Effect of lubricating oil on non-azeotropic refrigerant in flow boiling: an entropy generation analysis

Entropy generation analysis is a useful method for studying the relationship between heat transfer and pressure drop in flow boiling. The flow boiling process in an actual refrigeration system is inevitably affected by oil. Therefore, the effect of oil on entropy generation needs to be investigated. A new non-azeotropic ternary refrigerant mixture, R447A, was selected as working fluid. Subsequently, flow boiling entropy generation models of R447A with and without oil were established. The effects of individual parameters on entropy generation were investigated. The entropy generation per unit tube length under different conditions was proposed as the benchmark for comparison. The results show that there is an optimal mass flux corresponding to minimum entropy generation per unit tube length at a specific oil concentration, and optimal mass flux decreases with increasing oil concentration. The tube diameter also affects the optimal mass flux of the fluid with oil, which increases with decreasing tube diameter and is smaller than that without oil for the same diameter tube. When the oil concentration is 0-4.95%, the total entropy generation exhibits a decreasing trend. Therefore, within the scope of this research, the addition of oil is beneficial for reducing the entropy generation of heat exchangers.