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The Role of Swirl in the Flow Structure and Response of Premixed Flames



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Kallifronas, Dimitrios Pavlos 


Gas turbines have an important role in modern engineering and the development of low emission propulsion systems is a multi-dimensional challenge with no single solution. Combustion needs to be optimised and lean premixed combustion is an attractive approach to face this challenge. Unfortunately, lean premixed flames are often unstable as they may operate close to the flame blow off limit. Therefore, a suitable method of stabilisation is required. Swirlers or bluff bodies help in stabilising the flames, by creating recirculation zones of hot combustion products, and are common in practical applications. Another issue is the coupling of acoustic waves and heat release rate oscillations which create combustion instabilities. These instabilities can results is very large pressure fluctuations which may cause significant structural damage to the gas turbine.

Recent computational advances allow to predict the flame behaviour and characteristics with high accuracy, reducing the time period required and cost of product development. This is achieved through the use of complex turbulence and combustion models, and also, increased computational resources. The Large Eddy Simulation framework, where the large scales of turbulence are resolved and the small ones are modelled, is now widely used in the scientific community to explore complex flows that posed challenges in the past. This thesis employs this framework to focus on the influence of swirl on both the flow structure and the response of the flame subjected to acoustic perturbations.

To achieve those goals a range of bluff body based configurations with swirl numbers ranging from 0.30 to 0.97 are considered. Initially the recirculation zone and flow structure are compared under isothermal and reacting conditions. The recirculation zone created through vortex breakdown mechanisms is found to interact with the central recirculation zone of an upstream bluff body and this leads to a complex flow behaviour that depends on the blockage ratio and swirl number. In isothermal flows, as the swirl number or blockage ratio is increased, the vortex breakdown bubble moves upstream eventually merging with that central recirculation zone. The effect of heat release leads to considerable differences in the flow characteristics as the vortex breakdown bubble is pushed downstream due to dilatation. The critical swirl number, at which the vortex breakdown bubble and central recirculation zone merge, is observed to be higher in reacting flows for the same blockage ratio.

The flame describing functions of those swirling flames show typical characteristics involving gain minima and maxima in the frequency space and the swirl number can alter the frequencies at which they are encountered. It is then attempted to scale the flame describing functions using Strouhal numbers based on two different flame length scales. A length scale based on the axial height of the maximum heat release rate per unit length leads to a good collapse of the flame describing function gain curves. However, it is also observed that flow instabilities present in the flow can affect the flame describing function scaling leading to an imperfect collapse. Furthermore, it is found that when the forcing convective wavelength is comparable to the flame length, swirl can affect the non-linear characteristics of the flame by altering the flame roll-up mechanisms. This is related to the variation of the local swirl number in space and time. For frequencies where the convective wavelength is large compared to the flame length, the effect of swirl is small.





Swaminathan, Nedunchezhian


bluff body, CFD, Combustion, Combustion modelling, Computational Fluid Dynamics, FDF, Flame Describing Function, Flame Transfer Function, FTF, premixed flames, recirculation zones, Swirl, Swirling flows, Thermoacoustics, Thermo-acoustics


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Engineering and Physical Sciences Research Council (2275528)
Cambridge Trust Rolls Royce plc. EPSRC