Optimization strut-based fuel injection using multi-step hydrogen jets and air-assisted mixing in supersonic flow

Efficient fuel–air mixing remains one of the primary challenges in supersonic combustion chambers due to extremely short residence times and strong compressibility effects. This study presents a three-dimensional numerical investigation of hydrogen injection and mixing in a scramjet combustor equipped with a strut-based injector, focusing on the influence of injector geometry and auxiliary air injection. Three configurations—a single annular injector (Case N.1), a multi-step staged annular injector (Case N.2), and a flush concentric annular-slot injector (Case N.3)—were examined under identical injection surface areas and freestream conditions (Mach = 2, Ps = 1 atm) using ANSYS Fluent with the SST turbulence model and ideal-gas assumptions in a steady-state framework. The results reveal that injector configuration strongly governs the flow field, vortex formation, and hydrogen dispersion in the wake of the strut. The single annular injector exhibited strong jet penetration but weak lateral mixing, while the staged configuration produced multiple interacting shear layers and sustained vortical structures that enhanced turbulent diffusion and entrainment. The flush concentric design achieved broader near-wall fuel distribution with minimal total pressure losses but lower core mixing intensity. The introduction of an internal air jet intensified local shear interactions, increased circulation strength, and significantly improved overall mixing efficiency—most notably for the staged injector, which achieved the best balance between mixing performance and aerodynamic stability. The findings highlight the advantages of the multi-step staged injection strategy, demonstrating its potential to optimize hydrogen–air mixing in supersonic combustors while maintaining acceptable pressure losses. This approach provides valuable insights for the design of high-efficiency strut-based injectors in future scramjet propulsion systems.