Mixing Characteristics of Supersonic Jets Injected into a Pressurized Gas Environment

The transition toward carbon-neutral energy systems has accelerated interest in hydrogen-fueled combustion technologies, where efficient fuel–air mixing is essential for stable and clean combustion. In the present study, the mixing characteristics of under-expanded supersonic jets injected into a pressurized environment are numerically investigated using validated computational fluid dynamics simulations. Two nozzle configurations are examined: a straight nozzle and sudden-expansion nozzles with different expansion ratios and expansion locations. The governing compressible flow equations are solved using the rhoCentralFoam solver with the SST k–ω turbulence model. The numerical framework is validated against Sod’s shock tube solution and experimental data for under-expanded supersonic free jets. The results show that sudden-expansion nozzles significantly modify the shock-wave structure, jet penetration, and lateral spreading compared with the straight nozzle. Among the investigated configurations, nozzles with intermediate expansion-section lengths exhibited pronounced Mach-disk oscillations with a dominant frequency of approximately 10 kHz. The normalized supersonic core length decreased from 17.79 for the straight nozzle to 5.50 for the best-performing sudden-expansion configuration, while the normalized jet half-width increased from 0.82 to 1.70, indicating substantially enhanced mixing performance. The findings demonstrate that nozzle geometry strongly governs the trade-off between flow stability and mixing enhancement in high-pressure supersonic jets.