Visualizing Diffusion Flame Formation in the Wake of Partially Premixed Combustion

Results are presented on the formation of a diffusion flame in a methane nonpremixed jet following the propagation of partially premixed combustion. An initially nonreacting turbulent methane jet (Re=2700) in quiescent air is ignited at a downstream location $x/d=70.$ High-speed video images (125 and 250 Hz) were obtained that chart the evolution of the combustion process. Partially premixed flame propagation is witnessed as the combustion front moves upstream (downward) toward the nozzle exit. As the front propagates, the blue (premixed) character of the flame is diminished, the combustion region narrows, and the transition to diffusion-limited combustion along the stratified methane/air layer takes place. Before reaching the nozzle exit, axial wisps of blue flame emission are witnessed along the jet-edge near the fuel/air interface (i.e., at larger radii than the eventual diffusion flame boundary). Luminosity from soot is first apparent just upstream of an axisymmetric flame bulge as the diffusion flame forms, and within 100 ms, soot attains levels present in the steady-state turbulent diffusion flame. Images are presented portraying the phenomenon, and three regimes are proposed to characterize the propagation of combustion.