Experimental study on transition between ramjet and scramjet modes in a dual-mode combustor

Air-breathing propulsion has been regarded in recent years as the future solution for spacecraft launchers. Hypersonic flight of vehicles designed for horizontal take-off remains nevertheless a challenging task. Especially the propulsion unit must work over a broad range of flight Mach number: efficiency can be guaranteed only with a system integrating conventional and air-breathing engines and the latter have to be suitable for dual-mode operation. The Institute of Flight Propulsion of the Technische Universität München developed an air-breathing combustor concept, including a novel injection system. The combustion chamber consists of a constant cross section module hosting a strut injector followed by a diverging module which counteracts static pressure peaks consequent to combustion. Hydrogen and air are injected unlike-impinging into the strut wake, where a cylindrical pipe is inserted as flame-holder. The system allows creating a flow recirculation area. Combustion radicals are set free and promote further fuel reaction until a pilot flame stabilizes. Additional hydrogen or methane is injected through the strut sides, pre-mixes with the supersonic air flow (M=2.2) stoked to the combustor and is ignited by means of the pilot flame. The total temperature at the combustor entrance can be varied between 500K and 1200K. First results of wall static pressure measurements and Schlieren imaging demonstrated the possibility of dual-mode combustion. The transition between ramjet and scramjet modes showed to depend on the fuel total injection pressure. This paper presents the results of experiments carried out to define the fuel total injection pressure interval in which the transition occurs. Two injector versions of different thickness (i.e. 3mm and 5mm) have been used. This allowed evaluating the effects of the aerodynamic blockage due to the strut on the overall combustion process, particularly on the transition.