Different modes of nanoparticle migration at mixed convection of Al₂O₃–water nanofluid inside a vertical microannulus in the presence of heat generation/absorption

Intensity and direction of nanoparticle migration are able to tune thermophysical properties of nanofluids to improve the thermal performance of heat exchange equipments. The intensity strongly depends on nanoparticle diameter and cannot be actively controlled while different directions of nanoparticle migration are achieved with asymmetric heating. In the current study, the mixed convective heat transfer of Al₂O₃–water nanofluid inside a vertical microannulus is investigated theoretically considering different modes of nanoparticle migration. The model employed for the nanoparticle–fluid mixture is able to fully account for the effects of nanoparticle slip velocity relative to the base fluid originating from the thermophoresis (nanoparticle slip velocity due to temperature gradient) and Brownian motion (nanoparticle slip velocity due to concentration gradient). To consider surface roughness in the microannulus, Navier’s wall slip condition is employed at the solid–fluid interface. It is revealed that the asymmetric heating at the walls alters the orientation of nanoparticle migration and deforms the symmetry of the flow field. In addition, despite temperature-dependent buoyancy forces, concentration-dependent buoyancy forces have considerable effects on the flow fields and nanoparticle migration.