Evaporation into half-space: Kinetic simulation of Knudsen layer for monatomic gas

Tools for modeling phase change processes in confined spaces are necessary to estimate heat and mass fluxes impacted by micro-level effects. We develop and validate numerical models for the simulation of evaporation processes using the direct simulation Monte Carlo method and the S-model kinetic equation. The numerical results are compared with previous results obtained by Sone and Sugimoto using the Boltzmann-Krook-Welander kinetic equation. The method of moments for the solution of the Boltzmann equation is also applied. The profiles of macroscopic parameters in the Knudsen layer are analyzed in detail. A very good agreement among the results given by all the applied models is found for a Mach number up to 1. As the considered approaches differ only in the collision laws, the evaporation parameters (temperature, pressure ratios, and the evaporation rate) seem relatively insensitive to the collision law models. The analytical solutions for the marcoscopic profiles (temperature, velocity, density, and pressure) are in good agreement with the numerical results. Possible criteria for Knudsen layer thickness estimation are suggested and tested. The provided results could be used as reference data to test evaporation models.