Interfacial Temperature and Density Discontinuities for Phase-Change Heat Transfer With Non-condensable Gas

Abstract

In recent prior work, the author derived interfacial mass and heat flux conditions for phase-change processes. The mass flux condition is identical to the Schrage equation, but the additional heat flux expression enables one to couple the interface to the continua in both the liquid and the vapor phases and compute the interfacial temperature and density discontinuities. However, questions exist on how to treat phase change heat transfer in the presence of non-condensable gases. In this work, the author shows that the same set of interfacial conditions can be used to account for the presence of non-condensable gases. Although the mass flux of non-condensable gas is zero, their presence impacts the heat transfer. For evaporation, when the presence of the non-condensable gas is small, temperature and density discontinuities persist across the interface, as well as inverted temperature distributions. For condensation, however, no temperature inversion happens in the presence of a small amount of non-condensable gas and the interfacial temperature jump is significantly smaller. When a large amount of non-condensable gas is present, such as for evaporation into and condensation from air, the temperature discontinuities at the interface are significantly smaller and no temperature inversion happens. For evaporation driven purely by humidity difference, temperature inversion and discontinuity still exist. Results from this work will benefit the modeling of phase change processes in the presence of non-condensable gases, evaporative cooling in air, air-gap distillation, atmospheric water harvesting, and other applications.