PYROLYSIS ZONE STRUCTURE OF ALLENE, 1,3 BUTADIENE AND BENZENE SMOKE POINT DIFFUSION FLAMES

In an effort to further understand smoke point flames, species and temperature profiles were measured in the pyrolysis zones of diffusion flames using the heavily sooting fuels, allene, 1,3 butadiene and benzene. All three flames were at the smoke point and approximately the same visible flame height to provide a common basis for data comparison. The fuel was observed to be governed by a convective/diffusive balance in the pre-soot flame zone; fuel depletion along the axis is due to radial diffusion, not to local pyrolysis. Possible correlations of mixture fraction (a conserved scaler) with concentration and temperature are discussed. Flame results are interpreted based on flow reactor and shock tube pyrolysis and oxidation data. The benzene flame produced much lower concentrations of aliphatic intermediates than the allene and butadiene flames. The formation rate of benzene is approximately the same in the allene and butadiene flames, but the mechanism of formation is different. The experiments indicate: (1) further evidence for a pathway to form benzene through C3 species, which could occur in parallel to C4/C2 pathways, and (2) that aromatic compounds are essential to the soot formation process and aliphatic fragments, though necessary, may not have a controlling effect.