Control of High-Frequency Thermoacoustic Pulsations by Distributed Vortex Generators

Abstract

Axisymmetric and helical instabilities modes were identified in an experimental combustor. The low-frequency instabilities were associated with the external recirculation zone downstream of the dump plane and the central recirculation zone formed by vortex breakdown. High-frequency helical instabilities were excited by the small-scale vortices that were shed at the initial separating shear layer at high-power levels. 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 tests showed that, in addition to the effect on the initial vortices, the 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 that reached high-amplitude level in the baseline 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 two in a wide range of operating conditions. The results obtained in the laboratory combustor operating at atmospheric pressure were also confirmed in high-pressure combustion tests.