Ignition processes and modes excited by laser-induced plasma in a cavity-based supersonic combustor

Abstract

The ignition processes in an ethylene-fueled supersonic combustor excited by laser-induced plasma (LIP) were investigated experimentally under the condition of inflow Ma number of 2.92. The LIP excitation was implemented near the center floor of a flameholding cavity. Optical measurements, including simultaneous CH∗/OH∗ chemiluminescence imaging and Schlieren photography, were used to investigate the ignition processes. It is found that the CH∗ and OH∗ initiated by the LIP are mainly in the region between the cavity front wall and the LIP excitation site. The CH∗ and OH∗ are quenched rapidly, in particular at a low fueling rate. After a short delay, the distribution of CH∗ and OH∗ appears in the region between the rear wall of the cavity and the LIP excitation site, showing the onset of ignition therein. A stable flame was established in the shear layer between the downstream part of the cavity and the outer supersonic flow. It is concluded that the ignition processes excited by the LIP can be divided into a LIP initiation regime and a transient ignition reaction regime. Both the fueling rate and the LIP energy significantly affect the cavity ignition processes. Increasing the fueling rate or the laser energy can shorten the ignition processes in the cavity. A weak ignition mode and an intense ignition mode are postulated to explain the combustion behavior of the ignition processes in the cavity-based supersonic combustor.