Evaluation of Wettability of Solid Surface with Oil/Refrigerant Mixture

A part of lubrication oil stored in an oil sump of a refrigeration compressor becomes fine oil mist in a compressor shell. Although some of the oil mist is separated from refrigerant in the compressor shell, a certain amount of the oil mist is discharged with refrigerant into a refrigeration cycle. The oil circulated with refrigerant in the cycle degrades the heat transfer efficiency of heat exchangers, increases pressure drop in lines and the heat exchangers and sometimes reduces reliability of the compressor when an oil return from the cycle to the compressor is insufficient. It is therefore important to reduce the oil discharge from the compressor to the cycle. In order to decrease the oil amount discharged from the compressor, development of a superior oil separator should be done and many researches were focused on it. More basically, since the separation efficiency of the oil separator strongly depends on droplet size of the oil mist, factors associated with the droplet size such as generation mechanism of the oil mist in the compressor, the droplet behavior on solid surface in the compressor and surface tension of the oil in which the refrigerant is dissolved should be clarified. However, the surface tension of the oil mixed with refrigerant and wettability of solid surface with the oil/refrigerant mixture which influences the droplet behavior have not been reported sufficiently. In this study, the surface tension of the PAG oil/CO2 mixture is measured by a pendant drop method and the wettability of the solid surface is evaluated as the fundamental study for discussing the generation mechanism of the oil mist in the compressor. The measurements of the surface tension of the oil/refrigerant mixture and contact angle of the mixture on the solid surface are achieved under high pressure and temperature condition. It is found that the surface tension of oil/refrigerant mixture decreases steeply with increasing the refrigerant concentration in the oil. The contact angle on solid surface decreases with the refrigerant concentration due to the reduction of surface tension. The contact angle on Aluminum surface is smaller than that on the PTFE surface and decreases with elapsed time. With using Ohnesorge number which is a ratio of viscous force to inertia force and surface tension force to evaluate the oil droplet behavior on the metal surface, Ohnesorge number is estimated to be very small under discharge condition in the compressor and it suggest that the oil droplet of oil/refrigerant mixture will spread immediately on the metal surface