40 years with Swirl, Vortex, Cyclonic Flows, and Combustion

The combustion aerodynamics of such systems are always full of intrigue and unexpected results, sometimes beneficial, often not so. This paper will review the authors experience over the last 40+ years in this area, drawing common threads and experiences from a wide range of different systems. The major focus will be on the role of coherent and other structures in swirling flows and how they affect the performance of the described devices, especially swirl burners. How such features of the flow can couple with various possible modes of oscillation in combustors will also be discussed and comments made as to how future work can further elucidate the complexities of this flow and aid the design of improved burner in a wide range of different situations. Although the initial focus was on the precessing vortex core (PVC) and its characteristics, more recent work, especially with confined flows (ie combustion chambers/furnaces) has shown the high susceptibility of the central reverse flow zone (CRZ) to deformation and it's often time dependant and deformed structure, which has more effect on flame stability than the PVC. Swirl flow devices such as cyclone dust separators and vortex amplifiers are discussed as devices which can naturally oscillate without the presence of combustion, coupling occurring with naturally occurring pipe or volume resonances. Difficulties of operating at swirl numbers close to the vortex breakdown point are emphasised and how this can cause natural oscillations and 'jumps' in the characteristics. The role of predictive techniques such as LES or CFD/RANS with swirl burners will also be highlighted and the difficulties of effectively predicting what are time dependant flows.