GAS-FLOWS CAUSED BY EVAPORATION AND CONDENSATION ON 2 PARALLEL CONDENSED PHASES AND THE NEGATIVE TEMPERATURE-GRADIENT - NUMERICAL-ANALYSIS BY USING A NONLINEAR KINETIC-EQUATION

A rarefied gas between its two parallel plane condensed phases with different temperatures is considered, and the steady gas flows caused by evaporation and condensation on the condensed phases are investigated numerically by using the (nonlinear) Boltzmann-Krook-Welander equation. The flow properties are clarified for a wide range of the parameters (the Knudsen number, the ratio of the temperatures of the condensed phases, and the ratio of the saturation gas pressures at these temperatures). In particular, the phenomenon of the negative temperature gradient, which has extensively been studied for weak evaporation and condensation, is confirmed to occur also for nonweak evaporation and condensation. In the course of the numerical analysis, it is demonstrated that the linearized Boltzmann equation does not give the correct description of the problem for very small Knudsen numbers (i.e., Knudsen numbers comparable to or smaller than the flow Mach number), however weak the evaporation and condensation may be. In addition, a high-speed evaporating flow toward a perfectly absorbing wall is investigated as an extreme case of strong evaporation and condensation.