COMPRESSIBLE GAS IN POROUS-MEDIA - A FINITE-AMPLITUDE ANALYSIS OF NATURAL-CONVECTION

The theory of thermally driven convection of dry air in a porous mediu m is reviewed. The critical Rayleigh number for air is the same as for liquid, 4 pi(2), but the thermal gradient used is decreased by the ad iabatic gradient of air. Because of the differences in the physical pr operties of air and water, initiation of convection requires the produ ct of gradient and permeability to be thousands of times greater for a ir than for water. Finite amplitude analysis of the problem for Ra < 3 00 shows that: (1) the code predicts the onset of convection in an air filled porous medium; (2) at low thermal gradient, Ra vs Nu curves ar e nearly the same for air and water; (3) the slope of the Ra vs Nu cur ve matches well with experimental data reported by others for water; ( 4) time to steady state decreases approximately as the square root of Nusselt number. Copyright (C) 1996 Elsevier Science Ltd.