Numerical investigation on behaviors of under-expanded hydrogen jets: Influence of straight nozzle structure

In internal combustion engines, the use of hydrogen is considered as one of the most promising alternatives to 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. Nevertheless, hydrogen injection has to be operated at high pressure due to the low density of the hydrogen, which leads to the formation of highly under-expanded fuel jets and have a significant influence on the downstream mixture formation. Therefore, in order to achieve better hydrogen-air mixture, this work aims to numerically analyze high-pressure hydrogen spray behaviors, focusing on the effect of nozzle structure on the formation and the structure of the shock waves. The nozzle diameter ranged from 0.1 mm to 2.0 mm, with the ratio of nozzle length to diameter from 0.5 to 5. The nozzle pressure ratio varied from 5 to 10 with different combination of injection pressure and ambient pressure. Then, to provide insights regarding the dependence of the air-hydrogen mixing on the structure of straight nozzle the influence of the diameter and length of nozzle on the shock structure, axial penetration and radial spread of the under-expanded jet is investigated. Finally, a dimensionless correlation between the parameters of the under-expanded jet with the total pressure ratio and the geometry of nozzle and is established, which is significantly different from the correlations based on the large-hole nozzles. This work will provide a theoretical basis for the design of hydrogen injectors.