A new heat transfer model based on non-equilibrium film theory for annular flow condensation of binary zeotropic mixtures

Abstract

In recent years, zeotropic mixtures have been widely used as substitutes for pure fluids. Accordingly, their condensation properties have attracted increasing attention. However, it is a challenge to accurately describe the heat transfer performance of zeotropic mixtures in condensation, which is limited by the estimation of mixture effects, especially the mass transfer resistance therein. Focusing on this issue, the mass transfer due to the axial and radial concentration gradients in the vapor phase was considered, and a new heat transfer model for the annular flow of binary zeotropic mixtures in horizontal tubes was developed based on non-equilibrium film theory. In this model, the boiling point difference was introduced to approximate the effect of concentration gradients by combining with the vapor-liquid phase equilibrium properties of binary zeotropic mixtures. Secondly, equivalent specific heat capacities were defined for an analogy with single-phase heat transfer, which further simplifies the form of the model. The simplified model was evaluated with a consolidated database of 11 mixtures from 13 sources. The results show that the predictions of the model are in good agreement with the experimental results, with a mean absolute relative deviation (MARD) of 15.88 and 91.14% of points falling in the ±30% deviation band. Predictions were also performed for excluded data from other sources and satisfactory results were obtained with a MARD of 13.47%. Moreover, the mechanism of mixture effects is elaborated. It indicates that besides the commonly mentioned heat transfer degradation, there is also heat transfer enhancement for binary zeotropic mixtures condensation due to the opposite effects of the axial and radial concentration gradients.