Numerical study of the effects of pressure on soot formation in laminar coflow n-heptane/air diffusion flames between 1 and 10 atm

Abstract

Laminar nitrogen-diluted n-heptane diffusion flames burning in coflow air were numerically simulated at pressures from 1 to 10atm. Numerical simulations were performed by using a detailed reaction mechanism containing 175 species and 1086 reactions with PAH formation up to pyrene and a sectional soot model. The soot model consists of inception as a result of the collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition (HACA) mechanism, and soot particle coagulation and PAH surface condensation. Comparisons with available experimental data indicate that the numerical model reproduces successfully the influence of pressure on the flame structure and soot production. All the soot formation processes are enhanced with increasing pressure and HACA dominates the soot mass growth over the pressure range considered. PAH condensation is the most sensitive process to pressure whereas the inception and HACA processes exhibit similar pressure dependency. These trends are directly related to the pressure dependence of the molar concentrations of the species involved in soot formation. The increase in the molar concentrations of benzene and pyrene with pressure results from both physical and chemical effects. Propargyl recombination and interconversion of phenyl to benzene dominate the formation of benzene low in the flame over the pressure range considered, though the contributions of the addition of acetylene to n-butadienyl radical increases significantly with pressure in the flame centerline region.