Acoustic and intrinsic thermoacoustic modes of a premixed combustor

Premixed flames are velocity sensitive, i.e., they react to a velocity perturbation at the burner mouth, say, with fluctuations in heat release rate. Unsteady heat release generates acoustic waves that travel back from the flame to the burner mouth, where they modulate the velocity and thereby close an intrinsic thermoacoustic (ITA) feedback loop. The present paper demonstrates that corresponding ITA eigenmodes are in general important for the dynamics and stability of premixed combustion systems. It is shown that the complete set of eigenmodes of a combustor test rig should be interpreted as the sum of acoustic and ITA eigenmodes. A procedure is presented which allows to distinguish between eigenmodes that may be considered as acoustic modes driven by the flame, versus those resulting from ITA feedback (but influenced by the acoustic properties of the combustor). This procedure is based on a factorization of the dispersion relation of the thermoacoustic model. 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.