Coupling of diffusion flame structure to an unsteady vortical flowfield

A laminar methane-air diffusion flame is interacted with vortices of various sizes and strengths in order to better understand unsteady stretch and history effects on turbulent flames. The nitrogen-diluted fuel stream of a Wolfhard-Parker slot burner is acoustically forced, producing repeatable two-dimensional vortices that strain and curve the flame. Phase locked, planar laser-induced fluorescence (PLIF) diagnostics are used to quantify the response of the OH-radical to the vortex-induced stretch. Acetone PLIF images are used to clarify the relationship between the vortex structure and the flame. The results show that the vortex causes significant variations in the OH layer thickness. In particular, negative strain produces a doubling of the flame thickness. Such large increases in OH layer thickness are not predicted by the laminar flamelet model (LFM) because negative strain rates cannot be simulated using standard counterflow flamelet geometry. Local extinction of the OH layer due to high strain is observed near the flame base. Peak OH mole fraction levels vary considerably more than adiabatic LFM predictions. In particular, the peak OH decreases by a factor of two with downstream distance. This decrease is believed due to dilution of reactants by combustion products formed elsewhere in the flow. A simplified model is proposed, which shows the OH concentration is sensitive to product dilution through the scalar dissipation rate