Acoustic characterization of a partially-premixed gas turbine model combustor: Syngas and hydrocarbon fuel comparisons

In this work, the acoustic behavior of a combustion instability in a gas turbine model combustor was investigated as fuel properties, air flow rates, and burner geometry were varied. The dual-swirl burner, developed at DLR Stuttgart by Meier, was operated using syngas (H-2/CO), ethylene, propane, and methane. The frequency of the instability was found to vary significantly from 250 to 480 Hz. When the plenum volume and the exhaust pipe length and diameter were changed, the frequencies followed trends similar to a Helmholtz resonator. The variation of fuel type, flame speed, and air flow rate greatly altered the instability frequency and amplitude. These effects are not predicted by Helmholtz or organ tone acoustic theory. Higher frequencies were correlated with larger laminar burning velocities and higher air flow rates. The burner is a forced resonator, in which the flame oscillations couple with the flowfield to create convectively altered Helmholtz resonances. This suggests the need for an improved model of a forced Helmholtz resonator that includes flame properties. Alkane fuels displayed similar acoustic trends, but ethylene varied greatly from methane and propane. Syngas displayed different behavior than hydrocarbon fuels, even when the laminar flame speeds of the fuels were matched between ethylene and a syngas mixture. Flame characteristics such as anchoring, liftoff height, and shape appear to play a major role in the determination of instability strength and presence. With increasing hydrogen-content in the syngas-mixture, the flame transitions from a lifted to a fully anchored flame, resulting in a drastic decrease in the acoustic amplitude associated with non-resonating flames. Rayleigh indices show that flat flames create strong regions of thermoacoustic coupling compared to axially extended V-shape flames. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of Helmholtz and convectivemechanisms. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.