Drying of capillary porous media simulated by coupling of continuum-scale and micro-scale models

Abstract

In this work, we recall an isothermal version of the broadly accepted macroscopic continuum model of drying, and we impose flux boundary conditions at two interfaces, i.e., at the interface between the (gas-side) boundary layer and the (medium-side) dry region, as well as the interface between the dry and unsaturated regions which evolves freely during drying. Local relative humidity, local saturation as well as local transport parameters are computed from microscopic pore network simulations. This dataset is then employed to compute fluxes that couple the internal and external mass transfer in the continuum model. Decisive advantages of this approach over the classical method are that the continuity of the mass flux at the drying front and the porous medium surface is ensured and that the continuum model parameters are computed directly from pore network simulations – no need for any empirical correlation. Derivation of the macroscopic parameter functions from the pore network simulations for use in the continuum model raises issues of averaging intervals and treatment of the dataset. Operation of the continuum model is very sensitive upon the moisture transport coefficient in the totally or partially saturated zone of the porous medium. A hybrid method is introduced to control the effect of the sensitivity of the continuum model on the macroscopic parameters. By punctually adjusting the dataset in the high saturation period, the continuum model provides a stronger agreement with pore network simulations, which shows that the underlying transport phenomena are better preserved in the scattered dataset that the new hybrid method provides.