Molecular Study of Phase Change Phenomena of Methanol and Water Vapors in Wide Ranges of Vapor-Liquid Non-equilibrium State

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The boundary condition for the Boltzmann equation at a vapor-liquid interface is found to be, on the basis of molecular dynamics, 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, 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 alone.Numerical simulations of molecular gas dynamics are presented for the determination of condensation coefficient of methanol vapor from experimental data with a shock tube. The boundary condition for the Boltzmann equation is rewritten using the net condensation mass flux estimated by the shock tube experiment, and the evaporation and condensation coefficients are eliminated in the boundary condition. Thereby, the numerical solution of the vapor-liquid system is uniquely obtained for a given net condensation mass flux, and the instantaneous relation between the evaporation and condensation coefficients and vapor-liquid system can be examined from the mass conservation equation through the interface. The result shows that the condensation coefficient takes values from 0.7 to 0.9, which are dose to that of the evaporation coefficient evaluated in the molecular dynamics simulation.