Molecular simulation of evaporation mass flux during net evaporation/condensation

To examine the transport phenomena in a vapor–liquid two-phase system attributed to the phase change, a proper specification of the mass flux at a vapor–liquid interface is crucial. Since the mass flux induced by the phase change originates from the motion of molecules in the vicinity of the vapor–liquid interface, a continuum description such as the fluid dynamic based approach is inappropriate. An essential way to obtain this mass flux is the analysis of the Boltzmann equation with a certain boundary condition, that is, the kinetic boundary condition. In this study, we examined the definition and the estimation procedure of the evaporation coefficient, which is included in the kinetic boundary condition, at the vapor–liquid interface with phase change, especially at higher temperature for hard-sphere molecules. As the result, we confirmed that a conventional definition of the evaporation coefficient is accurate even if liquid temperature is higher. Moreover, we also confirmed that the evaporation coefficient is only the function of liquid temperature by counting the number of molecules passing through the two boundaries which are placed near the vapor–liquid interface.