A hierarchical modeling approach is presented for describing soot growth dynamics, encompassing reactive Molecular Dynamics (MD) simulations and a monodisperse particle dynamics model. Reactive MD is employed to investigate nucleation of soot nanoparticles during isothermal acetylene pyrolysis at 1200–1800 K. A “lumped” soot nucleation rate is determined by tracking the rate of formation of soot clusters at various fuel concentrations, following a power law dependency with the initial acetylene concentration. The MD-obtained soot nucleation rate is incorporated in a monodisperse particle dynamics model describing soot formation in laminar premixed methane flames. The soot volume fraction predicted by the monodisperse model with the MD-derived nucleation rate is in good agreement with measurements in a methane nucleation flame (φ = 1.95), showing significant improvement (3 orders of magnitude) compared to a semi-empirical nucleation rate. The MD-derived nucleation rate also performs well in the methane sooting flame (φ = 2.32), yielding soot volume fractions comparable to experimental measurements.

Chemical nucleation of carbonaceous nanoparticles is investigated during pyrolysis of n-heptane at high temperature (2200–2600 K) by reactive molecular dynamics (MD) simulations. The MD-derived n-heptane consumption rate is in agreement with kinetic modeling, validating the present work at high temperature and high fuel concentration conditions. The critical nucleus size is quantified by the free formation energy at 2200–2600 K for n-heptane concentrations ranging from 3 × 10²⁰−9 × 10²⁰ #/cm³. Increasing temperature leads to smaller critical size, starting from 59 ± 7 carbon atoms at 2200 K that decreases down to 33 ± 3 carbon atoms at 2600 K, while the fuel concentration hardly affects the critical nucleus size. The onset time of nucleation decreases exponentially with temperature, consistent with previous shock tube pyrolysis experiments. The nucleation rate is obtained by the rate of formation of critical and supercritical hydrocarbon molecules. An Arrhenius-type relationship between the nucleation rate and the process temperature is proposed, exhibiting a first-order dependency to the initial fuel concentration. The number density of carbon nuclei derived by this nucleation rate is four to five orders of magnitude higher than that obtained by kinetic models for soot nucleation by reactive polyaromatic hydrocarbon (PAH) dimerization. The present MD-derived nucleation rate provides a computationally efficient pathway to model carbonaceous nanoparticle formation dynamics without relying on individual chemical reaction rate constants or on computationally expensive PAH-based models.