59540 - Self-Excited High-Frequency Transverse Limit-Cycle Oscillations and Associated Flame Dynamics in a Gas Turbine Reheat Combustor Experiment 

This paper presents the investigation of high-frequency thermoacoustic limit-cycle oscillations in an experimental gas turbine reheat combustor featuring both auto-ignition and propagation stabilised flame zones at atmospheric pressure. Dynamic pressure measurements at the faceplate of the reheat combustor reveal high-amplitude periodic pressure pulsations at 3 kHz in the horizontal direction of the rectangular cross-section combustion chamber. This frequency corresponds to the first transversal mode in this orientation. Further analysis of the acoustic signal in terms of probability density functions of its bandpass filtered time trace and envelope amplitude shows that this is a thermoacoustically unstable condition undergoing limit-cycle oscillations. A sensitivity analysis is presented which indicates that these high-amplitude limit-cycle oscillations only occur with a particular geometric configuration and at certain operating conditions: namely high power settings with propane addition to increase auto-ignition propensity. This auto-ignition dependency is an early indicator that this instability could be the result of an auto-ignition based driving mechanism. The spatiotemporal flame dynamics are then investigated using CH* chemiluminescence, phase-locked to the dynamic pressure, captured from all lateral sides of the reheat combustion chamber. The top-down and rear views, both perpendicular to the orientation of the unstable mode, reveal strong heat release oscillations close to the chamber walls at the instability frequency. Axial movement of the flame tips in these regions is also observed. Both the heat release oscillations and the axial flame tip modulation occur in-phase with the acoustic pressure mode. This could be a further indicator of a driving mechanism resulting from coupling of the acoustic pressure oscillations and the pressure-dependant auto-ignition flame. Identification of high-frequency thermoacoustic driving mechanisms in reheat combustors is a key step towards the development of predictive approaches to mitigate the impact of such instabilities in future generations of gas turbine combustors with sequential combustion systems.