The onset of mixed convection in a porous layer heated with a constant heat flux

The onset of mixed convection in a fluid-saturated porous layer between two horizontal plates heated from below with a constant heat flux is investigated theoretically. In the present study, the Darcy model is applied and the resulting nonlinear equations are solved by using the finite volume method. In order to analyze the growth behavior of convective instabilities, the local growth rates of mean temperature and its fluctuations are examined. Based on the numerical results, characteristic distances to represent the onset of secondary flow and also the manifestation of mixed convection are clarified to a certain degree in comparison with available experimental data. The critical distance to mark the onset of intrinsic thermal instability is predicted from a new stability criterion. It is interesting that the propagation theory yields almost the same critical conditions as the numerical ones. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 2677-2683, 2006.