Modelling of soot formation and aromatic growth in laminar flames and reactor systems

Abstract

Soot particles formed and emitted from (e.g.) direct-injected diesel engines are dangerous to human health and legislative measures used to reduce emis- sions pose a technical challenge for manufacturers. Models of soot formation and oxidation may therefore be useful tools for developing engines and con- trol strategies. In the present work, a sectional soot model able to reproduce the soot particle size distribution (PSD) is applied to laminar premixed and diffusion flames as well as a reactor system. The soot PSDs in laminar premixed stagnation flow flames were found to be sensitive to the coagula- tion collision efficiency and a novel model was developed. The soot model under-predicted soot volume fraction levels when applied to a set of laminar ethylene and propane counter-flow diffusion flames and a sensitivity anal- ysis suggested further assessment of the formation of poly-cyclic aromatic hydrocarbons (PAH) was needed. The chemical reaction mechanism was subsequently assessed using species measurements from a laminar premixed benzene flame and selected parts of the reaction mechanism reviewed. Rea- sonable agreement was obtained, including for formation of PAHs. However, non-existing or insufficient oxidation paths of some PAH species, including pyrene, may contribute to over-predictions by the soot model during non- sooting conditions. Formation of PAHs in a laminar ethylene counter-flow diffusion flame was investigated next. The agreement between calculations and measurements was found to be reasonable for major, minor and single ring aromatic species. However, the calculated concentrations of all PAH species are under-predicted. The under-prediction of pyrene is comparable to the under-prediction of the soot volume fraction in some of the diffusion flames previously investigated, making the uncertainty of the PAH chem- istry a possible explanation. Future soot modelling research should therefore focus on investigating the PAH chemistry for different types of flames and fuels.