Spark ignition of a turbulent shear-less fuel-air mixing layer

A planar methane-air mixing layer with equal velocity in the two streams has been used to examine the ignition probability and the non-premixed edge flame speed following spark ignition. The mixing layer has approximately homogeneous turbulent intensity and lengthscale. Mean local mixture fraction has also been measured for the whole flow field. The ignition and subsequent flame propagation were visualized with a high-speed camera and the flame's edges in the upstream, downstream and cross-stream directions have been identified. The average rate of flame evolution in these directions allowed an estimation of the average absolute flame speed. Ignition probability contour of the mixing layer takes a V-shape, which matches the shape of the lean and rich flammability limits with a little discrepancy in the rich side. By subtracting the uniform mean velocity resulted in estimates of the mean relative edge flame speed. This quantity was approximately 2.5SL, where SL is the laminar burning velocity of stoichiometric methane-air premixed flames. The results are consistent with DNS of turbulent edge flames.This work has been funded by the European Commission through project "TIMECOP-AE" (AST5-CT-2006-030828). Thanks to Mr. I.A. Bahena Ledezma for assistance with the experimental techniques. S.F. Ahmed wishes to thank Qatar University for the support.Scopu