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

在分子动力学的基础上，发现汽液界面上Boltzmann方程的边界条件是汽相分子三个速度分量的三个一维Maxwell量与一个包含明确凝结系数的因子的乘积。垂直于界面的速度分量的麦克斯韦函数由液体温度表征，而切向分量的麦克斯韦函数由不同的温度决定，温度是界面能量通量的线性函数。发现冷凝系数是恒定的，且等于仅由液体温度决定的蒸发系数。用分子气体动力学数值模拟由激波管实验数据确定甲醇蒸汽的冷凝系数。用激波管实验估算的净凝结质量通量改写了Boltzmann方程的边界条件，并消除了边界条件中的蒸发系数和凝结系数。这样，对于给定的净凝结质量通量，汽液系统的数值解是唯一的，并且可以通过界面从质量守恒方程中考察蒸发系数和凝结系数与汽液系统的瞬时关系。结果表明，凝聚系数取值范围为0.7~0.9，与分子动力学模拟计算的蒸发系数取值一致。

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.