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Oil Retention and Its Effects on Pressure Drop and Heat Transfer in Microchannel Evaporators of Air Conditioning and Refrigeration Systems

Mulugurthi, Sarath Kumar
In Air-conditioning and Refrigeration systems, a small portion of the compressor oil circulates with the refrigerant through the cycle components. The oil circulating with the refrigerant flow retains in heat exchangers and affects its performance in terms of increased pressure losses and decreased heat transfer capacity.The purpose of this thesis is to experimentally investigate the oil retention and its effects on heat transfer and pressure drop of refrigerants and oil mixtures in microchannel type evaporators. In this work two different louvered-fin aluminum microchannel heat exchangers were tested in evaporator mode. The refrigerants R-410A and R-134a with Polyolester (POE) were considered with oil mass fraction ranging from 0.5 wt.% to 5.5 wt.%. The tests were run at a range of saturation temperatures which was divided into two sets; one set for air-conditioning application ranging from 33 to 48�F (0.5 to 9�C) and the second set for coolers and refrigeration systems with evaporation temperature ranging from 0 to 33�F (-18 to 0.5�C). Refrigerant entered the evaporator as saturated liquid and exited at as superheated vapor ranging from 5o F to 25o F (2.8?C to 13.9� C). Refrigerant mass flow rates were varied from 150 lbm/hr to 400 lbm/hr (0.019 kg/s to 0.05 kg/s). The results showed that the oil retention was strongly depended on the OMF and, at same OMF and saturation temperature, the oil retention increased if the refrigerant mass flux decreased. In addition, at same inlet conditions oil retention in microchannel evaporators increased if the superheat on the outlet side increased. The oil retention volume in the microchannel evaporators was measured up to 13 % of estimated internal volume of the evaporators. The oil decreased the heat transfer rate and increased the pressure drop. Its impact was also depended on oil mass fraction and the refrigerant mass flux. The effect of oil on heat transfer capacity was insignificant for OMF less than 1 wt. %. The refrigerant-side pressure drop across the microchannel evaporators increased by 10 to 25 percent when oil was present inside the heat exchangers and the OMF was in the range of 0 to 1 wt.%.