Mechanisms of formation and destruction of soot particles in a laminar methane-air diffusion flame

Abstract

A laser light scattering/extinction method has been used to examine the evolution of soot particles in a cylindrical laminar diffusion flame of methane and air at atmospheric pressure. The soot particle characteristics (volume fraction, mean diameter, number density) were plotted versus time along the streamline trajectories, assuming a monodisperse distribution of particle-sizes. Different steps of soot evolution have been deduced: (1) The soot formation phase consists of two steps, identical to those described by several authors in rich premixed flames: nucleation, then coagulation or aggregation; surface growth, which occurs simultaneously, is responsible for the main part of the total mass of soot. (2) Three steps have been observed in the destruction phase: -In the first step, the mean diameter increases while the particles number decreses; this phenomenon may be considered as an aggregation process or as an effect of the change of particle size-distribution. -In the second step, the mean diameter decreases but the number increases. According to Neoh's study in premixed secondary flames, this phenomenon has been attributed to a “break-up” of the aggregates; it occurs when about 75 percent of the soot mass has disappeared. -The last step is characterized by the decreases of diameter and number of particles. In the diffusion flame we have considered, the soot destruction phase takes about five milliseconds. The specific burnout rate has a maximal value of 10−3 g cm−2 s−1 which agrees with the order of magnitude previously measured in secondary premixed flames.