Analysis of High-Frequency Dynamics of a Reacting Jet in Crossflow Based on Large Eddy Simulation

Abstract

Distributed combustion systems have shown the potential to reduce emissions as well as increase load and fuel flexibility. A characteristic feature of such systems is a reacting jet in crossflow, which exhibits complex vortical structures. In this paper, a generic combustion chamber with elliptic reacting jets in crossflow is examined, operating under lean-premixed conditions at elevated pressure and exhibiting high-frequency transverse mode shapes. It can be seen that depending on the orientation of the elliptical shape of the jet to the crossflow, thermoacoustic modes can be suppressed. A multidimensional Fast Fourier Transform shows that low aspect ratios (major axis of the jet aligned with the crossflow) result in the mixed 1L1T mode of first longitudinal and first transverse structure, while this mode disappears at high aspect ratios. To get a more detailed insight into the different vortex systems of the various aspect ratios, Dynamic Mode Decomposition is applied. This modal decomposition technique reveals for low aspect ratios a shear layer mode that oscillates at a frequency close to the acoustic mixed mode. For this configuration, a mode representing a flapping motion is also identified. For high aspect ratios, the shear layer vortex increases its frequency and a higher-frequent mode appears in the acoustic spectrum.

Investigation of high-frequency thermoacoustic instabilities in a FLOX® burner using large-eddy simulation and multi resolution proper orthogonal decomposition

“…High-frequency pressure oscillations (i.e. frequencies above 1000 Hz [1]) appear to be exceptionally harmful as they can cause high-frequency vibrations in the mechanical structure of the combustion system which lead ultimately to a rapid fatigue of the material. An example of such destructive instabilities is the "screech" phenomenon in aero-engines [2].…”

“…A review on this topic with emphasis on gas turbine combustion is given in [3] and in particular for rocket engine combustion numerous LES studies exist on high frequency oscillations [4][5][6][7][8][9][10][11][12][13]. However, only few studies [1,[14][15][16][17] address to our knowledge explicitly the computation of self-excited, high-frequency instabilities (HFI) in gas turbine combustors. The present work extends one of these earlier works which is concerned with scale-adaptive simulations of HFI in a FLOX ® gas turbine burner [15].…”