Research On Supercritical CO₂ Floud Characteristic And Transcritical Cycle System

As more and more international research has been doing on natural working fluid CO2 transcritical cycle, scope enlargement of the CO2 transcritical refrigeration cycle's application, improvement of cycle performance and the promotion of its practicality all become the key problems of current concerning research. Through theoretical analysis in the thesis, combined with computer simulation and experimental research, research has been done with focus on characteristic of supercritical CO2 and transcritical cycle system.The equivalent temperature method is given in this paper and referring to the working condition of air conditioning system, improvement on system efficiency by employing heat regenerator, improving compressor efficiency, replacing throttling valve with expander ,two stage compression and optimizing cycle types are analyzed in the paper. From the performance analysis on CO2 transcritical two stage compression cycle, the optimum high pressure and the optimum media pressure can be obtained by calculating correlations for providing theoretical support for designed operation of the system.From the respect of molecular forces and molecular heat motion, the cause of formation of liquid surface tension, CO2 metastable characteristics, the bubble nucleation mechanism and phase change mechanism of supercritical fluid are analyzed respectively, the power for metastable phase changer and the percentage of this power in theoretical expansion power are calculated in the paper. By analyzing concerning properties of synthetic lubrication oil employed in CO2 transcritical cycle system, it is concluded that POE and PAG are the best ones for CO2 transcritical cycle system.In compressor and expander, leakage loss is the main factor of influencing their efficiencies. In this paper, various types of leakage model are generalized with focus on leakage of CO2 supercritical fluid in plate model. At the same inlet pressure, the temperature corresponding to the greatest change point of CO2 density accords with the temperature at which leakage mass comes to the greatest value. And by analyzing the influence of lubrication oil and leakage clearance onleakage, it is obtained that the content of lubrication oil has little influence on leakage, while the leakage clearance exert the greatest influence on leakage. By building up the leakage test rig of supercritical CO2, theoretical analysis of leakage model is verified, according to the experimental result, it is gained that the current theoretical models of supercritical fluid and supercritical gas, which has certain guidance for designing high efficiency compressor and expander in CO2 transcritical cycle.Based on the tube fine type heat exchangers in household air conditioners, models of both gas cooler and evaporator are established, and it is simulated that the heat transfer and pressure drop in both heat exchangers under various parameters. The experimental rig of CO2 transcritical air to air cycle system has been set up for performance testing of air conditioning system. Experimental research under various working conditions illustrate that the heat exchangers employed in the system have good performance and system matching meets the need of design requirement. Under current experimental condition, COP of the system can reach 3.0, which offers experimental data for the application of CO2 transcritical air conditioning system.