On the influence of liquid/vapor phase change onto the Nusselt number of a laminar superheated or subcooled vapor flow

Abstract

Based on a numerical approach, we propose in this study to characterize the interaction between a laminar boundary layer of a superheated or subcooled vapor flow and a static liquid pool at saturation temperature. For the purpose of this study, we define a canonical configuration that will help to improve our physical understanding of the interaction between a laminar flow and vaporization or condensation. By performing a full set of simulations sweeping the parameters space, correlations are proposed for the first time on the Nusselt number depending on the dimensionless numbers (the Reynolds number, the Prandtl number, the Jakob number and the density ratio) that characterize both vaporization and condensation. As attended, the Nusselt number decreases or increases in the configurations involving vaporization or condensation respectively. For high Jakob number, opposite trends are observed depending whether if vaporization or condensation is considered. Indeed a saturation of the heat flux happens in the first case, whereas a self-amplification of the heat flux occurs in the second one. Since the Nusselt number expressions are known, analytical expressions for the integrated heat flux exchanged at the liquid/vapor interface can be determined. Our study also takes interest to the behaviour of the viscous friction of the vapor flow on the liquid pool, which is weakly affected by the phase change, despite the important variation of the local flow structure due to evaporation or condensation. The physical mechanisms inducing all these phenomena are here discussed and clarified.