Aspects of the Two-Layer Model for Direct Contact Condensation of Steam on Wavy Falling Films

Detailed physical modeling of the direct contact heat transfer process of steam condensing on a falling liquid film is a very difficult task due to the complex hydrodynamics of the film. The present state of the art is restricted to film Reynolds number of the order of 100. On the other hand, empirical relations cannot offer any insight into the mechanism and features of heat transfer in liquid film. Phenomenological models are needed to bridge the gap between empirical relations and direct physical simulations. One of these models is the so-called two-layer model, which divides the falling film into a laminar conduction-dominated (substrate) layer flowing over the solid wall and a completely mixed layer representing the waves. This model is further developed here by focusing specifically on two aspects. First, the influence of temperature-dependent physical properties of the liquid on the structure and heat transfer characteristics of the substrate layer is studied, and typical results are presented in the absence of waves. Second, the relation of the main parameter of the model (i.e., thickness of the substrate layer) to local film state is discussed in detail. Generalized constitutive laws and an approach based on utilization of the experimental film thickness time series are proposed and discussed. The proposed procedures can be integrated to a generalized two-layer model for direct contact condensation.