Development And Application Analysis On Two Types Of Novel Transonic Ejector

Ejector provides threefold effects which are pressure lift,entrainment and mixing.It has many superiorities such as:compact mechanical structure and no moving part,in addition to the lowcost,easily maintained,tight.sealing,high reliability,long-life and strong adaptability regarding the working fluids.Owing to the advantages mentioned above,ejector has become the critical component in Multi-Effect Distillation with Thermal Vapor Compression(MED-TVC)system,Proton Exchange Membrane Fuel Cell(PEMFC)system and many other systems.In this thesis,two novel transonic ejectors are developed based on its basic theory and the flow field properties.The phenomenon inside the ejectors and the overall performance are discussed using mechanism analysis,model simulation and experimental study.A two-stage vacuum ejector is proposed and investigated systematically to solve the insufficient suction capacity issue of the vacuum system and to improve the desalination system performance.The detailed structure and geometric parameters are listed,and the mathematical model based on thermodynamics is established.An experimental investigation is conducted for comparison and verification of the study,and the simulation results are in acceptable agreement with the experimental data.It is shown that the two-stage vacuum ejector which designed in this study can reach a vacuum degree of 5.3kPa under the premise that the primary flow pressure is about 600kPa.The performance of the two-stage vacuum ejector is superior to the traditional ejector.In this thesis,the computational fluid dynamics(CFD)method is used for the ejector flow field analysis,and the inner flow distributions such as the turbulence,jet characteristics,shock wave,chock phenomenon and the "final shock wave" are presented.The operating mode of the two-stage ejector is presented and compared with the traditional ejector.Besides,the effect of the inter-stage connecting section between the two stage ejectors has been discussed.The matching principle between the two stage ejectors is the top priority in the ejector development due to the fact that it is related to the guarantee that the two-stage ejector can operate regularly.Hence,a new parameter named scale ratio(SR)is proposed during the two-stage ejector development process,and its optimum value has been obtain by analysis and comparison.The two-stage ejector structure based on the optimal SR can guarantee its best performance after the structural optimization.This parameter and its conclusion are useful for the design of the two-stage ejector in the future research.A multi-nozzle ejector used in PEMFC system for hydrogen supplement and recycle is proposed,and its key structural parameters are calculated based on the gas dynamic model and the operating parameters of the fuel cell.The design approach of the multi-nozzle ejector is provided together with the nozzle arrangement and detailed geometrical parameters.The experimental results show that the multi-nozzle ejector can greatly expand the primary flow range of the ejector on the premise that the hydrogen supplement pressure is relatively stable.The multi-nozzle ejector is aiming to solve the limitation problem of the fuel cell output power range which existed in the traditional ejector.It is shown that the fuel cell system with the multi-nozzle ejector can achieve a wide output-power range without a significant change in the primary pressure by simply switching the operating mode of the nozzles;moreover,the recirculation ratios are acceptable for the PEMFC system.The output-power can cover the range of 20-100 kW theoretically;in addition,a relatively stable anodic inlet pressure can be achieved.The performance of the fuel cell has been greatly improved compared with the traditional ejector.The CFD simulations are conducted to visualize and investigate the complex inner flow distribution of the multi-nozzle ejector.The effects of the nozzle deviation to the ejector performance and the turbulence phenomenon come forward have been investigated.The influence of the relative humidity have been discussed.It is shown that the humidity of secondary flow has a positive influence on the ejector performance.Higher relative humidity is beneficial to the hydrogen recirculation.The state of internal flow field and the primary flow influence under four different operating mode have been discussed and compared with the traditional ejector.