Stabilization of Hydrogen-Air Flames in Supersonic Flow

Abstract

The operating range of scramjet engines for supersonic flight includes combustor inlet conditions, particularly in the lower flight Mach number range, which does not allow thermal self-ignition of hydrogen-air flames. Therefore, other methods have to be applied in order to insure safe and reliable ignition of supersonic combustion. Besides other methods, a very efficient tool for supersonic flame stabilization at low temperatures and pressures is the use of recirculation zones. These zones can also be generated in various ways in supersonic flows, for example, by bluff-body-type flame holders, by suitable shaping of fuel injectors or can be observed in connection with fuel jets injected from the sidewalls.

The Damköhler number for flame stabilization by recirculation zones, introducing the fuel characteristics via the laminar burning velocity, is derived and was checked experimentally for hydrocarbon fuels and hydrogen. It has been applied to hydrogen-air diffusion flames stabilized by cylindrical flame holders in supersonic flows up to Mach numbers of 2.1. Characteristic fluid mechanic times for hydrogen-air flames have been measured for a broad range of equivalance ratios. These results can be used for the sizing of recirculation zones or flames holders for supersonic and subsonic flame stabilization of hydrogen-air flames.