Acoustic and intrinsic thermoacoustic modes of a premixed combustor

What is it about?

Thermoacoustic combustion instabilities are a severe challenge for the development of reliable, low-emission and flexible combustion technology for propulsion and power generation. These self-excited instabilities result from feedback between flow, flame and acoustics. Unless effectively controlled, they can lead to limited operating range, increased emissions, or even rapid destruction of a combustion systems

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Why is it important?

Intrinsic thermoacoustic (ITA) modes results from a flow/flame/acoustic feedback mechanism that has escaped attention until recently. The ITA feedback loop may be described as follows: An upstream velocity disturbance induces a modulation of the heat release rate, which in turn generates an acoustic wave traveling in the upstream direction, where it influences the acoustic velocity and thus closes the feedback loop. The resonances of this feedback dynamics, which are identified as ITA eigenmodes of the flame, have important consequences for the dynamics and stability of the combustion process in general and the flame in particular. The present paper shows that the complete set of thermoacoustic eigenmodes of a combustor is the aggregate of acoustic and ITA eigenmodes. A procedure is presented which allows to distinguish between eigenmodes that may be considered as acoustic "cavity modes" driven by the flame, versus those resulting from ITA feedback, and thus to some extent independent from the acoustic environment. Differences between the acoustic and intrinsic eigenmodes of a combustor test rig, in particular the corresponding mode shapes, are discussed. The paradoxical observation that increased acoustic losses at the boundaries may destabilize a combustion system is explained as an instability of the dominant ITA mode.

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