Reaction and Mixing-Controlled Combustion in Scramjet Engines

H2-fueled scramjet engines were tested under Mach 4 (M4) to Mach 8 (M8) flight conditions, and the local equivalence ratio and combustion efficiency were measured by gas sampling at the engine exit. Correlation between the local values of equivalence ratio and combustion efficiency showed that the M4 combustion was principally reaction controlled and the reaction and the mixing-controlled combustion coexisted in the M6 condition. The M8 combustion in the engines was rate controlled by the mixing of He. Comparison of engine performance in the air, supplied by a combustion heater and a storage heater, indicated that the performance strongly depended on the air-heating methods. The dependence of engine performance on air-heating methods could be explained by the finding that the M6 engine combustion was partially reaction controlled. The wall-heating rate and pressure distribution in the M6 tests also supported the shift from the partially reaction-controlled to the mixing-controlled combustion as the fuel rate was increased. The mixing-controlled combustion suggested weak facility dependence in the M8 condition. Because reaction in the M8 condition is not sufficiently fast, the main combustion region might be blown downstream in the engine. (Author)