Research On Heat Transfer Mechanism Of CO₂/Low-GWP Refrigerant Binary Mixtures Flowing In Micro-and Mini-Channels

Using heat pump system for heating is a reliable way to reduce the direct combustion of coal, solve environmental issues, and achieve the target of energy conservation and emission reduction. Therefore, natural refrigerant CO₂ heat pump system has received great attention. A binary mixture is achieved by mixing lowglobal warming potential（GWP） refrigerant with CO₂. Application CO₂/low-GWP refrigerant mixtures to heat pump can reduce the operation pressure, and improve the system thermodynamic performance compared with those of pure CO₂. Micro- and mini- channel heat exchanger performs with compact structure, efficient heat transfer performance, high-pressure bearing, and low refrigerant charge. In particular, it is suit for heat pump and air-conditioning system operating with high pressure, such as CO₂. In this paper, the binary mixtures of CO₂ with low-GWP are firstly screened out. Then, the heat transfer and pressure drop mechanisms of supercritical CO₂/R41 and subcritical CO₂/R32 flowing in micro- and mini-channels are theoretically and experimentally studied. This work can provide a theoretical basis for the design and optimization of CO₂/low-GWP refrigerant heat pump and air-conditioning system.Blends of CO₂/low-GWP refrigerant are evaluated to be used in heat pump water heater. A proper temperature glide of CO₂/low-GWP refrigerant performs a good thermal match with the temperature change of water as two pinch points appear in the gas cooler or condenser. Meanwhile, a maximum COP is obtained at a corresponding optimal discharge pressure for each component ratio. Temperature match in the evaporator plays a leading role for the whole system performance, because a proper thermal match in the evaporator can improve the system coefficient of performance（COP） and thermodynamic perfectibility. The systems using CO₂/R41 and CO₂/R32 achieve the best performance. The heat rejection process of CO₂/R41 is in supercritical status, and that of CO₂/R32 is subcritical. The CO₂/R41 system is robust and operates with low discharge pressure and GWP. In addition, the CO₂/R32 system runs with much higher COP, suitable temperature glide and lower discharge pressure.In order to verify the reliability of the experimental setup and the test method, the heat transfer and frictional characteristics of liquid ethanol flowing in multi-port extruded（MPE） tubes are studied. The frictional feature and turbulent heat transfer results are in good agreement with the classical theories, which lay a solid foundation for the experiments of supercritical and condensation heat transfer. Whereas, the heat transfer performance in laminar flow region deviates from the traditional theory. Consequently, considering the scaling effects of conjugated heat transfer, entrance effect, and variation in thermophysical properties, new correlations are developed. The relative error of the new correlation is within ±10%.For supercritical CO₂/R41 and pure R41 cooled in the single tube and MPE tube, the heat transfer coefficient（HTC） first increases and then decreases with the bulk temperature, and a maximum value is obtained at the temperature marginally higher than the pseudocritical temperature. The closer the pressure approaches to the critical pressure, the higher the maximum HTC is. Additionally, the maximum HTC decreases with the increase in R41 mass frictional. HTC increase with the reduction of heat flux at the temperature near the pseudocritical point, and the influences of heat flux become prominent as the tube hydraulic diameter decreases. The friction pressure drop increases predominantly near the pseudocritical temperature and decreases with the increase of pressure at the region far from the pseudocritical point. The supercritical heat transfer and frictional characteristics of CO₂/R41 and pure R41 are in accordance with those of supercritical pure fluid flowing in conventional tube. Finally, suitable prediction correlations are recommended.For condensation heat transfer and pressure drop of CO₂/R32 and pure R32 in single circular tube and MPE tube, the pressure drop increases linearly with the mass flux. Whereas, it decreases with the increase of condensation temperature, and the influences of condensation temperature become apparent as the mass fraction of CO₂（XCO₂） decreases. The frictional pressure drop decreases gradually as XCO₂ increases, and the influence of XCO₂ on pressure drop is more apparent at high mass flux. The pressure drop for the test section is calculated with the method of finite volume, and the test results are compared with the calculated values using two-phase frictional correlations. Then, reliable correlations are introduced for CO₂/R32 and pure R32 condensation frictional pressure drop calculation.A higher HTC is achieved at a higher quality and mass flux for CO₂/R32 and R32 condensation in single circular tube and MPE tube. The HTC first diminishes rapidly and then rises slowly, and a minimum value is obtained at XCO₂ of 55%. Condensation heat transfer models for pure fluid are screened, and suitable correlations are recommended to calculate the sensible thermal resistance. The contributions of sensible thermal resistance and mass transfer thermal resistance to the total condensation thermal resistance are analyzed according to the test data, and a new prediction model for mixture condensation HTC is proposed.