Numerical study on mixing augmentation mechanism induced by the gas-gas coaxial direct-flow shear injector in a supersonic crossflow

In the present study, the gas-gas coaxial direct-flow shear injector is employed in a scramjet to augment mixing between fuel and air. Two fuel injection methods, namely injecting hydrogen through the inner hole and air through the outer hole, and injecting air through the inner hole and hydrogen through the outer hole, were investigated to capture the mixing enhancement mechanism. Additionally, the area ratio (AR) between the outer and inner holes is examined to elucidate the underlying influences. Some parameters are also provided to evaluate the flowfield properties quantitatively, namely the mixing efficiency and the total pressure recovery coefficient. The obtained results predicted by the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two-equation shear stress transport (SST) k-ω turbulence model show that adoption of the gas-gas coaxial direct-flow shear injector can effectively improve the mixing efficiency and bring a slightly greater total pressure loss. The mixing speed and the mixing efficiency both increase with the increase of the AR. The mechanism for enhancing mixing involves the increase of the unstable Kelvin-Helmholtz (K-H) vortices within the flowfield, which is facilitated by the strong shear effect resulting from the velocity difference between the jets injected from the inner and outer orifices. The different vortical structures in the vicinity of the injector caused by the two injection strategies, along with their dynamic evolution, lead to significant differences in the flowfield properties.