CFD modeling of a vehicle exhaust laboratory sampling system: sulfur-driven nucleation and growth in diluting diesel exhaust

摘要

A new exhaust aerosol model CFD-TUTEAM (Tampere University of TechnologyExhaust Aerosol Model for Computational Fluid Dynamics) was developed. It isbased on modal aerosol dynamics modeling with log-normal assumption ofparticle distributions. The model has an Eulerian sub-model providingdetailed spatial information within the computational domain and acomputationally less expensive, but spatial-information-lacking, Lagrangiansub-model. Particle formation in a laboratory sampling system that includes aporous tube-type diluter and an aging chamber was modeled with CFD-TUTEAM.The simulation results imply that over 99% of new particles are formedin the aging chamber region because the nucleation rate remains at a high level inthe aging chamber due to low dilution ratio and low nucleation exponents. Thenucleation exponents for sulfuric acid in sulfuric-acid–water nucleationranging from 0.25 to 1 appeared to fit best with measurement data, which arethe same values as obtained from the slopes of the measured volatilenucleation mode number concentration vs. the measured raw exhaust sulfuricacid concentration. These nucleation exponents are very low compared to thenucleation exponents obtained from the classical nucleation theory of binarysulfuric-acid–water nucleation. The values of nucleation exponent lower thanunity suggest that other compounds, such as hydrocarbons, might have asignificant role in the nucleation process.