Numerical investigation on hydrogen mixing enhancement in a double-strut scramjet engine with circular cavity

Efficient fuel–air mixing is critical for stable combustion in scramjet engines, where residence times are only a few milliseconds. This study investigates hydrogen injection through a strut-integrated circular cavity under Mach 2 supersonic airflow using validated Reynolds-averaged Navier–Stokes simulations. Cavity depths from 1 to 10 mm were analyzed to evaluate their impact on shock–shear layer interactions and fuel distribution. In addition to conventional mixing efficiency and pressure loss, a new metric, the flammable zone coverage ratio (FZCR), is introduced to quantify the spatial extent of ignitable mixtures. Results show that a 3 mm cavity achieves the most favorable balance, with a 68 % improvement in mixing efficiency and ∼45 % flammable coverage at 0.3 m downstream, while incurring only ∼10 % total pressure loss. These findings highlight the role of cavity-induced flow structures in enhancing hydrogen dispersion and demonstrate FZCR as a practical indicator for scramjet combustor optimization.