Numerical Investigation of Injector Cap Design on Hydrogen Jet Characteristics

One of the most critical enablers of hydrogen internal combustion engines is achieving rapid injection and mixing of hydrogen into the combustion chamber. Optimal cap is actively being investigated to improve the injector performance without major hardware modifications. In this study, detailed computational fluid dynamics simulations using the Reynolds-averaged Navier-Stokes (RANS) turbulence model were undertaken to investigate the behavior of hydrogen jets with various cap designs mounted on a hollow-cone injector within a constant volume chamber. It was found that the implementation of a cap in general enhances mixture formation, leading to a higher proportion of lean mixture over time. Key parameters, such as the cap's inner volume and throat area ratio, directly influence the amount of hydrogen mass trapped within the cap. A smaller volume or larger throat area ratio results in less trapped hydrogen mass. Excessive enlargement of the cap's throat area can lead to a decrease in cap pressure, approaching the main chamber pressure, which can significantly reduce jet penetration length and mixing quality. Shock waves were observed at both the injector and cap exits, with their intensity varying based on the cap's internal pressure. In multi-hole caps, the jet angle played a crucial role in controlling penetration length, without considering plume-to-plume interactions. Increasing the number of holes reduced jet penetration length but enhanced mixing quality by speed up hydrogen mixing with the surrounding air. Dramatically increasing the number of holes could potentially transform the jet behavior into a traditional hollow-cone pattern due to stronger plume interactions. The findings suggest the potential for optimizing cap designs to enhance hydrogen injection efficiency. Further research is needed to identify the optimal design.