Mixing and combustion characteristics in a scramjet combustor with different distances between cavity and backward-facing step

Abstract

The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection. Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance. The cavity shear-layer mode, the lifted cavity shear-layer mode, and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5, 4.5, and 7.5. In both non-reacting and reacting flow fields, the numerical results are essentially in agreement with the schlieren photography, flame chemiluminescence images, and wall pressure, which verify the reliability of the numerical method. The results of non-reacting flow fields show that the Backward-Facing Step (BFS) can promote the flow separation downstream at a fixed distance. The more forward the separation position is, the larger the separation zone is in the non-reacting flow field. Furthermore, the larger the separation zone is, the higher the intensity of combustion in the reacting flow field is. A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor. The flame presents remarkable three-dimensional characteristics in the reacting flow fields. When dimensionless distance equals to 4.5, there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor, while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5. In the cavity flameholding combustors with a backward-facing step, the higher combustion intensity may bring much total pressure loss to the combustor. Thus, it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.