System identification of a large-scale swirled partially premixed combustor using LES and measurements

The forced response of a swirled partially premixed burner to acoustic excitations is computed with Large Eddy Simulations and compared to measurements performed at University of Karlsruhe. The configuration is a 1:1 representation of a prototype industrial burner provided by Siemens in which gases are injected through two complex-geometry swirlers. Thanks to hybrid meshes and parallel computations, the simulation integrates a large part of this complex geometry. The inlet of the largest swirler is numerically forced for three acoustic frequencies (80, 120 and 250 Hz) and the LES data is compared to experimental results in terms of combustion phase. LES also give access to the forced flow topology, revealing that the excitation creates large toroidal vortices which distort the flame surface and destabilise downstream of the flame. In addition to these toroidal vortices, LES also show that a precessing vortex core appears when excitation is started. Finally maps of combustion phase and response amplitude are extracted from LES and highlight the mechanisms controlling the flame behaviour under excitation. The phases obtained by LES are also compared to usual “convective phase” estimates used in Reynolds Averaged codes. This comparison shows that LES provide a better estimation of the phase between heat release and inlet velocity fluctuations.

Key Words:

• Gas Turbines
• Acoustics
• Large Eddy Simulations

PACS numbers:

• 00.00
• 20.00
• 42.10

11. Acknowledgments

The support of Siemens is gratefully acknowledged. Certain computations were performed at CINES (Centre Informatique National de l'Enseignement Superieur).

Notes

^‡ At low forcing frequencies, the flame response is quasi-steady. Moreover, LES of low frequency cases become expensive as characteristic physical times increase. This explains why LES were not performed at frequencies smaller than 80 Hz.

^§ Note that this normalisation is usually not available in “flight time” method, because the local n _w values of the flame transfer function are needed.