Re-initiation of soot surface growth in premixed counter-flow flames

Abstract

In order to contribute to the discussion of the active-site approach for surface growth reactions versus recovery of radicalic surface sites by reactions with species from the gas phase, experiments in counterflow, premixed flames have been performed. Soot particles are generated in premixed, fuel-rich ethyne/argon/oxygen flames counterflowing against nonsooting flames of the same components or lean carbon monoxide/argon/oxygen flames. The nonsooting flames produce a high level of hydrogen atoms combined with a variation of the concentration of other gaseous species so that surface growth by H abstraction from C−H sites at the surface of soot particles may be reinitiated in the stagnation region of the sooting flame. These flames are compared with single, premixed, sooting flat flames in which the soot volume fraction attains a final plateau f v ∞ due to the decay of surface growth. The experimental results clearly demonstrate that depending on the flow conditions in the counterflow flames, a second onset of soot formation is detected. This second onset of soot formation can be traced back to surface growth. The experimental findings are discussed by referring to results from modeling of these flames using the hydrogen-abstraction-hydrocarbon-addition (HACA) mechanism for surface growth. From this, a shift of the competing effects of surface growth and oxidation is identified to be responsible for the reinitiation of surface growth.