HIGH PERFORMANCE TRANSCRITICAL HEAT PUMP CYCLE USING CARBON DIOXIDE AS A REFRIGERANT

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Research and development of the expansion power recovery mechanism called an ejector, which is expected to improve the COP of carbon dioxide heat pumps, were performed. Based on the research result to a supersonic nozzle, the whole ejector system was actually designed, the actual ejector type heat pump incorporating it was manufactured, the performance test was performed, and analysis to the experimental result was performed. The following things were clarified by these research results. First, the performance of ejector in an ejector cycle is determined from the balance of the pressure increase due to the shock wave, which is produced at the mixing part and the diffuser, and the pressure loss characteristics covering the whole passage of the suction flow, from an ejector exit to the suction part of ejector through a gas-liquid separator, an expansion valve, and an evaporator. Moreover, the method of controlling the expansion valve, which is installed in suction line, for making COP optimal was clarified. The effect of the size of mixing part can be explained based on the above-mentioned balance, and the optimum size of the mixing part can also be predicted theoretically. Experimental results can be explained with sufficient accuracy by assuming nozzle efficiency and diffuser efficiency appropriately. The low viscosity of carbon dioxide showed that the length of mixing part did not have large effect on the efficiency.About the improvement in efficiency of the heat exchanger, which uses carbon dioxide as an operation medium, the performance of evaporation heat transfer was investigated using heat transfer tubes with 1-6mm ID. Heat transfer characteristics in case lubricant oil was included were also investigated. Following experimental correlations were obtained from the experimental results ; the pre-dry out heat transfer before dry out occurs, the quality when dry out generates, and the post dry out heat transfer after dry out generating.