Unsteady Natural Convection in a Porous Square Cavity Saturated by Nanofluid Using Buongiorno Model: Variable Permeability Effect on Homogeneous Porous Medium

Abstract

This paper investigated numerically a natural convection in a porous cavity saturated by nanofluide. The left and right wall of the cavity are maintained at the hot-cold temperature respectively, the other walls are adiabatic. The two-phase Buongiorno model has been adopted to take account Brownian and thermophoretic diffusion in order to demonstrate the spatial distribution of the local nanoparticles concentration. After following the temporal evolution of the different structures (0<τ<5), Numerical simulations are performed to explore the effect of density buoyancy (104<Ra<106), the permeability of the homogeneous porous medium (10-5<Da<10-2), on the hydrodynamic, thermal and mass behavior. An original motivation was also introduced in our work to examine the effect of linearly variable permeability along the opposite direction of the cavity by varying the initial Darcy number from (10-5<Da<10-2) and fixing the final Darcy number Daf =10-5. The dimensionless partial differential equations are solved using the finite element method. The effects of the governing parameters on heat transfer are analyzed. Results indicate that the stationary regime is formed after the unsteady regime at dimensionless time τ = 0.3. The movement of nanofluide is strongly influenced by thermal buoyancy forces and depends on the Darcy number, heat transfer is accentuated for the homogenous medium compared to this one with variable permeability. It is found that the convective flow in a homogeneous porous medium is considerably affected by the variation of permeability and consequently the heat transfer is reduced.