Investigation of the early stages in laser-induced ignition by Schlieren photography and laser-induced fluorescence spectroscopy

Laser ignition has been discussed widely as a potentially superior ignition source for technical appliances such as internal combustion engines. Ignition strongly affects overall combustion, and its early stages in particular have strong implications on subsequent pollutant formation, flame quenching, and extinction. Our research here is devoted to the experimental investigation of the early stages of laser-induced ignition of CH4/air mixtures up to high pressures. Tests were performed in a 0.9-l combustion cell with initial pressures of up to 25 bar with stoichiometric to fuel-lean mixtures using a 5-ns 50-mJ 1064-nm Nd:YAG laser. Laserinduced fluorescence (LIF) was used to obtain two dimensionally resolved images of the OH radical distribution after the ignition event. These images were used to produce an animation of laser ignition and early flame kernel development. Schlieren photography was used to investigate the laserinduced shock wave, hot core gas, and developing flame ball. We extend existing knowledge to high-pressure regimes relevant for internal combustion engines.