Two-dimensional Modeling Investigation And Flow Mechanism Analysis Of Ejectors Based On Shock Circle Definition

Ejectors are used in a wide variety of engineering applications, including refrigeration, fuel cell system, chemical engineering and aerospace because of their simple structure, cheep cost, easy operation and convenient maintain. As a boost, vacuum and mixing device, the ejector usually plays as a key role in system. Therefore, efficiency of all the ejector related industry areas can be increased by improving the ejector's performance.The flow and mixing process inside ejectors are complicated: shocks are generated by subsonic and supersonic flow, and the momentum and energy are exchanged through the mixing layer between the two flows. And the working characteristics of ejector and its operation mode are easily affected by the working fluid's properties, geometries and operating conditions. Most of the existing ejector models are based on 1D simplification. Since the velocity boundary layer is ignored, 1D models are inaccurate or complex. On the other hand, in the Computational Fluid Dynamics (CFD) researches, many of them are focused on the performance simulation and flow field analysis, but few are reported on the systemic research on the influence of key geometry parameters.Aiming at the research status of ejector, the modeling and working mechanism were investigated based on theoretical analysis, numerical simulation and experiment. Firstly, the"Shock circle"in ejector was defined. Three ejector models satifying differenet application areas were constructed based on the"Shock circle"definition. Then the ejector theory for the performance characteristics of fuel ejector in critical, sub-critical and backflow modes was propsed according to the geomtric and operation conditions of ejector in fuel cell system. Thirdly, CFD technique was applied in mechanism study of the ejector. There were more than 100 different ejectors and near 400 cases were analyzed and computed. Finally, an ejector based refrigeration experimental system was set up. A large number of cases were conducted and the results were used to validate the proposed ejector theoretical models and CFD models.Definition of"Shock circle"in the ejector is proposed for the first time. The secondary flow reaches sonic condition at the mixing chamber inlet when the ejector is working at the critical mode."Shock circle", which is a think mixing layer of the primary and secondary flows, located at the inlet of the mixing chamber. Position of the"Shock circle"is the key location inside the ejector for the ejector performance and operation mode depend on the fluid states there directly."Shock circle"definition provides the theoretical fundament for the 2D modeling theory of ejector.Two velocity equations are proposed: one is the 2D exponential velocity equation and the other is 2D linear equation. The fisrt one meets the velocity distribtion of the turbulent flow in the pipe. Compared to the existing 1D models, the proposed 2D model can reveal the actual velocity. The second velocity equation is more suitible of analysis and calculaion. The constructed model suits for both dry vapor and wet vapor fluids. It is concluded that fluid like the R11 which has a small slope of its isentropic curve can be treated as the dry vapor. This treatment can simplify the calculation procedure and keep the model accuracy.A one-equation model for the on-line control and optimization of ejector is derived based on"Shock circle"definition and the lumped parameter method. The model is very simple that has only one algebraic equation and three constant parameters. The parameter identification method is given in detail. A new determination method of two key primary flow pressures in the fuel ejector is presented. The two pressures PPE and PPC separate the ejector performance into backflow, sub-critical and critical modes. The ejector works in the critical mode that is the precondition for the safe running of the fuel cell system. The two pressures can be used to estimate the operation modes and monitor the performance of ejector and anode gas recirculation cycle.CFD technique is applied to investigate the ejectors in the refrigeration and fuel cell systems. Seven main geometry parameters: nozzle diameter, mixing chamber diameter, included angle of mixing chamber inlet, nozzle exit position, nozzle divergent part length, mixing chamber length and diffuser length are systemically analyzed. The influence rule of ejector geometries and operating conditions on performance is revealed, which can provide as the guideline for the optimal design of ejector geometry. In addition, The pressurized and entrainment principle of ejector is analyzed in the microscopic mechanism aspect. The physical meaning and reasonability of"Shock circle"is studied.An ejector based refrigeration experiment rig is set up, including hardware arrangement and measurement and control system design. Many experimental data are obtained by varying the primary flow pressure, back pressure and secondary flow pressure. Comparisons show that the proposed ejector theoretical models and CFD models agree with the experimental data fairly well, and the linear-velocity model has the best simulation accuracy.The"Shock circle"based 2D ejector theory, not only provides the theoretical fundament for establishing high accuracy ejector models that can be applied in the geometry optimization and performance evaluation of ejectors in the refrigerating and fuel cell systems, but also the proposed"Shock circle"definition, modeling theory and simulation method can conveniently extend to the ejectors in other applications.