Optimization and performance investigation of confocal twin-nozzle ejector for PEMFC hydrogen supply and recirculation system under actual variable operating conditions

Abstract

The actual operating boundary conditions under different Proton Exchange Membrane Fuel Cell (PEMFC) power outputs are considered to evaluate the performance of a confocal twin-nozzle ejector and optimize its geometric parameters through the experimentally verified 3D Computational Fluid Dynamics model. The effects of primary and outlet pressures, anode gas relative humidity (RH), and nitrogen mole fraction (XN2) on its recirculation ratio are investigated. Besides, two key geometric parameters, mixing chamber diameter Dm and length Lam, are optimized under variable operating conditions. The results show that the two-nozzle (TN) mode can operate over a wider range of outlet pressure variation than single-nozzle (SN) mode, and an optimum primary pressure exists for both two modes. Considering the overall performance of SN and TN modes over the entire investigated PEMFC power range, the optimal Dm and Lam are 5.50 and 22.00 mm, respectively. The recirculation ratio at the rated power of 84 kW is improved by 14.10% compared to the initial structure. The ejector's total recirculation capability is positively correlated with both RH and XN2, while the hydrogen recirculation capability is negatively correlated with them. The impact of RH on the recirculation ratio of confocal twin-nozzle ejector is consistent with that of the conventional nozzle ejector. This work may provide useful ways for designing novel-type nozzle ejectors to expand the narrow operating range of ejectors used for PEMFC hydrogen supply and recirculation systems.