Insights into evaporation from the surface of capillary porous media gained by discrete pore network simulations

Abstract

Reliable prediction of the drying rate from the surface of a porous medium remains a scientific challenge due to complex interactions between the gas-side boundary layer and porous medium structure. One way to tackle this issue is to opt for a mesoscale pore network model. Two parameter functions (i.e. surface vapor pressure–surface saturation, and the surface vapor transport coefficient–surface vapor concentration relationship) are assessed from the pore network modeling, which can be used in the frame of a continuum model to couple the porous medium with the boundary layer. Moreover, emendation factors are introduced to link the normalized evaporation rate from the wet part of the porous medium surface with the surface wetness or surface saturation. Their evaluation shows that the normalized evaporation rate contributed from wet patches tends to surface saturation with increasing intensity of drying, whereas a similar but weaker trend can be observed with surface wetness. This correlation provides, especially in case of enhanced and non-isothermal drying processes, a simple method for coupling the wet surface region with the gas-side boundary layer in a two-equation continuum model. We have also evaluated the evaporation rate from the wet surface region by inserting pore network modeling data into a modified version of Schlünder's model. The results are not encouraging the derivation of continuum model boundary conditions according to Schlünder, neither for the whole medium surface nor for its wet part.