Numerical Identification of a Premixed Flame Transfer Function and Stability Analysis of a Lean Burn Combustor

Combustion instabilities represent a long known problem in combustion technology. The environment-friendly lean premixed gas turbines exhibit an increased risk of occurrence of thermo-acoustically induced combustion oscillations. In the present work the stability of a lean premixed swirl-stabilized combustor, experimentally studied at Technische Universitӓt of Munich, has been investigated. The complex interaction between the system acoustics and the turbulent swirling flame is studied using unsteady CFD simulations with Flamelet-Generated Manifolds combustion model. Results were validated against experimental data. Perturbations are introduced in the system imposing a broadband excitation as inlet boundary condition. The flame response to the perturbation is then computed and described exploiting system identification techniques. The identified Flame Transfer Function (FTF) shows quantitative agreement with experiment for amplitude and phase, especially for the low frequency range. At higher frequencies the phase prediction slightly deteriorates while the gain is still well described. The obtained results are implemented into a finite element model of the combustor in order to analyze the stability of the system. Results are compared with available experimental data showing a satisfactory agreement. The advantage introduced by a more sophisticated model for FTF is further evidenced comparing the results with those obtained with analytical formulation found in literature.