Flameholding characteristics and flame behaviors of a scramjet model combustor with two-stage fuel injection for low flight Mach numbers

In this study, two types of fuel injection methods were applied to a scramjet model combustor equipped with a dual-cavity flameholder and a burned-gas injection to achieve operation at a low flight speed corresponding to a flight Mach number of approximately 3. The first method was single-stage fuel injection, in which hydrogen gas as fuel was injected from a location 60 mm upstream of the combustor inlet. The second method was two-stage fuel injection, in which hydrogen gas as fuel was injected from the downstream cavity floor in addition to the single-stage fuel injection. Flameholding limits, pressure profiles, and high-speed OH chemiluminescence images in the scramjet model combustor for both injection methods were experimentally investigated. The experimental results showed that the two-stage fuel injection significantly improved the flameholding limits compared with those of the single-stage fuel injection. Furthermore, observations of the high-speed OH chemiluminescence images revealed four types of ignition processes in the two-stage fuel injection cases. Among these, the ignition process initiated by the second-stage fuel was unique to the two-stage fuel injection configuration. The flame of the second-stage fuel ignited even when the first-stage fuel could not. Subsequently, the flame of the second-stage fuel propagated upstream and ignited the first-stage fuel. The flame was initially formed from the second-stage fuel as a result of the impingement between the high-temperature burned-gas jet and the second-stage fuel plume.