High-Frequency Instabilities in Cylindrical Flame Tubes: Feedback Mechanism and Damping

Thermoacoustic instabilities are a major concern in gas turbine combustion chambers today. In the last decades research interest in thermoacoustic instabilities has focused on low frequencies. The feedback mechanisms related to longitudinal modes are for the most part understood. Transverse modes, though, have not been studied to a large extent in the past. However, interest has been rising in the last few years. But little is known about the thermoacoustic feedback of high-frequency instabilities. Our previous publications characterized the flow and the flame at the eigenfrequency of high-frequency instabilities. There, a feedback mechanism was derived from the experimental results and discussed: the acoustic velocity leads to a periodic displacement of the flame resulting in a positive contribution to the Rayleigh criterion. Thus, the thermoacoustic feedback couples to the acoustic velocity, but not to the pressure or a periodic vortex formation. Different means can be derived from the model to influence high-frequency instabilities: Helmholtz dampers are used to shift the onset of instabilities to increased thermal power. With loudspeakers naturally stable operating points are excited. Stopping the excitation and evaluating the signal, decay rates are analyzed. Decay rates — i.e. stability margins — are compared for different operating conditions. Switching from perfect premixing to technical premixing, the radial profile of the fuel-to-air ratio can be changed. The influence of a lean core flow compared to a homogeneous mixture on the feedback is investigated and its impact on the instabilities is compared to the model. The observations reflect, what is predicted by the model. Velocity coupling, at least a significant part of the feedback mechanism for transverse high-frequency instabilities, is supported by the experimental results.