Self-excited longitudinal and azimuthal modes in a pressurised annular combustor

Abstract

A new laboratory scale pressurised annular combustion experiment is introduced and used to generate selfexcited longitudinal and azimuthal instabilities. The experiments are operated at mean pressuresranging from approximately 2 to 3 atmospheres in order to maintain a well defined acoustic boundary at exit. A range of operating conditions is studied parametrically, and it is observed that at high equivalence ratios, the flame stabilisation location propagates upstream, significantly altering the flame structure. The change in flame stabilisation location promotes a transition from a dominant longitudinal to a dominant azimuthal instability. Investigation of the azimuthal instabilities highlights a rich array of frequency content, with significant amplitude pressure and heat release responses observed for not only the fundamental (n = 1), but also higher harmonics (n = 2, 3). These higher harmonics are also shown to exhibit distinct characteristic modal dynamics, shown through probability density functions of the spin ratio. The flame dynamics for three distinct operating states, corresponding to longitudinal modes at two different stabilisation locations, and one corresponding to strong azimuthal modes are studied. These highlight the difference between longitudinal and azimuthal modes, and demonstrate the presence of significant higher harmonic content. The characterisation of both longitudinal and azimuthal modes in a pressurised laboratory scale annular combustor for the first time provides a unique opportunity for understanding the nature of such instabilities in practically relevant configurations.