Abstract: To investigate the influence of composition shift on the heat exchange characteristics of zeotropic refrigerants during the phase change in the heat exchanger，and understand the variance of temperature difference between refrigerant and heat transfer fluid, corresponding theoretical analysis is carried out. Based on the temperature pinch points and void fraction model, the heat exchanger is divided into some control volumes and a mathematical model of refrigerants condensation and evaporation is built up. Under a given work condition, the composition shift in the heat exchanger using R407C is calculated and furthermore, the nonlinear relationship between temperature and enthalpy with considering the composition shift is obtained. The variance of temperature difference between refrigerant and heat transfer fluid in the heat exchanger is foundand compared with that without considering the composition shift. The results show that the occurrence position of pinch points and the irreversible gain of entropy are significantly influenced by the composition shift, and it should be taken into consideration in the design of heat exchanger.

Key words: Composition shift, Heat exchange, Phase change, Pinch points, Zeotropic refrigerants