On the blow‐off correlation for swirl‐stabilised flames with a precessing vortex core

Abstract

Large eddy simulations of turbulent swirl-stabilised flames gradually approaching blow-off conditions in a gas turbine model combustor are undertaken. The global equivalence ratio of the flames is reduced by increasing and decreasing the air and fuel flow rates respectively. The filtered reaction rate for partially premixed combustion is modelled using a presumed joint probability density function flamelet model. The average position of the flame is closer to the fuel nozzle when the global equivalence ratio is decreased, contrary to what is expected. Comparisons are made with simultaneous particle image velocimetry, and acetone and OH planar laser induced fluorescence imaging for the corresponding simulated case. It is demonstrated that the mechanisms leading to local extinction are well captured in the simulation and insufficient mixing induced by the precessing vortex core leads to local extinction. Further analysis of the simulations shows that including heat loss effects within the modelling is important for flames near or approaching blow-off. The non-adiabatic simulation influences the re-stabilisation of the flame after lift-off and shows that the flame leading edge follows the rotation of the PVC. A blow-off correlation based on the Damköhler number is proposed using the PVC rotation frequency and a chemical time scale. Analysis shows that the simulated flames respond to the correlation and including non-adiabatic flamelets is important for flames approaching blow-off.