Flow boiling characteristics and flow pattern visualization of refrigerant/lubricant oil mixtures

A comprehensive review of flowboilingcharacteristics and flowpatternvisualization of refrigerant/lubricantoilmixtures is presented in this paper. First, various parameters influenced by the lubricantoil in convective boiling of refrigerants, such as mass velocity, vapor quality, oil concentration and geometric characteristics of the heat transfer tube are discussed. The effects of the unavoidable introduction of the lubricantoil on the thermodynamics properties of a refrigerant are described. Then, a review of the main experimental studies of flowboiling of refrigerant/lubricantoilmixtures is presented and also describes research with halocarbons, carbon dioxide, hydrocarbons and ammonia. There is no agreement among these studies regarding the effect of the oil in the evaporator, with studies showing an increase or decrease in the heat transfer coefficient. However, in relation to pressure drop, all the results presented the same trend, increasing the pressure drop with increasing oil concentration. Next, the flowpatterns of refrigerant/oilmixtures are illustrated together with a selection of video images. It is possible to notice the difference in frothing formation with respect to the particular refrigerant and tube geometry. Some predictions of oil effects on the heat transfer coefficient and pressure drops based on the mixture physical properties are then presented and the trends compared to data. Finally, some suggestions for future work are given.