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 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 subharmonic (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.