Molecular structures in flames: A comparison between SNapS2 and recent AFM results

Abstract

Models capturing the growth of polycyclic aromatic compounds (PACs) and nanoparticles in combustion have always faced a large degree of uncertainty due to the paucity of detailed direct experimental validation. In particular, data on molecular structures, chemical composition, size, cross-linking, and aliphatic chains is still very limited. In the past few years, we have developed an atomistic code, SNapS2, that models the formation of PACs in combustion conditions, providing information on the chemical and structural evolution of these compounds. In this paper, we present a detailed analysis of the compounds formed in a premixed ethylene-air flame that was previously characterized experimentally by AFM (Commodo et al. 2019). The comparison is based on molecular structures and extends to other properties of PACs. The results demonstrate that SNapS2 is able to capture the vast array of PACs in terms of a large variety of functional groups. The simulations, confirmed to a large extent by the experimental data, show the presence of different types of oxygenated (e.g., phenol, ketone) and cyclic (acenaphthylene-type, acenaphthene-type, and fluorene-type five-membered rings) functional groups. Moreover, the structures that were not observed in the simulations (indane-type five-membered rings or six-membered rings containing oxygen) are missing due to the lack of formation pathways, highlighting the fact that important kinetic mechanisms are still missing in the literature. Finally, we observed that a preponderant number of high molecular mass PACs are curved, which may have an impact on their aggregation and sampling. Overall, while some discrepancies remain due to the inherent limitation of the model and the AFM techniques, this work demonstrates the unique capabilities of the SNapS2 code to provide insights on the structures and chemical pathways of PACs.