The prediction of binary zeotropic mixtures in-tube flow condensation – A generalized non-equilibrium heat transfer model

As the demand for environmentally friendly and high-performance refrigerants grows, accurate prediction of condensation heat transfer in binary zeotropic mixtures has become critical for the design of compact and efficient heat exchangers. In this study, a generalized non-equilibrium heat transfer model is developed to predict the condensation behavior of such mixtures under annular flow conditions. The model is based on film theory and incorporates mass transfer resistance induced by both axial and radial concentration gradients in the vapor phase. Unlike traditional models, it introduces two iterative correction mechanisms, interface temperature and equivalent heat flux applied across three thermal regions- the vapor core, the interface mixture, and the condensate layer. The framework incorporates a range of annular flow correlations to ensure flexibility and applicability across various binary blends. A key strength of the proposed model lies in its ability to track evolving temperature and concentration gradients throughout the condensation process, offering detailed insights into the thermal resistance mechanisms and composition shifts of the more volatile component. The model was validated against 871 experimental data points spanning multiple refrigerant pairs and operating conditions. Among the tested correlations, Shah (2009) exhibited the highest accuracy, with over 92 % of predictions within ± 30 % deviation from experimental data. Comparative analysis with existing pure fluid, equilibrium, and non-equilibrium models demonstrates the superior performance and generality of the proposed approach. The model provides a robust and practical tool for accurately predicting heat transfer coefficients in binary zeotropic mixtures, offering valuable guidance for the design of next-generation thermal systems.