Effect of injection distance on supersonic combustion under different combustion modes

The effect of injection distance on supersonic combustion under different modes in an ethylene-fueled scramjet combustor at Mach 2.52 is experimentally investigated. The study is based on three injectors at injection distances of 42, 21, and 7 mm. As the injection distance increases, a higher equivalence ratio is required to complete the transition from the Scram to Dual mode. The increase in injection distance will cause the reaction zone to move downstream in the Scram mode. However, the opposite is true in the Dual and Ram modes. In addition, the increase in injection distance will intensify the reaction zone oscillation under all modes. Moreover, in the Scram mode, shortening the injection distance will increase the heat release and the oscillation of heat release intensity. In the Dual mode, although shortening the injection distance cannot enhance the heat release, it will decrease the oscillation of heat release intensity. In the Ram mode, increasing the injection distance will enhance the heat release and will hardly increase the oscillation of heat release intensity. Considering the combustion heat release and oscillation, the injection distance should be shortened in the Scram and Dual modes and increased in the Ram mode.