Formation of Turbulent Eddies in Jet Diffusion Flames

Abstract

The transition from laminar to turbulent mode of an ethylene jet flame was investigated using the two-dimensional instantaneous photography of turbulent eddies by a laser-light sheeting method. Observations were made of the eddies around the break point in the fuel flow and the flame. The results show that in the laminar region the fuel flow is curved due to instability in the shear layer, whereas the outer soot layer has little curvature because of the high viscosity in the hot layer. In the transient region, eddies generated in the fuel flow deform the outer soot layer. Numerical calculations were performed to predict fluid motions due to interaction between density and pressure gradients in the flame boundary. The results show that the pressure gradient in a medium of varying density generates the vorticity along the flame. Deformation and stretching of the flame boundary take place once the vorticity becomes stronger than the dissipation due to viscosity.