KINETIC-THEORY APPROACH TO INTERPHASE PROCESSES

Using kinetic theory approach, the fluid- and thermodynamics aspects o f a vapor next to its interphase surface is studied under conditions o f arbitrarily strong interphase processes in single component systems. The physical domain considered is a boundary layer known as the Knuds en layer, a few molecular mean free paths thick, in which the vapor ad apts to given external equilibrium conditions. It is shown that the li miting flow conditions in strong evaporation closely corresponds to th e sonic state. Expressions for the mass transfer are derived both in t he general and in the linear cases, and comparisons are made with the classical Hertz-Knudsen and Schrage formulae which are both shown to b e incomplete. The vapor is found to be far from its saturated state, b oth at the interphase surface and at the border of the continuum regio n. The coupling between the Knudsen layer and this latter region is sh own by an example of evaporation into a shear flow.