Effect of liquid films on the drying of porous media

The effect is studied of capillarity-driven viscous flow through macroscopic liquid films during the isothermal drying of porous materials. A mathematical model that accounts for viscous flow in a 2-D pore network, through both the liquid films and the bulk liquid phase, is presented. The results are compared with previous works that do not account for the effect of films and with experimental findings by other authors. It is shown that film flow is a major transport mechanism in the drying of porous materials, its effect being dominant when capillarity controls the process, which is the case in typical applications. By contrast, viscous flow in the bulk contributes negligibly. The results are then generalized to drying under an applied temperature gradient. (C) 2004 American Institute of Chemical Engineers.