Numerical Investigation on Behaviors of Under-Expanded Hydrogen Jets: Influence of Straight Nozzle Geometry

In internal combustion engines, hydrogen is considered as one of the most promising alternatives to replace fossil fuels and reduce CO₂ emissions. In such a context, traditional injectors for hydrocarbon fuels are currently being tailored to be used with hydrogen, or a single-hole/multi-hole cap mounted at the injector tip was used to obtain better mixing and air utilization. Nevertheless, the hydrogen injection can be accompanied by the formation of highly under-expanded jets and will significantly influence the downstream mixing process. Therefore, in order to achieve a better understanding on hydrogen-air mixture, this work aims to numerically investigate the influence of the nozzle geometry on the jet behaviors in the near nozzle region. The nozzle diameter ranges from 0.1 mm to 2.0 mm and the nozzle length is from 1mm to 2mm. The injection pressure ranges from 10 bar to 70 bar. As the boundary condition varied, differences in both the internal flow of different nozzle structures and the development of external jets through these nozzles were found, as well as the close connection between the internal flow and the external under-expanded jet development. This work will provide a theoretical basis for the design of hydrogen injectors.