Effects of Sedimentation of Nanoparticles on Flow, Heat and Mass Transfer of Al2O3-water Nanofluid in a Cavity

In this study, effects of transport mechanisms of nanoparticles such as: sedimentation, Brownian motion, and thermophoresis in natural convection of Al2O3-water nanofluid as a twocomponent mixture on flow field, heat transfer and mass transfer during a period of thirty days have been investigated numerically. Left and right walls of the cavity are hot and cold, respectively. The temperature difference between two vertical walls is 8 K. In order to consider the variations of the volume fraction of nanoparticles versus time, an experimental model during a period of thirty days have been applied. The momentum, energy and mass transfer equations have been solved using the finite volume method. The initial volume fractions of nanoparticles are 0.0025, 0.0077 and 0.013, the ultrasonicator dispersion time is 1, 2 and 3 hours, and Rayleigh number range is from 102 to 105. The results show that in low Rayleigh number (102 and 103), during a period of time and sedimentation of nanoparticles, the heat transfer enhancement coefficient (E) is reduced. In Ra=104 and 105 with φb=0.0025, E is decreased as time passed. In φb=0.0077, the value of the E has a critical point due to the increasing thermal conductivity and viscosity coefficient, so that before the critical point, the E has increased and then has decreased. In φb=0.013, the E is increased over time. It is also observed that including the effect of sedimentation velocity is increased the thickness of mass boundary layer and the Nusselt number are reduced.