Transient Helium Jet Evolution Using High Speed Schlieren Imaging

The accelerating global shift towards decarbonised energy systems has positioned hydrogen as a highly promising carbon-free fuel. This study comprehensively investigates the macroscopic characteristics and temporal evolution of vortex ring trailing helium jets, serving as a surrogate for hydrogen, injected into a quiescent ambient environment using high-speed Schlieren imaging. This research addresses critical insights into fuel-air mixing dynamics essential for optimising hydrogen direct injection (DI) internal combustion engines. Analysis of helium jet tip’s topology revealed a three-stage evolution from an initial pressure-insensitive phase, dominated by pressure wave structures, to a momentum-driven, vortex-dependent growth stage, then to a fully developed stage. Specifically, the lower-pressure cases showed increased Kelvin-Helmholtz instability and distinct head vortex pinch-off at the final stage. Jet tip velocities transitioned from initial high, rapid pressure wave development speeds to a momentum-controlled phase, with lower-pressure jets exhibiting greater fluctuations and susceptibility to Kelvin-Helmholtz instabilities effects. Jet width growth initially mirrored across pressures due to vortex ring expansion before diverging into a turbulent mixing regime, notably displaying a transient width reduction as internal ring structures dissipated. The jet angle stabilised around 32°, with higher injection pressures resulting in slightly narrower angles due to enhanced axial momentum. Overall, jet area growth was significantly faster and larger at higher injection pressures, confirming their superior mixing potential. These findings provide crucial insights into the interplay of injection parameters, vortex dynamics, and turbulent processes, advancing the fundamental understanding necessary for optimising fuel-air mixture formation and combustion efficiency in hydrogen DI engine development.