The use of lean, premixed fuel and air mixtures is a common strategy to reduce NOx emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in phase in a feedback loop. This study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor where the fuel can be injected inside the fuel injector to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the mixture entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. CH\* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for quantifying the relative phase of pressure and heat release rate oscillations, suggest that the coherence between equivalence ratio oscillations and the vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics. Implications of this work on both experimental research and industrial nozzle design are discussed. 


The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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