Swirling flow is often employed in gas turbine combustion chambers for the sake of improving flame stability. Swirling flow induces not only recirculation zones but also large coherent structures, which show close relationship with flow dynamics and combustion instability. The flow dynamics including precessing vortex core (PVC) in simple swirlers is extensively studied, while the flow instability characteristics in a multiswirler combustor are not fully reported. In this paper, large eddy simulation (LES) of nonreacting turbulent swirling flow is conducted in a multiswirler burner, which comprises a pilot stage and a main stage. Flow dynamics in the multiswirler combustor are analyzed based on phase-averaged evolution of instantaneous flowfield. LES results are compared with particle image velocimetry (PIV) data in terms of mean and root mean square (RMS) velocities. Proper orthogonal decomposition (POD) is employed to identify the coherent structures in the multiswirling flow. Results show that LES results are in good agreement with particle image velocimetry (PIV) data. Main stage and pilot stage flow interact with each other generating highly turbulent swirling flow. PVC is successfully captured at the boundary of main recirculation zone (MRZ) in the pilot stage with a dominant frequency of 1915 Hz. The PVC leads to periodic azimuthal flow instability. POD analyses for the velocity fields show dominant high-frequency modes (modes 1 and 2) in the pilot stage. However, the dominant energetic flow is damped rapidly downstream of the pilot stage that it has little effect on the main stage flow.

Thermoacoustic instability is a major issue in developing high-efficiency low emission gas turbine combustors. In order to predict the amplitude of limit cycle oscillation, an understanding of the amplitude dependent response of the flame, i.e. the nonlinear response, to large acoustic excitation is needed. In the present study, the nonlinear response of a low-swirl CH4/air premixed flame to acoustic excitation is experimentally studied. Amplitude dependences of flame dynamic at 75 Hz and 195 Hz are discussed in detail over a wide range of excitation level. Experimental results show the gain of flame describing function of the low-swirl flame has a peak value at 65 Hz and a local minimum at 105 Hz which is caused by the destructive (out of phase) and constructive (in phase) of the axial and azimuthal velocity fluctuation. At low perturbation level, flame heat release fluctuation is in linear relationship with the normalized velocity driving level. Heat release fluctuation begins to saturate at a certain level which depends on the driving frequency. The low-swirl flame oscillates mainly in the axial direction at 75 Hz while it is in the radial direction at 195 Hz. The non-linear flame heat release response is a result of combination effect of flame rollup process and harmonic responses.

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