Transient effects of fuel-air mixing in a partially-premixed turbulent swirl flame

Unsteady interactions of flow, fuel-air mixing and reaction in a lean partially-premixed turbulent swirl flame are investigated using simultaneous particle image velocimetry and planar laser-induced fluorescence of OH and acetone with a repetition rate of 10 kHz. The flame is operated with methane and air in a gas turbine model combustor at ambient temperature and pressure. Transport and mixing of fuel is visualized by fluorescence of acetone, which is added with 9% by volume to the methane. The dominant unsteady flow structures are a precessing vortex core (PVC) in the shear layer of the inner recirculation zone and the lower stagnation point (LSP) at the flame root. The measurements show that fuel and air are largely separated at the inlet of the combustion chamber, but are then strongly mixed by the PVC. A well-mixed zone of unburned fuel and air is formed around the vortex, and then ignited by recirculating burned gas. At the flame root, the PVC induces periodic changes in the composition of the unburned gas, which varies between pure air and well-mixed fuel and air. Reaction is locally quenched at the LSP when fresh air without fuel is present, and re-ignition takes place when mixed fuel and air arrive at the boundary of recirculating burned gas. Generally, the results show that the enhancement of fuel-air mixing induced by the PVC contributes significantly to the stabilization of the flame, and that the flame dynamics can only be properly understood when an analysis of transient mixing mechanisms is included.