Numerical study of evaporation and condensation phenomena by molecular dynamics and kinetic theory of gases

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The evaporation from and condensation on a liquid or solid surface have long been an important subject of fundamental researches in physics of fluids. Molecular gas dynamics (rarefied gas dynamics) can give an accurate description of the behavior of vapor adjacent to its condensed (liquid or solid) phase. This has actually been accomplished by solving the Boltzmann equation with a kinetic boundary condition at an interface between vapor and its condensed phase. However, the physical appropriateness of the kinetic boundary condition has never been verified, and the parameter called condensation coefficient cannot be determined in the framework of the molecular gas dynamics. In this research, we examined its physical validity by the numerical method of molecular dynamics (MD), numerical analysis of the initial and boundary value problem of Gaussian-BGK-Boltzmann equation, and experiments using a shock tube. As a result, the boundary condition for the Boltzmann equation at a vapor-liquid interface is found to be the product of three one-dimensional Maxwellians for the three velocity components of vapor molecules and a factor including well-defined condensation coefficient. The Maxwellian for the velocity component normal to the interface is characterized by the liquid temperature, as in a conventional model boundary condition, while those for the tangential components are prescribed by a different temperature, which is a linear function of energy flux across the interface. The condensation coefficient is found to be constant and equal to the evaporation coefficient determined by the liquid temperature only.