Soutenance de thèse Thomas Keiser
jeudi 31 janvier à 14 h 00 – Amphithéâtre Nougaro
This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.
In the second configuration the cause of flow rotation is a hydrodynamic instability, namely the Precessing Vortex Core (PVC) in an industrial fuel injection system. Experimental measurements and Large Eddy Simulation (LES) show that the non-reacting flow within the primary injector is superimposed by a PVC. The hydrodynamic instability furthermore is investigated by a Linear Stability Analysis (LSA). Both local an BiGlobal approaches are applied and compared regarding their respective results. Both experimental, LES and LSA results demonstrate that mounting a central rod in the interior of the primary injector stabilizes the PVC. In addition, the same industrial injector is investigated for a reacting flow via LES. The results demonstrate, that the flame stabilizes the flow concerning the PVC. It is shown by a BiGlobal stability analysis, that the density gradient in the mean flame front has a significant dampening effect on the instability. Finally, the impact of the central rod is investigated also for reacting flows. It is shown, that the central rod only marginally effects the global flame shape, but has a beneficial effect on flame anchoring in the lean regime. Both cases, the one with rod and the one without rod are compared also by a LSA concerning the stability of the PVC. Results suggest, that the rod significantly increases the dampening of the PVC. This could lead to decreased turbulence levels in the flow, prevent flame quenching and therefore explain the beneficial influence of the rod regarding flame anchoring.
Jury : P. Bruel, R. Vicquelin, K. Oberleithner, JM Chomaz, M. Juniper, L. Selle and T. Poinsot