On applying an external-flow driven mass transfer boundary condition to simulate drying from a pore-network model

Abstract

A novel method to simulate drying from a pore-network model is presented. Due to the evaporative mass-flux going out from the network as well as the outside airflow, a vapor mole-concentration boundary layer forms around the network. Concentration Boundary Layer Thickness (CBLT) and consequently the mass transfer coefficient are usually assumed constant in the literature. However, CBLT varies during drying owing to the changes in the outgoing mass flux from the network. To investigate the variation in CBLT and its dependence on convective effects due to the airflow, a numerical code based on the finite volume method has been developed to simulate drying characteristics of a pore-network. Laminar, steady state and two-dimensional Navier–Stokes equations as well as the vapor mole-concentration equation with both the convective and diffusive terms are discretized and solved according to the SIMPLE algorithm. A 20×20 two-dimensional pore-network after including cylindrical throats with circular cross sections is considered to model slow (isothermal) evaporation. Due to the geometry of throats and the network size, the effects of liquid film, viscous and gravitational forces are not considered. Therefore the capillary effect remains the dominant force for liquid transport inside the pore network. The drying algorithm applied inside the pore-network is the same as the one proposed by Metzger et al. (2007) [14], and it is verified that the convective effects are negligible within the gas phase inside the network. The effect of air flow over the exposed surface of the network causes a sudden decrease in evaporation rate from the pore-network after a short time period. This confirms the existence of the first drying period, as addressed in previous publications (e.g. Metzger et al. [14]). Also, a unique CBLT that grows along the length of the pore-network has been obtained. It is shown that the thickness of the concentration boundary layer decreases with time everywhere along the top surface of the network; therefore the assumption of constant CBLT that is used in the literature is merely a rough estimate. The present model provides a more accurate analysis of the isothermal evaporation from the porous structures exposed to laminar external air flow.