The Acoustic Self-Excitation of a Bluff-Body Stabilised, Premixed Flame

Abstract

The objective of this investigation in a laboratory combustion system was to understand why a premixed propane flame is acoustically excited with particular acoustic boundary conditions, in particular regions of the frequency spectrum. h was demonstrated that the phase of the heat release at various heights in the flame relative to the burner exit velocity indicated a disturbance with a time-delay related to convection by the mean flow from the moment that it left the burner mouth. This gave rise to both positive and negative contributions to acoustic energy flux (the Rayleigh integral) from different heights in the flame, in some cases over as many as seven cycles. Thus. some thermo-acoustic eigen-frequencies were not strongly excited. even when local contributions were high. Conversely, the coincidence of a Helmholtz resonance (low acoustic pressure fluctuations associated with high acoustic velocity fluctuations) in the premixture supply system with a combustion chamber quarter-wave resonance (high acoustic pressures) made it impossible to stabilise the flame.

The thermo-acoustic response of a premixed swirl burner

“…The convection velocities are 2.0 AE 0.3 times the nominal exit velocity for all the configurations in which there was a modest swirler retraction, increasing to 2.7 due to the higher proportion of unswirled premixture and combustion near the axis with further retraction. When the swirler was seated, however, the earlier expansion also gave rise to an increased average convection velocity, of 3.5 U. Observations on a Bunsen flame by Buechner et al [4] apparently needed no such correction to the exit velocity, while those on an annular jet showed convection to be slightly slower than the mean burner exit velocity [15].…”

“…Most of the data in the investigations already mentioned were obtained by recording chemiluminescent emission as a marker of the heat release. This principle was also used here, building on the advanced techniques and analysis which have previously been deployed on a bluff-body stabilized flame [15,16]. These are briefly described and then the analysis of the acoustics is outlined, before the presentation of the results for the various flames.…”