Comparison of soot models for reacting sprays in diesel engine-like conditions

Abstract

Stringent emission legislations and growing health concerns have contributed to the evolution of soot modeling in diesel engines from simple empirical relations to methods involving detailed kinetics and complex aerosol dynamics. In this paper, four different soot models have been evaluated for the high temperature, high pressure combusting dodecane spray cases of engine combustion network (ECN) spray A which mimics engine-relevant conditions. The soot models considered include an empirical, a multistep, a method of moments based, and a discrete sectional method soot model. Two experimental cases with ambient oxygen volume of 21% and 15% have been modeled. A good agreement between simulations and experiments for vapor penetration and heat release rate has been obtained. Quasi-steady soot volume fraction contours for the four soot models have been compared with experiments. Contours of the species and source terms involved in soot modeling have also been compared for a better understanding of soot processes. The empirical soot model results in higher magnitude and spread of soot due to a lack of modeling framework for oxidation through OH species. Among the four models studied, the multistep soot model has been observed to provide the most promising agreement with the experimental data in terms of distribution of soot and location of peak soot volume fraction. Due to a two-way coupling of soot models, the detailed models predict an upstream location for soot as compared to the multi-step soot model which is one way coupled. A significant difference (of an order of magnitude) in the concentration of PAH (polycyclic aromatic hydrocarbons) precursor between multistep and detailed soot models has been observed because of precursor consumption due to the coupling of detailed soot models with chemical kinetics. It is recommended that kinetic schemes, especially those concerning PAH, be validated with experimental data with a kinetics-coupled soot model.

Investigating the Effects of Chemical Mechanism on Soot Formation Under High-Pressure Fuel Pyrolysis

“…One contributor is that some kinetic mechanisms and soot models are tuned to capture PAH formation under only oxidizing engine conditions (Wang et al, 2014), which can negatively affect their performance under pure pyrolysis conditions. Additionally, Duvvuri et al (2021) pointed out that kinetic mechanisms are sometimes used without a coupled soot model during validation in which they are used to reproduce the species concentrations from shock tube experiments. Because the formation and destruction of PAH species are tied to the destruction and formation of soot, failing to use a coupled soot model during validation could lead to mechanisms with reduced formation of PAH species.…”

“…Computational Fluid Dynamics (CFD) simulations can capture these length and time scales, while also providing a means of understanding the time-history of short lived and intermediate species (Duvvuri et al, 2019). There have been several studies through the Engine Combustion Network that have leveraged the extensive experimental database and explored the use of a number of different turbulence models (Skeen et al, 2016;Chishty et al, 2018) and soot models (Duvvuri et al, 2021;Ong et al, 2021).…”