Progress on Forcing Pulsations for Acoustic, Thermoacoustic or Flow Control Purpose in a Pressurised Vessel by Means of a Siren

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 air-jet sheared periodically 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 because of aggressive flow conditions such as hot air under elevated pressure as well as the presence of impurities in the gas.

The siren is used for laboratory applications, and its technology is a promising candidate for effective flow control on real machines. 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. Regarding control applications, it has the possibility to act out of phase for damping purposes in the low-frequency domain (phase control), or to transfer the acoustic energy to a higher sub-harmonics less harmfull than the problematic one (modal control).

This work is a follow-up to paper GT2011-45071 where a siren model was introduced with the capacity to vary the amplitude of pulsation independently from the frequency. The technical details are described and discussed in the paper.

In order to extend the range of operation to frequencies met by precessing vortex cores in a burner or tangential instabilities in an annular combustor, the performance of the apparatus was adapted to visit frequencies up to 5 kHz. For fast pressure probe and accelerometer calibration purpose, the flow pulsation levels can achieve up to 155dB SPL. Two modes of operation are presented: the blow-down operation with the siren placed upstream from the pressurised test cell is excited, and the discharge configuration where the siren is placed downstream from the test cell or derives a part of the pressurised air in the plenum. The last part of the paper focuses on multi-modal excitation, where more than one frequency peak is being produced, and the exited frequencies create a kind of a chord. The novelty in this process is the possibility not only to detect resonances based on the peak of acoustic pressure measured at some point, but also to get an idea of its location based on the sudden phase-shift happening when a resonator catches up and takes over the lead as a noise generator.