Effects of the electric field on soot formation in combustion: A coupled charged particle PBE-CFD framework

In this article, a coupled PBE-CFD framework has been proposed to study counterflow non-premixed flames and soot formation under an external electric field . This framework integrates the population balance equation (PBE) for nanoparticle dynamics into an in-house CFD solver for the multicomponent reactive flows. Different electric properties have been considered in this model. An ion mechanism used in both fuel-rich and fuel-lean combustion is combined with a detailed chemistry for neutral gaseous species and small-size aromatics to retain the full chemistry. In order to model soot particles carrying charges and the movement of the reacting fluid medium in the electric field, a second PBE for the production and transport of charges on soot particles is introduced for the first time and incorporated into the original PBE for the number density of particles. Also, the electric force for the gas mixture is included in the momentum equations. The electric drift velocities for ions and soot particles are also considered in the transport equations of ions and the PBE of soot particles, respectively. The simulations have shown that the presence of the electric field modifies the stagnation plane of the counterflow flames and reduces the soot formation in both rich-fuel and lean-fuel conditions in agreement with experimental observations. The application of the soot particle charging model, accompanied by a proper electric correction factor on the nanoparticle processes of nucleation and surface growth, significantly improves the stability of the flame structure. The introduction of the electric correction factor reveals that the suppression of soot formation in an electric field is mainly caused by the inhibited chemical reactions of the PAH nucleation and particle surface growth, which is more important than the electric drift of the charged particles. Reducing the critical size of the particle charging process enhances the electric drift of nascent soot, thus lessening its subsequent evolution.