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. PLIF measurements on the horizontal (x-z) plane confirmed the existence of intensely burning fuel-air pockets in the interaction zone. These pockets burn intermittently resulting in continuous evolution and annihilation of the flame surfaces in the flame-flame interaction regions, which eventually leads to fluctuations in the heat release rate. The combined effect of these heat release fluctuations gets coupled with natural frequencies of the combustor, which drives the self-excited thermoacoustic instability modes. In the third part, we present experimental results that provide insights into the flow-flame dynamics leading to flame blowoff inside a model gas turbine combustor housing three interacting swirling premixed flames. In particular, we focus on the mechanism of intermittent extinction-reignition phenomena and the final blowoff event i.e., complete flame extinction. The dynamics of the system close to blowoff conditions is monitored using simultaneous pressure and multi-kHz OH* Chemiluminescence measurements. These measurements reveal that there exists low frequency, large amplitude oscillations due to the large-scale extinction and reignition events occurring inside the combustor. sPIV and OH-PLIF techniques are used to probe the flow-flame interactions, especially the ones which are responsible for the flame extinction along the shear layers and the mechanisms of flame reignition.