Design criterion of critical mode ejector for PEMFC hydrogen supply and recycle system

Most studies on hydrogen ejectors focus on enhancing recirculation capability and extending the working range through structural optimization or innovation. Its wide application is limited by its performance deterioration dramatically due to its operating point slipping into the subcritical region under dynamic or fluctuating conditions. However, the criterion for ensuring and evaluating its operational stability is still lacking. Therefore, this study proposes a design criterion for stable and efficient hydrogen ejector, and a critical mode ejector is designed under the criterion. The internal flow characteristics, recirculation capability, and operational stability of the designed ejector are analyzed using an experimentally validated 2D axisymmetric CFD model. The findings indicate that the increase in primary flow pressure may decrease the critical recirculation ratio (ω⁎), but effectively increase the critical back pressure (pc⁎). In addition, the ω⁎ increases roughly linearly as secondary flow pressure rises, while the pc⁎ is less affected by it. The ω⁎ is sensitive to anode gas relative humidity and increases with it, while its effect on pc⁎ can be disregarded. Considering both the recirculation capability and operational stability, the designed critical mode ejector can operate within a power range of 38.86–86.59 kW, with the corresponding stability margin of 5–90 kPa and minimum ω⁎ above 0.72. The designed ejector not only meets the requirement of ω but works in the critical mode over a wide power range compared to other hydrogen ejectors reported in the literature. Compared to ω⁎, pc⁎ should have a higher priority in designing hydrogen ejectors. This research may contribute to designing stable and efficient ejectors used in PEMFC and promote its wide application.