Mechanisms of convective heat transfer of nanofluids

Research on nanofluids has progressed rapidly since its enhanced thermal conductivity was identified about a decade ago. Much evidence shows that the enhancement of convective heat transfer is much higher than that of thermal conductivity only. The mechanism of such enhancement, however, is still unclear. This work reviews the mechanisms of convective heat transfer of nanofluids in a single channel, and identifies two most likely mechanisms: the modification of effective properties and the migration of nanoparticles under flow conditions. A numerical simulation based on a combined Euler and Lagrange method is investigated in this work to illustrate the feature of nanoparticle migration and the drawback of the effective property approach. The motion of discrete nanoparticles is determined by the Lagrangian trajectory method based on the Newton's second law that includes influence of the body force, various hydrodynamic forces, and the Brownian and thermophoresis forces. The coupling of discrete particles with continuous flow is realized through the modification of the source term of the continuous equation. It concludes that the two-phase flow nature of nanofluids, especially the nanoparticle migration and the resultant non-uniform particle and effective property profile, needs to be considered to properly model the convective heat transfer.