The importance of reversibility in modeling soot nucleation and condensation processes

Given the upcoming EURO 6 regulations, which include limits on particle number density (and hence size) for soot emissions from land vehicles, soot models must be capable of accurately predicting soot particle sizes. Previous modeling work has demonstrated the importance of the relative strengths of nucleation and condensation in predicting soot primary particle size; however, fundamental models still rely on tunable constants for modeling both processes, which limits predictive capability. Recent investigations into nucleation and condensation processes suggest that both processes are not thermodynamically favored to occur from 5-ringed PAHs, yet 5-ringed PAHs have been experimentally observed in abundance within nascent soot particles. This contradiction leads to the understanding that nucleation and condensation from 5-ringed PAHs is plausible, although they are likely highly reversible processes. A fundamental reversible model for nucleation and condensation is developed through the use of statistical mechanics and the results from several recent works. The model is highly sensitive to both the binding energy and the vibration frequencies created during the nucleation and condensation processes, although reasonable values are obtained through an extensive literature review. A model for tracking the PAHs on the surface of soot particles is developed, which allows for the calculation of the reverse rate of nucleation and condensation. The inclusion of reversibility in the nucleation and condensation subroutines enables the model to accurately reproduce all relevant soot morphological parameters determined experimentally for the atmospheric pressure, laminar, ethylene-air Santoro flame. This is due to more accurate partitioning of PAH mass through the nucleation and condensation processes. The developed reversible model represents an advancement in fundamental soot formation modeling by replacing tunable constants with fundamental physics. (C) 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.