Physics-Based Flame Dynamics Modeling and Thermoacoustic Instability Mitigation

The objectives of this work are (i) to investigate the coupled unsteady heat release mechanisms responsible for thermoacoustic instabilities under di erent ame anchoring con gurations, (ii) to develop reduced-order models to predict the dynamic ame response, and (iii) to develop passive instability mitigation strategies by modifying the dynamics of the ame anchoring zone. The two di erent anchoring con gurations investigated were the wake-stabilized ames and the perforated-plate stabilized ames. In order to investigate the wake-stabilized ames, experiments were performed in an atmospheric pressure, backward-facing step combustor. In this con guration, the dynamics are primarily governed by the ame-vortex interactions, whereas the equivalence ratio oscillations have minor impacts. Depending on the equivalence ratio, inlet temperature and the fuel composition, the combustor operates under di erent dynamic modes. The operating conditions at the transition between di erent modes were predicted by developing a model based on the acoustic and vortex time scales. A passive control strategy involving injection of steady air ow near the step in the cross-stream or streamwise directions were tested. Both injection con gurations were able to suppress the instability under some operating conditions at which the unsteady interactions between the ame and the wake vortex were eliminated. The cross-stream air injection method eliminates these interactions by generating a new, steady recirculation zone upstream of the step. On the other hand, the streamwise air injection method eliminates these interactions by directly stabilizing the ow dynamics in the unsteady recirculation zone. In order to investigate the perforated-plate stabilized ame dynamics, a theoretical model was developed to predict the dynamic heat-release response to inlet velocity oscillations. In this con guration, the dynamics are driven by the coupled e ects of the ame-wall interactions and the ame-acoustic wave interactions, generating burning velocity and ame area oscillations. The model predictions under di erent operating conditions were compared with the experiments and good agreement was obtained. As the heat loss to the plate increases, the plate's surface temperature rises, the ame temperature decreases, and the burning velocity 3 oscillations become more signi cant. In order to verify and relax some model assumptions, two-dimensional simulations were performed in the same con guration utilizing a detailed chemical kinetic mechanism and allowing the heat transfer between the gas and the perforated-plate. The primary results of the simulations support the conclusions of the theoretical model, and show the signi cant impact of the heat transfer to the plate on both the steady ame characteristics, and the unsteady dynamic ame response. Thesis Supervisor: Ahmed F. Ghoniem Title: Ronald C. Crane (1972) Professor