The Structure of Jet Diffusion Flames

Abstract

This paper presents the structural characteristics of free, round, jet diffusion flames as obtained using a new 2D laser sheet lighting visualization technique referred to as the RMS (Reactive Mie Scattering) method. The results of analyzing photographs and high speed movies of flames using the RMS method are discussed in terms of the visible flame structure. The fuel veloCity is varied from 0.16 to 17 m/s. The presence of large toroidal vortices formed outside the visible flame zone have been known for many Years but their importance in determining the dynamic structure of free jet diffusion flames has not been fully appreciated. The influence of the outer vortices on flame structure is prevalent for near laminar and transitional flames and diminishes for near turbulent flames. They are believed to result from a Kelvin-Helmholtz type instability formed by a buoyantly driven shear layer. They appear to be responsible for flame flicker defined by the separation of the flame tip or the oscillations of the flame surface and for determining the shape of the mean, rms, and pdf radial profiles of temperature. Vortex structures have also been observed inside the visible flame zone. In transitional flames established by a contoured nozzle, these structures are shown to be on the scale of the 10 mm diameter nozzle, toroidal, and coherent for a long distance downstream. However, they may have only a minimal impact on the mean temperature characteristics of transitional flames. Their impact on the visible flame structure of near turbulent flames is large. At high fuel velocities, coalescence of the large vortices appear to be correlated with the formation of small 3D vortices which are randomly distributed in size and space. Collisions of the small vortices with the visible flame front produce small localized flamelets which are responsible for the wrinkled appearance of the visible flame surface. The localized stretching of the flame surface is believed to invoke finite rate chemistry effects. Indeed, collisions are observed where the flame stretch is large enough to cause localized holes to form in the flame surface. This appears to occur when the radial velocities of the inner vortices are large. Holes formed near the lip of the jet are postulated to be one mechanism that induces flame lift-off.KeywordsShear LayerFlame StructureFlame SurfaceContour NozzleVisible FlameThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.