Combustion and blow-out behavior in a cavity flame holder of Mach 2.5 n-dodecane-fueled scramjet model combustor

The supersonic combustion mechanism was experimentally investigated using a Mach 2.5 n-dodecane fueled ramjet/scramjet dual-mode combustor with a cavity flame holder at stagnation pressure of 1021 kPa and a stagnation temperature of 1847 K. Heated n-dodecane was injected from the cavity closeout ramp. Optical diagnostics, including CH* and OH* chemiluminescence and Laser induced breakdown spectroscopy (LIBS), were employed. During steady combustion, the cavity shear-layer stabilized combustion was established autonomously without any assistance of the torch igniter. The local equivalence ratio was measured by moving the breakdown points using a 3-axis high precision motorized stage with measurements of CH* chemiluminescence in the cavity flame holders. Additionally, measurements were performed at fixed positions at 30 Hz as the fuel injection rate increased gradually. The results indicated the intensive OH* chemiluminescence emissions near the lower wall near the closeout ramp during the stable cavity shear-layer combustion. The equivalence ratio in the lower region of the flame was sufficiently rich, exceeding 2, while the flame region with high OH* and CH* signals was stoichiometric or lean. A steep gradient in the equivalence ratio was observed across the flame. During the transient combustion, as the fuel injection rate increased gradually, the equivalence ratio within the cavity increased. Additionally, the equivalence ratio within the cavity was high on the step side and decreases toward the ramp side. Near the flame regions exhibiting strong CH* chemiluminescence signals, the equivalence ratio approached unity. When the flame left the measurement point, the equivalence ratio exceeded 2, showing a substantial gradient across the flame. The flame tip was located near the fuel rich region exceeding 2. The findings of this study provide valuable insights for the design of kerosene-fueled scramjet combustors. Furthermore, the steady cavity flame demonstrated here has potential applications as a piloted flame for staged combustions.