Study On Supersonic Expansion,Flow Characteristics And Structure Optimization Of CO₂ Two-Phase Ejector

In order to achieve the dual carbon target,it is necessary to accelerate the optimization of energy architecture,which brings great challenges to energy saving applications of thermal systems.CO₂ejectors are widely used in thermal systems because of their advantages of simple structure,convenient operation,good adaptability to two-phase operating conditions and excellent heat transfer characteristics.In this paper,by studying the multi-physical mechanism of the mixing process of CO₂two-phase ejector,the coupling relationship between different flow mechanisms was discussed,and then the global model of the ejector describing the internal flow was established.Firstly,in order to suppress the influence of expansion wave and improve the ejector coefficient,the structural design of the ejector absorption chamber is optimized.Finite difference method is used based on characteristic line method,a supersonic expansion model of CO₂two-phase ejector mixed initial active flow is proposed.The relationship between the expansion angleβand the section ratio A_t/A_m,the experimental ejection coefficientμand the expansion ratio P_d/P_sis analyzed,and the correlation formula is fitted by the least square method.The correctness of the model is verified by comparing with the experimental data.The decrease of nozzle outlet area leads to the increase of active flow expansion angle,which increases with the increase of ejector expansion ratio and experimental ejector coefficient.Secondly,in order to promote the mixing of the active flow and the pilot jet,the friction loss of the two streams in the mixing process is reduced.A two-fluid model was used to analyze the relationship between the parameters of the active flow and the jet flow in the mixed diffuser,and the coupling relationship between the different flow mechanisms was studied.The mixing characteristics and performance variation rules of the ejector were obtained.It is found that in the mixed diffuser,as the two streams of fluid flow towards the outlet of the diffuser,the temperature and speed of the active flow along the diffuser are higher and lower,and the temperature and speed of the jet flow along the diffuser are lower and faster.The temperature and velocity of the two streams converge more and more.The mass fraction of active flow and gas phase increases gradually,while the mass of jet lead decreases gradually.Finally,in order to convert more kinetic energy into pressure energy and improve the performance of the ejector,the structure of the ejector hybrid diffuser was optimized.Based on the study of CO₂two-phase ejector mixed diffusing chamber constructed by two-fluid model,the structural optimization model of ejector mixed diffusing chamber was established,and the optimal length of the mixing chamber and the range of diffusing angle of the diffusing chamber were obtained.Through the above work,it is helpful to deepen the understanding of the mixed flow law in the two-phase ejector,guide the structural design and performance optimization of the ejector,expand the practical application of the CO₂two-phase ejector and strengthen the thermal performance of the ejector refrigeration system.