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

In a preceding paper (Brouwers and Chesters, Int. J. Heat Mass Transfer35, 1–11 (1992)) possible supersaturation in a film and in the bulk of a binary mixture has been discussed. In the present analysis the exact conditions for fog formation and the magnitude of the fogging and superheated regions in the film are first determined. Next, the governing equation of diffusion and energy (coupled with the saturation condition) of the fog layer is solved numerically. An evaluation of various mixtures of water vapour and air illustrates the substantial effect of fog formation on heat and mass transfer rates. Subsequently, a thorough asymptotic analysis of the fog layer's governing equation yields an excellently matching approximation solution. Furthermore, this solution leads to analytical film model correction factors for the combined effects of fog formation and injection/suction on transfer rates. Finally, the fog film model is applied to channel flow of a binary mixture. This approach provides new procedures for the computation of condensers and evaporators, allowing both fog formation in the film (affecting transfer rates) and/or in the bulk (affecting the incremental balances of mass and energy).