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Vibro-acoustical instabilities induced by combustion dynamics in gas turbine combustors

Pozarlik, Artur
Publication year: 2010

Abstract: The lean premixed combustion suffers from a high sensitivity to thermo-acoustic instabilities which may occur in a combustion chamber of a gas turbine. The high level of acoustic excitation is hazardous to the combustion chamber walls (liner). The situation is even worse when mutual interaction between thermo-acoustic instabilities and liner vibration is present; then both processes may enhance each other. This behaviour reduces the life time of the gas turbine significantly. Therefore, the possibilities of thermo-acoustic instabilities to appear and their interaction with vibrating walls must be
predicted in advance to avoid combustion system destruction. This multi-phenomena interaction is presented and studied in this thesis. The experimental and numerical techniques are employed to investigate the interaction between coupled fields. The experimental part of the study is done on the laboratory scale combustion test rig, which mimics the combustion conditions as encountered in the full scale gas turbine. Experiments are performed at operating conditions, which differ with respect to power and absolute pressure, using two different liner configurations. The obtained results are used for validation of the numerical models. In the fluid-structure interaction analysis (FSI), the thermo-acoustic instabilities are correlated with walls vibration using partitioning approach. Here, two numerical solvers applying CFD (Ansys-CFX) and FEM (Ansys-Multiphysics) are employed to calculate phenomena occurring in the fluid and structural domain, respectively. These solvers exchange information about mechanical loads and structural displacement every time step through the interface connection created
between them. Both one-way and two-way data transfer is studied. For the acousto-elastic analysis (AE) a hybrid approach is used. First the combustible flow is calculated by CFD and latter a pressure data from the near-flame region is transferred to FEM code as the input conditions. This solution allows solving
acoustics inside the combustion chamber more precise than the FSI model, but in costs of only one-way interaction between pressure waves and flame. Additionally, a modal analysis of acoustic, structural and coupled modes is performed. The results of the numerical investigations have shown a good agreement with experimental data. Both models were able to predict correctly the frequencies of thermo-acoustic instabilities and liner vibration.