Flashback of stratified swirl flames in a model combustor with an axial swirler and a centerbody were investigated at 1 atm and 3 atm. The global equivalence ratio for the flashback conditions is kept at 0.63 and 0.85 for atmospheric and elevated pressure experiments respectively. The bulk velocity of the fuel-air mixture during the flashback is set to 2.5 m/s for all experiments. Stratification in the flow is achieved by injecting methane through the fuel-ports on the swirler vanes. The nature of stratification is assessed by conducting planar laser induced fluorescence (PLIF) experiments in non-reacting flows. Acetone was used as fuel tracer which allowed the calculation of spatial distribution of equivalence ratio in the mixing tube. The ensemble-averaged distribution confirmed the radial stratification in the mixing tube. Instantaneous PLIF measurements showed intermittent presence of fuel rich structures close to the center-body. For flashback experiments, time-resolved three-component PIV and simultaneous chemiluminescence imaging at 4kHz were applied to investigate the interaction between the flame and the stratified flow. The global behavior of flashback was characterized by a large flame tongue swirling around the center-body during upstream propagation. The flame tongue stopped propagating at an intermediate location and kept swirling around the center-body. A comparison with fully premixed flashback showed that the stratified flame was more wrinkled and the radial extent of stratified flame brush was larger than the fully premixed cases. Reacting flow experiments at elevated pressure showed highly wrinkled flame surface due to the presence of turbulence and local variation in equivalence ratio.

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