SOOT FORMATION IN WEAKLY BUOYANT ACETYLENE-FUELED LAMINAR JET DIFFUSION FLAMES BURNING IN AIR

The structure and soot properties of weakly buoyant, acetylene-fueled, laminar jet diffusion flames were studied experimentally for combusti on in air at pressures of 0.125-0.250 atm. Properties along the axis, where soot processes are similar to behavior within nonbuoyant diffusi on flames, were emphasized. The following measurements were made: soot volume fractions using laser extinction, temperature using both therm ocouples and multiline emission, soot structure using thermophoretic s ampling and analysis by transmission electron microscopy, concentratio ns of major gas species using sampling and analysis by gas chromatogra phy, and velocities using laser velocimetry. As distance increased alo ng the axis of the present acetylene-fueled flames, significant soot f ormation began when temperatures exceeded roughly 1250 K, and ended wh en fuel equivalence ratios decreased to roughly 1.7, where the concent ration of acetylene became small. This behavior allowed observations o f soot growth and nucleation for acetylene concentrations of 6 x 10(-6 )-1 x 10(-3) kg-mol/m(3) and temperature of 1000-2100 K. Over this ran ge of conditions, soot growth rates were comparable to past observatio ns of new soot in premixed flames, and after correction for effects of soot oxidation yielded essentially first-order growth with respect to acetylene concentrations with a negligible activation energy, and an acetylene/soot collision efficiency of 0.53%. Present measurements of soot nucleation rates also suggested first-order behavior with respect to acetylene concentrations but with an activation energy of 32 kcal/ gmol and with rates that were significantly lower than earlier estimat es in the literature. Nevertheless, uncertainties about effects of soo t oxidation and age on soot growth, and about effects of surface area estimates and translucent objects on soot nucleation, must be resolved in order to adequately define soot formation processes in diffusion f lames.