Shock-wave-enhancement of the mixing and the stability limits of supersonic hydrogen-air jet flames

TLDR: In this article, a supersonic, non-premixed, jet-like flame was stabilized along the axis of a Mach-2.5 wind tunnel, and wedges were mounted on the sidewall in order to interact oblique shock waves with the flame.

Abstract: A supersonic, non-premixed, jetlike flame was stabilized along the axis of a Mach-2.5 wind tunnel, and wedges were mounted on the sidewall in order to interact oblique shock waves with the flame. Schlieren photographs show how the interaction occurs, and measurements quantify how the flame length and the flame blowout limits are affected by the shocks. An optimum shock-interaction location was investigated by adjusting the wedge position. It was found that shock waves enhance the fuel-air mixing such that flame lengths decreased by 20% when an optimum shock location and shock strength were chosen. Enhanced mixing resulted, in part, because the shocks turn the flow and induce radial inflows of air into the fuel jet. A Mach disk sometimes occurs, which appears to split the reaction zone into two parts and severely distorts the flame shape. Substantial improvements in the flame stability (i.e., changes in the blowout limits) were achieved by properly interacting the shock waves with the flame-holding recirculation zone. The reason for the significant improvement in flame stability is believed to be the adverse pressure gradient caused by the shock, which can elongate the recirculation zone. Excessive shock strength (or poor shock placement) caused thermal choking to occur, and the flame base moved upstream of the fuel tube exit, leading to dangerously high wall heat transfer rates. Optimization of the mixing and stability limits requires a careful matching of the shock-flame interaction location and the shock strength. The experimental results show that the best mixing and stability correspond to 10° wedges placed at an upstream position (4 d F ) such that the primary shocks create radial inflow near the flame base and interact with the recirculation zone. This upstream wedge position also allowed the second (recompression) set of shocks to provide radial inflow near the flame tip.