Thermoacoustic instabilities in axisymmetric annular combustors are generally coupled by degenerate azimuthal modes, resulting in periodic oscillations of standing, spinning or mixed nature. Symmetry breaking due to the presence of an azimuthal mean flow splits the degenerate thermoacoustic eigenvalues, resulting in pairs of counter-rotating modes with distinct frequencies and growth rates. In the present study, experiments have been performed using an atmospheric annular system, in which twelve ducts mimic the burners, and twelve identical electro-acoustic feedback (EAF) loops mimic the thermoacoustic coupling due to the unsteady flame response to acoustic perturbations. Each EAF-loop is composed of one microphone and one loudspeaker plugged into a duct. The pressure signal measured by the microphone is amplified and sent to a real-time controller (RTC), where the signal is modulated by a transfer function and a nonlinearity, which reproduce the linear and nonlinear response of the flame, respectively. We use an *n*-*τ* model with a low-pass filter, for various values of the gain, *n*, and the time delay, *τ*. The RTC output signal is amplified and sent to the loudspeaker, which generates acoustic fluctuations in the duct.  The absence of combustion processes allows us to perform experiments with a negligible level of noise, to easily vary the thermoacoustic coupling, and to close the annular cavity downstream with a hard wall, in order to have a reflective boundary condition acoustically similar to that of a chocked outlet. To introduce symmetry breaking due to azimuthal mean flow effects, we use twelve fans located in the annular chamber between each pair of annulus-duct connections, thus preserving the discrete rotational symmetry of the system.

We investigate the standing/spinning nature of the oscillations as a function of the azimuthal-flow Mach number, and how the stability of the system is affected by the mean flow. Experiments are performed at various values of the Mach number by varying the fan input power, for two different initial conditions. The first is an initial state without oscillations: the EAF is initially switched off, and then suddenly turned on at a given power during the experiment. In the second set of experiments, instead, we track the amplitude and stability of the limit cycle oscillations in a quasi-continuous fashion, by slowly increasing the Mach number from 0 to 0.025.  The process is then reversed, with the input power decreased towards 0, to study hysteresis effects.

It is found that spinning, standing or mixed modes can be encountered at very low Mach number. Instead, increasing the mean velocity tends to promote a spinning direction. At sufficiently high Mach number, periodic oscillations are only of spinning nature. For some values of the gain and time-delay, the initial conditions have a significant impact on the final state of the system. Interestingly, it is found that the presence of a mean azimuthal flow tends to reduce the amplitude of the oscillations. In some cases, this leads to a full suppression of the thermoacoustic oscillations, whereas in other cases another mode is destabilised by the presence of the mean flow and the oscillations persist, but at a different frequency. We discuss our findings in relation to existing theoretical models based on a low-order wave-based representation of the thermoacoustic interaction and the method of averaging for investigating nonlinear saturation effects.


Effect of an Azimuthal Mean Flow on the Structure and Stability of Thermoacoustic Modes in an Annular Combustor Model With Electroacoustic Feedback

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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