Swirl stabilized premixed flame analysis using of LES and POD
Abstract: For environmental and human health reasons, the regulations on the emissions from combustion devices are getting more and more strict. In particular, it is important to abate the emission of pollutants deriving from combustion in gas turbines. A solution is the use of swirl stabilized, lean premixed combustion. Besides the beneficial effects, there are still some issues related to instabilities and a full, clear understanding of the dynamics of swirling flows and flames has not been reached yet. This is where the contribution of this thesis lie. In this work, advanced techniques, namely Large Eddy Simulation (LES), Proper Orthogonal Decomposition (POD), and optical diagnostics, have been applied to analyze swirl stabilized flames, relevant to gas turbine applications. A simple geometry combustor, the “Lisbon”burner, useful for fundamental studies, was simulated by LES. The dynamics of a forced swirling flame are successfully captured, allowing to characterize the influence of the Precessing Vortex Core (PVC) on the flame stabilization and its interactions with axial fluctuations. However, real applications are typically characterized by more complex geometry. Special attention was then paid to the study of a realistic burner, the “Triple Annular Research Swirler (TARS)”. Detailed LES of this aeroengine-like fuel-injector, including the upstream portion of it, shed some light on the experimentally observed asymmetry of the flow. The flow through the fuel-injector, un-accessible to experiments, was clarified and detailed. Differences and similarities with academic simple geometry swirl burners were also highlighted. For reacting conditions, the LES formulation was able to explain the peculiar stabilization mechanism in a case where the Central Recirculation Zone was destroyed by thermal expansion, to capture dual behavior/hysteresis phenomena, to describe the dynamics of a lean flame and its interaction with the PVC. Throughout the thesis, POD analysis highlighted large scale structures and flame fluctuations of several combustors contributing to the understanding of the dynamics of swirl stabilized flames. It was shown how POD can relate to conditional averaging and in particular to phase averaging. A POD based phase averaging procedure was used to study thermo-acoustic oscillations. Applied to experimental data obtained with a simple and relatively cheap set-up, but complex geometry and flow, it opens possibilities for application on industrial rigs, enabling phase averaging with a priori unknown period. The concept of Extended POD was expanded to combustion applications highlighting the correlations between flow and flame dynamics. For both numerical and experimental data, it gave new insights into flames and their correlation with the flow field.
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