Ignition Transients in a Scramjet Engine with Air Throttling Part II: Reacting Flow

The analysis presented in Part I of this study is extended to investigate ignition transients and flame development in an ethylene-fueled scramjet engine. Unheated gaseous ethylene is transversely injected into the combustor upstream of a recessed cavity flameholder. Immediately after the ethylene-air mixture reaches its steady state in the combustor, a hotspot igniter is activated in the cavity to initiate chemical reactions. Cases both with and without air throttling downstream of the cavity during the engine startup stage are examined in detail. Successful ignition can only be achieved with the aid of air throttling under the present flow conditions. As a consequence of the backpressurization by the throttling air, a shock train is generated in the isolator, which then decelerates the high-speed main stream, enhances the fuel mixing efficiency, and increases the temperature and pressure in the combustor. Chemical reactions are intensified and produce sufficient heat release to maintain a flow environment conducive to flame stabilization. A self-sustaining mechanism is thus established between the flow and flame development. Stable flames are achieved even after the deactivation of air throttling. The predicted pressure distribution along the entire flowpath agrees well with experimental measurements.