Natural Convection in Horizontal Porous Layers: Effects of Darcy and Prandtl Numbers

Natural convection in horizontal porous layers heated from below is studied by employing a formulation based on the Brinkman-Forchheimer-extended Darcy equation of motion. The numerical solutions show that the convective flow is initiated at lower fluid Rayleigh number Ra(f) than that predicted by the linear stability analysis for the Darcy flow model. The effect is considerable, particularly at a Darcy number Da greater than 10(-4). On the other hand, an increase in the thermal conductivity of solid particles has a stabilizing effect. Also, the Rayleigh number Ra(f) required for the onset of convection increases as the fluid Prandtl number is decreased. In the stable convection regime, the heat transfer rate increases with the Rayleigh number, the Prandtl number, the Darcy number, and the ratio of the solid and fluid thermal conductivities. However, there exists an asymptotic convection regime where the porous media solutions are independent of the permeability of the porous matrix or Darcy number. In this regime, the temperature and flow fields are very similar to those obtained for a fluid layer heated from below. Indeed, the Nusselt numbers for a porous medium with k(f) = k(s) match with the fluid results. The effect of Prandtl number is observed to be significant for Pr(f) < 10, and is strengthened with an increase in Ra(f), Da, and k(s)/k(f). An interesting effect, that a porous medium can transport more energy than the saturating fluid alone, is also revealed.