On the Effect of Lubricant on Pool Boiling Heat Transfer Performance
This paper presents a brief overview of pool boiling heat transfer performance of refrigerant/lubricant mixtures. Various parameters affecting the heat transfer coefficient, such as viscosity, surface tension are examined. It is known that, during evaporation process, the lubricant accumulates on the surface since the refrigerant is preferential to evaporate. Hence, excess lubricant enrichment on the surface results in a thin lubricant excess layer and a thermal boundary layer, which influence the heat transfer performance, either enhancement or degradation. The effect of lubricant concentration on pool boiling is not consistent due to different working conditions. Some major parameters causing the inconsistency includes surface active components in the oil that changes the surface tension appreciably, such as local variations in concentration and surface tension gradient on growing bubbles; enhanced stirring in the boundary layer detaching bubbles; number of boiling sites; foaming on the heating surface and the like. Generally, increasing the oil concentration continuously degrades the heat transfer performance for highly interrupted surfaces. But the deterioration effect of lubricant on the HTC for plain and integral tubes occurs at a comparatively high oil concentration. At a lower concentration, the HTC can be higher or lower than the oil free refrigerant, depending on the complex interactions amid surface geometry, lubricant, saturation temperature, heat flux, foaming, etc. Yet a higher viscosity and a lower surface tension of lubricant normally cast positive influence on HTC. It appears that foaming may be beneficial to enhance pool boiling and a higher heat flux or a lower saturation temperature also provide positive influence on HTC of refrigerant lubricant mixtures.