Soot formation in hydrocarbon/air laminar jet diffusion flames

Abstract

Soot processes along the axes of round laminar jet diffusion flames were studied, considering ethane, propane, n-butane, ethylene, propylene, and 1,3-butadiene burning in air at pressures of 25–99 kPa. Measurements included soot volume fractions, temperatures, soot structure, concentrations of major gas species and gas velocities. As distance increased along the axes of the flames, significant soot formation began when temperatures reached roughly 1250 K and fuel decomposition yielded acetylene, and ended when hydrocarbon concentrations became small at fuel-equivalence ratios of roughly 1.14. Soot growth rates were higher than earlier observations within acetylene-fueled laminar jet diffusion flames and premixed flames, which were correlated in terms of acetylene concentrations alone; this effect was attributed to either parallel soot growth channels due to the presence of significant concentrations of light hydrocarbons other than acetylene (mainly ethylene and methane) or to enhanced soot surface reactivity caused by the presence of these hydrocarbons. The present and previous soot growth data are best correlated using parallel acetylene and ethylene collision efficiencies of 0.0030 and 0.014, respectively. In contrast, present soot nucleation rates were correlated as a first-order acetylene reaction alone, with reaction parameters nearly identical to earlier findings in laminar acetylene/air diffusion flames, e.g., an activation energy of 35 kcal/gmol.