Stochastic Simulation of Soot Formation Evolution in Counterflow Diffusion Flames

Abstract

Soot generally refers to carbonaceous particles formed during incomplete combustion of hydrocarbon fuels. A typical simulation of soot formation and evolution contains two parts: gas chemical kinetics, which models the chemical reaction from hydrocarbon fuels to soot precursors, that is, polycyclic aromatic hydrocarbons or PAHs, and soot dynamics, which models the soot formation from PAHs and evolution due to gas-soot and soot-soot interactions. In this study, two detailed gas kinetic mechanisms (ABF and KM2) have been compared during the simulation (using the solver Chemkin II) of ethylene combustion in counterflow diffusion flames. Subsequently, the operator splitting Monte Carlo method is used to simulate the soot dynamics. Both the simulated data from the two mechanisms for gas and soot particles are compared with experimental data available in the literature. It is found that both mechanisms predict similar profiles for the gas temperature and velocity, agreeing well with measurements. However, KM2 mechanism provides much closer prediction compared to measurements for soot gas precursors. Furthermore, KM2 also shows much better predictions for soot number density and volume fraction than ABF. The effect of nozzle exit velocity on soot dynamics has also been investigated. Higher nozzle exit velocity renders shorter residence time for soot particles, which reduces the soot number density and volume fraction accordingly.

Gas-Phase Synthesis of Coronene through Stepwise Directed Ring Annulation

“…Sophisticated analyses of carbonaceous chondrites such as Allende and Murchison revealed the presence of PAHs, including coronene (C 24 H 12 ), presumably formed in circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars along with planetary nebulae as their descendants . The formation mechanisms in these high-temperature circumstellar environments such as the hydrogen abstraction–acetylene addition (HACA) mechanism , are assumed to mirror those in hydrocarbon-rich flames, where coronene has been observed in flames of small hydrocarbons (methane, ethylene) and of gasoline surrogates ( n -heptane, iso-octane, toluene). − In sooting hydrocarbon flames, coronene (C 24 H 12 ) has been dubbed as a critical precursor in soot nucleation processesa kinetic bottleneck in the formation of carbonaceous nanostructures. − …”

“…These models incorporate gas-phase reactions forming PAHs up to coronene (C 24 H 12 ) followed by soot nucleation and aggregation via PAH dimerization . While traditional PAH growth processes have focused on the HACA mechanism, , more recent models have incorporated resonance-stabilized free radicals (RSFRs) as well as the hydrogen abstraction–vinylacetylene addition (HAVA) and phenyl addition–dehydrocyclization (PAC) pathways. − ,,− However, even these refinements were not able to replicate the observed fractional abundances of coronene (C 24 H 12 ) in, for example, ethylene flames, with modeled coronene abundances falling short by up to an order of magnitude compared to observed data. , Therefore, the inability to replicate the fractional abundances of coronene (C 24 H 12 ) in combustion flames implies that critical reaction pathways to coronene (C 24 H 12 ) are not properly incorporated in the models. Therefore, an advanced experimental and computational protocol is required to investigate elementary gas-phase reactions yielding coronene (C 24 H 12 ) at elevated temperatures.…”