Laser-induced ignition of a methane-oxygen turbulent shear layer*

Laser-induced optical breakdown of a gas is a versatile means of depositing energy and seeding ignition in a combustible mixture. The expanding plasma kernel generates a complex flow that can affect subsequent flame growth, though the details of these dynamics in a turbulent and otherwise inhomogeneous flow are not fully understood. We analyze the flow and ignition dynamics in a subsonic methane-oxygen turbulent shear layer using direct numerical simulation. The vorticity fluctuations in the hot kernel are significantly attenuated by its supersonic expansion, whereas the fluctuations immediately behind the resulting shock wave do not exhibit a comparable change. At the flow conditions considered, the vorticity generated by the deposition alone is weaker than that of the turbulence, indicating that the hydrodynamic development of the kernel is dominated by the shear flow. The initial high temperature leads to rapid dissociation into monatomic radical species, which persist as the kernel becomes distorted and corrugated and cools to the flame temperature. Comparison with a corresponding unsheared, yet non-premixed, configuration indicates that vorticity generated due to the fuel-oxidizer density gradient also leads to distortion of the flame kernel.