Numerical Investigation Of PEMFC Anode Ejector And A New Volute Design

In order to avoid local fuel starvation,excess hydrogen with more than 1stoichiometric ratio is required to be supplied to the fuel cell system.Thus,the exhaust gas of the fuel cell contains a portion of the unreacted hydrogen gas,which will cause fuel waste and environmental pollution.The anode recirculation system can recirculate the exhaust gas and mix the exhaust gas with the pure hydrogen from the hydrogen tank.The humid and hot exhaust gas can humidify and heat the pure hydrogen directly,resulting in an improvement of the stack performance and fuel utilization.As a typical method of the anode recirculation system,the ejector has become the research hotspot due to its outstanding features,such as high reliability and without parasitic power.The purpose of this study is to optimize the structure of the ejector for the 130 k W PEMFC system.A one-dimensional(1D)ejector design model is developed by using MATLAB solver and validated by experiments.With this model,the relevant structural parameters such as the diameter of the nozzle of the ejector,the diameter of the mixing tube are determined on the base of fuel supply requirements and working conditions of the 130 k W PEMFC system.Based on the key parameters obtained from the 1D ejector design model,a three-dimensional numerical model of the ejector was established by the CFD method to investigate the component distribution,working condition,and dynamic response.The three-dimensional(3D)model has overcome the defects of the1 D model in studying the parameters such as the angle and the diameter of the secondary flow inlet tube.Understanding the effects of the working conditions helps to prevent the occurrence of water vapor condensation and reveals the influence of nitrogen and water vapor in the ejector.The characteristics of the ejector in the dynamic response are found through modeling the transient process.Finally,the new structure design of the secondary flow inlet tube is carried out,and the fluid is guided by the volute inlet tube to improve the mixing of the primary flow and the secondary flow.The performance of the ejector is optimized by the structural design.The results show that the transient response of the ejector during stack power variations can be classified into two periods: the primary flow impact period and the mixed flow impact period.If the inlet relative humidity of the secondary flow is higher than 85%,water vapor condensation is possible to happen at the ejector outlet region,leading to the fuel supply instability.Besides,the hydrogen entrainment ratio decreases with the increase of nitrogen mass fraction.If the pressure difference between the primary and the secondary flow is less than 10 k Pa,the ejector will occur backflow.A series of optimized design parameters are proposed to improve the performance of the ejector to reach maximum entrainment ratio.The volute ejector has a higher performance,wider working range,better humidify ability,more uniform temperature distribution,and a more stable flow supply than traditional ejector.