Free stream investigations on methane combustion in a supersonic air flow

An important task in the research on supersonic combustion is the comparison of different kinds of fuel. Due to its high specific impulse, hydrogen has often been selected for ramjet and scramjet applications. Nevertheless, in recent years the interest has been focused on alternative fuels, such as methane and ethylene. At the Institute of Flight Propulsion of the Technische Universität München research on hydrogen combustion in a supersonic air flow has been carried out for more than a decade both in free stream and inside a scramjet combustor. Since the beginning of 2003 the direct connected test facility has been re-organized to investigate feasibility and efficiency of methane combustion in the Mach 2.2 air flow, corresponding to transition between ramjet and scramjet operation of the real engine. Additionally as electrical air pre-heating did not allow for simulating conditions representative of the ones in flight, catalytic pre-combustion of hydrogen has been optimized for air vitiation, to attain temperatures up to 1000K in the test section. The strut injection system has been adapted to the new requirements: brass has been replaced by a high temperature nickel alloy. The changes made in the experimental setup are detailed in this paper. As for the previous investigations on hydrogen supersonic combustion, to better understand mixing and burning processes the experiments have so long been performed in free stream. The stability of a high subsonic hydrogen pilot flame has been investigated under the new flow conditions and for different ignition methods. Methane has been injected for the main combustion through a series of holes on the strut sides to obtain a partially premised diffusive flame. First experiments outlined difficulties in igniting and stabilizing the pilot flame and consequently the main combustion. Further investigations confirmed the possibility of lean, supersonic methane combustion.