SHOCK-TUNNEL EXPERIMENTAL STUDY OF COMBUSTION ENHANCEMENT METHODS FOR A HIGH-MACH-NUMBER SCRAMJET

Based on the JF-24 high-enthalpy shock tunnel in Institute of Mechanics, Chinese Academy of Sciences, the current paper performed direct-connect combustion tests of a high-Mach-number scramjet engine to study high-Mach-number combustion enhancement methods and fuel types’ effects. The test-section was a circular cross-section scramjet combustor with cavity structures, and fuel injectors were arranged in the isolator. Hydrogen and ethylene fuels were severally used in current tests at the same equivalence ratio of 0.7. Fuel injection utilized two different test-section configurations without and with small struts, respectively. Some injection holes of the latter configuration were located on the strut tops. For each configuration, two adjacent rings of injecting holes were arranged for single-ring and dual-rings injections, respectively. Test results demonstrated that stabilized combustion performances of hydrogen and ethylene fuels in hypersonic flows under a Mach number Ma_\textf\text = 10 flight condition. Meanwhile, compared to the single-ring fuel injection method, dual-rings fuel injections and adding injections on small-strut tops were beneficial for combustion enhancements. The reason was speculated that interactions of adjacent fuel jets and shock/separation structures probably could improve fuel-air mixing, and additional fuel injection on small-strut tops meant more available air for mixing. Under the same combustion enhancement methods of dual-ring injections and additional small-strut top injections, hydrogen fuel generated better thrust performance than ethylene fuel, while their combustion efficiencies were similar. This was possibly because that the hydrogen fuel had a higher caloricity, and thus it could generate more heat release. Besides, test also verified that under the current high-enthalpy high-speed inflow condition, combustion heat release was controlled by fuel-air mixing processes, and meanwhile the upper limits of heat release was limited by high-temperature dissociation effects. This was because that heat release led to decreases of local flow speeds and increase flow temperatures. Consequently, high-temperature dissociation endothermic reactions would be more remarkable, resulting in decrease of heat release.