Pore-scale effect on evaporation from a porous surface

Evaporation from porous surfaces often plays an essential role in nature and industrial applications such as comet sublimation, transpiration, micro pumps, and membrane-based cooling devices. Evaporation generates a layer with a highly nonequilibrium gas flow next to the liquid surface, and this layer is called the Knudsen layer. Understanding the microscopic transport phenomenon in the Knudsen layer is of great importance since it governs the evaporation mass flux. In this work, we analyzed evaporation flows from two-dimensional porous surfaces and clarified the pore-scale effect on the evaporation mass flux. To cover a wide range of pore scales (i.e., the pore spacing and diameter), we employed the low-variance deviational simulation Monte Carlo (LVDSMC) method, which can reduce the computational cost by typically two orders of magnitude for small Mach number conditions. The evaporation mass flux was evaluated for porous surfaces with the same pore opening fraction but different pore spacing. The simulation results showed that the evaporation mass flux increases as the pore spacing increases. To explain the pore scale dependence obtained from the numerical simulations, we examined the pressure distribution within the Knudsen layer. In addition, we constructed models for two limiting cases where the pore spacing and diameter are comparable or far larger than the mean free path of gas molecules.