A computational study of ethylene–air sooting flames: Effects of large polycyclic aromatic hydrocarbons

Abstract

An updated reduced gas-phase kinetic mechanism was developed and integrated with aerosol models to predict soot formation characteristics in ethylene nonpremixed and premixed flames. A primary objective is to investigate the sensitivity of the soot formation to various chemical pathways for large polycyclic aromatic hydrocarbons (PAH). The gas-phase chemical mechanism adopted the KAUST-Aramco PAH Mech 1.0, which utilized the AramcoMech 1.3 for gas-phase reactions validated for up to C2 fuels. In addition, PAH species up to coronene (C 24 H 12 or A7) were included to describe the detailed formation pathways of soot precursors. In this study, the detailed chemical mechanism was reduced from 397 to 99 species using directed relation graph with expert knowledge (DRG-X) and sensitivity analysis. The method of moments with interpolative closure (MOMIC) was employed for the soot aerosol model. Counterflow nonpremixed flames at low strain rate sooting conditions were considered, for which the sensitivity of soot formation characteristics to different nucleation pathways were investigated. Premixed flame experiment data at different equivalence ratios were also used for validation. The findings show that higher PAH concentrations result in a higher soot nucleation rate, and that the total soot volume and average size of the particles are predicted in good agreement with experimental results. Subsequently, the effects of different pathways, with respect to pyrene-or coronene-based nucleation models, on the net soot formation rate were analyzed. It was found that the nucleation processes (i.e., soot inception) are sensitive to the choice of PAH precursors, and consideration of higher PAH species beyond pyrene is critical for accurate prediction of the overall soot formation.

Effect of soot model, moment method, and chemical kinetics on soot formation in a model aircraft combustor

“…The change in the chemical mechanism from NBP to KAM1.0 consistently increases soot volume fraction for all the soot models considered, consistent with previous studies in laminar flames [22] . While the qualitative volume fraction profiles are the same, there is 50-200% increase in soot volume fraction.…”

“…• The KAM1.0 mechanism produces more soot mass compared to the NBP mechanism for all statistical approaches considered, which is consistent with laminar flame results [22] . While the nucleation and condensation source terms are much higher with KAM1.0 compared to NBP, the final soot mass fraction changes by less than a factor of two.…”

“…On the other hand, recent nucleation models including higher PAHs up to coronene (A7) have shown improved soot prediction in laminar counterflow flames [21,22] . Moreover, the predictions were also found to be sensitive to the flow configuration used [2,26] .…”

“…The NBP mechanism includes PAH chemistry up to four aromatic rings (pyrene), and the KAM1.0 mechanism includes PAH chemistry up to seven aromatic rings (coronene). Both mechanisms have been validated against canonical laminar flames [22,30,38] .…”

“…While several models are available, the two main concepts are based on (a) an acetylene-based inception model [4,17] and (b) a polycyclic-aromatic hydrocarbon (PAH) based nucleation model [18][19][20][21][22] . Both models have been used extensively to simulate a variety of turbulent sooting flames [5,7,[23][24][25][26] with a range of predictive success.…”