Effects of Surface Tension on Film Condensation in a Porous Medium

In the process of film condensation in a porous medium, the thermodynamics of phase equilibria requires the existence of a two-phase zone lying between the liquid and the vapor regions. In the two-phase zone, solutions of the conservation equations indicate a boundary-layer profile for the capillary pressure. The liquid zone is analyzed using three models, which assume either slip or no slip at the wall and Darcy velocity or no shear at the interface with the two-phase zone. The results show that the condition of no slip at the wall must be satisfied in all cases except where the thickness of the liquid zone is much larger than the characteristic boundary layer in the porous medium. At the interface with the two-phase zone, the assumption of no shear is more realistic than that of an imposed Darcy velocity, in conjunction with no-slip condition at the wall. Comparisons with experiments suggest that the drag on the liquid film due to surface tension is significant for permeabilities lower than 10⁻⁷ m². A dimensionless group, characterizing viscous flow due to surface tension forces, is introduced in this study.