Identification of coherent structures in distributed swirl-stabilized wet combustion

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Towards achieving sustainable and decarbonized power, biomass to electricity is an attractive pathway. To that end, the humidified gas turbine cycle is a promising technology. Recirculated steam which contains low-grade heat can be used to replace part of the air flow. This immediately reduces power loss in the compressor and increases specific power output, benefiting higher electrical efficiency compared to dry cycles. With high steam content, wet combustion leads to the so-called flameless combustion (FC) or colorless distributed combustion (CDC) which is presently investigated using large eddy simulation and a detailed finite rate chemistry method. Further insight regarding the coherent structures is obtained. Proper orthogonal decomposition method is applied on both the velocity and the heat release field aiming to explore the in-depth dynamic of flow-flame interaction in a swirl burner. Our results are the first reporting two distinct sets of helical coherent structures. A higher frequency mode or structure at Strouhal number St ~ 0.7 is caused by the vortex shedding, and a lower frequency mode at St ~ 0.1 corresponds to the off-central motion of an intermittently occurring precessing vortex core (PVC). With high steam content (hence very distributed reaction regime), more frequent occurrence of the marginally stable PVC is observed. The wet flame local extinction is evidenced to be an important driver towards the promotion and suppression of the PVC structure. Compared to the very energetic flow-flame interaction in conventional flames, FC or CDC flames undergo less fluctuations.