An experimental study of supersonic hydrogen-air flames for scramjet applications.

Abstract

Supersonic hydrogen-air jet flames were investigated experimentally in order to measure the general flame properties and to investigate the effect of shock waves. This experiment was the first reacting flow experiment interacting with shock waves. The general properties measured were flame length, blowout limits, heat release patterns, static pressure distributions, stagnation pressure losses, thermal choking limits, and nitric oxides levels. An improved Mach 2.5 supersonic combustor was built. The supersonic flames are round to be significantly shorter than corresponding subsonic flames, for the same velocity and density ratios. This difference implies that for axisymmetric jet geometries with combustion, mixing rates are larger in the supersonic case. One possible reason for the relatively short supersonic flames is that the radial velocity, which controls entrainment, can be altered by compression/expansion waves that are inherent in the jet geometry. Shock effects were investigated by changing shock strength and position with particular emphasis on the lengths and stability limits. It was found that shock waves enhance the fuel-air mixing such that flame lengths decreased by 30% when an optimum shock location and shock strength was chosen. Enhanced mixing resulted, in part, because the shocks turn the flow and induce radial inflows of air into the fuel jet. Substantial improvements in the flame stability were achieved by properly interacting the shock waves with the flameholding 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. Optimization of the mixing and stability limits requires a careful matching of the shock-flame interaction location and the shock strength. The results show that the best mixing and stability corresponds to 10$\sp\circ$ wedges placed at an upstream position (4.0 d$\sb{F})$ 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.