A computational study of the thermal ionization of soot particles and its effect on their growth in laminar premixed flames

The effect of thermal ionization on the growth of soot particles has been analyzed by detailed kinetic modeling of a low-pressure premixed acetylene flame. The detailed kinetic model considers the oxidation of fuel, the formation and growth of polycyclic aromatic hydrocarbons, and particle inception, coagulation, as well as mass growth via surface reactions. A numerical method has been developed, which considers the production of charged particles by thermal ionization as well as coagulation and surface reactions of these particles. The enhancement of coagulation by collisions between charged-charged and charged-neutral particles is rigorously accounted for in the numerical model. The particle size distribution functions for both neutral and charged particles were solved using the method of moments. The computed relative soot volume fractions for neutral and charged soot particles were compared to measurements and found to be in good agreement with them. The results show also that omitting of thermal ionization of soot particles does not lead to significant errors in the simulation of soot formation in the acetylene flame, as long as the nature of the surface reactions between charged particles and gaseous molecules remains the same as that for neutral particles. This result can be generalized to most laboratory laminar premixed and counterflow diffusion flames with flame temperatures not exceeding 2100 K