High-Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors—Part II: Modeling and Analysis

Abstract

This paper deals with high-frequency (HF) thermoacoustic instabilities in swirl-stabilized gas turbine combustors. Driving mechanisms associated with periodic flame displacement and flame shape deformations are theoretically discussed, and corresponding flame transfer functions (FTF) are derived from first principles. These linear feedback models are then evaluated by means of a lab-scale swirl-stabilized combustor in combination with part one of this joint publication. For this purpose, the models are used to thermoacoustically characterize a complete set of operation points of this combustor facility. Specifically, growth rates of the first transversal modes are computed, and compared against experimentally obtained pressure amplitudes as an indicator for thermoacoustic stability. The characterization is based on a hybrid analysis approach relying on a frequency domain formulation of acoustic conservation equations, in which nonuniform temperature fields and distributed thermoacoustic source terms/flame transfer functions can be straightforwardly considered. The relative contribution of flame displacement and deformation driving mechanisms–i.e., their significance with respect to the total driving–is identified. Furthermore, promoting/inhibiting conditions for the occurrence of high frequency, transversal acoustic instabilities within swirl-stabilized gas turbine combustors are revealed.

Spontaneous and explicit symmetry breaking of thermoacoustic eigenmodes in imperfect annular geometries

“…(2017) and of Hummel et al. (2017), the response of a non-compact swirled flame to transverse acoustic excitation was investigated for the case of high frequency thermoacoustic instabilities. However, since a simple model, which only involves the acoustic pressure load across the burners, allows us to satisfactorily reproduce and explain the main experimental observations, we do not include the additional possible effect of the azimuthal acoustic velocity in our flame response model.…”

“…O'Connor & Lieuwen 2011; Acharya et al 2014;Saurabh & Paschereit 2017;Smith et al 2018;Li et al 2019) where combinations of axial and transverse acoustic forcing of compact swirled turbulent flames were considered. Besides, it can be noted that, in the work of Berger et al (2017) and of Hummel et al (2017), the response of a non-compact swirled flame to transverse acoustic excitation was investigated for the case of high frequency thermoacoustic instabilities. However, since a simple model, which only involves the acoustic pressure load across the burners, allows us to satisfactorily reproduce and explain the main experimental observations, we do not include the additional possible effect of the azimuthal acoustic velocity in our flame response model.…”

Impact of the heat release distribution on high-frequency transverse thermoacoustic driving in premixed swirl flames

“…During the review, the subject of this work was continued and extended in Berger et al, 18 and Hummel et al 19,20 …”

Injector-coupled transverse instabilities in a multi-element premixed combustor

“…21,22 The local heat release profile, which directly impacts the local Rayleigh integral, determines which of these mechanisms play a dominant role in the driving of transverse instabilities. [23][24][25] Transverse to longitudinal coupling is another pathway that has been widely studied. In this mechanism, the fluctuating transverse pressure field in the combustion chamber causes axial pressure disturbances at the end of the injector.…”

“…21,22 The local heat release profile, which directly impacts the local Rayleigh integral, determines which of these mechanisms play a dominant role in the driving of transverse instabilities. 23–25…”