Numerical study of water droplets in hydrogen recirculation ejectors for proton exchange membrane fuel cells

Gas ejectors play a vital role in recirculating anode hydrogen within proton exchange membrane fuel cells. Previous studies primarily use single-phase flow computational fluid dynamics to design ejectors, neglecting water phase changes and compromising accuracy. Here, we develop a two-phase flow model incorporating droplet injection in the secondary flow and a water condensation model to analyze the ejector's behavior. Validated by previous experiments in six different conditions, our two-dimensional model captures dynamic interactions between the gas and liquid water phases, leading to predict entrainment ratio more accurately, with an average deviation of 3.08% compared to 24.04% for the single-phase model. Additionally, simulations have been done for six different cases comparing different degrees of humidification of the secondary hydrogen flow. Water droplet growth increases gas temperature and pressure in the mixing chamber while reducing velocity, lowering the entrainment ratio by over 30%. Injecting a certain amount of droplets into the secondary flow can effectively improve the efficiency of the mixing chamber. Integrating a heat exchanger in the hydrogen supply line increases overall temperature and decreases water condensation. This study provides an in-depth understanding of water phase behavior, further optimizes the hydrogen ejector, and improves the accuracy of its simulation.