Near-field jet characteristics of a single-hole medium-/low-pressure hydrogen injector

Green hydrogen (H 2 ) becomes an important alternative fuel for decarbonizing internal combustion engines (ICEs). Optimization of air-hydrogen mixture is a significant challenge to achieve stable combustion in H 2 ICEs. In the paper, Schlieren optical diagnostic technique was used to reveal the near-field spray characteristics of a single-hole, medium/low-pressure H 2 injector under various injection and ambient pressures and Helium (He) is used as a surrogate. Image processing code was developed to obtain critical jet parameters, including jet tip penetration, width, angle, and area. The cycle-to-cycle variation (CCV) of jet behavior on thirty repeated experiments was assessed using the coefficient of variation (COV) and the normalized dimensionless “ Ins number ”. The results indicated that the near-field jet could be distinctly divided into three stages: a smooth teardrop-like shape at early stage, a transitional wavy mushroom-head structure at mid stage, and a stable triangular-tower structure at later stage. Different stages of near-field jet exhibited varying CCV characteristics. Compared to jet tip penetration, the CCV in jet width and angle closely approached the proposed “absolute variation” line, playing a crucial role in the jet area CCV. This research provides essential data for the development of H 2 jet models, facilitating the design optimization of H 2 injectors. • First study on near-field jet structure and variability of medium-/low-pressure hydrogen injectors • Near-field H 2 jet shows three stages: teardrop-like, mushroom-head, and triangular tower • Jet tip penetration increases with injection pressure but decreases with ambient pressure • Cycle-to-cycle variation of jet width and angle mainly affects jet area, approaching "absolute variation" line