Flow boiling heat transfer characteristics of zeotropic refrigerant/oil mixtures in horizontal smooth tube

In the Organic Rankine Cycle (ORC), the lubricant is inevitably mixed with the working fluid, which affects its flow boiling heat transfer characteristics in the evaporator. In this study, the flow boiling heat transfer char-acteristics of zeotropic fluid R141b/R245fa at different polyester oil (POE) concentrations were experimentally investigated. Meanwhile, the effects of heat flux, mass flux and vapor quality were also investigated. The results show that both the mass flux and vapor quality can increase heat transfer coefficient (HTC) and frictional pressure drop (FPD), while the heat flux decreases the HTC and increase the FPD. The maximum promotion of HTC is achieved at an oil concentration of 2.3% compared to other concentrations. Except for the oil concen-tration of 3%, the FPD of R141b/R245fa-POE mixture increases with the oil concentration. Additionally, the effects of lubricant oil on the HTC and FPD of refrigerant/POE mixture were discussed. The Schager-R22/150-SUS correlation can well predict the enhancement factor of HTC and penalty factor of FPD. The comprehen-sive factor is always greater than 1, so POE can enhance the overall heat transfer performance of refrigerant-POE mixture.