Kinetic theories of evaporation

Abstract

Theories of evaporation and condensation which apply to a wide range of disequilibrium conditions must be based on assumptions regarding the form of the velocity distribution function of vapor molecules at the surface. However, the distribution function describing vapor molecules at the surface and moving away from it is not the same as that which applies to the emission of molecules from the condensed phase. A number of previous treatments of the half-space evaporation problem are shown to be incorrect and an improved solution, taking into account the evaporation coefficient, is presented.

Vacuum Evaporation of Pure Metals

“…According to Algie, [12] however, the distribution function describing vapor molecules moving away from the surface is not the same as for the emission of molecules from the condensed phase. Algie [12] extended Crout's approach [13] for evaporation (where vapor near the evaporating surface is considered as non-isotropic) and obtained an expression for the…”

“…[16] Considering the evaporation coefficient a as unity, g = 1.667 is obtained. Algie [12] indicated that for weak evaporation, g ¼ a 1À0:355a , and this gives g = 1.55 for a = 1. Koffman et al [14] applied the kinetic theory approach with continuum flow in gas between a hot liquid surface and a cold liquid surface.…”

“…According to Algie, [12] of the molecules which would leave the surface in the absence of surface constraints, only the fraction a actually leaves the surface. Algie does not seem to specify how to determine a.…”

“…Ytrehus and Østmo obtained g = 1.668 for weak evaporation, in which the results are linearized into the simple formulae, Eq. [12]. Considering the above studies under weak evaporation conditions (S fi 0), T g % T, Eq.…”

“…[12] with g = 2. [12,14] This differs by a factor of 2 from the Hertz-Knudsen equation. [3] Evaporation in a semi-infinite space has been studied through the development of a linear kinetic theory of condensation and Fig.…”