Effects of hydrodynamics on soot formation in laminar opposed-jet diffusion flames

Abstract

Effects of flow (hydrodynamic) properties on the presence of soot in hydrocarbon-fueled laminar opposed-jet diffusion flames were studied experimentally at atmospheric pressure, emphasizing effects of velocities normal to the flame sheet. These velocities were varied for conditions corresponding to combustion in air by transferring nitrogen from the oxidizer stream to the fuel stream, which increases the stoichiometric mixture fraction of the flame and causes the stagnation plane of the flow to shift toward the fuel-rich side of the flame sheet. Fuels considered included acetylene, ethylene, ethane, propylene, propane, and 1-3 butadiene. Present measurements consisted of the critical strain rates for the flames to contain soot (the soot extinction limit) and for the flames to extinguish (the flame extinction limit). It was found that increasing the stoichiometric mixture fraction causes a progressive reduction of the critical strain rates for both flame and soot extinction; however, their ratio increases, and even becomes unbounded in most instances to yield a permanently-blue-flame regime. The results suggest that soot formation in nonpremixed flames can be controlled by varying velocities normal to the flame sheet. Nevertheless, definitive conclusions along these lines require evaluation of effects of corresponding variations of fuel and oxygen concentrations on soot formation when velocities normal to the flame sheet are changed by varying stoichiometric mixture fractions for laminar opposed-jet diffusion flames.