Theoretical And Experimental Research On Heat Transfer Of CO₂ And CO₂ With PAG Mixture Flow Boiling Inside Tubes

The natural refrigerant CO₂ has a very good application prospect and also is a favored choice in the process of searching alternative refrigerants. The key problem of putting it into practice is improving the efficiency of CO₂ trans-critical refrigeration cycle and designing the high efficient and more compact heat exchangers. Generally, there are always being some oil in the refrigeration cycle, especially in the heat exchanger, the oil adheres to the wall of the heat exchanger and affects the heat transfer characteristics.The flow boiling heat transfer characteristics of mixture of CO₂ and PAG oil study was mainly investigated by combining theoretical analysis, EES simulation and experimental study.The thermo-physical properties of CO₂ in the sub-critical region were analyzed, and compared with those of conventional refrigerant (R22, R290, R410a, R134a) in the evaporation temperature ranging from-20℃to 20℃, pointing out that CO₂ flow boiling in tubes was much advantaged. The critical gas entrainment rate of CO₂ was significantly lower than that of other four refrigerants and showed that CO₂ heat transfer process was more intense in CO₂ flow boiling process. The paper judged nucleate boiling as the main form of CO₂ flow boiling in tubes after the calculation of nucleate boiling suppression dryness.Developed the model of CO₂/PAG mixture flow boiling heat transfer, and made simulation calculation after analyzing and comparing several heat transfer correlations, getting the heat transfer coefficient with the evaporation temperature, heat flux, mass flow and the dryness of import and export to provide necessary theoretical basis for the experiment.The experiment of CO₂/PAG oil mixture flow boiling heat transfer was designed and conducted. The experimental results showed that the oil had a great impact on heat transfer, and with the oil increased, the heat transfer coefficient was decreased to a constant. low oil content have a great impact on heat transfer coefficient.