A siren is a robust fast-valve that generates effective flow pulsations and powerful noise levels under well-controlled conditions. It operates under the inlet flow conditions of a gas turbine combustor. Its principle is based on a sonic air jet periodically sheared by a cogged wheel rotating at a given speed. It is used as an alternative to loudspeakers in combustion laboratories when the use of these is made difficult by aggressive flow conditions, such as hot air under pressure, possibly containing impurities. It is also a serious candidate as an effective flow actuator to be deployed on power gas turbine fleets. The authors have gathered more than twenty years of knowledge on siren technology. This pulsator was originally developed for research on thermoacoustics. By scanning through a given frequency range, one detects the acoustic resonance of specific parts of the combustor assembly, or possibly triggers a combustion instability during a sensitivity analysis of a flame to small perturbations. In 2010, Giuliani et al. developed a novel siren model with the capacity to vary the amplitude of pulsation independently from the frequency. In this contribution, the physics, the metrics, and the resulting parameters of the pulsator are discussed. Technical solutions are unveiled about visiting large frequency ranges (currently 6 kHz) and achieving elevated pressure fluctuations (150 dB SPL proven, possibly up to 155 dB SPL) with a compact device. A multimodal excitation is available with this technology, one idea being to dissipate the acoustic energy on nearby peaks. The contribution ends with a summary of the applications performed so far and the perspective of an industrial application.
A siren is a robust fast-valve that generates effective flow pulsations and powerful noise levels under combustor field conditions. Its principle relies on a sonic jet sheared periodically by a cogged wheel rotating at a given speed. Developed for experiments in combustion stability, it offers an alternative to loudspeakers when the use of these is made difficult because of aggressive flow conditions (e.g. elevated conditions of pressure and temperature, presence of impurities in the gas). While the siren was designed for laboratory applications, its technology is a promising candidate for effective flow control on gas turbine fleets. By scanning through a given frequency range, one detects the acoustic resonance of specific parts of the combustor assembly and identifies a zone of combustion instability during a sensitivity analysis where the flame is exposed to calibrated perturbations. Regarding control applications, it can act out of phase for damping purposes in the low-frequency domain (phase control), or transfer the acoustic energy to a higher and less harmful sub-harmonic (modal control). Since a chocked nozzle is involved, the siren’s actuation is decoupled from the flow condition downstream, which is convenient for control. Following-up to previous works where a siren model was introduced with the capacity to vary the amplitude of pulsation independently from the frequency [GT2011-45071], this paper describes into details new features as follows. First, the performance of the apparatus was improved to visit frequencies in the 0–6 kHz range so that the operation is extended to frequencies met by precessing vortex cores in a burner or tangential instabilities in an annular combustor. Next, the discharge configuration is tested and validated as an alternative to the conventional blow-down operation with the siren placed upstream of the pressurised test cell. There, the siren is placed downstream of the test cell, deriving a small part of the pressurised air in the plenum. Finally, a new calibration procedure for fast pressure probes and accelerometers is presented and tested. A multi-modal excitation is introduced where more than one frequency peak is being produced and where the excited frequencies create a kind of a chord. In conclusion the siren is recommended as an effective flow controller, and as a broadband and powerful calibrator.
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