Flame dynamics and unsteady heat release rate of self-excited azimuthal modes in an annular combustor

This paper presents an experimental study into the structure and dynamics of the phase-averaged heat release rate during self-excited spinning and standing azimuthal modes in an annular combustion chamber. The flame response was characterised using two methods: high-speed OH∗ chemiluminescence imaged above the annulus to investigate the structure of the phase-averaged fluctuations in heat release rate, and high-speed OH-PLIF measured across the centreline of two adjacent flames to investigate phase-averaged flame dynamics. Two-microphone measurements were obtained at three circumferential locations to determine the modes and the amplitude of the velocity fluctuations. It was found that the flame responds differently to spinning and standing wave modes. During standing wave modes, the amplitude of the unsteady heat release rate of each flame (sector) varied according to its location in the mode shape with maximum fluctuations occurring at the pressure anti-nodes and minimum fluctuations occurring at the pressure nodes. At the pressure anti-nodes, peak fluctuations result from the production of flame surface area by axisymmetric flame motions caused by the modulation of flow at the burner inlet by the pressure fluctuations. However, at the pressure nodes, an anti-symmetric, transverse flapping motion of the flame occurred producing negligible unsteady heat release rate over the oscillations cycle via the mechanism of cancellation. During spinning modes, the structure of the heat release rate was found to be asymmetric and characterised by the preferential suppression of shear layer disturbances depending on the spin direction.