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The effect of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame
; Guo, Hongsheng
Smallwood, Gregory J.
National Research Council Canada; NRC Institute for Chemical Process and Environmental Technology
Combustion Theory and Modelling
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.