DescriptionA siren is a robust fast-valve that generates effective flow pulsations and powerful noise levels under well-controlled conditions. It is designed for the operating conditions at the inlet of a pressurised 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 gather more than twenty years of know-how on siren technology. The 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 2~kHz, possibly 5~kHz) and achieving elevated pressure fluctuations (146~dB SPL proven, possibly up to 155~dB SPL) with a compact device. Different modes of operations are described. The blow-down operation is the most effective one, where the siren is placed upstream from the pressurised test cell. As a comparison, a configuration in discharge is shown: the siren is placed downstream from the test cell or derives a part of the pressurised air in it. The second configuration is a convenient strategy for the control of the acoustic boundaries in the combustor because it does not relies on a separate source of compressed air anymore, but it leaks periodically a part of the plenum air that can be derived further down the machine e.g. for cooling purpose.
In laboratories, the siren can be used as a calibrator for fast-pressure probes or accelerometers. Another application is to force flow pulsation and trigger in well-controlled manner thermoacoustic instabilities on research flames. In the industry, the intention is to equip gas turbine combustors with an effective flow modulator for control purpose, that has the possibility to pass through some critical frequencies with a zero-gain. One application is the displacement of the acoustic energy from a resonant frequency to one of its a sub-harmonics, where the sub-harmonic is not harmful. A more immediate application is to generate a powerful and calibrated sound and check-out the response and sensitivity drift of all mounted pressure sensors and accelerometers with time, without dismounting them, and possibly during operation.
The contribution ends-up with a summary of the applications performed so far as well as with the work-in-progress using this device.
|Period||19 Nov 2019|
|Event title||The European Virtual Institute for Gas Turbine Instrumentation: Conference on Gas Turbine Instrumentation|
ASJC Scopus subject areas
- Energy Engineering and Power Technology