Abstract

Dry Low Emissions (DLE) systems are well-known to be susceptible to thermoacoustic instabilities. In particular, transverse, spinning modes of high frequency can lead to severe hardware damage in a matter of seconds. The thermoacoustic response of an engine is specific to the combustor geometry, operating conditions and difficult to reproduce at the lab-scale. In this work, details of high frequency dynamics observed during the early development phase of a new DLE system are provided, where a multi-peaked spectrum was noticed during rig-testing. Beginning with an analysis of the measured pressure spectra and the natural resonance frequencies of three different concepts, an analytical model of the clockwise and anti-clockwise transverse waves was fitted to the experimental data using a non-linear curve fitting approach to produce a simple yet useful understanding of the phenomena. A flamelet-based Large Eddy Simulation (LES) of the entire system was used to complement this analysis and confirm the mode shapes using dynamic mode decomposition (DMD). Both approaches independently identified a spinning second order mode as the dominant one in the high frequency regime. The LES indicated that precessing vortex core dynamics tend to impose a deformation (in the circumferential direction) on the outer shear layer vorticity magnitudes. Flame anchoring in this shear layer is suggested as a possible cause for the onset of instability. With regard to modeling sensitivities, it is shown that sub-grid scale combustion modeling has a strong impact on predicted amplitudes. Ultimately, a thickened-flame model with a modified efficiency function provided consistent results.

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