A picture of soot particle inception

Abstract

We have shown in earlier work that the ultimate soot loading in a sooting premixed flame is determined largely by processes that occur before the growth region, even though 90% of the soot mass is contributed by C2H2 surface growth. In this work we make the first measurements of how early soot formation kinetics vary with equivalence ratio. We find that the particle inception rate does not by itself control the ultimate soot loading. Molecules which grow to be larger than about 15 carbon atoms early in the flame form a thermodynamically stable pool of soot precursors. The coagulation of the heaviest of these precursors, which may have molecular weights near 1000, contributes significantly to inception, and inception ceases when they are depleted. Species with somewhat lower molecular weight coagulate with the newly formed soot particles (surface growth) until they, too, are depleted. Oxidation—primarily by O2—competes with chemical growth and coagulation in determining the amount of soot exiting the particle inception zone.