Laser-induced spark ignition of pulsed methane jets in homogeneous and isotropic turbulence

Abstract

Reliable and controllable ignition is of significant importance for combustion applications. While electrical spark has been the dominant source for forced ignition for several decades, laser ignition shows great potential as an alternative. The advantages of laser ignition include that it is non-intrusive, provides greater flexibility in ignition location and more accurate control over ignition timing; it is also suitable for multi-point ignition and less energy-demanding under high pressure conditions. Among the four mechanisms of realizing laser ignition, laser-induced spark ignition is most commonly adopted; this is due to its freedom in wavelength selection and simplicity of implementation. While laser-induced spark ignition has been extensively investigated under premixed conditions, few studies have been performed under non-premixed conditions, particularly in the presence of ambient turbulence. The present study, therefore, experimentally investigated laser-induced spark ignition of non-premixed methane jets, which were injected in pulses into an air environment with controllable homogeneous and isotropic turbulence without mean flow. Jet mixing and flame kernel development, which are important for the ignition process, and the ignition probability were systematically examined. Mixing behaviour of isothermal jets with low Reynolds numbers within ambient turbulence was characterised in the near field by measuring instantaneous 2D concentration field using Planar Laser-induced Fluorescence technique. It was found that the ambient turbulence was able to enhance jet mixing and boost jet entrainment; these effects became more significant at higher level of ambient turbulence, lower jet Reynolds number and increased distance downstream from the nozzle exit. In the presence of ambient turbulence, self-similarity was achieved in the radial distribution of ensemble-averaged mole fraction but not for mole fraction fluctuations. Development of flame kernel in non-premixed methane jets was examined up to 10 ms after the ignition laser pulse by imaging the flame kernel luminosity. The influence of jet Reynolds number, laser pulse energy, direction of incident laser beam and ambient turbulence was characterised. Both the projected area and spatially integrated luminosity of the flame kernel grew in the examined time window approximately following a power law. While ambient turbulence showed no clear impact on flame kernel size and propagation speed of flame kernel edges, it was able to enhance the fluctuations of flame kernel location. The jet Reynolds number had a two-stage influence on flame kernel growth in the examined time window, while lower laser pulse energy and parallel ignition (laser beam parallel to jet axis) tended to slow down kernel development throughout the examined stage. Ignition probability in pulsed methane jets was evaluated at various locations for a range of laser pulse energies, levels of ambient turbulence and two different ignition timings, and correlated with instantaneous local mixture composition, measured with Laser-induced Breakdown Spectroscopy technique. Higher laser pulse energy enhanced ignition probability until it reached a saturation level. For a certain laser pulse energy, when the spark was small, ignition probability was mainly determined by the local mixture composition and the heat loss from and the stretch of the spark introduced by ambient turbulence. However, the influence of spark size had to be considered if it became large relative to the jet width, since the spark could expand and reach flammable regions that assisted the formation of flame kernel. The correlation of ignition probability and instantaneous local methane mole fraction followed a similar trend, regardless of the level of ambient turbulence and ignition timing; ignition probability first increased with local methane mole fraction and then levelled off for a certain range of local methane mole fraction before dropping for further increase of methane mole fraction. However, ambient turbulence was found to narrow the range of local methane mole fraction over which ignition probability remained high.