A comparative study of laser-induced gas breakdown ignition and laser ablation ignition in a supersonic combustor

Cavity ignition was achieved in a supersonic combustor by laser-induced gas breakdown (LIGB) and laser ablation (LA). High-speed photography and schlieren imaging were employed to characterize the ignition processes. schlieren imaging was also applied to visualize LIGB and LA in quiescent air. The input energy was approximately 200 mJ. The distance between the focal point of the convex lens and the target plate (d) varied from 10 mm above the target plate (d = 10 mm) to 5 mm below the target plate (d = −5 mm). The experiments performed in quiescent air indicated that the plasma of LA (d = −5 mm) was relatively small. Compared to LIGB, the expansion of the hot plume resulting from LA was less significant because the plate prevented the formation of the third lobe. The primary shock wave of LA propagated faster than that of LIGB because of high pressure ratio between the plasma and the ambient air. In the supersonic combustor with d = 5 or 10 mm, the laser induced a plasma by LIGB at the focal point and induced another plasma by LA at the cavity floor. These two tests were the combination of LIGB ignition and LA ignition. LA ignition was achieved in the test with d = −5 mm. The ignition process of the test with d = 10 mm was the slowest because of severe plasma quenching at the focal point. For the early stage of ignition, the flame kernels of the tests with d = 5 and −5 mm were different. However, the differences gradually vanished after the flame kernels reached the cavity leading edge. The ignition times of these two tests were almost the same. Therefore, the plasma resulting from LA was as effective as the plasma generated by LIGB in terms of ignition. Laser ablation ignition was a promising ignition method in a supersonic combustor. (paper)