SCRAMJET COMBUSTOR MODE TRANSITION BY CONTROLLING FUEL INJECTION DISTRIBUTION

Dual-mode scramjets are able to expand the operable Mach number range of the simplescramjet through manipulation of a thermal throat. Using the thermal throat, the scramjet can operate in either thermally-choked mode or supersonic combustion mode. The transition between these two events is still not very well understood. Past research has shown natural combustor mode transition to be highly unstable and characterized by frequent mode hopping. Long timescales associated with combustor mode transition also increase the potential for combustion dynamic events to occur. Due to the high level of hysteresis present in these events, designing a way to precisely controlling mode transition timing proves to be an ongoing challenge. The present research seeks to use a distributed fuel injection method to control and better understand combustor mode transition behavior. This study was performed using a laboratory-scale, direct-connect scramjet combustor. The facility simulated Mach 5 flight conditions using vitiation to match the enthalpy conditions necessary and to recreate typical isolator and combustor flowfields characteristic of the dual-mode scramjet. A supersonic nozzle was employed to achieve an isolator inlet Mach number of 2.0. For the reacting flow tests, gaseous hydrogen was injected through one to four injectors using a distributed fuel injection scheme while keeping a global equivalence ratio of 0.52 constant. Various imaging diagnostics and wall pressure measurements were used to better study the relationship between combustor behavior and the number of fuel injectors. The findings revealed that combustor mode operation has a significant effect on combustor performance, as indicated by the pressure rise, axial heat release distributions, and local flowpath Mach number for these cases. The deduced heat release for the single injection case showed that most of the heat release occurs near the cavity flame-holder leading to a relatively large pressure jump causing a premature transition to thermal choking. In the case of distributed injection, heat release occurs more evenly across the expanding portion of the combustor, which prevents early transition to thermal choking. Active control of the combustor mode transition event is demonstrated through the use of a fuel injection distribution and scheduling system as well as fast-response solenoid valves. In the active control cases, the global equivalence ratio was maintained at 0.52, but using the active control system the combustor was able to bidirectionally switch between stable, thermally-choked mode and stable, scramjet mode. Furthermore, actively controlled mode transition occurs at much faster timescales than what was observed for natural mode transition. This allows for the potential to actively control on demand combustor mode transition in a real world dual-mode ramjet-scramjet combustor through appropriately scheduled fuel injection distribution