Using internal air injection for fuel mixing enhancement of annular hydrogen jet behind an inclined strut in a supersonic combustor: computational study

This study investigates the aerodynamic and fuel mixing characteristics of an inclined strut-based injection system for scramjet combustors, focusing on the effects of strut angle and internal air jet assistance. Four configurations were evaluated: annular hydrogen injection with and without internal air four multi-jet, implemented at strut angles of 15° and 7.5°. A series of numerical simulations were conducted to analyze the flow structures, shock interactions, temperature fields, hydrogen mass fraction distribution, and vorticity strength (circulation) downstream of the strut. Results show that increasing the strut angle from 7.5^∘ to 15^∘ significantly enhances fuel-air mixing by producing stronger shocks, larger separation zones, and more energetic vortices. At far downstream (x = 40 mm), mixing efficiency improvements of + 8% for coaxial injection and + 29% for annular injection were observed for the larger strut angle. Coaxial injection consistently outperformed annular injection for the same geometry, with gains up to + 35%. Air-assisted injection further increased near-field mixing, especially for the 7.5^∘ strut, by boosting initial circulation strength by ~ 10% and accelerating plume dispersion. Circulation analysis confirmed that stronger initial vortical fields correlate directly with faster mixing, with the 15° strut sustaining the highest vortex strengths even without air-assist. The findings demonstrate that strut geometry is the dominant factor controlling the mixing flowfield, while air-assist injection provides substantial benefits for smaller strut angles.