| With the increasingly serious of energy and environmental problems,the mixed refrigerants have been widely used in the field of low grade energy recovery utilization,refrigeration and air conditioning.The temperature slip characteristic of non-azeotropic mixed working medium during phase transformation can reduce the irreversible loss of heat transfer process and improve the efficiency of thermodynamics cycle.Thermodynamics cycle is the main tool of energy conversion.Since the working medium is the energy carrier of thermodynamic cycle,knowing the thermophysical properties of working medium is important to design and optimize the thermodynamic cycle.Meanwhile,the energy conversion of thermal system is based on evaporation and condensation,and the phase transition of working medium is the basis of heat exchange,which is a key thermophysical process in heat exchanger.Therefore,it is necessary to conduct an in-depth research on evaporation and boiling phenomena of mixed working medium on heated surfaces,explore the factors affecting heat transfer efficiency,and seek ways to improve heat transfer efficiency.Due to evaporation occurs at the vapor-liquid interface at the nanometer level,and the heterogeneous nucleation on the surface is also involved in the nanoscale driving mechanism.Therefore,molecular dynamics simulation method was adopted in this paper to study the thermodynamic properties and transport characteristics of non-azeotropic mixture working medium at the vapor-liquid interface,and the relationship between condensation coefficient and potential energy with kinetic energy was established.Then,the evaporation and boiling behavior of non-azeotropic mixture working medium with different heating temperatures was studied,and the optimal ratio of working medium that can promote the heat transfer efficiency was analyzed.At the same time,based on the proposed evaporative selectivity parameters,the different evaporative capacities of different working medium in the mixed working medium were revealed.Through the evaporation and boiling simulation of the mixture for lubricating oil and R1234ze(E),the internal mechanism of the deterioration of heat transfer that caused by lubricating oil was revealed.Finally,the influence of surface wettability on evaporation and boiling process of working medium was discussed,and the combined surfaces with different wettability were constructed.Through the change of Gibbs free energy during the nucleation process,the nucleation mechanism is revealed and a theoretical model is constructed,which can predict the critical wettability parameters that promote the growth of bubbles.In this paper,Ar/Kr is used as a simple non-azeotropic mixture model for verification and mechanism research,and then the properties of R1234ze(E)/R32 non-azeotropic mixture are discussed.The main conclusions of this paper are as follows:(1)The liquid-vapor equilibrium model of non-azeotropic mixtures was established to study the thermophysical properties of the mixtures at gas-liquid interface.The thermodynamic properties and transport characteristics of mixture working medium at liquid-vapor interface are revealed.Based on the collision flux and residence time of molecules in the liquid,the condensation coefficient of the simulated systems was calculated.It was found that the condensation coefficient decreased with the increase of temperature.And the condensation coefficient increased with the increase of the mole fraction of R32.Meanwhile,the relationship between condensation coefficient and potential energy with kinetic energy was established,and it was found that the greater the ratio of potential energy difference to kinetic energy,the greater the condensation coefficient.(2)Based on molecular dynamics method,a model of evaporation and boiling of non-azeotropic mixture working medium on heated surface was established,and the evaporation mechanism at vapor-liquid interface and nucleation mechanism on heated surface were revealed.The evaporation selectivity parameter was first proposed to compare the evaporation capacity of different components in mixed working medium.The results show that the evaporation capacity of Ar atom is stronger than that of Kr atom.The existence of Kr atom can reduce the evaporation capacity of Ar atom,especially when the mole ratio of Ar and Kr is 1:1.This is because the system of Ar:Kr=1:1 can effectively reduce the temperature of gas-liquid interface.By analyzing the kinetic energy of the fluid and the interaction energy of the fluid molecules with the substrate,the system can be divided into three regions: the first adsorption layer region,the second adsorption layer region and the nucleation region.Then,a model of evaporation and boiling for mixture working medium on Cu surface was calculated by molecular dynamics method.The results show that nucleation and flim boiling can be observed simultaneously in pure R32,R32:R1234ze(E)=3:1 and R32:R1234ze(E)=1:1systems when the heating temperature of substrate is 360 K.Film boiling will form a gas film near the heated substrate,affecting the heat transfer of the substrate and leading to the deterioration of heat transfer in the system.However,nucleation boiling only occurs in the mixed system with R32:R1234ze(E)=1:3.Therefore,adding R1234ze(E)into the R32 system can effectively prevent the occurrence of heat transfer deterioration.R32:R1234ze(E)=1:3 system can not only prevent the film boiling of R32,but also has the best ability to cool the substrate.(3)The evaporation and boiling model for the mixture of refrigerant R1234ze(E)and lubricating oil PEC4 on Cu surface was established.The results show that,with the increase of the mass fraction of lubricating oil PEC4,PEC4 molecules will replace R1234ze(E)molecules to adsorb on Cu surface,thus forming oil film on Cu surface and reducing the heat transfer capacity of the liquid-solid interface.The concept of Kapitza thermal resistance length is introduced to evaluate the heat transfer capacity of the system.The calculation shows that the effect of temperature jump on heat transfer will decrease with the increase of heating temperature.The mixing of PEC4 and R1234ze(E)will weaken the heat transfer capacity of the system.(4)The evaporation and boiling models of non-azeotropic mixtures on different wettability surfaces are established,and the internal mechanism promoting the growth of bubble is revealed.A method for predicting the critical wettability promoting the growth of bubble is proposed.It is pointed out that the nucleation of strongly hydrophobic surfaces are generated earlier than those of weakly hydrophobic surfaces in the combined wettability surface.And this earlier generated nucleation will make the hydrophilic and strong hydrophobic mixed surface system have stronger boiling heat transfer ability than the hydrophilic and weak hydrophobic mixed surface system.It is found that there are three main types of nucleation in the boiling process of working fluid: growth type,stable type and collapsing type.Among them,the growing bubble gradually grows with the heating of the substrate;the stable bubble remains stable with the heating of the substrate and reaches a state of thermal equilibrium;the collapsed bubble first increases and then decreases with the heating of the base,and periodically becomes larger and decrease.The variation of Gibbs free energy relative to the initial free energy of the system during the nucleation of three types of nucleation is studied.The results show that there is an energy barrier hindering the growth of the bubble during nucleation.When ΔG of the bubble is equal to the energy barrier,the size of the bubble remains stable.When the ΔG of the bubble is larger than the energy barrier,the bubble can break through the energy barrier and continue to grow.However,when theΔG of the bubble is smaller than the energy barrier,the bubble will become smaller rapidly after heating.Finally,the heat transfer equilibrium equation is established for the stable nucleation,and the critical wettability angle of the bubble is predicted at different heating temperatures. |
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