Thermoacoustic instability in a sequential combustor: Large eddy simulation and experiments

This paper presents a large eddy simulation (LES) of a generic sequential combustor that was operated at atmospheric pressure and that features a thermoacoustic instability at 145 Hz. The full test rig consisting of a first stage plenum, burner and combustion chamber, of an air dilution section and of a sequential stage combustor was modeled. A compressible reactive 3-D LES was performed with semi-detailed chemistry accounting for different combustion modes which are all involved in the sequential combustor flame dynamics. These modes are flame propagation at ambient and elevated temperatures, and autoignition. The simulation is compared to experiments that featured hydroxide planar laser-induced fluorescence (OH-PLIF), OH∗ chemiluminescence optical diagnostics, and acoustic pressure measurements. Both flames show strong oscillatory dynamics that are remarkably well captured by the LES. The mode shape of the self-excited instability extracted from the LES pressure signals is also in very good agreement with the experiment. An analysis of the LES sequential flame dynamics shows that autoignition lengths in the mixing section are strongly modulated due to self-sustained out-of-phase oscillations of mixing temperature and axial velocity, which lead to strong oscillations of the reaction zone location.