Large Eddy Simulation of Thermoacoustic Oscillations in an Annular Combustor Under EGR Conditions: Mode Decomposition Analysis

Exhaust gas recirculation (EGR) is a key technology to improve the efficiency of carbon capture in industrial gas turbine systems. However, lean combustion under elevated EGR levels can significantly increase the susceptibility to thermoacoustic instability. In this study, large eddy simulations (LES) are performed on a full-annular gas turbine combustor model for Solar Turbines’ Taurus 60 engine, in comparison with a $60^\circ$ sector combustor model. The LES results reveal that increasing EGR leads to a more diffuse and unsteady flame structure, accompanied by enhanced flame fluttering. A power spectrum density (PSD) analysis of the pressure signal of the high EGR case identifies a dominant longitudinal instability at $St=0.2$ in both models, while an additional mode at $St=0.62$ is captured exclusively in the full-annular model. These dominant frequencies show good agreement with the experimentally observed instability bands. Dynamic mode decomposition (DMD) is then applied to pressure snapshots at various locations. The DMD analysis reveals that the low frequency mode corresponds to a spatially uniform beating pattern, while the higher frequency mode exhibits a circumferential pattern. This circumferential mode can be only captured using the full-annular combustor geometry, underscoring the importance of full-domain simulations in accurately predicting thermoacoustic instabilities.