Experimental and Theoretical Investigation of Oil Retention in a Carbon Dioxide Air-Conditioning System

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In a closed loop vapor compression cycle, a small portion of the oil circulates with the refrigerant flow through the cycle components while most of the oil stays inside the compressor. The worst scenario of oil circulation in the refrigeration cycle is when large amounts of oil become logged in the system. Each cycle component has different amounts of oil retention. Because oil retention in refrigeration systems can affect performance and compressor reliability, it receives continuous attention from manufactures and operators. Thus, the objective of this dissertation is to develop and use a method to experimentally and theoretically investigate the oil retention behavior in a refrigeration system on a component by component level.

The test facility for the oil retention study mainly consists of a refrigeration loop and an oil loop. An oil injection-extraction method was developed to measure the oil retention at each component of the cycle. As the oil circulation ratio increases, the oil retention volume in the heat exchanger and suction line also increases. 16% and 10% of the total oil amount charged initially is retained in heat exchangers at 5 wt.% of oil circulation ratio for the refrigerant mass flux, 290 kg/m2s and 414 kg/m2s, respectively. The effect of oil on pressure drop was found to be most profound at high vapor qualities where the local oil mass fractions are the highest.

An analytical model for the annular flow pattern to estimate the oil retention was developed. According to the analysis of CO2 and oil flow in the suction line, the interfacial friction factor should be expressed as the function of CO2 gas Reynolds number as well as the dimensionless oil film thickness. Furthermore, an empirical interfacial friction factor based on experimental results was developed. All simulation results for the suction line are bounded by ± 20% from experimental results. In the case of heat exchangers, void fraction models were used to estimate the oil retention. Due to the changing oil properties, the heat exchangers were divided into segments. Then the oil retention volume in the heat exchangers was calculated from the oil fraction and the length of the corresponding segment. Void fraction models by Hughmark (1962) and Premoli et al. (1971), show good agreement with current experimental results of oil retention at the evaporator and the gas cooler, respectively. Simulation results at the evaporator and the gas cooler are bounded by ± 20% of experimental results.

To minimize the oil retention in system components, several design guidelines are suggested.