refrigerant, lubricant, distribution, validated model
The paper presents a semi-empirical model to predict refrigerant and lubricant inventory in both microchannel condenser and plate-and-fin evaporator of an air conditioning system. In the model, heat exchanger is discretized into finite volumes. Temperature, pressure and mass inventory are calculated by applying heat transfer, pressure drop and void fraction correlations to these volumes respectively. Refrigerant and lubricant are treated as a zeotropic mixture with a temperature glide. As refrigerant evaporates or condenses, thermophysical properties are evaluated accordingly with the change of lubricant concentration. Oil is assumed to be retained in refrigerant oil mixture at concentration that is changing during phase change in condenser and evaporator. Experimental data with R134a and PAG oil indicated 20% agreement with the model of refrigerant and oil retention in the evaporator. However, in the condenser, lubricant mass was consistently under-predicted while refrigerant mass was predicted same as in evaporator (within 15% error). Moreover, the lubricant under-prediction becomes more significant at higher Oil Circulation Ratio (OCR). The analysis showed that the lubricant was separated from the flow in the condenser header and starts to accumulate in the bottom channels. The temperature profile in the infrared image supports this hypothesis, as the temperature of the bottom channels was much lower.