Soot formation in weakly buoyant acetylene-fueled laminar jet diffusion flames burning in air

Abstract

The structure and soot properties of weakly buoyant, acetylene-fueled, laminar jet diffusion flames were studied experimentally for combustion in air at pressures of 0.125–0.250 atm. Properties along the axis, where soot processes are similar to behavior within nonbuoyant diffusion flames, were emphasized. The following measurements were made: soot volume fractions using laser extinction, temperatures using both thermocouples and multiline emission, soot structure using thermophoretic sampling and analysis by transmission electron microscopy, concentrations of major gas species using sampling and analysis by gas chromatography, and velocities using laser velocimetry. As distance increased along the axis of the present acetylene-fueled flames, significant soot formation began when temperatures exceeded roughly 1250 K, and ended when fuel-equivalence ratios decreased to roughly 1.7, where the concentration of acetylene became small. This behavior allowed observations of soot growth and nucleation for acetylene concentrations of 6 × 10 −6-1 × 10 −3 kg-mol/m 3 and temperatures of 1000–2100 K. Over this range of conditions, soot growth rates were comparable to past observations 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 estimates in the literature. Nevertheless, uncertainties about effects of soot 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 flames.