Analysis of Flow-Flame Interactions in a Gas Turbine Model Combustor Under Thermo-Acoustically Stable and Unstable Conditions

Abstract

Flow-flame interactions in a swirl-stabilized gas turbine model combustor are investigated using doubly-phase-resolved analysis of high-repetition-rate laser and optical measurements. Three flames were studied, each of which exhibited self-excited thermo-acoustic oscillations of different amplitude ranging from fairly stable to highly unstable combustion. High-repetition-rate stereoscopic particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence measurements were performed at a sustained repetition rate of 5 kHz, which was sufficient to resolve the thermo-acoustic combustor dynamics. Using spatio-temporal proper orthogonal decomposition, it was found that the flow-field contained several simultaneous periodic motions: the reactant flux into the combustion chamber periodically oscillated at the thermo-acoustic frequency, a helical precessing vortex core (PVC) circumscribed the burner nozzle at a frequency set by the global flow rate, and the PVC underwent axial contraction and extension at the thermo-acoustic frequency. The amplitude of the axial PVC dynamics increased with increasing thermo-acoustic oscillation amplitude. The global heat release rate fluctuated at the thermo-acoustic frequency, while the heat release centroid circumscribed the combustor at the difference between the acoustic and PVC frequencies. This latter motion was caused by the axial PVC dynamics. Hence, the three-dimensional location of the heat release and heat release fluctuations depended on the interaction of the PVC with the flame surface. This motivated the compilation of doubly-phase-resolved statistics based on the phase of both the acoustic and PVC cycles, which showed highly repeatable periodic flow-flame configurations. These doublyphase-resolved statistics were used to reconstruct the dynamics of the three-dimensional periodic flow structures and flame surface oscillations over the thermo-acoustic cycle. It was found that the majority of the heat release oscillations at the thermo-acoustic frequency were caused by oscillations in the reaction layer length. By filtering the instantaneous reaction layers at different scales, the importance of the various flow-flame interactions affecting the flame length was determined. The greatest contributor was large-scale elongation of the reaction layers associated with the fluctuating reactant flow rate, which accounted for approximately 50% of the fluctuations. The remaining 50% was distributed between fine scale stochastic corrugation and large-scale corrugation due to the PVC.