Laser-induced optical breakdown and shock waves in combustible mixtures near flammability limits

Optical breakdown and shock wave generation precede the emergence of a self-sustained combustion wave during laser-induced ignition. Understanding the breakdown process in reactive and non-reactive gases is crucial to developing quantitative predictive models of successful laser-induced ignition. In this study we use a focused 532 nm Nd:YAG laser to induce optical breakdown in air and combustible mixtures of methane/air and biogas/air in a constant volume reactor. The resulting shock wave propagation is characterized by means of high-speed Schlieren imaging and interpreted according to the blast wave theory. Differences between the shock dynamics of reacting and non-reacting gases are brought out, thereby shedding light on the contribution of heat release to the overall shock dynamics in the case of reacting gases. Further, visualization of these early processes using interferometry highlights differences in the evolution of the laser-induced plasma near lean flammability limits.