Effect of Acoustic Excitation on Lean Blowoff in Turbulent Premixed Bluff Body Flames

The lean blowoff characteristics of a premixed air-methane flame were investigated in a ducted combustor with a bluff body according to acoustic excitation. The blowoff equivalence ratio increases with the Reynolds number and changes depending on the extent of the recirculation zone. Using the relation between the Damkohler number and the Reynolds number, it was confirmed that the flow velocity at the downstream tip of the bluff body and the laminar flame speed are decisive blowoff factors. Although a periodic flame hole appeared far from the blowoff only with acoustic excitation, the blowoff observed by OH radical chemiluminescence occurred using a similar process regardless of the excitation. The recirculation zone collapses and the flame becomes small when it is close to the blowoff. Then, the flame is locally extinguished downstream from the bluff body and the recirculation zone completely collapses. Eventually, the unburned gas does not ignite and the flame is extinguished. The blowoff equivalence ratio rapidly increases at specific acoustic excitation frequencies. This was investigated using proper orthogonal decomposition analysis, the two-microphone method, and phase-lock particle imaging velocimetry measurement. Resonance occurs when the excitation frequency approaches the harmonic frequency of the combustor and it increases the velocity fluctuation in the combustor and the infiltration velocity of the unburned gas in the shear layer of the recirculation zone. Consequently, because the burning velocity must have a larger value corresponding to the enhanced mixture velocity for a sustained flame, the blowoff occurs at a higher equivalence ratio.