Incorporation of species interdiffusion in boundary layer modelling of filmwise condensation with non-condensable gases

Steam condensation in the presence of non-condensable gases involves species interdiffusion. Yet this effect is generally neglected in the literature without a clear justification. We first examine the effect of species interdiffusion in the laminar regime. We consider a binary mixture of a condensable and a non-condensable gas in a differentially heated square cavity. The phenomena described by the numerical simulation are natural convection and condensation, which we model with a Stefan velocity. Examination of various cases shows that species interdiffusion can contribute significantly to the heat transfer when the mass flux is large and when the difference in the heat capacity of the two species is not small. In addition, we examine the influence of Dufour and Soret effects and find that both can be neglected for an air-steam mixture. We then consider the turbulent regime. As a first step, we determine the prediction abilities of low Reynolds models such as the Chien and Launder-Sharma formulations by comparing them with experimental results. We show that the models are able to reproduce successfully the modification of the velocity profile due to wall suction and to estimate correctly the temperature profile. The mass fraction is not as well recovered, but there are reasons to believe that this could be at least partly due to a bias in experimental measurements. As a second step, we build generalized wall functions to capture the different phenomena characterized by spatially-varying physical properties. Our method is grounded on a wall equilibrium model that leads to a system of ordinary differential equations. It relies on the mixing length assumption with the incorporation of a Van Driest damping factor. Results obtained with the method are confronted with low Reynolds model calculations and experimental measurements for three types of boundary layers: a purely kinetic boundary layer with suction/blowing, a boundary layer with suction and heat transfer, and a boundary layer with coupled heat and mass transfer. We determine how the damping factor depends on the wall suction. Finally, we establish that including the effect of interdiffusion is essential to obtain a good agreement of the wall function with experimental results. (author)