Effects of high shear on the structure and thickness of turbulent premixed methane/air flames stabilized on a bluff body burner

The effects of preferential transport and strain on the scalar structure (profiles of major species, elemental ratios, and equivalence ratio) of turbulent premixed bluff-body stabilized flames are examined using lineimaged Raman/Rayleigh/CO-LIF diagnostics combined with crossed-planar Rayleigh imaging to determine the 3D flame orientation. Comparison of the experimental measurements with laminar flow calculations shows strong effects of preferential diffusion on the flame structure and the product state in lean and rich flames. Measurements of the flame orientation show a strong correlation between the flamefront normal angle and the strength of the preferential transport effects. As the flame-front angle decreases (that is increasing the reactant velocity, or decreasing the distance from the surface), the coupling between the preferential diffusion through the flame brush and the recirculation region is increased, enhancing the preferential transport effects. Flame thickness measurements show good agreement (within 3%) with the laminar calculation for fuel lean flames, and .,slow. ( 2.0 m/s reactant velocity) fuel rich flames. The measured thickness for the .,fast. (9.4 m/s reactant velocity.) fuel rich case is 2.4 times larger than the predicted value at 10 mm from the surface and shows a strong dependence on the flame normal angle.