Laser Imaging in the Stabilisation Region of Turbulent Lifted Flames

Abstract

The stabilisation region of turbulent lifted flames of methane has been investigated using simultaneous laser imaging of Rayleigh scattering, Raman scattering and OH·LIF. These images are used to derive mixture fraction, temperature, fuel mass fraction and OH concentration images. This enables the mixing field and the reaction zone of the lifted flames to be studied in detail. The images reveal that the oscillation in the lift off height of these flames may be caused by large scale vortices in the fuel jet pushing out into the reaction zone and causing entrainment of ambient coflowing air around the base of the reaction zone subsequently shifting it further downstream. It is proposed that the flame is stabilised through a combination of premixed propagation into fresh mixture and extinction due to the interaction of large scale turbulent structures in the jet. The large scale structures cause extinction by drawing ambient airin the base of the stabilisation region and convecting the reaction zone downstream. The longer mixing times encountered with the reaction zone in a downstream location allow thefluid upstream of the flame to become partially premixed resulting in propagation of the flame upstream into the partially premixed fluid.