Thermoacoustic instabilities in gas turbine engines are of increasing interest for design of future low-emission technology combustors. Numerical methods used to predict thermo-acoustic instabilities often employ either fully compressible resource-intense Computational Fluid Dynamics (CFD) simulations, or two-step approaches where source terms extracted from CFD simulations are supplied to subsequent Computational Aero Acoustics (CAA) simulations. The present work demonstrates an analysis of self-excited thermoacoustic instabilities using a hybrid method based on runtime coupling of incompressible CFD and CAA simulations. A prior implementation for the computation of combustion noise through one-way coupling of CFD and CAA solvers has been extended to a two-way coupling that allows acoustics to influence the flow field. The resulting hybrid method couples acoustic and convective physical phenomena by using mean flow field quantities and the thermoacoustic energy source term obtained from the solution of the Navier-Stokes equations in the low Mach number limit as an input for the Acoustic Perturbation Equations (APE). The fluctuating acoustic quantities pressure and velocity obtained from the solution of the APE are in turn included in the solution of the low Mach number Navier-Stokes equations, influencing the convective dynamics of the flow field. The suitability of the present implementation to capture thermoacoustic feedback is demonstrated through analysis of the well-known self-excited instability observed in a Rijke Tube. The results are presented and discussed in comparison with a fully compressible reference solution.

During development of modern lean burn aero engine combustors, prediction and characterization of injector thermoacoustic response plays an important role. The SCARLET (SCaled Acoustic Rig for Low Emission Technologies) test rig fulfills this need by providing acoustically defined upstream and downstream conditions under realistic operating conditions to characterize the acoustic response of full-scale injectors in a single sector setup. This work presents results from the complementary numerical analysis campaign that aims to reproduce the acoustic scattering behaviour as measured in the SCARLET rig. As a first step, non-reacting flow simulations were performed using fully compressible CFD simulations and the CAA (computational aero acoustics) method to compute upstream and downstream acoustic fields. The results obtained are then post-processed based on a multi-microphone method to yield the acoustic scattering matrix, which follows the employed experimental post-processing strategy. Results obtained from both methods show satisfactory agreement with measurements. Geometrical simplifications and their implications on scattering behaviour are investigated to achieve a trade-off between accuracy and geometrical complexity. Both methods are compared in terms of their inherent advantages and disadvantages. Limitations associated with employed models and possible improvements are discussed.

Thermoacoustic instabilities in gas turbine engines are of increasing interest for design of future low-emission technology combustors. Numerical methods used to predict thermo-acoustic instabilities often employ either fully compressible resource-intense Computational Fluid Dynamics (CFD) simulations, or two-step approaches where source terms extracted from CFD simulations are supplied to subsequent Computational Aero Acoustics (CAA) simulations. The present work demonstrates an analysis of self-excited thermoacoustic instabilities using a hybrid method based on runtime coupling of incompressible CFD and CAA simulations. A prior implementation for the computation of combustion noise through one-way coupling of CFD and CAA solvers has been extended to a two-way coupling that allows acoustics to influence the flow field. The resulting hybrid method couples acoustic and convective physical phenomena by using mean flow field quantities and the thermoacoustic energy source term obtained from the solution of the Navier-Stokes equations in the low Mach number limit as an input for the Acoustic Perturbation Equations (APE). The fluctuating acoustic quantities pressure and velocity obtained from the solution of the APE are in turn included in the solution of the low Mach number Navier-Stokes equations, influencing the convective dynamics of the flow field. The suitability of the present implementation to capture thermoacoustic feedback is demonstrated through analysis of the well-known self-excited instability observed in a Rijke Tube. The results are presented and discussed in comparison with a fully compressible reference solution.