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

A scramjet model combustor comprising a dual-cavity flameholder and a burned-gas injection torch was investigated to clarify the effects of two-stage injection on the pressure profiles and flame behaviors. Experiments corresponding to low-flight Mach number conditions were performed, and a burned-gas torch was employed for fuel ignition. For the first and second stage fuels, the mass flow rates were varied, whereas the total mass flow rate was maintained at 2.5 and 3.5 g/s, respectively. Shadowgraph images showed that the two-stage fuel injection generated an oblique shock at the leading edge of the downstream cavity owing to impingement between the mainstream and second-stage fuel jet. Pressure profiles showed that two-stage injection improved the combustion efficiency and unstart characteristics. Furthermore, the pressure rise downstream of the second-stage fuel injection was comparable under the same flow rate of the second-stage fuel, even though the first-stage fuel flow rate increased. High-speed OH chemiluminescence images showed that the flame of the first-stage fuel ignited the second-stage fuel. This ignition occurred because of collision between the second-stage fuel jet and the high-temperature gas burned from the first-stage fuel. The flame was then independently stabilized in each cavity. By contrast, the first-stage fuel ignited at the leading edge of the upstream cavity. This ignition process yielded reasonable results owing to a large flow separation occurring therein. Furthermore, a re-ignition process, such as the second-stage flame re-igniting the first-stage fuel, was observed. The standard deviation of the OH chemiluminescence intensity indicated that the second-stage flame was more stable than the first-stage flame. This implies that a two-stage injection may improve flameholding performance. In summary, two-stage injection can improve combustion efficiency and unstart characteristics and may improve flame stability.