Numerical Simulation And Optimization Of CO₂ Two-phase Ejector Based On Cavitation And Boiling Model

With the increasingly serious environmental problems,traditional refrigerant CFCs and HFCs with greenhouse effect are facing a huge alternative pressure.The natural working fluid CO2 is considered as one of the most promising alternative refrigerants because of its high efficiency and environmental safety.And its performance in the transcritical cycle is outstanding.The use of ejectors to replace throttling devices in transcritical CO2 refrigeration cycles not only reduces throttling losses,but also effectively improves system cycle efficiency.In order to further improve the performance of the system,the internal flow mechanism,and geometry of the ejector of its core components were studied.First of all,the shock effect that exists inside the ejector has an important effect on the performance of the ejector,and the existing one-dimensional theoretical model cannot achieve the capture of the shock.For this deficiency,in the process of non-equilibrium phase change.supersonic flow and shock waves phenomena et al.,a CFD model of C02 two-phase ejector was built to capture the shock characteristics of the supersonic nozzle.The Cavitation and boiling phase transition models was used to simulate the phase change of fluids.Compared with the experimental results in the literature,the established model could accurately predict the performance of the ejector,the maximum error of the entrainment ratio was 4.3%.Five RANS turbulence' models have been compared,as a result,the SST k-co model shows better performance.Secondly,the model was used to further analyze the influence of critical back pressure on jetting performance and its relationship with shock wave characteristics.The results show,the ejector has a critical back pressure,when the back pressure is lower than the critical value,the velocity of the mixed fluid reaches the speed of sound at the inlet of the diffusion chamber,and a fouling occurs.At this time,the entrainment ratio is maintained at the maximum value and the change of back pressure has no obvious effect on the shock wave chain.As the pressure of the outlet increases,the length of the shock chain gradually becomes shorter and the strength gradually weakens.At last,the model is used to study the geometry on the ejector performance and analyze its relationship with shock waves.The results showl)with the change of the diffusion angle,there is a balance between friction loss and turbulent kinetic energy loss,corresponding to the optimal value of the diffusion angle.2)Under certain conditions,there is an optimum mixing chamber length and diameter to maximize the entrainment ratio,increasing the length of mixing section with a fixed mixing section diameter would increase the length of the shock chain.But when the optimum mixing chamber length is exceeded,the intensity of the shock wave begun to decrease;when the length of the mixing chamber is constant,decreasing the diameter of the mixing chamber will weaken the shock intensity at the outlet of the working nozzle and increase the shock at the inlet of the diffusion chamber,while excessively increasing the diameter of the mixing chamber will reduce the function of the mixing chamber,losing pressure lift effect.The model and method helps to interpret the experiment results and shows clear potential for its utility as design instrument of two-phase flow of CO2 ejector.