In most vapor-compression refrigeration systems, oil is added into the compressor for lubrication. However, it is inevitable that a portion of oil escapes from the compressor and circulates throughout the system due to the mutual solubility between the refrigerant and oil. The presence of circulating oil would affect the characteristics of heat transfer, pressure drop and mass retention in system. In addition, a large amount of retention of oil outside of the compressor might cause insufficient lubrication of the compressor, and eventually lead to compressor failure. The objective of this thesis is to experimentally and numerically investigate the transient refrigerant and oil distribution in a residential heat pump water heater (HPWH) system. In the experiments, R134a is used to pair with POE 22 oil as the working fluid. Quick Closing Valve Technique (QCVT) is employed to localize refrigerant and oil into each component of the system. Remove and Weigh Technique (RWT) is then used to measure the refrigerant mass, with an uncertainty about 0.17% of total refrigerant charge. The retained oil mass in each component, except for the compressor, is determined by Mix and Sample Technique (MST), of which the uncertainty is about 0.15% of total oil charge. Five experiments are conducted to cover a full heating process of five hours. The experimental data shows the retention of refrigerant is mainly determined by the internal volume and refrigerant density in the component. The retention of oil is found depending on the velocity of liquid refrigerant-oil mixture. A linked EES-CFD system model has been developed to simulate the transient system performance of the HPWH unit. Experimental data is used to validate this model. A retention model has also been established to analyze the local refrigerant and oil distribution in the heat exchangers.
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Transient distribution of refrigerant and oil in a residential heat pump water heater system
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