Investigation of flame structures in turbulent partially premixed counter-flow flames using planar laser-induced fluorescence

This paper reports on investigations of structural turbulent flame characteristics using planar laser-induced fluorescence (PLIF) of hydroxyl (OH) radicals. Turbulent, counter-flowing methane/air flames with different equivalence ratios spanning from premixed to partially premixed are investigated. Additionally, Reynolds numbers of these configurations are varied and reach from stable to extinguishing flames. The main objective of this study is to extract topological quantities from spatial OH distributions such as area and local thickness of the OH layer, flame brush, or contours approximating the profile of the stoichiometric mixture fraction. The post-processing procedure applied to extract these quantities from single-shot OH PLIF images is described. It is shown that OH areas shrink with increasing Reynolds number whereas lengths of stoichiometric contour lines are nearly unaffected. This indicates that flame extinction is driven by large scale eddies rather than small scale wrinkling. In addition to also performed quantitative laser Doppler anemometry and Raman/Rayleigh measurements of the same flames, topological studies are helpful in viewing complex turbulent-chemistry interaction. Latter data are essential for validation purposes of large eddy simulations (LES) that predict transient flame movement and thereby spatial OH distributions.