The prediction of the limit-cycle amplitude of thermoacoustically unstable practical gas turbine combustion systems remains a challenge for the gas turbine industry. The present study uses an experimentally obtained Flame Describing Function (FDF) for the determination of the thermoacoustic oscillation frequency and amplitude. In contrast to other studies, which investigated perfectly premixed laminar or marginally turbulent flames, this study deals with a highly turbulent swirl flame with spatial and temporal fuel–air unmixedness in an order of practical interest. A partially premixed swirl-stabilized flame is investigated at a Reynolds number of approximately 35 000. The Multi-Microphone-Method is used to determine the amplitude dependent transfer function of the flame as well as the transfer function of the burner and the acoustic response of the boundary conditions. The results are compared to OH* chemiluminescence measurements, which show a significant deviation in terms of the flame transfer function gain due to equivalence ratio fluctuations. The measured transfer functions are incorporated into a thermoacoustic modeling framework to determine frequency and amplitude of the self-excited limit cycle oscillation. Measurements were made for various lengths of the exhaust gas tube to verify the results for different frequencies and amplitudes. Good agreement is found for the entire range of combustor lengths investigated. The error between model and experimental results is thoroughly assessed.

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