Molecular dynamics simulations of incipient carbonaceous nanoparticle formation at flame conditions

Abstract

Particle nucleation remains one of the most intriguing steps in the process of particle formation in flames. Although stacking/clustering of polycyclic aromatic hydrocarbons (PAHs) has been identified as a key step in the nucleation process, uncertainties remain about the size of the PAHs undergoing clustering, the organisation of the PAHs in the forming clusters and the factors affecting the relative amounts of order and disorder present in the texture organisation. A molecular dynamics approach is presented to study the influence of the molecular weight and structure of PAHs on the nucleation and coagulation processes and on the morphology of the formed clusters. Two different types of macromolecules have been analysed: pericondensed aromatic hydrocarbons (PCAHs) and aromatic aliphatic linked hydrocarbons (AALHs), known to be present in flame environments depending on flame temperature, equivalence ratio and fuel chemical structure. Clustering of six organic molecules has been analysed at two extreme temperatures, namely 500 K and 1500 K. The six molecules have been chosen on the basis of their molecular weight (pyrene and coronene) and molecular structure (single molecules, covalent-bonded dimers and covalent-bonded trimers). Nucleation of single pyrene molecules is negligible at low temperature and practically absent at high temperature. Conversely, nucleation of single coronene molecules involves 33% of the initial molecules at low temperature. Molecules within the clusters are highly mobile at flame temperatures and just 5% of the initial molecules are in clusters. The presence of covalent bonds among the aromatic sub-units improves clustering and growth, both for pyrene and coronene sub-units. Looking at the structure and the fractal dimensionality of the formed clusters, an enhancement of the disorder of the clusters is observed in the case of the AALHs (dimers and trimers) with an increased number of molecules not arranged in parallel planes. The same behaviour is found for the higher temperature.

Soot formation in laminar counterflow flames

“…Similar conclusions were reached by a recent molecular dynamic study, showing that pyrene dimerization at 1500 K is almost impossible [237]. It was further estimated that a 3.6 bar partial pressure of pyrene is required for 1% pyrene to be converted to dimers at 1500 K, while in practical hydrocarbon flames, the volume fractions of pyrene are generally on the order of 10 -6 .…”

Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics

“…Molecular dynamics uses classical mechanics to study the movement and behaviour of molecular systems over time and provides valuable information about molecular interactions and ar-rangements of nascent soot particles (Totton et al, 2012;Chen et al, 2014b,a;Iavarone et al, 2016;Grancic et al, 2016;Chung and Violi, 2011;Elvati and Violi, 2013;Bowal et al, 2018).…”

Mechanical Properties of Soot Particles: The Impact of Crosslinked Polycyclic Aromatic Hydrocarbons

“…All the above mechanisms are believed to take part in the soot formation process, but their relative importance or feasibility at different conditions and temperatures is still unclear. Previous studies utilising different atomic simulation methods, such as density functional theory (Zhang et al, 2014), molecular dynamics (MD) (Schuetz and Frenklach, 2002;Totton et al, 2012Totton et al, , 2010Chung and Violi, 2011;Iavarone et al, 2017;Pascazio et al, 2017;Mao et al, 2017Mao et al, , 2018, Monte Carlo (Rapacioli et al, 2005) and well-tempered metadynamics simulations Violi, 2018, 2013) have shown that the physical interactions between medium-sized PAH are not strong enough to stabilise clusters at high temperatures, and only aromatic molecules as big as circumcoronene are able to cluster at temperatures where soot forms (≈1500 K). Understanding the degree of crosslinking in soot particles could then be important for determining which mechanisms are involved in their growth and potentially help in understanding their inception process.…”