Quasi-two-dimensional ejector model for anode gas recirculation fuel cell systems

Abstract

An anode gas recirculation ejector plays an important role in the performance and service life of fuel cells. This study establishes a new quasi-two-dimensional ejector model by considering the flow phenomena of the boundary layer, shock train, and mixing layer. The two-dimensional flow field of the mixing process of the primary and secondary flows is calculated using the compressible turbulent shear layer development theory. The effects of both compressible flow friction and nozzle flow separation caused by the boundary layer are considered. Furthermore, the dynamic pressure loss and mixing acceleration caused by the shock train are calculated in the model. The mixing-length theory proposed by Prandtl is used to calculate the momentum transfer between fluids, and a method for calculating the viscous dissipation of the mixing layer is presented. In addition, the effects of the flow phenomena are introduced into the governing equations through source terms. Finally, an experimental system is built to test the ejector performance under room-temperature, high-temperature, and wet-secondary-flow conditions. The performance and flow parameters obtained using the proposed model are verified through experiments and CFD simulations. The results show that the quasi-two-dimensional ejector model can accurately predict the overall performance and axial flow parameters of the ejector. The root-mean-square errors of the mass flow rates of the primary and secondary flows are 2.29% and 8.78%, respectively. The two-dimensional flow field obtained using the quasi-two-dimensional ejector model is in good agreement with the CFD simulation results. The development of the mixing layer affects the axial parameters and performance of the ejector because it influences the momentum transfer, friction loss, and viscous dissipation of the flow field. This study is of significance for accurately predicting the ejector performance as well as for optimizing the design of the ejector for anode gas recirculation fuel cell systems. This contributes to the efficient operation of fuel cells and simplification of supporting auxiliary systems, which has energy and economic benefits. The proposed ejector model can also be extended to ejectors in other fields.