A three-dimensional flow and heat-transfer theoretical model was established for a 5-mm micro-fin tube to explore the heat-transfer and flow characteristics of supercritical CO2/R41 therein under different pressures, mass fluxes, and components. The research attempts to provide reference for selecting components of working media and setting the pressure and mass flux in different application scenarios. Results show that the closer the temperature of the working medium to the critical temperature, the larger the local convective heat transfer coefficient (CHTC). The CHTC at the critical temperature is 8 to 16 times higher compared with that at the non-critical temperature. The maximum CHTC is greater and the temperature corresponding to the maximum CHTC is lower when the mixed working medium is at a pressure closer to the critical pressure. The maximum CHTC under 7.0 MPa is twice that at 8.0 MPa. As the mass flux increases from 400 to 800 kg/(m2?s), the CHTC at the non-critical temperature increases by 1.7 times, while the comprehensive evaluation results of heat transfer and pressure drop decrease significantly. When the CO2 fraction increases from 20.5% to 75%, the maximum CHTC is increased by 2.6 times.

This research establishes 5 mm three-dimensional (3-d) flow and heat transfer microfin tube theoretical models with three different geometric structures. Using these models, the thermal-hydraulic performances of supercritical CO2/R32 in microfin tubes with different structures at various working conditions were investigated. The influences of each of three factors (pressure, mass flow, and microfin tube structures) on the thermal-hydraulic performance of CO2/R32 were evaluated respectively. Furthermore, orthogonal tests were undertaken to obtain the optimized combination of overall thermal-hydraulic performance. Results indicate that: the more the temperature of working media approximates to the critical temperature, the bigger the local convective heat transfer coefficient. Compared to non-critical temperatures, the convective heat transfer coefficient at critical temperature shows an eight-fold increase. The closer the pressure of the mixed working media is to the critical pressure, the greater the maximum convective heat transfer coefficient (CHTC) and the lower the temperature corresponding to the peak point, among which, the maximum CHTC under 7.5 MPa is three times as large as that at 8.5 MPa; the CHTC increases with increasing mass velocity, generally showing a linear relationship; through calculating the most optimal combination of thermal-hydraulic performance evaluation using orthogonal tests, the maximum CHTC is determined to be 96 kW/(m2?K).

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