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Abstract. Soot inception by acetylene pyrolysis at 1350–1800 K is investigated using reactive molecular dynamics. The composition and chemical structure of soot precursor molecules formed during inception are elucidated. During soot inception, increasing the process temperature leads to faster depletion of C2H2 molecules and faster formation of C2H3, C2H4, C2H6, CH4, and C2 with the concurrent appearance of H2 molecules. Small molecules consisting of 1–5 C atoms (C1–C5) are formed due to reactive collisions and grow further to larger hydrocarbon compounds consisting of 6–10 C atoms. At initial stages of inception, prior to the formation of incipient soot, three-member rings are formed, which are associated with the formation of compounds with fewer than 10 C atoms. Once incipient soot is formed, the number of C1–C10 compounds and the number of three-member rings drop, while the number of five- and six-member rings increases, indicating that the formation of larger rings is associated with the growth of soot clusters. The chemical structure of soot precursor molecules obtained by bond order analysis reveals that molecules with up to 10 C atoms are either linear or branched aliphatic compounds or may contain three-member rings fused with aliphatic components. Molecules with more than 10 C atoms often exhibit structures composed of five- or six-member C rings, decorated by aliphatic components. The identification of molecular precursors contributing to soot inception provides crucial insights into soot formation mechanisms, pinpointing potential pathways of soot formation during combustion. 

Incipient soot particles obtained from a series of reactive molecular dynamics simulations were studied to understand the evolution of physical, chemical, and morphological properties of incipient soot. Reactive molecular dynamics simulations of acetylene pyrolysis were performed using ReaxFF potential at 1350, 1500, 1650, and 1800 K. A total of 3324 incipient soot particles were extracted from the simulations at various stages of development. Features such as the number of carbon and hydrogen atoms, number of ring structures, mass, C/H ratio, radius of gyration, surface area, volume, atomic fractal dimension, and density were calculated for each particle. The calculated values of density and C/H ratio matched well with experimental values reported in the literature. Based on the calculated features, the particles were classified in two types: type 1 and type 2 particles. It was found that type 1 particles show significant morphological evolution while type 2 particles undergo chemical restructuring without any significant morphological change. The particle volume was found to be well-correlated with the number of carbon atoms in both type 1 and type 2 particle, whereas surface area was found to be correlated with the number of carbon atoms only for type 1 particles. A correlation matrix comparing the level of correlation between any two features for both type 1 and type 2 particle was created. Finally, based on the calculated statistics, a set of correlations among various physical and morphological parameters of incipient soot was proposed. 

A series of reactive molecular dynamics simulations is used to study the internal structure of incipient soot particles obtained from acetylene pyrolysis. The simulations were performed using ReaxFF potential at four different temperatures. The resulting soot particles are cataloged and analyzed to obtain statistics of their mass, volume, density, C/H ratio, number of cyclic structures, and other features. A total of 3324 incipient soot particles were analyzed in this study. Based on their structural characteristics, the incipient soot particles are classified into two classes, referred to as type 1 and type 2 incipient soot particles in this work. The radial distribution of density, cyclic (5-, 6-, or 7-member rings) structures, and C/H ratio inside the particles revealed a clear difference in the internal structure between type 1 and type 2 particles. These classes were further found to be well represented by the size of the particles, with smaller particles in type 1 and larger particles in type 2. The radial distributions of ring structures, density, and C/H ratio indicated the presence of a dense core region in type 2 particles. In contrast, no clear evidence of the presence of a core was found in type 1 particles. In type 2 incipient soot particles, the boundary between the core and shell was found to be around 50%–60% of the particle radius of gyration. 

Modeling thermal radiation in combustion environments can be extremely challenging for two main reasons. First, the radiative transfer equation (RTE), which is the cornerstone of modeling radiation in such environments, is a five-dimensional integro-differential equation. Second, the absorption and scattering coefficients of molecular gases and particulates prevalent in combustion environments oscillate strongly with the wavenumber (or wavelength), i.e., the medium is strongly nongray, requiring the solution of the RTE for a large number of wavenumbers. This article reviews the progress that has been made in this area to date with an emphasis on the work performed over the past three decades. Progress in both deterministic and stochastic (Monte Carlo) solutions of the RTE is reviewed, in addition to the review of the treatment of the spectral properties of gases, soot, and fuel droplets that dominate combustion environments, i.e., spectral or nongray models. The application of the various state-of-the-art nongray models and RTE solution methods to flames (particularly turbulent), fires, combustors, and other combustion systems are summarized along with a critical discussion of the pros and cons of the models and methods. Finally, the challenges that remain in modeling thermal radiation in combustion systems are highlighted and future outlooks are shared. 

Soot or black carbons are combustion-generated carbonaceous nanoparticles formed during the incomplete combustion of hydrocarbon fuels. The complexity of hydrocarbon systems often makes it difficult to investigate the fundamentals of soot formation experimentally. To address this, this study uses reactive molecular dynamics simulations with reactive force field (ReaxFF) potentials. The current work focuses on the formation and evolution of soot during acetylene pyrolysis. The analysis provides insights into the physicochemical aspects of soot formation and the maturation of incipient soot particles. In this work, we focus on the evolution and interdependence of features such as the number of carbon atoms, number of aromatic rings, mass, C/H ratio, the radius of gyration, atomic fractal dimension, surface area, volume, and density. Based on the physicochemical features, two distinct classes of nascent soot can be observed. These are termed type-1 and type-2 particles. The type-1 particles show significant morphological evolution, while the type-2 particles show chemical restructuring without significantly changing the morphology. Qualitative correlations of various degrees are also observed between some of these morphological features.