Experimental and Theoretical Analysis of Refrigerant Absorption in Lubricant Oil

Abstract

Refrigerant absorption and mixing in lubricant oil are important in the design of compressors and refrigeration systems. Experimental and modeling work is reported on absorption of refrigerant vapor through the top interface of an initially stagnant layer of pure lubricant oil. Depending on the refrigerant-oil pair, the liquid refrigerant is heavier than the oil, and mixing is enhanced due to natural mass convection. This Rayleigh-unstable behavior is typical of HFC refrigerants, such as R-134a. In contrast, in systems where the liquid refrigerant is less dense than the oil (for example, HC refrigerants, such as R-600a and R-290), liquid density instability near the interface does not occur and absorption takes place by molecular diffusion alone. A test rig consisting of a transparent test section through which absorption is observed was specially constructed for the present experiments. During each experimental run, the overall mass of refrigerant in the test section was kept constant and, as a result, the pressure decreased with time. Tests were conducted for R-134a and R-600a with three polyolester (POE) oils of distinct viscosity grades, and the effect of the initial height of the oil layer (aspect ratio) on the refrigerant absorption rate (i.e., pressure change rate) was assessed. Two modeling approaches were pursued to predict the experimental data: a simple semi-empirical correlation and an integral control volume model. Good agreement with the experimental data was obtained with both methodologies.