Research On The Pool Boiling Heat Transfer Characteristics And Mechanisms Of R32+R1234yf Mixtures

The new hydrofluoroolefins(HFO)refrigerants,represented by R1234 yf,exhibit outstanding environmental performance,but certain limitations exist in the thermal performance and safety of HFO.Mixed refrigerants have the superiorities of complementary performance and flexible design,which are expected to play an important role in various thermal systems.In-depth understanding of the pool boiling heat transfer characteristics and mechanisms of mixed refrigerants is essential for the design and optimization of relevant thermal systems.However,there are still urgent issues to be addressed in the current research on pool boiling heat transfer of mixed refrigerants,such as lack of experimental data,limited performance of prediction models,and ambiguous perception of heat and mass transfer mechanisms.Taking R32+R1234yf mixtures as the objective working fluid,a systematic research on the 3 key aspects of boiling heat transfer,namely boiling heat transfer characteristics of the working fluid,single bubble growth on the heating surface,and interactions between adjacent bubbles,has been carried out by combining experimental study and numerical simulation.A visualization experimental system applicable to pool boiling of mixtures was established.By designing and processing insertable platinum resistors and sampling tubes,direct and real-time online measurement of the temperature and component concentration of the working fluid was realized.By designing and processing the artificial nucleation site,controllable bubble growth was realized.The reliability of the experimental apparatus and method was verified through validation experiments.Pool boiling heat transfer experiments of mixed refrigerants on a flat surface were conducted.The boiling curves and heat transfer coefficients of different test refrigerants were analyzed in combination with the visualization images of bubbles.The heat transfer deterioration of mixtures was highlighted.It was found that the vapor-liquid equilibrium properties and the nonlinearly varying physical properties jointly affected the boiling heat transfer capacity of mixtures.The performance of several common correlation models in the literature was evaluated,and two of them were further modified considering the effects of component concentration and heat flux.The prediction accuracy of the modified models significantly improved.Bubble growth experiments of mixed refrigerants on the surface with artificial nucleation site were conducted.The bubble dynamic behaviors of pure and mixed refrigerants were compared and analyzed.Compared with pure refrigerants,the bubble growth constant decreased and the growth exponent increased for mixed refrigerants,but the detachment conditions were similar.It was found that the heat transfer deterioration of mixtures was closely related to the reduction in bubble growth rate.By introducing a series of characteristic parameters such as characteristic diameter,characteristic time,effective Jacob number,and a dimensionless factor representing mass transfer resistance,a dimensionless correlation model for the bubble growth of mixtures with good calculation accuracy was proposed.A CFD numerical model was established for the bubble growth of mixtures,and the reliability of the developed model was verified by comparing the bubble growth curves obtained from simulations and experiments.In terms of single bubble growth on the heating surface,the influences of component concentration on the interfacial heat transfer,microlayer heat transfer,and total heat absorbed by the bubble were analyzed.It was found that both single bubble heat transfer and macroscopic pool boiling heat transfer deteriorated the most with R32 mole fraction of about 0.4.For the growth process of adjacent bubbles on the heating surface,the characteristics of hydrodynamic,thermal,and compositional interactions between bubbles under two sliding modes,namely repulsion and aggregation,were investigated.It was found that bubble aggregation would limit the replenishment and evaporation of R32 in the microlayer.