Driving mechanisms for laser ignition of a gaseous oxygen–methane shear-coaxial injector in a rocket combustor

A statistical investigation was performed on a laser-ignited single element gaseous oxygen–methane shear-coaxial injector in a model rocket combustor. The laser focal point was positioned at 21 discrete locations across a total of 474 ignition tests. Laser energy was deposited during the steady state flow of oxygen and the transient introduction of methane. A point-biserial statistical analysis was performed to ascertain that the ignition outcome had no correlation with changes in inlet conditions. This enabled the study of the ignition transient affected exclusively by laser focal location and the underlying flow field. Two-dimensional velocity statistics of the pre-ignition flow field were obtained using particle image velocimetry. High-speed schlieren and OH* chemiluminescence imaging captured density gradients and heat release during ignition tests. Four distinct mechanisms inherent to the ignition process were investigated: (1) laser energy deposition, (2) hydrodynamic ejection, (3) shear layer mixing, and (4) flame growth. A variety of properties (kernel/ejecta position and velocity, flame location and size, etc.) were extracted from diagnostic images. Phenomena controlling ignition outcome at multiple deposition locations were identified. Near the injector face, plasma-kernel asymmetry and the resulting strength of the hydrodynamic ejecta controlled ignition outcome more strongly. Farther downstream from the injector face, the instantaneous position of the turbulent reactant-jet shear layer relative to the kernel and ejecta were more dominant. Stochastic fluctuations in the pre-ignition recirculation zone and shear layer velocity fields were found to influence shear layer mixing time and flame growth trajectory, affecting test outcome throughout all steps of the ignition process.