A Generalized Diffusion Layer Model for Condensation of Vapor With Noncondensable Gases

The diffusion layer model for condensation heat transfer of vapor with noncondensable gases was originally derived on a molar basis and developed from an approximate formulation of mass diffusion, by neglecting the effect of variable vapor–gas mixture molecular weights across the diffusion layer on mass diffusion. This is valid for gases having a molecular weight close to that of the vapor or for low vapor mass transfer rates, but it may cause serious error if a large gradient in the gas concentration exists across the diffusion layer. The analysis herein shows that, from the kinetic theory of gases, Fick’s law of diffusion is more appropriately expressed on a mass basis than on a molar basis. Then a generalized diffusion layer model is derived on a mass basis with an exact formulation of mass diffusion. The generalized model considers the effect of variable mixture molecular weights across the diffusion layer on mass diffusion and fog formation effects on sensible heat. The new model outperforms the one developed by Peterson when comparing with a wide-ranging experimental database. Under certain limiting conditions, the generalized model reduces to the one developed by Peterson.