Kinetic Approach to the Evaporation and Condensation Problem
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概要
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Phenomena caused in a vapor gas concerning evaporation from, or condensation onto a liquid (or solid) surface in contact with its pure vapor are investigated by using the Monte Carlo method. The formation of the Knudsen layer adjacent to the interphase and the growth of the hydrodynamic or continuum region outside the layer as time advances are simulated based on the kinetic theory of gases. The results are found to simulate these phenomena with sufficient accuracy. It is shown that there are two possible types of the transient developments of the vapor field according to the physical property of the evaporating substance. Several physical quantities in the hydrodynamic region in a quasi-steady state are computed and compared with a number of analytical and experimental data. The agreement of the present results with them is qualitatively satisfactory. A set of the slip condition at the interphase is presented to give the boundary conditions at the phase surface in hydrodynamic approaches. It is confirmed that the kinematic effects of binary collisions among vapor molecules on the mass flux rate are not so serious except in the case of very strong evaporation. This fact seems to indicate that the evaporation-condensation coefficient is close to unity provided that no condensation in the form of tiny droplets occurs in the vapor and that the mass accommodation (molecular absorption) coefficient of the phase surface is one. The quasi-steady state in the hydrodynamic region cannot be accomplished in both cases of the limiting condensation, where only condensation occurs but no molecules are spontaneously evaporated and reflected at the interphase, and of extremely strong evaporation, where the local Mach number in the continuum region exceeds unity. Some discussions on the feature of strong evaporation are also presented here.
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