Onset of Darcy-Benard convection using a thermal non-equilibrium model

In this paper we use a two-field model for the separate modelling of the solid and fluid phase temperature fields in a fluid-saturated porous medium, and, in particular, we consider how the onset criterion for convection in a horizontal layer is affected by the adoption of such a model. In general we find that both the critical Rayleigh number and wavenumber are modified by the presence of thermal non-equilibrium effects. It is shown that the well-known result of Lapwood [Proc. Cambridge Philos. Soc. 44 (1948) 508] which corresponds to local thermal equilibrium (LTE), is recovered when taking the thermal equilibrium limit of the non-equilibrium analysis. We also present asymptotic solutions for both small and large values of H the inter-phase heat transfer coefficient, H, and compare this with the numerical solutions. For intermediate values of H we find that the critical wavenumber is always larger than pi, the critical value for the LTE case. In some cases this critical wavenumber may be very large compared with pi. (C) 2002 Published by Elsevier Science Ltd.,