Spatial and temporal evolution of laser-induced flame kernels in a methane jet injected into air

•Early-stage evolution of laser-induced flame kernel in a fuel jet into air.•Impact of laser pulse energy and beam orientation relative to jet’s axis on kernel evolution.•Kernel size and propagation speed increase with laser pulse energy.•Strong dependence of kernel shape, size, and propagation speed on beam orientation.•Kernel extends far beyond flammable region due to spark-induced gas motions. We report the first measurements of the evolution of laser-induced flame kernels in a methane jet (nozzle diameter D = 2 mm, injection velocity U = 20.0 m/s) as a function of laser energy (E = 50–80 mJ) and beam orientation, along either the jet’s axial or radial direction. Kernels were initiated at x = 10 D axially and r = −1 D radially, for high probability of successful ignition, where the mean mole fraction of methane was 0.19. Flame kernel evolution within t = 1–10 ms after ignition laser pulse was quantified for kernel projected area, luminosity, shape, location, and propagation of kernel edges. Within t = 1–3 ms, the kernel extended beyond the jet’s nominal flammable regions for both orientations, attributed to mixture transport by the spark-induced gas motions. For the radially-orientated beam, these motions led to a three-lobe structure of the kernel: this structure was absent for the axially-orientated beam. Development after t = 5 ms was primarily along the jet’s nominal flammable regions for both orientations. The kernel’s projected area and integrated luminosity grew according to a power law, regardless of the pulse energy and beam orientation; both variables increased approximately linearly with E, but reduced by half when orientation changed from radial to axial. Fluctuations of the kernel’s location reduced with increased E, and were insensitive to the beam orientation. For the radially-orientated beam with E = 80 mJ, the kernel’s r-minus edge first moved away from, then towards, and eventually away from the jet axis, probably driven by spark-induced gas motions; as E reduced this gradually changed to consistent motion away from the axis. For the axially-orientated beam, this edge moved only after t = 5 ms, away from the axis. The speeds of the kernel’s x-minus and x-plus edges, moving relative to the flow, were estimated from the speeds measured in laboratory coordinates. That of the x-minus edge was about five to six times methane’s stoichiometric laminar burning velocity (SL) for both beam orientations; that of the x-plus edge was slightly highe