Blowoff Characteristics of Bluff-Body Stabilized Conical Premixed Flames in a Duct with Upstream Spatial Mixture Gradients and Velocity Oscillations

An experimental study of flame blowoff phenomenon in a bluff body stabilized flame confined in a cylindrical duct is presented. Blowoff equivalence ratios were determined for two approach velocities and three different upstream equivalence ratio profiles (uniform, inner, and outer fuel enrichment) in the absence and presence of imposed upstream velocity oscillations. The results were compared with those for the unconfined flame configuration as well. It is found that the blowoff equivalence ratios exhibit somewhat different trends for different approach velocities. For the uniform mixture profile, blowoff equivalence ratio first increases with increasing excitation frequency and then decreases at higher frequencies for the approach velocity of 5m/s. For the 11m/s approach velocity, the trend is different, and the blowoff equivalence ratio continuously increases with increasing excitation frequency. The blowoff equivalence ratios are higher for the confined flame configuration. An empirical correlation of the blowoff data for the uniform mixture profile is presented in terms of Damkohler, Reynolds, and Strouhal numbers. Finally, analysis of the CH* chemiluminescence signal as blowoff condition is approached shows emergence of low-frequency oscillations that are connected with intermittency of the localized flame weakening and/or extinction near the flame stabilization region.