A two equation continuum model is developed to simulate the mass transfer in drying porous media. The main goal is to capture the so called non equilibrium effect To this end , we opera te in a regime where the liquid phase is immobile so that non equilibrium mass exchange between liquid and vapor phase dom inates. The formulation of the model relies on an upscaling technique. This notably permits to formula te the non local equilibrium phase change term on a firmer basis. The upscaling also indicates that there is no reason to consider an enhancement factor in the vapor diffusion model. The macroscopic model parameters are determined from pore network drying simulations. The same simulations are also used as a reference to compare with the predictions of the non local equilibrium continuum model. The solu tion of the two equation continuum model proves that this model simulates the non local equilibrium effect with reasonable accuracy. Also, the simulations indicate that the non local equilibrium effect is especially significant at the porous medium surface.

The coupling boundary condition to be imposed at the evaporative surface of a porous medium is studied from pore network simulations considering the capillary regime. The study highlights the formation of a thin edge effect region of smaller saturation along the evaporative surface. It is shown that this thin region forms in the breakthrough period at the very beginning of the drying process. The size of this region is studied and shown to be not network size dependent. This region is shown to be the locus of a non-local equilibrium effect. The features lead to the consideration of a coupling boundary condition involving an interfacial mass transfer resistance and an external mass transfer resistance. Contrary to previous considerations, it is shown that both resistances depend on the variation of the saturation, i.e. the fluid topology, and the size of the external mass transfer layer, i.e. the mass transfer rate. This is explained by the evolution of the vapor partial pressure distribution at the surface which becomes increasingly heterogeneous during evaporation and depends on both the evolving fluid distribution in the interfacial region and the mass transfer rate. However, the geometric effects due to the configuration of the fluids can be separated from rate effects that arise due to the nonequilibrium mass transport.