OH Radical Imaging in a DI Diesel Engine and the Structure of the Early Diffusion Flame

Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location and nature of the flame zones within the combusting fuel jet have been largely unstudied. To address this issue, planar laser-induced fluorescence (PLIF) imaging of the OH radical has been applied to the reacting fuel jet of a direct-injection diesel engine of the "heavy-duty" size class, modified for optical access. An Nd:YAG-based laser system was used to pump the overlapping Q₁9 and Q₂8 lines of the (1,0) band of the A➙X transition at 284.01 nm, while the fluorescent emission from both the (0,0) and (1,1) bands (308 to 320 nm) was imaged with an intensified video camera. This scheme allowed rejection of elastically scattered laser light, PAH fluorescence, and laser-induced incandescence. OH PLIF is shown to be an excellent diagnostic for diesel diffusion flames. The signal is strong, and it is confined to a narrow region about the flame front because the threebody recombination reactions that reduce high flame-front OH concentrations to equilibrium levels occur rapidly at diesel pressures. No signal was evident in the fuel-rich premixed flame regions where calculations and burner experiments indicate that OH concentrations will be below detectable limits. Temporal sequences of OH PLIF images are presented showing the onset and development of the early diffusion flame up to the time that soot obscures the images. These images show that the diffusion flame develops around the periphery of the downstream portion of the reacting fuel jet about half way through the premixed burn spike. Although affected by turbulence, the diffusion flame remains at the jet periphery for the rest of the imaged sequence. The images also show many details of the diffusion flame structure including its upstream extent. Finally, the location and nature of the diffusion flames are discussed with respect to previously reported soot and fuel distributions.