Lean blowoff behavior of asymmetrically-fueled bluff body-stabilized flames

Bluff-body stabilized flames were studied in an enclosed, asymmetrically-fueled duct with a two-dimensional triangular flame holder. Acetone laser-induced fluorescence was used to characterize the fuel distribution for both uniform and non-uniform fuel profiles. Flame dynamics were captured with high-speed chemiluminescence imaging during stable operation and near blow off conditions for three cases with varying fuel–air gradients across the flame holder. Particle imaging velocimetry was used to measure the velocity field. It was discovered that for a given velocity, increased fuel profile asymmetry caused an increase in the blowoff equivalence ratio, produced greater vortex shedding coherence, and for lower velocities resulted in dynamic coupling between the heat release and the duct acoustics. High-speed imaging of the acoustically uncoupled cases revealed the same flame blowoff process as previously observed in uniformly fueled cases. The blow off process in the acoustically coupled cases was dominated by acoustically influenced velocity straining the flame adjacent to the wake stagnation zone causing local extinction and rapid entrainment of reactants into the recirculation zone. From the Mie scattering images gathered for PIV, density transition contours were extracted and used as flame contours to calculate local aerodynamic strain rates and curvature. Statistics revealed conditional relationships between the local strain, wake geometry and fluid mechanics.