EFFECT OF LARGE-SCALE TURBULENT STRUCTURES ON THE LIFT-OFF BEHAVIOR OF TURBULENT-JET DIFFUSION FLAMES

This paper describes an experimental investigation into the time-depen dent lift-off behaviour of flames stabilised in an axisymmetric turbul ent jet. Reactive MIE scattering techniques and high-speed CCD photogr aphy were employed to investigate the behaviour of lifted flames and t he flow field in the near-nozzle region of isothermal jets, attached f lames and lifted flames. With the flow-visualisation techniques it was observed that the lowest point of the base of the lifted flame rotate s around the periphery of the jet, and this rotary behaviour was confi rmed by temperature fluctuations at the base of the lifted flame. From visualisation of various entrainment patterns of the ambient air into the jet fluid, the rotary behaviour is seen to give rise to azimuthal variations in the instant lift-off height. The rotary behaviour of th e lifted flame base sets up large-scale helical vortices in the isothe rmal region of the lifted flame, in the near field of the nozzle. In t his study it is also suggested that both the lobe size and the local j et diameter should be considered for determining the large-scale mixin g time in connection with the large-scale structural mixing model of t he lifted flame. Obviously, the physical mechanism of the lifted flame is not yet sufficiently well understood; the present investigation wa s therefore performed in order to provide more detailed understanding of the lift-off behaviour in relation to large-scale turbulent structu res.