Numerical simulation of front lobe formation in laser-induced spark ignition of CH 4/air mixtures

Abstract

Initial flame-kernel development in laser-induced spark ignition has been investigated numerically for methane/air mixtures. Laser energy absorbed by the spark has been simulated accounting for spatial distribution, especially, asymmetric absorption in the laser-beam direction. Results showed that the shock wave generated from the expansion of high-pressure initial kernel propagates rapidly having nearly spherical shape and leaving behind a high-temperature region. A torus-like shape flame kernel propagated radially and a front lobe was formed, which propagated back toward the laser source. These kernel structures were developed due to vortical motions generated by the interaction of pressure field and flow, which stems from the asymmetric deposition of laser energy in the direction of laser. The front lobe was separated from the torus for the lean mixtures, while the lobe and torus were connected for the stoichiometric mixture. The front lobe was extinguished at a later stage for the lean mixture at subatmospheric pressure. The calculated results agreed well with experimental observations.