Abstract
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.
Highlights
This paper is about active control of combustion stability in gas turbines
The flame transfer function, or the combustor acoustic response under cold flow conditions are determined experimentally using the siren as an exciter and a set of fast-pressure transducer distributed along the combustor measuring the core’s response to the excitation
The different options for flow forcing, the specifics, the dimensioning details, and the successive features of a siren were described in detail
Summary
This paper is about active control of combustion stability in gas turbines. In opposition to batch-cycle internal combustion engines, continuous flow machines, such as a gas turbine, are supposed to operate a steady flow conditions at the different stations of the Joule-Brayton cycle. Small perturbations can trigger a large-amplitude fluctuation of each of these quantities, where the steady flow becomes pulsed in the combustor, where the local pressure fluctuation is such that vibrations and narrow-peak sounds are being generated, and the flame front deforms and moves periodically at the same pace possibly nearer to the walls. The sum of it all greatly impairs the performance of the machine. The siren can be used as a calibrator for fast-pressure probes or accelerometers Another application is to force flow pulsation and trigger, in a well-controlled manner, thermoacoustic instabilities on research flames. They happen due to an inconvenient coupling of dynamic parameters under some operating conditions of the machine
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