Ionic Mechanisms of Soot Nucleation in Premixed Flames
Although numerous chemical mechanisms have been postulated for the formation of soot in hydrocarbon combustion, none have received quantitative support. Understanding of this process is complicated by the large number of species involved and by the phase change associated with nucleation.
An experimental program is underway at AeroChem to determine the role of ionic processes in soot formation in premixed hydrocarbon flames. The hypothesis being tested is that ions produced via chemi-ionization serve as nuclei for initial formation of soot particles. Rapid ion-molecular kinetics, rapid structural rearrangements, and thermochemistry favorable toward growth to larger species are characteristics of the ion chemistry, in agreement with observations about the soot formation process.
Mass spectrometric measurements of flame ion concentration profiles have been made in low pressure rich and sooting acetylene/oxygen and benzene/oxygen flames. As both flames are made increasingly fuel rich and approach sooting the predominant ion C3H3 + is replaced by large positive aromatic ions with mass greater than 300. It is argued that these ions are the soot precursors.
Similarities and differences betweeen the ion spectra of rich and sooting acetylene and benzene flames are discussed along with possible reasons for dual maxima in ion concentration profiles observed in sooting flames. A detailed chemical mechanism considering both neutral and ionic flame chemistry is being developed for which it is necessary to estimate the thermochemical parameters for large ionic species. Preliminary results of computer simulations of the chemical kinetics using this scheme are presented. The results are consistent with an ionic mechanism of soot formation.
KeywordsEquivalence Ratio Flame Front Soot Particle Premix Flame Soot Formation
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