Analysis of Gas Flows with Phase Change on the Basis of Molecular Gas Dynamics

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1. We investigated the flow of a gas between its two condensed phases of different temperatures caused by evaporation and condensation on the basis of an accurate numerical analysis of the linearized Boltzmann equation for hard-sphere molecules. By this analysis, we clarified the steady behavior of the gas, in particular, the parameter range for which the negative temperature gradient phenomenon (the phenomenon that the temperature gradient in the gas is in the direction from the hotter condensed phase to the colder) appears, for the entire range of gas rarefaction.2. We investigated high-speed flows of a gas evaporating from or condensing on its plane condensed phase on the basis of the nonlinear Boltzmann-Krook-Welander equation (a model Boltzmann equation). By an accurate numerical analysis based on a finite-difference method, we showed that the steady evaporating flow is possible only when there is no gas motion along the condensed phase. We also found the relation among the parameters of the condensed phase and of the gas at infinity which allows the steady condensing flow on the basis of a great amount of the numerical data. The relation is completely different depending on whether the velocity conponent normal to the condensed phase at infinity is subsonic or supersonic.3. We derived systematically from the Boltzmann equation the correct fluid-dynamic system (the fluid-dynamic equation and its boundary condition) which describes the steady flow of a gas around its condensed phase of an arbitrary smooth shape, on the surface of which strong evaporation or condensat ion is taking place, at the level of classical fluid dynamics. We applied this system to some problems of practical importance.