In this work the thermoacoustic analysis of a full annular aero-engine combustor developed by AVIO is presented and discussed. The system is equipped with an advanced PERM (Partially Evaporating and Rapid Mixing) injection system based on a piloted lean burn spray flame generated by a pre-filming atomizer. Combustor walls exploit multi-perforated liners to control metal temperature; these devices are also recognized as potentially effective sound absorbers, thus they could be used for both wall cooling and damping combustion instabilities. The performed analysis is based on the resolution of the eigenvalue problem related to an inhomogeneous wave equation which includes a source term representing heat release fluctuations using a three-dimensional FEM code. A model representing the entire combustor has been assembled including all the acoustically relevant geometrical features with particular attention to the acoustic effect of multi-perforated liners which were modelled by assigning to the corresponding surfaces an equivalent internal impedance. Different models for multi-perforated liner impedance have been assessed and evaluated by numerically reproducing an experimental test rig developed at the University of Florence to measure the adsorption properties of cylindrical perforated walls. Different simulations of the full annular combustor with and without the presence of the flame have been performed analysing the influence of the multi-perforated liners. Comparisons with available experimental data showed a good agreement in terms of resonant frequencies in the case of passive simulations. On the other hand, when the presence of the flame is considered, comparisons with experiments showed the inadequacy of FTFs commonly used for premixed combustion and thus the necessity of an improved FTF, more suitable for liquid fueled gas turbines where the processes related to droplet evolution could play an important role in the coupling between acoustic field and heat release fluctuations.
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