A reaction pathway for nanoparticle formation in rich premixed flames

Aromatics growth beyond 2-, 3-ring PAH is analyzed through a radical-molecu le reaction mechanism which, in combination with a previously developed PAH model, is able to predict the size distribution of aromatic structures for med in rich premixed flames of ethylene at atmospheric pressure with C/O ra tios across the soot threshold limit. Modeling results are in good agreemen t with experimental data and are used to interpret the ultraviolet absorpti on and the light scattering measured in flames before soot inception. The m odel shows that the total number concentration of high molecular mass aroma tics and the different moments of the size distribution are functions of bo th the PAH and H-atom concentrations, two quantities which have different t rends as functions of the residence time and the C/O ratio. Regimes of near ly stoichiometric or slightly rich premixed combustion are dominated by rea ctions between aromatics which lead to the formation of particles with size s of the order of 3 to 4 nm. At higher C/O ratios the formation of nanopart icles is less efficient. Particles with sizes of the order of 2 nm are pred icted in flames at the threshold of soot formation, whereas particles with sizes around 1 to 1.5 nm are predicted in fully sooting conditions. (C) 200 1 by The Combustion Institute.