Experimental and Numerical Investigations of Flames Stabilized by Swirl Flow and Bluff-body:Flame Structures and Flame Instabilities
Sammanfattning: Combustion and its control are essential to our existence on this planet since we knew it. Nowadays, the largest share of the world’s electricity and most of our transportation systems are powered by combustion. In addition, industrial processes also rely heavily upon combustion. In most industrial combustion systems, combustion occurs under turbulent flow conditions that can produce combustion instabilities. These are problematical since they can result in oscillations in thrust, low- or high-cycle fatigue of system components, flame blowoff or flashback, and oscillations in combustion efficiency together with high emission levels or even damage to the combustion systems. Thus, flame stabilization is of fundamental importance in the design, the efficient performance and the reliable operation of the combustion systems.Experimental and numerical investigations of swirl and bluff-body stabilized flames are presented in the thesis. Both premixed and diffusion flames are investigated in detail. Different parameters including the operating conditions, the burner geometries, fuel injection strategies are examined. High-speed PIV, high-speed PLIF and intensified CH' chemiluminescence, together with thermography and corresponding image analysis methods are adopted in the experimental work. In addition, DES and LES turbulence models on the basis of ANSYS Fluent and OpenFOAM, respectively, are employed in the numerical studies. The doctoral work is concerned above all with flame structures and flame instabilities.For swirl stabilized lean premixed flames, the operating conditions, including the total mass flow rate and the equivalence ratio of the reactants all play an important role in determining flame structures and lean blowout limits of swirl stabilized lean premixed flames. The geometries of the confinement, including the cross-sectional shape and the cylindrical confinement diameter, strongly affect the swirl stabilized flame structures and flame dynamics. Proposals are made regarding the mechanisms behind the forming of swirl stabilized lifted M-shape flames and corresponding flame dynamics. Fuel injection strategy employed also affects the characteristics of low-swirl stabilized flames, such as the time-averaged flame structures and the flame oscillations that occur. An axial fuel injection strategy leads to a more compact flame and a leaner blowout limit than a tangential fuel injection method does. The flame oscillation frequency near lean blowout is affected when fuel injection strategy is altered. When the global equivalence ratio decreases, flame dynamics frequency is reduced until the occurrence of flame lean blowout.A central fuel or air jet through the bluff-body axis makes the bluff-body stabilized premixed flame less stable. When a central fuel injection takes place, the flame is found to be lifted off from the bluff-body, with a circular motion of the flame tip along the outer edge of the bluff-body. The injection of a central fuel or air jet results in a higher lean blowout limit. At the same time, the temperature on the upper surface of the bluff-body becomes lower.Different patterns of bluff-body stabilized diffusion flames are presented: the recirculation zone flame, the stable jet diffusion flame, the lifted flame, the split-flashing flame being included here. The position of a bluff-body in relation to the annular channel exit affects the instabilities of the diffusion flame, particularly when the annular air flow velocity is high. Mounting the bluff-body downstream of the annular channel exit makes it able to better stabilize the flame. The flame stabilization is achieved by the recirculation bubble that was adjacent to the outer wall of the bluff-body. A diffusion flame stabilized by a combination of swirl and bluff-body is studied both experimentally and numerically. The effects both of a bluff-body on a swirl-stabilized diffusion flame and of swirl on a bluff-body stabilized diffusion flame are investigated. Diffusion flame ‘flashback’ is studied, the mechanisms behind it being proposed. When a larger bluff-body is employed, air driven recirculation zone is found to be located further upstream near the burner exit. The flame is found to be better stabilized by use of a larger bluff-body and/or a stronger swirling flow.
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