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STUDY OF PRECESSING VORTEX CORE DURING VORTEX BREAKDOWN USING LES AND POD

Christophe Duwig
Department of Energy Science, Division of Fluid Mechanics, Lund University, LTH, Box 118, 22100 Lund, Sweden; and Haldor Topsøe, A/S, DK-2800 Lyngby

Laszlo Fuchs
Department of Mechanics, KTH, CICERO, SE-10044 Stockholm; Division of Fluid Mechanics Lund University Lund, 22363, Sweden

Abstract

Modeling and understanding the vortex breakdown is a key issue of modern Lean Premixed Combustors. The main difficulty of the problem is the unsteady behavior of this type of flow: Large structures resulting from vortex breakdown and the swirling shear-layers, affect directly the flame stabilization leading to heat-release fluctuations and combustion instabilities. Consequently, one needs to capture and understand turbulent coherent structures dynamics for designing efficient burners. This task is particularly challenging since it deals with capturing coherent motions within a chaotic system. This requires the use of state-of-the art numerical and experimental techniques.
The present work focuses on the numerical study of isothermal vortex breakdown using Large Eddy Simulation (LES) for obtaining a 4D description of the flow. A sensitivity analysis and further comparisons with experimental data indicate that the LES tool captures accurately the flow. The LES results are then processed for capturing and identifying the coherent structures. The characteristic frequencies are analyzed. Also with this respect good agreement with data from the literature is obtained. The large scale vortices have been visualized providing a good insight into the unsteady flow pattern. Finally, Proper Orthogonal Decomposition (POD) has been applied to the 4D LES data in order to identify the contribution of different modes. The presence of the Precessing Vortex Core (PVC) corresponding to a pair of helical structures is identified and is highlighted.