Study of the vortex breakdown in a conical swirler using LDV, LES and POD

Authors: Duwig, C., Fuchs, L.F., Lacarelle, A., Paschereit, O.C.
Document Type: Conference
Pubstate: Published
Volume: ASME IGTI   ASME GT2007-27006
Year: 2007


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 and should be done using state-of-the art numerical and experimental techniques. The present work focuses on the experimental and numerical study of iso-thermal vortex breakdown in a conical swirler. Experimental investigations were performed with 2D Laser Doppler Velocimetry (LDV) and Hotwire Anemometry at the outlet of the combustor model. Averaged velocity fields and RMS values are showing a strong central recirculation zone. In addition, characteristic frequencies of the flow have been exhibited showing the strong influence of large scale turbulent fluctuation on the flow pattern. These measurements showed also the impact of different outlet geometries on the strength and position of the coherent structures of the flow. Further, Large Eddy Simulation (LES) has been used to obtain a 4D description of the flow. Comparison with LDV profiles showed a good agreement, indicating that the LES tool captures accurately the flow. The LES results were then processed for capturing and identifying the coherent structures. Firstly, characteristic frequencies were analyzed. Here also a good agreement with the experimental data was achieved. Secondly the cores of the vortices were visualized providing a good insight into the unsteady flow pattern. Finally, Proper Orthogonal Decomposition (POD) was applied to the 4D field in order to identify the contribution of different large scale fluctuation modes. The presence of the Precessing Vortex Core (PVC) and of a strong helical structure and were therefore captured.