Effects of nanoparticle migration and asymmetric heating on mixed convection of TiO2-H2O nanofluid inside a vertical microchannel

The effects of nanoparticle migration on mixed convection of titania/water nanofluid inside a vertical microchannel have been investigated numerically via Runge-Kutta-Fehlberg method. A modified two-component heterogeneous model is employed for the nanofluid in the hypothesis that the Brownian motion and the thermophoresis are the only responsible mechanisms for nanoparticle migration. Because of small dimensional structures of microchannels, a linear slip condition is considered at the boundaries, which appropriately represents the non-equilibrium region near the interface. To impose different temperature gradients, the heat flux ratio of the right to the left wall (epsilon) is investigated in three different situations, namely the adiabatic right wall (epsilon = 0), unequal heat fluxes at the walls (epsilon < 1) and equal heat fluxes (epsilon = 1). It is revealed that the asymmetric thermal boundary condition affects the direction of nanoparticle migration and distorts the symmetry of the velocity and temperature profiles. In the rich nanoparticle concentration region, the viscosity and the local conductivity increase, which lead to a stronger conduction and a weaker convection rate. Also, it is found that splitting the total amount of heat Mix on the walls unevenly, is the most efficient way to enhance the heat transfer rate in the vertical microchannels. (C) 2014 Elsevier B.V. All rights reserved.