Self-ignition and transition to flame-holding in a rectangular scramjet combustor with a backward step have been investigated experimentally and numerically in order to clarify whether these are dominated by the near-field phenomena or the far-field phenomena. Hydrogen fuel was injected perpendicularly into the Mach 2.0 high-enthalpy airflow downstream of the step. The details of the flowfield were captured by a three-dimensional full Navier-Stokes numerical code with a large-eddy simulation turbulence model and a detailed chemical reaction model. The characteristics of self-ignition and transition to a bulk flame are explained by the Damkoehler number (Da). Da in the combustor is lower than unity right after the fuel injection due to the low pressure level, and the flowfield in the combustor is essentially reaction-limited. Therefore, fast chemical reaction is allowed only in the hot boundary layer and the recirculation zone behind the step. Although self-ignition was observed either in the near-field or in the far-field boundary layers according to the condition of the mixture, intensive chemical reaction was not observed outside the boundary layer at the early step because of the very low Da and therefore the transition to the bulk flame was difficult. However, the transition to the bulk flame was achieved by the propagation of the shock wave system which increased the Da by 1 or 2 orders of magnitude and turns the flowfield to mixing-limited. The shock wave was triggered by the slight increase of the pressure level in the far field. The generation and the propagation of the shock wave were significantly affected by the combustor geometry and the heat release in the far field; thus, the transition was essentially dominated by the far-field phenomena.
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