Scale Adaptive Simulation of a thermoacoustic instability in a partially premixed lean swirl combustor

Abstract

The thermoacoustic oscillation of a turbulent, swirl-stabilized, partially premixed flame in the PRECCINSTA gas turbine model combustor is analyzed by means of a Scale Adaptive Simulation (SAS) method. In the critical regions of the combustor, the SAS features a fine spatial resolution and thus corresponds to a Large Eddy Simulation (LES). Two combustion models are applied, a simple eddy dissipation model and a detailed finite rate chemistry (FRC) model. A previous LES for the same combustor by Franzelli et al. (2012) indicated that the acoustic impedance of the fuel supply plays a critical role. Therefore in the present work, the fuel channels and fuel plenum are included in the computational domain and thereby the fuel inlet impedance is inherently taken into account. The resulting fields of velocity, temperature and mixture fraction fit well to experimental data with a slightly better agreement for the detailed FRC model. For both combustion models the computed frequency of the thermoacoustic oscillation is close to the experimental value, whereas its amplitude is significantly overestimated by about 15 dB in comparison to measurements. The reason for this overestimation is analyzed using an additional measurement where acoustic damping due to vibrating side walls is suppressed. For the latter experiment both frequency and amplitude agree well with CFD results, which indicates that acoustic damping effects must be carefully taken into account for validation of CFD. The 3D time-resolved simulations further provide detailed insights into the interaction of flow and mixing in the swirler, which leads to a convective time-lag between oscillations of velocity and equivalence ratio in the flow of unburned gas that largely affects the heat release response of the flame. Taken together, the results show that SAS computations can accurately reproduce frequency and amplitude of thermoacoustic oscillations of turbulent partially premixed flames in a gas turbine combustor provided that proper modeling of fuel supply and acoustic boundary conditions is applied.