Condensation and droplet characteristics in hydrogen recirculation ejectors for PEM fuel cell systems

The hydrogen recirculation ejector plays a pivotal role in proton exchange membrane fuel cell systems. Nevertheless, a comprehensive grasp of the intricate two-phase flow characteristics within hydrogen recirculation ejectors remains elusive. This investigation delves into the condensation and droplet behaviors of an ejector designed for a 100 kW fuel cell stack. The analysis relies on a two-phase flow model that takes into account both homogeneous and heterogeneous condensation. The findings reveal that homogeneous condensation nuclei manifest within the mixing chamber when the stack power surpasses 31 kW. The average median diameter of homogeneous droplets at the ejector outlet measures 0.367 μm, whereas that of heterogeneous droplets stands at 1.265 μm, signifying a 26.5 % augmentation compared to their initial size. Additionally, two crucial factors impact droplet size: residence time and subcooling degree. At the 100 kW condition, the droplet residence time is a mere 0.272 ms, yielding a meager droplet flow rate of merely 1.79 mg/s. In contrast, at the 53 kW condition, the downstream of the mixing chamber exhibits the maximum subcooling degree, resulting in the maximum droplet flow rate of 22.12 mg/s. A temperature elevation of 2.68 K at the ejector outlet is observed due to the latent heat of condensation. Furthermore, condensation has a marginal impact on the entrainment ratio, with the most significant reduction being 4.18 %, averaging at 1.42 %.