Film models for transport phenomena with fog formation: the classical film model

In the present analysis the classical film model (or film theory) is reviewed and extended. First, on the basis of a thorough analysis, the governing equations of diffusion, energy and momentum of a stagnant film are derived and solved. Subsequently, the well-known correction factors for the effect of suction/injection on mass, heat and momentum transfer are derived. Next, employing global balances of mass, energy and momentum, the film model is applied to channel flow. This application yields a new expression for the pressure drop and hence it is compared extensively with experimental and theoretical results of previous investigators, yielding good agreement. The onset of fog formation in a binary mixture, both in the transferring film and/or in the bulk, is explained graphically with the help of the relation between temperature and vapour mass fraction and the saturation line of the vapour.