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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 50430 (45)
- University: Sharif University of Technology
- Department: Aerospace Engineering
- Advisor(s): Farshchi, Mohammad
- Abstract:
- Lean premixed combustion is highly susceptible to thermoacoustic instabilities. Such instabilities are compounded in low swirl flames by the fact that low swirl flames are stabilized aerodynamically at a relatively considerable distance downstream of the burner rim, such stabilization mechanism makes the flame more prone to both vertical and acoustic perturbations. The main objective of the present thesis is to evaluate effects of acoustic and hydrodynamic perturbations on low swirl flame features. To such aim, first, low swirl flame characteristics and stabilization mechanism are studied using large eddy simulations along with thickened flame combustion model. The obtained results show that the combustion heat release decreases the local swirl number of the flow field. Investigations also reveal that the combination of the swirling motion and combustion heat release temporally imposes a transient recirculation zone at the post flame regions. Results aslo show that combustion heat release has the effect of decreasing the instability amplitude and vortex shedding frequency. Investigations show that in addition to the swirling motion, baroclinic torque and vortex stretching have considerable effect on the premixed flame stabilization in low swirl flows. In the second part of the thesis, the burner inlet velocity profiles are perturbed periodically using a sinusoidal forcing function with different amplitudes and frequesncies to evaluate low swirl flame dynamic and response to external excitations. Obtained results show that external flow excitations increase the local swirl number upstream of the flame front. Such increase in the local swirl number is at maximum value when the low swirl flame is excited at the dominant frequency of the flow field, which in turn induces a vortex breakdown and hence a central recirculation zone. Phase-locked results shows that the size and strength of the recirculation zone are highly phase dependent. In the last part of the present thesis, a consice summary of the investigations carried out in sabbatical program at IIT Madras are addressed. The main objective of such investigations is to evaluate low swirl flame characteristics under external acoustic excitations using both experimental methods (OH Chemiluminescence, OH Planar Laser Induced Fluorescence, Particle Image Velocimetery, Hot Wire Anemometery) and numerical simulations (using large eddy simulations)
- Keywords:
- Combustion Instability ; Large Eddy Simulation (LES) ; Low Swirl Flame Interference ; Acoustic Excitation
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