Soot formation in strained diffusion flames with gaseous additives

Abstract

The effects of various gaseous additives on soot formation in strained diffusion flames are reported. The additives N2, Ar, He, H2, and CO were introduced with fuels C2H4, C3H8, and C4H10, and were selected to isolate the effects of dilution, temperature, preferential diffusion, and active chemical participation resulting from the additive. Special emphasis was placed on understanding the mechanisms by which CO and H2 addition influence soot inception. Measurements were made of the limiting strain rate for complete suppression of soot, i.e., the soot-particle inception limit, Kp, in the counterflow diffusion flame. Some laser-extinction measurements of soot volume fraction were also made in the coflow flame to determine the applicability of the results to this geometry. The addition of inerts to the fuel decreases the sooting limit due to the reduction in fuel concentration and temperature. Concentration modification due to preferential diffusion enhances the suppressive effect of He, causing it to be the most effective additive considered. The behavior of the reactive additives is more complex. The addition of H2 increases flame temperature but decreases Kp for the fuels considered. Preferential diffusion is partially responsible for this behavior, however direct chemical suppression may also play a role in the strongly suppressive effects of this additive. The chemical role of H2 is discussed in the context of Frenklach's H abstraction/C2H2 addition model for PAH formation. Carbon monoxide addition to C2H4 results in a monotonic decrease in Kp that is primarily a consequence of dilution. For CO addition to the alkanes there is initially an increase in Kp followed by a decrease for XCO > 0.5, suggesting a small chemical enhancement. Coflow results tend to support these findings: For C2H4 the results are consistent with dilution while for C3H8 a small chemical enhancement combined with suppression due to dilution nets a weak suppression of soot formation. This finding, that CO can enhance inception chemistry in alkanes, requires further study.