The interaction of combustion pressure oscillations and liner vibrations

Gas turbine combustors have at industrial scale a thermal power released by combustion of 1 to 400 MW. As the flames in these combustors are very turbulent, the combustion generates high levels of thermo acoustic noise. Of crucial importance for the operation of the engine is not the noise emitted, but its structural integrity. This may be at hazard when the combustor liner starts to vibrate in a mode linked to the thermo acoustic noise. This is even more likely when the combustion noise changes to an unstable closed loop feed back system. Another dangerous situation may arise when there is a two way interaction between the combustion oscillations and the liner vibration. For these reasons the understanding of transient combustion and its coupling with wall vibration in a typical gas turbine combustion chamber is of prime interest. This phenomenon is investigated in the project FLUISTCOM in both experimental and numerical work. In the project a liner was designed with a thin, flexible section with a significant amplitude response on changes in the pressure field caused by the combustion oscillations. Numerical calculations of eigenfrequencies and eigenmodes were performed, followed by transient numerical calculations of the transient combusting flow within the combustion chamber with the use of CFX-ANSYS. The flame investigated was a 1.5 bar, 150 kW premixed natural gas flame. Solutions for the pressure field obtained during numerical computations of the combustor flow were collected and implemented in the structural code (Ansys) as surface loads on the liner side. Results show the one way response of the liner structure as a result of the transient pressure generated by the combustion of the gas flow. The paper will present the predicted results on the combustion field, the accompanying oscillating pressure field, and the induced structural vibration of the combustor liner as predicted by the finite element structural code.