The instantaneous spatial structure of the recirculation zone in bluff-body stabilized flames

This paper focuses on resolving the spatial structure of the recirculation zone in turbulent non-premixed flames stabilized on a bluff body. The Raman-Rayleigh-LIF technique is used to perform joint, spontaneous, and quantitative imaging of mixture fraction, temperature, and the hydroxyl radical, OH. The quantitative accuracy of the images is established by comparison with single-point measurements collected in similar environments. It is found that double reaction zones exist simultaneously within the main recirculation region. However, at conditions far from blowoff, combustion occurs mainly in the outer edge and in relatively thin layers. As the jet-to-coflow velocity ratio is increased, the occurrence of the double reaction structures becomes more frequent, indicating a shift of the reaction zone toward the center of the main vortex. Localized extinction is observed downstream, but not within, the recirculation zone as blowoff is approached. Regular, vortical coherent structures are detected on the outer (air side) edge of the recirculating region, but these do not play a significant role in the fuel-air mixing process.