Numerical studies on wide-operating-range ejector based on anodic pressure drop characteristics in proton exchange membrane fuel cell system

The applicable operating range of the ejector is limited in the proton exchange membrane fuel cell system, although the ejector recirculates the unused hydrogen reliably without consuming any parasitic power. In this study, the ejector's Computational Fluid Dynamics model is established coupled with the stationary characteristic equation of the hydrogen ejector, which is derived by utilizing the anodic pressure drop formula. The model is capable of evaluating the entrainment performance in the overall operating range. The major geometric parameters are then optimized to promote the entrainment performance and extend the operating range, including the nozzle diameter (D-n), the mixing tube diameter (D-m), the mixing tube length (L-m), and the primary nozzle exit position (NXP). It is found that the ejector hydrogen entrainment performance is sensitive to D-m/D-n and the optimal value range is 3-3.54. The entrainment ratio curve shows different changing tendencies along with D-m/D-n, separated by 3.54. The optimal Lm/Dm is confirmed, but the value increases with the primary flow rate. The hydrogen entrainment 'ratio decreases dramatically in the whole operating range when NXP is above the optimal value range. In addition, the effect of anodic operating pressure is investigated, and the performance reduction under lower pressure is mainly attributed to the higher water vapor content in the secondary flow. The adverse effect of anodic water flooding on the ejector performance is also quantified. This study supplies ways to extend the applicable operating range and helps the parameter design of wide-operating-range ejector.