The Visible Structure of Turbulent Jet Diffusion Flames: Large-Scale Organization and Flame Tip Oscillation

Abstract

High speed visual observations of free turbulent jet diffusion flames are presented. Computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution. Two fuels were used, acetylene and ethylene, to span Ihe range from the momentum-driven to the buoyancy-driven regime. The data show that, for all flow regimes, the flames exhibit a large-scale organized motion in the far-field of the jet as manifested by a progression of organized structures up through the length of the visible flame. Occasional pairings of structures are also observed. These structures eventually become the flame tip, which burns out in a quasi-periodic manner. Under no conditions was a random, small-scale breakup and disappearance of the flame tip ever observed. In all cases the speed of the organized structure is seen to be constant and equal to 12 ± 2° of the jet exit velocity. The constancy of speed occurs in spite of the axial decay of the centerline velocity and is believed to result, in part, from the requirement of the flame to burn from the outer edges of the jet towards the jet centerline with increasing downstream distance, thus compensating for axial velocity decay. The characteristic time of the flame tip burnout event, for all flow regimes, is found to be in good agreement with the local large-scale time estimated from the visible jet width and the centerline velocity. We conclude that the organized component of the jet motion exists for most jet flames reported in the literature, and is responsible for flame tip oscillation, but that its role has been overlooked.