In the CO_2 heat pump system, Poly alkyl glycol (PAG) oil is commonly used for lubrication of the compressor, while it is reported that the PAG oil has influence on the heat transfer performance due to its immiscibility against CO_2 within its working condition. Experimental work has been carried out to investigate the heat transfer performance of fin and tube heat exchangers using CO_2 as a refrigerant with taking PAG oil into account. From previous study it was found that the heat transfer performance decreased significantly in both evaporator and gas-cooler conditions when oil was mixed. The present work presents experimental results on three types of air-cooled heat exchangers with smooth and inner grooved tubes. It was found that the deterioration ratio of heat transfer performance with oil was different depending on the inner surface geometry of the grooved tubes. To understand this, flow visualization inside these tubes has also been carried out through transparent section made of glass, which can withstand high pressure. It was confirmed that the oil behavior inside tubes was related to heat transfer performance. Heat transfer performance can be improved by using inner grooved tubes with the optimal pattern to remove oil away from inner surface. These tubes are effective to develop high performance heat exchangers for the CO_2 heat pump system.
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