On the relevance of surface growth in soot formation in premixed flames

The role of surface growth mechanisms in particle mass accumulation was investigated in rich, premixed, ethylene/air flames from non-sooting to moderately sooting conditions using in situ optical diagnostics and predictions from a detailed chemical kinetic model. Particles formed just after the flame front, which are transparent to the visible light but absorb in the UV range, have been detected in large amounts in nonsooting flames and earlier in the flame than soot particles in sooting-flames, using UV-visible light optical diagnostics. For C/O < 0.8, the amount of visible-transparent particles accounts for the total mass of soot detected later in the flame, indicating that surface growth processes are negligible and that soot formation is a rearrangement of the carbonaceous material already present in the form of smaller particles. Furthermore, predictions from the kinetic model, which does not include surface growth reactions, agree well with experiments for C/O < 0.8. The model is able to predict the total carbon contained in particles observed in the non-sooting flames and in slightly sooting flames, as well as that observed up to the onset of soot formation in richer flames. For C/O > 0.8, additional soot growth mechanisms need to be included in the mechanism to account for the amount of soot observed in the later part of the flames. Interestingly enough, this late growth mechanism occurs almost simultaneously with a strong increase in the coagulation rate of the particles, thus indicating that both effects are related and are probably due to a major change in the chemical nature of the particle surface.