Characterisation of Large Coherent Structures in a Swirl Burner under Combustion Conditions

Dry Low NOx combustors based on the use of Lean Pre-mixtures and Swirling Flows are widely used in gas turbine combustors to produce systems with low emissions, although the efficiency of reducing emissions is dependant on the techniques used for premixing. Swirling flows are recognized as one of the best systems for stabilizing high intensity flames giving reduced maintenance, increase of turbulent flame speed, wide stability limits and ability to produce low emissions. However, most practical swirling flows are highly complex, three dimensional and time dependant in nature and contain complex coherent structures which are essential to the flame stabilization process. Even though the study of these types of flows has been extensive for several decades , some of the fundamental mechanisms are still barely understood owing to their complexity. Numerical simulations have also tried to explain the development of different regimes, but the extremely complex nature of the former has shown a variety of results whose lack of time dependant validation leaves the results open to question. Due to the high complexity of these flows, parameters such as Re, S, φ and geometry play a crucial role in the development of these structures, interrelation that has been barely studied. This paper thus adopts an experimental approach to characterize large coherent structures in swirl burners under combustion conditions so as to reveal the effects of swirl, Re and φ in a number of geometries. Aided by different techniques such as High Speed Photography (HSP), CH* Photo Chemiluminescence (CH* CL) and Particle Image Velocimetry (PIV), the recognition of several structures was achieved in a 100 kW Swirl Burner model. Several varied, interacting, structures developed in the field as a consequence of the configurations used.