Investigation of thermoacoustic instability in sequential combustor during first stage lean blow-off

The pressing need to increase the operational and fuel flexibility of gas turbines has recently led to the development of new combustor architectures such as constant-pressure-sequential-combustion systems. In this study, the thermoacoustic instabilities near the lean blow-off limit of the first stage in a generic sequential combustor was investigated. High-speed OH chemiluminescence was used to visualize the lean-premixed first stage swirled flame and the second stage autoignition flame, and piezo-sensors were employed to record the acoustic pressure around the first-stage blow-off condition. Two characteristic frequencies for the thermoacoustic oscillations were identified: one at 144 Hz when flames in both stages existed in the combustor and the other at 84 Hz after the blow-off of the first stage flame. A calculation of the acoustic modes of the sequential combustor, using a Helmholtz solver, showed that these two frequencies both corresponded to the second eigenmode of the combustion chamber. The sequence of thermoacoustic events associated with the first stage flame blow-off was unraveled for this generic setup, and this analysis could support interpreting the corresponding complex spectral signature in case such an event would occur in a practical system.