Modeling of soot particle size distributions in premixed stagnation flow flames

Abstract

Soot formation in five laminar premixed ethylene flames was studied computationally in order to investigate the ability of a sectional model to reproduce measured soot particle size distributions (PSDs). The flames were modelled using a burner-stabilised stagnation flow configuration due to the influence of the sampling probe. In the vicinity of the probe, the residence time is relatively long and the temperature reduced. It was found that under such conditions the coagulation efficiency for small (<10nm) particles becomes important due to the combination of a relatively low nucleation rate with a significant collision frequency. In previous studies, the impact of the probe was only accounted for by shifting the position of the measured PSDs. A model for the collision efficiency was introduced with a particle size dependent interpolation between a lower limit, based on the Lennard–Jones potential, and an upper limit, based on collision stabilisation caused by the surrounding gas. The developed model is consistent with a low collision efficiency for PAH coagulation and explains the measured high number densities of small particles. The size of particles affected by a reduced collision efficiency were found to decrease with increasing flame temperature. The effect is consistent with temperature dependent internal carbonisation processes changing the polarisability of particles leading to a strengthening of attractive forces and enhanced collisional stabilisation. The calculated PSDs were found to be in reasonable agreement with measurements.