials, enhanced conduction because of aggregation, and nanoscale convection induced by the Brownian motions of the nanoparticles. We report here the results of experiments on mass transfer in nanofluids, which were carried out in an attempt to help elucidate the mechanism(s) responsible for enhanced thermal transport. The results indicate a more than one-order-of-magnitude increase in effective mass diffusivity, for a nanofluid consisting of 20-nm Al₂O₃ nanoparticles in water, relative to that of pure water. For some nanoparticle volume fractions, the mass transfer appears non-diffusive, but in all cases the mass transfer is greater than that in pure water. The mass transfer enhancement is maximized, for this system, at a volume fraction of 0.5%. The enhanced mass transfer suggests that nanoscale convection is one of the primary mechanisms giving rise to increased transport in nanofluids.