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The effect of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame

National Research Council Canada; NRC Institute for Chemical Process and Environmental Technology
Published in:
Combustion Theory and Modelling
Pages :
NPArC #:
Soot; Preferential diffusion; Laminar diffusion flame; Lewis number; PAH
Environment Monitoring Technologies; Technologies de surveillance de l'environnement
LII - Nanoparticle Diagnostics and Characterization; LII - Diagnostic et caractérisation des nanoparticules
Environment Monitoring Technologies Program; Programme des technologies de surveillance de l'environnement
The influence of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame was investigated by numerical simulation using three different transport property calculation methods. One simulation included preferential diffusion and the other two neglected preferential diffusion. The results show that the neglect of preferential diffusion or the use of unity Lewis number for all species results in a significant underprediction of soot volume fraction. The peak soot volume fraction is reduced from 8.0 to 2.0 ppm for the studied flame when preferential diffusion is neglected in the simulation. Detailed examination of numerical results reveals that the underprediction of soot volume fraction in the simulation neglecting preferential diffusion is due to the slower diffusion of some species from main reaction zone to PAH and soot formation layer. The slower diffusion of these species causes lower PAH formation rate and thus results in lower soot inception rate and smaller particle surface area. The smaller surface area further leads to smaller surface growth rate. In addition, the neglect of preferential diffusion also leads to higher OH concentration in the flame, which causes the higher specific soot oxidation rate. The lower inception rate, smaller surface growth rate and higher specific oxidation rate results in the lower soot volume fraction when preferential diffusion is neglected. The finding of the paper implies the importance of preferential diffusion for the modeling of not only laminar but maybe also some turbulent flames.
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