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 .…”

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 .…”

Modeling Thermal Radiation in Combustion Environments: Progress and Challenges

“…The determination of the spectral optical properties of soot remains a challenge due to the complexity of soot processes and an incomplete understanding of soot composition and morphology. However, recent explorations in ab initio soot modeling [213][214][215][216] may provide a promising outlook for detailed soot optical property modeling. One major challenge facing the research community is the ability to keep up with newer versions of the spectroscopic databases.…”

A Review of Recent Research Results on Soot: The Formation of a Kind of Carbon-Based Material in Flames

“…Xu et al (Xu et al, 2020) found that cyclopentane, with a fivemembered ring in the molecule, generated more soot than cyclohexane in counterflow diffusion flames because more odd carbon radicals (cyclopentadienyl and allyl, which are efficient aromatic precursors) were produced in cyclopentane flames. The aliphatic bond between the aromatic sub-units enhances the clustering of PAHs and decreases the dissociation effect due to increasing temperature (Iavarone et al, 2017). Influence of oxygenated compounds concentration on sooting propensity was studied by Abboud et al (Abboud et al, 2017).…”

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 .…”

Modeling Thermal Radiation in Combustion Environments: Progress and Challenges

“…The determination of the spectral optical properties of soot remains a challenge due to the complexity of soot processes and an incomplete understanding of soot composition and morphology. However, recent explorations in ab initio soot modeling [213][214][215][216] may provide a promising outlook for detailed soot optical property modeling. One major challenge facing the research community is the ability to keep up with newer versions of the spectroscopic databases.…”

A Review of Recent Research Results on Soot: The Formation of a Kind of Carbon-Based Material in Flames

“…Xu et al (Xu et al, 2020) found that cyclopentane, with a fivemembered ring in the molecule, generated more soot than cyclohexane in counterflow diffusion flames because more odd carbon radicals (cyclopentadienyl and allyl, which are efficient aromatic precursors) were produced in cyclopentane flames. The aliphatic bond between the aromatic sub-units enhances the clustering of PAHs and decreases the dissociation effect due to increasing temperature (Iavarone et al, 2017). Influence of oxygenated compounds concentration on sooting propensity was studied by Abboud et al (Abboud et al, 2017).…”

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 .…”

Modeling Thermal Radiation in Combustion Environments: Progress and Challenges

“…The determination of the spectral optical properties of soot remains a challenge due to the complexity of soot processes and an incomplete understanding of soot composition and morphology. However, recent explorations in ab initio soot modeling [213][214][215][216] may provide a promising outlook for detailed soot optical property modeling. One major challenge facing the research community is the ability to keep up with newer versions of the spectroscopic databases.…”

A Review of Recent Research Results on Soot: The Formation of a Kind of Carbon-Based Material in Flames

“…Xu et al (Xu et al, 2020) found that cyclopentane, with a fivemembered ring in the molecule, generated more soot than cyclohexane in counterflow diffusion flames because more odd carbon radicals (cyclopentadienyl and allyl, which are efficient aromatic precursors) were produced in cyclopentane flames. The aliphatic bond between the aromatic sub-units enhances the clustering of PAHs and decreases the dissociation effect due to increasing temperature (Iavarone et al, 2017). Influence of oxygenated compounds concentration on sooting propensity was studied by Abboud et al (Abboud et al, 2017).…”