Combustion enhancement characteristics of backward-facing step in an axisymmetric scramjet

The supersonic combustion enhancement characteristics of a combined combustor configuration consisting of a backward-facing step and a cavity are investigated numerically in this paper. Numerical simulations are conducted using a hybrid RANS/LES method. Numerical validation is performed on an axisymmetric cavity supersonic combustor as the baseline configuration. The numerical results agree well with the experimental measurements. On this basis, this paper proposes a combined configuration of a backward-facing step and a cavity, introducing a connecting step upstream of the cavity to achieve combustion enhancement. By comparing the two configurations, it is found that the addition of the backward-facing step regulates the heat release distribution, shortens the premixing process, and lifts the boundary layer at the intersection zone of strong shock waves. In the combined configuration, the inflow decelerates under strong compression waves, resulting in extended fuel residence time, elevated vortex stretching, raised jet penetration boundary, all of which promote fuel mixing. Combustion initiates earlier in the combined configuration, with the step and its upstream region serving as key hot product zones. Premixed combustion dominates the flame combustion mode, and subsonic combustion prevails upstream of the step. The combustion efficiency is improved. Meanwhile, this improvement is accompanied by an increase in total pressure loss. In the combined configuration, the transport and fuel entrainment capabilities of large-scale vortex structures in the upstream region of the cavity are enhanced, leading to faster reaction rates in local areas. Most of the combustion occurs in the wrinkled flamelet mode and corrugated flamelet mode, with a small portion in the thin reaction zone mode.