Effect of Distributed Fuel Injection on Model Scramjet Combustor Performance

Direct-connect ground testing of a model scramjet combustor is performed in order to explore the effects of distributed fuel injection on supersonic combustor performance. The test rig is designed to experimentally simulate the isolator and the combustor flowfields in a dual-mode scramjet. The isolator-combustor model rig is connected directly to a vitiated airflow facility in the laboratory, which simulates an enthalpy-matched Mach 4.7 flight condition. Using a supersonic nozzle, the isolator entrance Mach number is kept at 2.0 throughout the testing runs. For the combustor section, hydrogen fuel is supplied either through a single injector or via multiple injectors using a distributed fuel injection system. Resulting combustor performances are compared for the single-injector case, for the two-injector case, and for the four-injector case, while holding the total fuel flow rate constant and the overall equivalence ratio unchanged at 0.5. Various flow visualization and wall pressure measurements are combined to evaluate the resulting combustor behavior as a function of number of fuel injectors. The results show that the combustor performance is drastically affected, including changes in the mode of combustor operation, the amount of pressure rise, the axial heat release distribution, and the local flowpath Mach number. The resulting heat release in the single-injector case is concentrated mostly near the cavity flame-holder, leading to relatively large pressure increase and facilitating a relatively early transition to thermal choking. For the distributed fuel injection case, the heat release is also widely distributed over the expanding section of the combustor, thus preventing a premature transition to thermal choking. The combustor entrance Mach number for this case remains relatively high and the wall pressure is observed to increase slowly toward the downstream. The results open up the possibility of actively scheduling fuel injection distribution to control the scramjet combustion process and potentially avoid unwanted regimes of combustor operation, such as the large-amplitude combustion dynamics observed during some mode transition processes.