Implementation of two multiphase flow methods in modeling wetting of microporous hydrophobic membranes

Pore wetting phenomenon plays a critical role in a porous media and is critical in various processes. For instance, liquid entry pressure (LEP) is one of the critical characteristics of hydrophobic membranes used in membrane distillation (MD) processes. In this study, pore-scale models were developed to assess the accuracy of two multiphase flow computational fluid dynamics (CFD) methods, as modeling tools for predicting two-phase flow in microporous MD membranes. Finite element method (FEM)-based phase field (PF) method (which was applied in the COMSOL package) and finite volume method (FVM)-based volume of fluid (VOF) method (which was applied in Star-CCM+) were the selected CFD tools for the implementation. The boundary conditions of the models were first set based on the experimental procedure for measuring the LEP, as given in the literature. Then, the models were used to capture the LEP under the gradually increased water pressure. Critical tuning of CFD parameters of each tool (such as mesh size, mesh type, and interface thickness) was conducted to investigate their influence on the LEP prediction accuracy and the water/air interface representation at the pore entrance. CFD model results were presented and compared with both experimental LEP data and the calculated value using the Young-Laplace equation (YLE). Both CFD tools were capable of capturing the water/air interface. LEP result from the VOF model showed good agreement with the experimental data, but the PF model overestimated the LEP value closer to the theoretical YLE value. For both approaches, the adjustment of the interface thickness was critical. In the VOF method, a realistic interface thickness could be achieved by adjusting both mesh size and time step simultaneously. In contrast, PF simulations were less mesh sensitive. The accuracy of the VOF model was better due to its mass conservation condition at the interface.