Influence of flow field scaling on flashback of swirl flames

In this paper, the effect of geometrical scaling on the onset of flashback into a cylindrical premixing zone of a swirl flame is investigated. We discriminate two types of flashback. In the first type of flashback the flame propagates upstream inside an already present axial recirculation zone. This flashback is caused by turbulent burning along the vortex axis (TBVA(1)) and is controlled by flame extinction inside the recirculation zone. The second type of flashback is caused by combustion induced vortex breakdown (CIVB2). This type of flashback is characterised by the aerodynamic influence of the combustion heat release that leads to propagation of the axial recirculation zone and the flame in upstream direction. To study the effects of geometrical scaling on the flow fields and the two types of flashback, the operation of two geometrically scaled burners are compared at equal Reynolds number. By this method it is possible to observe the flashback phenomena in similar swirl flow fields but with different turbulent scales affecting the combustion process. To check flow field similarity and to indentify the flashback type, the non-reacting and reacting flow fields have been examined by planar particle imaging velocimetry and simultaneous recording of the flame luminescence. It is shown that geometrical scaling of the burner shifts the equivalence ratio at which flashback occurs and that this shift is different for the two types of flashback. Consistency and inconsistency with known scaling and stability criterions is discussed. Analysing the fluid dynamics and turbulent combustion gives a first explanation of why CIVB and TBVA ate affected differently by geometrical scaling at constant Reynolds number which is in good agreement with the experimental observations. (C) 2009 Elsevier Inc. All rights reserved.