Hydrodynamic suppression of soot emissions in laminar diffusion flames

Abstract

Effects of flow (hydrodynamic) properties on soot formation and oxidation in nonpremixed hydrocarbon/air flames were studied, emphasizing conditions where effects of buoyancy are small in coflowing laminar jet diffusion flames. Effects of air/fuel-stream velocity ratios were of particular interest; therefore, the experiments were carried out at reduced pressures (0.19-0.50 atm) in order to minimize effects of flow acceleration due to buoyancy. Test conditions included acetylene, propylene, and 1,3-butadiene flames burning in air with air/fuel-stream velocity ratios in the range 0.4-6.7. Measurements included laminar smoke-point properties, as well as the following flame structure properties: soot volume fractions, temperatures, soot structure, concentrations of major gas species, and velocities. The measurements showed that laminar smoke-point flame lengths can be increased, and soot emissions possibly suppressed entirely, by increasing air/fuel-stream velocity ratios. The flame structure measurements suggest that the mechanism of this effect involves the magnitude and direction of flow velocities relative to the flame sheet. In particular, large air/fuel-stream velocity ratios cause soot to form in cool and fuel-rich gases, inhibiting soot nucleation, and then to be drawn directly toward the flame sheet with a limited residence time, inhibiting soot growth, so that capabilities to complete the oxidation of soot are enhanced and the tendencies to emit soot are reduced.