Flame stabilization analysis of a premixed reacting jet in vitiated crossflow

The flame stabilization behavior of a premixed ethylene-air jet injected normal to a hot vitiated crossflow (JICF) was studied experimentally using simultaneous hydroxyl (OH) planar laser induced fluorescence (PLIF), formaldehyde (CH2O) PLIF, and particle image velocimetry (PIV). Pixel-by-pixel multiplication of OH and CH2O fluorescence signals was conducted to estimate the reacting JICF flame front. The simultaneous PLIF-PIV measurements allowed for an in-depth study of the interaction between the flame and the flowfield. The flame structure was divided into two branches, a windward and leeward flame branch. The unsteady windward flame exhibited both attached and lifted flame behavior, while the leeward flame branch remained consistently attached at the jet exit. In some cases, formaldehyde signal was observed upstream of the windward flame base, suggesting the build-up of a radical pool due to mixing between the jet reactants and hot crossflow. Both flame branches were anchored in the jet shear layer, but with increasing distance from the jet exit the flames moved inside the shear layers. Small scale vortices caused local wrinkling of the flame front. The windward flame was observed to wrap around the large-scale vortices that formed along the jet shear layer. The large-scale structures distorted the flame front but the associated strain-rate was typically lower than that imparted by the small-scale structures. The leeward flame edge aligned with regions of high principal extensive strain-rate and high dilatation. On the other hand, the windward flame edge was located in regions where principal extensive and principal compressive strain rate magnitudes were high and dilatation was low. The results suggest that auto-ignition is the dominant flame stabilization mechanism for the unsteady windward flame and premixed flame propagation is the more dominant stabilization mechanism for the leeward flame branch.