Investigation of refrigerant dissolution into thin oil film during compression process.

Refrigerant dissolution into refrigeration oil considerably decreases the viscosity of refrigeration oil. The dissolution of refrigerant into the oil in a compression chamber during a compression process is generally neglected because it is thought that the compression process is instantaneous and the refrigerant cannot be dissolved into the oil film in such a short time. However, if the oil film is very thin, the dissolution of refrigerant may decrease the oil viscosity. In this study, the dissolution of refrigerant into thin oil film was examined by designing a test device that uses an optical-fiber probe (OFP) method to evaluate the transient change in refrigerant concentration in oil during the compression process. The OFP can detect changes in a returned light intensity by refractive index change due to refrigerant dissolved in the oil film. Since more light is reflected from the interface of the oil film as the oil film thickness decreases, which causes the intensity of the returned light to increase, a mirror is installed above the OFP to eliminate the reflected light from the interface. As a result, the returned light intensity from the OFP is not affected by the oil film thickness, even in conditions where the film is very thin. The dissolution process of the refrigerant into the oil film was successfully detected in the oil film, whose thickness ranged from 0.23 to 0.51 mm. In this thickness range, the refrigerant dissolution during the compression process can be ignored because the dissolution time is much longer than the compression time. Furthermore, the dissolution of R32 into the oil film was found to be faster than that of R410A. Based on the estimation of the refrigerant dissolution into the thin oil film by a one-dimensional diffusion model, it is suggested that the refrigerant dissolution during the compression process affects the oil viscosity when the oil film thickness is less than approximately 0.01 mm.