Investigation of Mean-Flow Effects on Tubular Combustion Chamber Thermoacoustics Using a Burner Transfer Matrix Approach

The paper presents a methodology to account for local mean-flow effects on thermo-acoustic instabilities to improve typical thermo-acoustic calculations generally performed under the zero-Mach number assumption. A 3D FEM model of a simplified combustor is solved in COMSOL Multiphysics with the pressure acoustics module. The Helmholtz equation is used to model the combustor and the classical k-τ model for the Flame Transfer Function (FTF) is adopted. In order to account for local non-zero Mach number effects in the burner region, the burner is replaced with its transfer matrix (BTM), computed through the aero-acoustics module considering an assigned mean-flow, and which implicitly takes into account the mentioned effects. The obtained matrix is inserted in the FEM model of the simplified combustor. The BTM ability to represent local mean-flow effects and the impact on the resonant frequencies and their growth rate is then evaluated comparing the results with those provided by an in-house 1D code solving the linearized Navier-Stokes equations in the presence of a mean flow.