Study On Flow Condensation Characteristics And Heat And Mass Transfer Mechanisms Of R134a/r245fa Zeotropic Mixture

With the development of micro-mechanical manufacturing technology and the improvement of the application requirements of micro-equipment,the micro-electro-mechanical system shows a broad application prospect,and the research of micro-scale flow and phase change heat transfer is of great scientific significance.Flow condensation is the key thermo-physical process in the heat management system.The study of heat transfer characteristics and mechanism during flow condensation in micro scale has important guiding significance for the design and application of micro-scale condensing heat exchanger.Refrigerant is the main working medium used in the thermal energy management system.The development of mixed refrigerants with environmental friendly and superior performance is of great significance to the research of tackling climate change and reducing greenhouse gas emission.The application advantage of zeotropic mixture is of great significance to improve the performance of heat management system,and also provides a new direction for heat transfer research in micro-scale field.At present,the relevant research of the flow condensation of zeotropic mixture is mainly focused on the conventional channels,but it is relatively limited at the micro-scale.Based on the above background,the research of flow condensation characteristics and heat and mass transfer mechanism of binary zeotropic mixture R134a/R245 fa has been carried out theoretically and experimentally,including four parts: The heat and mass transfer model of binary zeotropic mixture during annular flow condensation in micro-channel,the experimental study of the zeotropic mixture during flow condensation in the micro-channel,the liquid film characteristic of the zeotropic mixture during annular flow condensation in the micro-channel and the mass transfer in the vapor phase of the zeotropic mixture during stratified flow condensation in the rectangular channel.Based on the double-film theory,the flow condensation models of zeotropic mixture in circular and rectangular micro-channels were established respectively to grasp the basic condensation law of zeotropic mixture.The influences of composition,mass flux,saturation pressure,wall sub-cooling temperature and pipe diameter on the the temperature difference fraction and diffusion flux fraction in the vapor phase,liquid film thickness(meniscus radius)and condensation heat transfer coefficient were studied.The condensation characteristics of zeotropic mixture in circular and rectangular micro-channels were compared.It was found that the temperature difference fraction,the diffusion flux fraction and the liquid film thickness affected the condensation heat transfer coefficient of zeotropic mixture.The heat transfer coefficient of the zeotropic mixture in the rectangular micro-channel was almost twice as much as that of the circular micro-channel at the beginning of condensation.For the flow condensation of the zeotropic mixture in the rectangular micro-channel,the zeotropic mixtures with larger surface tension difference and higher boiling point component corresponding to the larger surface tension should be selected to strengthen the suction effect of the surface tension in the corner region.A visualization experiment on zeotropic mixture during flow condensation in micro-channel were carried out and the effects of the composition,mass flux,saturation pressure and wall sub-cooling temperature were studied.Considering the micro-scale effect,composition and temperature glide characteristic,the flow pattern and transition characteristics of zeotropic mixtures with different compositions under different conditions were observed and analyzed,and the “annular-intermittent” flow pattern transition criterion was proposed.The heat transfer and degradation characteristics of the zeotropic mixture during flow condensation were thoroughly discussed.According to the heat transfer degradation characteristics of the zeotropic mixture,the heat transfer coefficient prediction method was proposed.The influence of factors on the frictional pressure gradient was analyzed,and the prediction method of the frictional pressure gradient was also proposed.Combining the visual photography and the Laser Focus Displacement Method(LFDM),the liquid film thickness measurement experiment of zeotropic mixture during flow condensation in micro-channel was carried out.It was found that the temperature glide have no obvious effect on the average liquid film thickness.The shear force at the vapor-liquid interface mainly affected the fluctuation frequency of the transient liquid film thickness,and the liquid film thickness determined the fluctuation range.The increase of mass flux weakened the thinning effect on the average liquid film thickness.As the mass fraction of R134 a increased,the effect of mass flux on the liquid film thickness increased.Based on the momentum equation of laminar liquid film,a simple method for predicting the mean liquid film thickness during annular flow condensation in micro-channel was proposed.Combining the encapsulation probe and gas chromatography technology,the mass fraction of each component of zeotropic mixture during flow condensation in rectangular channel was measured.It was found that the composition,the temperature glide,the mass flux and the wall sub-cooling temperature all had effects on the mass fraction along the passage and the mass transfer in the vapor phase.Taking into account the thermophysical properties,temperature glide and working conditions,a method for predicting the mass transfer resistance in the vapor phase of zeotropic mixture during stratified flow condensation was proposed.