Spark ignition of turbulent non-premixed flames

This thesis investigates the spark ignition of various turbulent non-premixed flames namely, jet, counter-flow and bluff-body flames. This detailed fundamental study of spark ignition aims to provide useful information for solving the high-altitude relight problem in the aviation gas turbine. A specially designed ignition unit has been built. Different spark parameters and flow conditions have been examined to study their effects on the ignition probability defined as successful flame establishment. The ignition probability results have been correlated with the measured or estimated flow velocity and mixture fraction. The whole ignition and flame propagation events have been visualized by a high-speed camera and OH-PLIF. In the jet flames, it was found that after an initially spherical shape, the flame took a cylindrical shape with a propagating edge upstream. The probability of successful ignition P_ign increases with high spark energy, thin electrode diameter and wide gap, but decreases with increasing dilution of the jet with air. The flame kernel growth rate is high when the ignition probability is high for all parameters, except for jet velocity. Increasing the jet velocity decreases the ignition probability at all locations. The estimated net propagation speed relative to the incoming flow was about 3 to 6 laminar burning velocities of a stoichiometric mixture S_L. In the counter-flow flames, it was found that the flame spread as an edge flame with a large scatter in its radial position. P_ign decreased with bulk velocity, which suggests that the local strain rate can be detrimental to ignition so that, even with the strongest spark tested, ignition could not be achieved at a bulk velocity about 90% of the extinction velocity. P_ign was greater than zero even in regions well into the fuel and air streams where the mixture fraction fluctuations were virtually zero, giving zero probability of finding flammable mixture at the spark location. The estimated edge flame speed relative to the radial flow is higher than S_L for the premixed flame and is less than S_L for the non-premixed flames.