Convective drying in thin hydrophobic porous media

A pore network (PN) model is developed to explore drying of a thin hydrophobic porous medium bounded with a gas purge channel. The PN is composed of cubic pore bodies connected by cylindrical pore throats. At the interface between a pore throat and body, a sudden geometrical expansion exists. When a meniscus advances to this interface, it will be pinned first until the pressure across the meniscus increases to a critical value. This phenomenon is called the capillary valve effect. Because of this effect, two types of invasion into pore bodies are discerned, i.e. bursting and merging invasion. The developed PN model with the capillary valve effect is validated against the experimental results. For the drying case of dominant merging invasion, a drying front of finite width is stably receded. But when bursting invasion dominates, gas invasion is a random process; the drying process can be characterized by three regimes: a surface evaporation period, a constant rate period, and a falling rate period. The total liquid saturation for transition from the constant to falling rate period is close to that at which the total area for vapor transport is maximal between the partially filled pores and their neighboring empty pores.