16 September 2011
This paper deals with several measurement methods for oil circulation ratio (OCR) in CO2 heat pump systems in which immiscible oil (PAG) is used. These methods can be
used to investigate the effects of OCR, and the effect of an oil separator on the system performance. The visual or oil-droplet method (OCR measurement method by measuring the numbers of oil droplets and the time required) proposed by Gao and Honda, was modified for higher mass velocities and higher OCR conditions. The results agree well with those measured by the oil-volume method (OCR measurement method by measuring the oilvolume
and the time required). The heat-balance method for measuring OCR was also proposed, and the results show that the method is reliable for high OCR conditions although
the accuracy was about ±1 mass%. Measurement of oil concentration using the densimeter method was also conducted, and results showed that oil concentration is different from OCR
in a system with immiscible oil.