Large Eddy Simulation of Turbulent Swirling Flows in Combustor Related Geometries

Sammanfattning: This thesis deals with the physics of turbulent swirling flows. Large eddy simulation (LES) method is used to investigate the vortex breakdown process, the precessing vortex core (PVC), and the effect of swirl number and flow field configuration on swirling flows. The study is based on an open source CFD code, extit{OpenFoam}. Turbulent swirling flows are widely used in combustion devices such as internal combustion engines and gas turbine combustors to promote fuel/air/hot gas mixing so that a compact combustor can be designed and to provide combustion stabilization. The aims of the thesis work are to gain deeper understanding on the structures of the vortex breakdown and PVC and to develop and to evaluate simulation methods for predicting turbulent swirling flows. The work is focused on isothermal turbulent swirling flows in gas turbine related geometries. The LES approach is first applied to simulate turbulent swirling flows in several experimental rigs where laser doppler velocimeter (LDV) and particle imaging velocimeter (PIV) data are available. The experimental data are used to evaluate the performance of LES models and the requirement of grid resolution and then they are used as baseline cases for further exploration of the flow physics with respect to variations in the flow conditions and geometrical configurations. In all cases it is found that the LES results are comparable to the experimental results and the sensitivity of LES results to the sub-grid models is not high provided that a sufficient grid resolution is used. Several criteria for the assessment of LES accuracy are examined. It is found that the vortex breakdown structure in a combustor is sensitive to the swirl number in the inflow. In a given combustor geometry when swirl number increases above a critical level vortex breakdown is shown to occur. Further increase the swirl number leads to the onset of PVC and the drastic change in the structures of the recirculation zones. The coherent structures associated with vortex breakdown becomes unsteady. The onset of vortex breakdown and the unsteady mode are analysed using dynamic mode analysis (DMD) method. For a given swirl number it is found that when the combustor outlet geometry is changed the vortex breakdown structures can be significant affected. When the outlet area is gradually decreased a center bubble type vortex breakdown is replaced by an annular recirculation zone, and the center vortex core becomes more dynamic. This is expected to have a significant influence on the way the flame is stabilized in the combustor. Examination of the LES results indicates that the transition from the center bubble recirculation to the annular recirculation structures is a manifestation of the coupling of the tangential momentum and the pressure gradient.