  In this work, the thermo-acoustic instabilities of a gas turbine model
combustor, the so-called SFB606 combustor, are numerically investigated using
Large Eddy Simulation (LES) combined with tabulated chemistry and Artificial
Thickened Flame (ATF) approach. The main focus is a detailed analysis of the
thermo-acoustic cycle and the accompanied equivalence ratio oscillations and
their associated convective time delay. In particular, the variations of the
thermo-chemical state and flame characteristics over the thermo-acoustic cycle
are investigated. For the operating point flame B ($P_{th}=25\,$kW), the burner
exhibits thermo-acoustic instabilities with a dominant frequency of 392Hz, the
acoustic eigenmode of the inner air inlet duct. These oscillations are
accompanied by an equivalence ratio oscillation, which exhibits a convective
time delay between the injection in the inner swirler and the flame zone. Two
LES, one adiabatic and one accounting for heat losses at the walls by
prescribing the wall temperatures from experimental data and Conjugated Heat
Transfer (CHT) simulations, are conducted. Results with the enthalpy-dependent
table are found to predict the time-averaged flow field in terms of velocity,
major species, and temperature with higher accuracy than in the adiabatic case.
Further, they indicate, that heat losses should be accounted for to correctly
predict the flame position. Subsequently, the thermo-chemical state variations
over the thermo-acoustic cycle for the enthalpy-dependant case are analyzed in
detail and compared with experimental data in terms of phase-conditioned
averaged profiles and conditional averages. An overall good prediction is
observed. The results provide a detailed quantitative analysis of the
thermo-acoustic feedback mechanism of this burner.
