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

Abstract

This paper describes an experimental investigation into the time-dependent lift-off behaviour of flames stabilised in an axisymmetric turbulent jet. Reactive MIE scattering techniques and high-speed CCD photography were employed to investigate the behaviour of lifted flames and the flow field in the near-nozzle region of isothermal jets, attached flames and lifted flames. With the flow-visualisation techniques it was observed that the lowest point of the base of the lifted flame rotates around the periphery of the jet, and this rotary behaviour was confirmed 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 the lifted flame base sets up large-scale helical vortices in the isothermal region of the lifted flame, in the near field of the nozzle. In this study it is also suggested that both the lobe size and the local jet diameter should be considered for determining the large-scale mixing time in connection with the large-scale structural mixing model of the lifted flame. Obviously, the physical mechanism of the lifted flame is not yet sufficiently well understood; the present investigation was therefore performed in order to provide more detailed understanding of the lift-off behaviour in relation to large-scale turbulent structures.