Enhanced performance of a gas-turbine combustor using miniature vortex generators

• Published in: Proceedings of the Combustion Institute Vol. 29; no. 1; pp. 123 - 129
• Main Authors: Paschereit, Christian Oliver, Gutmark, Ephraim J.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 2002
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/S1540-7489(02)80020-7

Several modes of instabilities were identified in an experimental combustor. The axisymmetric lowfrequency instabilities were associated with the external recirculation zone downstream of the dump plane. The helical low-frequency instabilities were related to the central recirculation zone formed by vortex breakdown. High-frequency helical instabilities were excited when the power level was increased or when the air inlet temperature was reduced. At these conditions, the high-frequency mode dominated the thermoacoustic instability. The high-frequency oscillations were excited by the small-scale vortices that are shed at the initial separating shear layer and are related to the Kelvin-Helmholtz flow instability in this region. Miniature vortex generators were installed at the circumference of the burner's exit to interfere with the rollup of these vortices through the induction of streamwise vorticity. The streamwise vorticity interacted with the circumferential vorticity, causing azimuthal deformation and excitation of high-order azimuthal modes that disrupted the formation of coherent circumferential vortices. The tests showed that in addition to the effect on the initial vortices, the entire process that leads to the formation of large-scale vortices through pairing and vortex merging was disrupted. Thermoacoustic instabilities that are excited by the periodic heat release due to the presence of coherent vortices were thus avoided in both the high-and low-frequency ranges. The effect was particularly significant in the high-frequency oscillations, which reached high-amplitude level in the plain burner and were suppressed by up to 28 dB by the miniature vortex generators. At the same time, low-frequency instabilities were reduced by 50%. Emissions of NO x were reduced by a factor of 2 in a wide range of operating conditions and CO and unburned hydrocarbons were slightly reduced.