Thermal versus acoustic response of velocity sensitive premixed flames

Premixed flames respond to velocity perturbations with fluctuations in heat release rate ("thermal response"), which in turn generate acoustic perturbations ("acoustic response"). The latter may subsequently influence the velocity field in such a manner that feedback leads to self-excited thermoacoustic instability. The present paper investigates interrelations between the thermal and the acoustic responses of premix flames. The analysis is formulated such that it properly represents the underlying causality of acoustics-flow-flame-acoustics interactions. A flame-intrinsic feedback loop is revealed, which is quite independent of the acoustic environment of the flame, i.e. the acoustic impedances of plenum and combustor. The eigenmodes of this flame-intrinsic feedback loop coincide with poles of the acoustic scattering matrix of the flame. The corresponding frequencies, where the acoustic response is maximum, are in general quite different from frequencies where the thermal response is strong, i.e. where the flame transfer function exhibits "excess gain". Even more remarkable, the intrinsic flame modes may result in thermoacoustic instabilities without lock-on to one of the acoustic eigenmodes of the combustor. Experimental results from two combustor test rigs with laminar conical as well as turbulent swirl flames are scrutinized and are found to confirm our analysis. In particular, unstable modes are identified that are strongly related to flame-intrinsic feedback.