Abstract
In this paper we investigate self-excited azimuthal modes in an annular combustor with turbulent premixed bluff-body stabilised flames. Previous studies have shown that both swirl and equivalence ratio influence modal dynamics, i.e. the time-varying nature of the modes. However, self-excited azimuthal modes have not yet been investigated in turbulent flames without bulk swirl, which do not generate any preferential flow in either azimuthal direction, and may therefore lead to different behaviour. Joint probability density functions of the instability amplitudes at various flowrates and equivalence ratios showed a strong bi-modal response favouring both ACW and CW spinning states not previously observed. Operating conditions leading to a bi-modal response provide a unique opportunity to investigate whether the structure of the global fluctuating heat release rate of self-excited spinning modes in both directions exhibit similar dynamics and structure. This was investigated using high-speed OH* chemiluminescence images of the annular combustor and a new rotational averaging method was applied which decomposes the spinning components of the global fluctuating heat release rate. The new rotational averaging, which differs from standard phase-averaging, produces spatial averages in a frame of reference moving with the spinning wave. The results show that the structure of the fluctuating heat release rate for spinning modes is highly asymmetric as characterised by large, crescent shaped regions of high OH* intensity, located on the far side of each flame, relative to the direction of the azimuthally propagating pressure wave. In comparison with interacting swirling flames, these results indicate that the previously observed radial asymmetry of OH* fluctuations may be introduced through advection by local swirl.
Highlights
Thermoacoustic instabilities remain a crucial impediment in the design and operation of low emission combustors in aeroengines and gas turbines for power generation
In this paper we have demonstrated experimentally that self-excited azimuthal modes can be excited in an annular combustor using turbulent bluff body flames without swirl, enabling us to better understand the time-varying nature of azimuthal modes and the structure of the fluctuating heat release rate
Simultaneous pressure measurements and high speed OH∗ chemiluminescence were obtained for a range of operating conditions and showed that self-excited instabilities occur over a wide range of conditions
Summary
Thermoacoustic instabilities remain a crucial impediment in the design and operation of low emission combustors in aeroengines and gas turbines for power generation. Other investigations in annular combustors with turbulent swirling flames give rise to modal dynamics where, at a given operating point, the excited mode exhibits time-varying phase and amplitude, switching back and forth between spinning, standing and mixed modes [1,2,3,4,8,10]. In theoretical studies it was suggested, that this mode switching is related to the asymmetries in the heat release rate or flow [11,12,13]
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