Detailed modeling of laminar film condensation from zeotropic binary mixtures in a vertical tube

Abstract

Zeotropic binary mixtures have the potential to be suitable replacements for phased-out refrigerants. In this work, a detailed numerical model of film condensation from a binary mixture is developed as a potential tool to assist evaluating the performance of such replacements. Downward co-current laminar film condensation of a zeotropic binary mixture inside a vertical tube is modelled numerically using the full two-dimensional parabolic governing equations in both phases. The model predicts the condensation behavior of a binary mixture at any composition with properties as functions of the local pressure, temperature, and composition. Detailed results of the heat and mass transfer and the local velocity, temperature, and mass fraction in the entire domain are produced as well as the axial variation of the pressure and the film thickness. Detailed results for zeotropic mixtures of R290/R600A and mixtures of R23/R116 with an azeotrope are presented. A non-monotonic variation of the vapor mixture axial velocity is observed during the axial development of the film and the depletion of the vapor mass flow. It was found that the local heat transfer coefficient depended mainly on the liquid thermal conductivity and the film thickness. For the two refrigerant pairs studied in this work, the local heat transfer coefficient increased with the mass fraction of the less condensable component. The difference in the condensation rates of the constituting components are quantified with and without an azeotrope.