Dynamics of Actuated and Interacting Swirl Premixed Flames in Model Gas Turbine Combustors

Abstract

Lean premixed combustion is preferred in gas turbines because of the reduced (NOx) emissions. However, the combustors operating in lean premixed regime could suffer from problems like flame flashback, flame blowoff and thermoacoustic instabilities. In the first part of this work, we have designed and developed a novel technique to mitigate the self-excited thermoacoustic instabilities inside a lab-scale combustor. The mitigation strategy is realized by rotating the otherwise static swirler, which is primarily meant for stabilizing the lean premixed flame. The proposed strategy is tested over a range of bulk flow velocities, mixture equivalence ratios, and swirler rotation rates for validating the robustness of this concept. A prominent reduction in the fundamental acoustic mode amplitude by about 25 dB is observed with this control technique for the cases that are studied. The physical mechanism responsible for the instability mitigation due to the rotating swirler is investigated by observing the distinct changes associated with the reacting flow field using Particle Image Velocimetry. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate strongly positive Rayleigh indices regions (and eventually the acoustic energy source) to render quiet instability mitigated swirling flames. In the second part, we have extended the studies to a more realistic combustor design, comprising three interacting swirl premixed flames, arranged in-line in an optically accessible hollow cuboid test section, which closely resembles a three-cup sector of an annular gas turbine combustor with a very large radius. Multiple configurations with various combinations of swirl levels between the adjacent nozzles and the associated flame and flow topologies have been studied. We observe that, for the cases where adjacent flames interact, there exists a dominant mode of oscillation whose amplitude is 30 dB more for 30-45-30 case as compared to the 45-45-45 configuration...