On the flame stabilization mechanism of rearwall-step cavity flameholder under nitrogen throttling

The flame stabilization mechanism was investigated under the conditions of a Mach number of 2.52, a stagnation temperature of 1650 K, a stagnation pressure of 1.30 kPa, and a mass flow rate of 0.69 kg/s. Specifically, the study was conducted with nitrogen throttling (accounting for 3.4% of the inflow mass flow rate) and utilizing a rearwall-step cavity as the flameholder. The control experiments without throttling were performed under the same equivalence ratios of 0.20, 0.24, 0.28, 0.32, 0.36, and 0.40. The static pressure distribution, schlieren images, and CH* chemiluminescence images were employed to elucidate the combustion stabilization mechanism. As the equivalence ratio increases, the “Scram-Dual” and “Dual-Ram” processes exhibit intermittency under the no-throttling condition. Herein, “Scram-Dual” denotes the transitional phase between scram mode and dual mode, while “Dual-Ram” refers to the transitional stage between dual mode and ram mode. Under nitrogen throttling, by contrast, the processes tend to stabilize, with combustion oscillations effectively suppressed. The combustion zone can be subdivided into two distinct regions: the shear layer zone, characterized by stable flame intensity, and the jet wake zone, which exhibits strong oscillations that vary with the combustion mode. Throttling enhances flame stability, particularly within the shear layer zone. In contrast, flame instability in the jet wake zone is primarily influenced by flame geometry, as an increase in the windward area intensifies the blowoff effect induced by the inflow. In the dual mode, flame instability tends to intensify as the equivalence ratio increases. In the ram mode, by contrast, the effect of throttling is significantly diminished, and flame oscillations in the jet wake zone reach their peak, with negligible differences observed between the no-throttling and nitrogen-throttling conditions.