Ignition delay in diffusive supersonic combustion.

Abstract

This paper deals with the study of the zone located near the injector exit of an idealized supersonic combustion burner using hydrogen as fuel. The ignition delay length is calculated considering a constant-pressure, free mixing layer, where fuel and oxidizer mix without appreciable concentration change or heat release resulting from chemical reactions, although radicals, mainly atomic hydrogen, are produced. The chemical kinetics scheme is reduced to one over-all chemical reaction, and the presence of radical-introduced in the mixing zone from outside is taken into account. Dissociation at the injector outer boundary layer is considered in this paper, as the main radical-producing mechanism. The field near the injector is further complicated by the recirculation due to adverse pressure gradients induced by the wake-like configuration that appears at low ignition rates and by the aerodynamic effect of the heat released from the flame, once it has been established. This recirculation improves the mixing process. It will be shown that temperature of the injector outer wall and, to a lesser extent, pressure, injector length, and the conditions outside of the boundary layer control the ignition process. The corresponding ignition delay lengths are comparable to the length of the near-wake existing under low injection rate conditions.