Intrinsic thermoacoustic modes in an annular combustor

Thermoacoustic instabilities originate from the interaction between the unsteady heat release rate associated with flames and the acoustic field of a combustor. It was recently shown that thermoacoustic feedback can be established even without acoustic resonances, giving rise to so-called intrinsic thermoacoustic modes. While these modes have been investigated in single-flame configurations, their occurrence in annular combustion chambers, with multiple flames arranged around the circumference, has not been characterized yet.

We compute the spectrum of an annular combustor with a 3D thermoacoustic Helmholtz solver. The configuration is similar to those commonly found in gas turbine engines, and we use a generic flame response model. In addition to the observations in previous studies, we find numerous intrinsic modes in the frequency range covering the first few acoustic modes. Most of the intrinsic modes are organized in clusters, at multiples of the inverse flame response time delay. The structure of the intrinsic spectrum is shown to be linked to cut-off effects of the non-propagating azimuthal modes. The study indicates that intrinsic modes are generically present in annular combustors, even in large numbers, but have eluded previous studies for numerical reasons.