Modern stationary gas turbines potentially suffer from thermoacoustic instabilities due to emission constraints and the resulting need for lean premixed combustion. In the development process, thermoacoustic characteristics are assessed with combustion tests employing acoustic excitation. In this work an actuator principle is developed that allows for high amplitude, phase accurate excitation at multiple frequencies simultaneously, while being able to operate in the harsh environment of an industrial combustion test rig. It is based on a proportional valve for gas flow modulation that injects pressurized air into a nozzle-like section downstream of the combustion chamber. In this section the main air flow is accelerated, which simulates the first turbine stage. Injection of modulated air causes blockage of the exhaust gas flow, generating high sound pressure amplitudes. As a demonstration of the effectiveness of the proposed approach, the actuator is used to control combustion instability with a closed-loop algorithm. A reduction of over 55 % of the oscillation amplitude is achieved. Additionally, it is shown that the modulated air injection inside the nozzle throat generates similar sound pressure amplitudes as a siren-type actuation with the same stationary blockage level. The amplitude can be further increased by inclining the injection axis towards the incoming main air flow.

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