Experimental studies of transient and limit cycle thermoacoustic oscillations in a lean premixed swirl stabilized combustor

Abstract

The lean premixed mode of operation of gas turbine combustors has been developed as a means of meeting the NO$_{x}$ emission reduction regulatory requirements. However, this technology is inherently susceptible to triggering of thermoacoustic instabilities, wherein the thermal and the acoustic field couple into a self amplification feedback loop that eventually results in the establishment of high amplitude dynamic pressure and heat release rate oscillations, a dynamic state termed limit cycle. Despite lengthy research efforts, it has not been possible to fully identify the reason behind the triggering events leading to the establishment of thermoacoustic instabilities. The thesis aims to increase the understanding of the combustion phenomena involved in the triggering and the perpetuation of these instabilities. Experiments are conducted in a model swirl stabilized combustor, burning premixtures of air with methane, propane and blends of hydrogen enriched methane. The operational conditions feature lean equivalence ratios at turbulent Reynolds numbers. The silica windows of the model combustor grant optical access to a highly confined swirl stabilized flame. High speed CH$^*$ chemiluminescence imaging and Particle Image Velocimetry and low speed OH Planar Laser Induced Fluorescence are employed, in conjunction with non linear analysis, proper orthogonal decomposition and dynamic mode decomposition. The analysis shows that upon increasing the resistance of a mixture to extinction either by enriching the equivalence ratio or by increasing the hydrogen molar concentration, the transition from quiescence into limit cycle Period-1 thermoacoustic oscillations is presaged by intermittent high amplitude bursts of dynamic pressure and heat release rate, due to a subcritical Hopf bifurcation. The intermittent bursts are attributed to the interaction of the flame with the Precessing Vortex Core (PVC). The limit cycle is perpetuated due to oscillations of the flow imposed strain rate, causing periodic local extinction of the flame at the root close to the centerbody. The relative ratio of the flow imposed over the mixture extinction strain rate dictates the limit cycle flame anchoring topologies, which in turn affect whether the PVC is excited or suppressed during the limit cycle regime. When excited, the PVC is responsible for a period doubling limit cycle bifurcation, wherein the PVC time scale is superimposed on the fundamental acoustic time scale. The flame front curvature, extracted via OH Planar Laser Induced Fluorescence, indicates that the flame becomes more wrinkled during a high amplitude burst. The findings also suggest that the limit cycle oscillations are reflected on the flame front curvature, with the extent of flame wrinkling being commensurate with the heat release rate.