Entrainment and Flame Geometry of Fire Plumes

Abstract

This study concerns the flame structure and fire plume entrainment of natural gas diffusion flames on 0.10, 0.19 and 0.50 m. diameter burners. The heat release rates ranged from 10 kW to 200 kW. Flame heights based on high speed photography and eye averages show a transition in the dependence of flame height on a dimensionless heat addition parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of these flames whereas for short flames initial geometry becomes important. Another prominent feature of these flames is the presence of large scale ring vortex-like structures which are formed close to the burner surface more or less regularly. It is found that these structures are responsible for the fluctuations of the flame top. Entrainment measurements spanned heights starting very close to the burner surface to distances about six times the average flame heights. Experiments indicate the presence of three regions; a region close to the burner surface where plume entrainment rates are independent of the fuel flow (or heat release) rates; a far field region above the flame top, where a simple point source model correlates the data reasonably well; and a not so well-defined intermediate region where entrainment seems to be similar to that of a turbulent flame with plume-like characteristics. It is also found that the disturbances in the ambient atmosphere will greatly enhance the fire plume entrainment. Finally, a theoretical study of a steady, buoyant, diffusion flame indicated the importance of the puffing in the entrainment process.