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Thermoacoustic Modification of Test Rigs

In the development of new gas turbines, low emissions and increased efficiency are two of the main goals. Modern gas turbines therefore rely on lean premixed combustion, which has the downside of potentially rendering the combustion process unstable. So-called thermoacoustic instabilities are high amplitude pressure fluctuations that can arise when pressure fluctuations and unsteady heat release of the flame are in constructive interference. These instabilities can deteriorate the efficiency as well as the emissions and can be the reason for fatigue failures.

Therefore, susceptibility towards thermoacoustic instabilities needs to be assessed during the development of new combustor systems, which can thus far not be done in a reliable fashion without any experimental investigations. As the susceptibility of a burner system towards thermoacoustic instabilities needs to be found for the real engine, the test rig has to resemble the engine more or less depending on the type of investigation. If the system is partially modelled, the test rig may be of a generic type, which simplifies the implementation of sensors and actuators. In the project at hand the geometry of the test rig is kept close to the engine design.

Measurements for determining thermoacoustic stability properties require acoustic actuators that can generate high sound pressure amplitudes. Pressure sensors are used to measure the acoustic field upstream and downstream of the flame, which e.g. is the basis for a stability analysis. Sensors and actuators need to provide high accuracy on one hand and have to be robust enough for the harsh environment inside the test rig on the other hand. Furthermore, the acoustic boundary condition of the combustion chamber shall be actively modified, to directly assess susceptibility towards thermoacoustic instabilities. To that end, a phase-accurate actuator needs to be developed.

The project is carried out in cooperation with Siemens AG.

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