| Evaporation phase transition is one of the important energy transfer modes of the fourth generation working fluid hydrofluoroethylene in the refrigeration cycle and organic Rankine cycle(ORC).Therefore,revealing the evaporation mechanism of pure hydrofluoroethylene working fluid and its mixture is of great significance to improve the thermal efficiency of the cycle system and reduce the design cost of the evaporator.Nowadays,the research on the interfacial phase transition of hydrofluoroethylene working fluid mainly focuses on experimental research.It is difficult to reveal the interfacial evaporation mechanism and corresponding driving mechanism of pure hydrofluoroethylene working fluid and its mixture from the perspective of experiment,and the molecular dynamics simulation method based on intermolecular interaction is expected to solve this problem.The typical hydrofluoroethylene pure working fluid R1234ze(E)and its mixture R32/R1234ze(E)are studied in this paper.Combined with non-equilibrium and equilibrium molecular dynamics simulation methods,the interfacial evaporation process of hydrofluoroethylene refrigerant R1234ze(E)is investigated.By analyzing the properties of vapor-liquid interface,heat transfer resistance and intermolecular interaction,this paper discusses the influence mechanism of isomers,mass ratio of mixed working fluid and surface wettability on the evaporation process of R1234ze(E).Firstly,the interfacial evaporation behavior of isomers R1234 yf,R1234ze(E)and R1234ze(Z)is studied.The results show that there is a dynamic stable period in the evaporation process of isomers,during which the evaporation rate remains basically unchanged.The evaporation rate of R1234 yf is the largest,followed by R1234ze(E)and R1234ze(Z).In the stable phase,thermal resistance can be obtained from temperature difference and heat flux.The total thermal resistance of R1234ze(Z)system is greater than that of R1234ze(E)system and R1234 yf system.The thermal resistance of solidliquid interface accounts for only a small part of the total thermal resistance.The thermal resistance of vapor-liquid interface is the main reason for the difference of total thermal resistance of isomers.Among the three systems,R1234 yf system has the largest area of vapor-liquid interface,which provides a larger heat and mass transfer surface for evaporation.R1234 yf has the smallest interaction energy between liquid molecules,so its diffusion resistance in the liquid film is the smallest,resulting in the smallest evaporation heat transfer resistance and energy barrier of R1234 yf liquid molecules.Secondly,the interfacial evaporation behavior of R32/R1234ze(E)pure working fluid and its mixture is studied.The results show that in the R32/R1234ze(E)mixture,R32 molecule which is easier to evaporate increases the evaporation rate of R1234ze(E)molecule which is not easy to evaporate.The smaller the mass percentage of R32 in the system,the greater the total thermal resistance.The thermal resistance between solid and liquid is the smallest,the thermal resistance of liquid is the second,and the evaporation thermal resistance of vapor-liquid interface is the largest.The solid-liquid interface thermal resistance decreases with the decrease of R32 mass percentage,while the liquid thermal resistance and vapor-liquid interface evaporation thermal resistance increase with the decrease of R32 mass percentage.The thickness of the vapor-liquid interface region decreases with the decrease of the mass percentage of R32.The intermolecular attraction of R32 is smaller than that of R1234ze(E),which reduces the attraction and diffusion resistance of R1234ze(E)molecule in the mixture,resulting in the evaporation heat and mass transfer resistance of the mixture being less than that of R1234ze(E)pure working fluid.Finally,the evaporation behavior of R1234ze(E)pure working medium on different wettability surfaces is studied.The results show that the surface wettability affects the three stages of interfacial evaporation of hydrofluoroethylene working fluid R1234ze(E).Strong surface wettability reduces the duration of the "initial" stage of evaporation and improves the "stable" evaporation rate of evaporation,but increases the duration of the "inhibition" stage of evaporation.The enhancement of surface wettability enhances the interaction force between solid and liquid,making R1234ze(E)working fluid at the solid-liquid interface present a "solid like" state,which reduces the temperature jump and heat transfer resistance of the solid-liquid interface,and also increases the possibility of "non evaporable layer". |
