Laser-Induced Plasma Ignition in a Cavity-Based Scramjet Combustor

Experimental investigations of laser-induced plasma (LIP) ignition processes were conducted in an ethylene-fueled scramjet combustor with a rear-wall-expansion cavity. The results were obtained under the inflow conditions of Ma = 2.92 supersonic inflow with a stagnation pressure of 2.6 MPa and a stagnation temperature of 1530 K. CH* chemiluminescence recorded at a flame rate of 50 kHz was used to characterize the ignition and flame stabilization processes. Large-eddy simulations were carried out to characterize the mixing process and the flow structure before LIP. It was found that, with the LIP ignition in the foreside of the cavity, the initial plasma flame kernel first anchored at the cavity leading edge and then moved away along the cavity leading edge toward the high-speed shear layer, where the flame could be stabilized or quenched, depending on the global equivalence ratio. On the other hand, after a LIP ignition in the rearward of the cavity, the initial flame in the rearward of the cavity was propagating along the cavity floor to the foreside of the cavity, and eventually to the shear layer where a stable flame could be established. The numerical results showed that a more successful LIP ignition in a cavity-based scramjet combustor could be achieved inside the subsonic zones and in the rear side of the cavity. The ignition process could be characterized as four stages: an initial plasma ignition stage; followed by the plasma-quenching stage, the reignition stage, and, finally, the stable flame stage.