Flows of a Vapor due to Phase Change Processes at the Condensed Phases with Temperature Fields as their Internal Structures

摘要

Transient to steady motions of a vapor caused by the evaporation and condensation processes occurring at the condensed phases placed in parallel have been studied based on the Boltzmann equation of BGK type. As the internal structures of the condensed phases, the temperature fields are taken into account. Because of this, the temperatures of the interfaces become unknown parameters and, therefore, the condition of the continuity of energy flow across the interface has to be imposed simultaneously with the conditions so far used for the cases with no internal structures. This extra condition gives great difficulty in the numerical simulations but this has been surmounted by a simple method developed earlier in our laboratory. The present analysis has also incorporated a certain kind of imperfectness of the interface in the boundary conditions by the introduction of a simple parameter, called the imperfectness parameter here, first proposed by Wortberg and his colleague. The results obtained describe appropriately the development of the transient flow fields due to the processes of evaporation and condensation at the interfaces across which the continuous energy flows are taking place. Some of the features worth to be mentioned are that 1) a certain value of the latent heat parameter gives the maxima of the mass and energy flows. This fact, which is newly found here, is due to the coupling effects of the latent heat parameter and the existence of the internal structures of the condensed phases; 2) the negative temperature gradient phenomenon, a well-known phenomenon at steady state in weak evaporation and condensation problems between the two condensed phases having no internal structures, seems to be non-existent in the present case with internal structures; 3) the negative mass flow phenomenon, first noticed and discussed by Sone and Onishi, seems to be non-existent at steady state but this surely manifests itself in a short period of time during the transitional state of the flow fields.