Transient laser-induced ignition kernels in supersonic flow

Simulation predictions with quantified uncertainty are made for ignition of an axisymmetric slot jet on a centerbody injecting hydrogen into a M=2.7 crossflow that is seeded by the optical breakdown of a focused laser (LIB). The LIB dissociates the local mixture into elemental species, generates vorticity, and heats the gas above 10⁴ K, activating reactions within ≈10^-9 s and producing the ignition kernel. The flow near the kernel is complex: turbulent, compressible, separating boundary layers, non-premixed fuel and oxidizers, and velocities induced by the LIB exceeding 10³ m/s. As a prediction testbed, blind comparisons are made with corresponding experiments, and they focus particularly on the kernel's position, size, and orientation based on the light intensity emitted by excited hydroxyl radical. In the simulations, this comparison entails using a detailed kinetics model and simulating the same data-acquisition characteristics (e.g. frame rate and exposure time). Depending on the breakdown site, the kernel orients and convects in a primary direction with the flow. The initially sub-millimeter-scale kernels are three-dimensional, elongate along their principal axis, and grow by over a factor of ten as ignition ensues.