Criteria for flame holding in H 2-fueled scramjet engines

Abstract

Although self-sustaining combustion has often been termed auto ignition in studies on scramjet combustors, it should be interpreted as a flame-holding phenomenon rather than ignition. Criteria for flame holding in H2-fueled scramjet engines have been discussed by approximating the recirculation zone in the combustor with a perfectly stirred reactor. The influence of mass flow rates and reactor volumes on flame holding can be summarized by a critical reaction time in the recirculation zone. It was found that the reaction time was proportional to pressure, with P−13 for H2 reactions, and the recirculation zone should be occupied with reactants having equivalence ratios in the range 0.4<<3. The flow time was evaluated from the turbulent exchange rate over the backward-facing steps, and it was proportional to the size of the steps and struts. Comparison between the reaction and the residence times in the step case yielded a new criterion for flame holding. It indicated that the step with 4 mm height, the design of which was based on component studies, was insufficient to anchor the flame in the Mach 4 (M4) flight condition. This explained why a flame stabilized on the top wall of an engine failed to propagate to the cowl side of the engine in the M4 tests. Because burning pressure is low and convective velocity becomes high, flame holding is till crucial in the 4 mm step case under flight conditions at March 6 and 8. Consequently, flameholding struts are necessary, in addition to the step and the flame tends to elongate from the combustor to cause imperfect combustion. A competition between the slow heat release and the high convection velocity dominates the performance of scramjet engines in flight at the higher Mach numbers.