Heat transfer and pressure drop of CO2/R32 mixture in mini-channel

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In this study, we investigated the condensation heat transfer characteristics of the CO2/R32 mixture in a mini-channel with a diameter of 2 mm. The experimental conditions were as follows: mass flow rate of 50–400 kg·m−2·s−1, vapor quality of 0–1, and a condensation temperature of 35–45 °C. The experimental data were then compared with the heat transfer correlations. The higher the CO2 content, the smaller the annular flow area. The convective heat transfer coefficient decreases rapidly and then increases slowly with an increase in CO2. The minimum value was obtained when the CO2 content was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapor quality and mass flow rate conditions correspond to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer have low prediction results for CO2/R32, and the model of Shah is recommended as the prediction model. The experimental results of the condensation pressure drop were analyzed, and the results showed that the pressure drop increased linearly with the mass flow rate but decreased with an increase in the condensation temperature. The lower the CO2 content, the more clear the effect of the condensation temperature. The correlation of Kim and Mudawar is recommended for the CO2/R32 two-phase condensation friction pressure drop. This study provides theoretical support for the analysis of the condensation heat transfer mechanism of a CO2/R32 mini-channel.