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

Abstract

The characteristics of premixed flame closed to lean blowoff were investigated in a ducted combustor with the bluff body. The combustor has a long duct and a rectangular cross-section. The bluff body is in the shape of an equilateral triangle. Methane was used as fuel, which was injected transversely into the cross-flow.The blowoff equivalence ratio increased with the Reynolds number, and the blowoff equivalence ratio changed depending on the extent of the recirculation zone. Shortly before the blowoff, the unburned gas strongly penetrated into the recirculation zone, resulting in a dramatic change in the heat release in the vicinity of the bluff body. This change can be used as a precursor to predict blowoff. The blowoff occurred in a similar process regardless of the excitation. The blowoff equivalence ratio was increased by the intensity of the excitation and rapidly increased at particular frequencies of excitation. The POD analysis revealed that the frequencies with a high blowoff equivalence ratio corresponded to the peak frequencies of the shear generated vortex of the bluff body. This result suggests that the resonance phenomenon of the excitation and the shear generated vortex is the cause of increase in the blowoff equivalence ratio. Nomenclature d = width of the bluff body f ex = frequency of the acoustic excitation f sh = frequency of the shear generated vortex ̇a = inlet air mass flow rate R 2 = correlation coefficient Re d,lip = Reynolds number based on d and U lip t bo = time to blowoff U lip = tip velocity of the bluff body Φ bo = blowoff equivalence ratio