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In the current work, a comprehensive computational fluid dynamics (CFD) model is developed for an accurate description of the transport and transformation of automotive particulate matter (PM) in monolithic reactors. The model accounts for the developing gas flow, the evaporation of hydrocarbons (HCs) from the particles, and the adsorption of HCs in the washcoat, as well as motion, shrinkage, and deposition of particles in the channel. The comprehensive CFD model is used to validate a simplified tanks‐in‐series approach with a conceptual model for PM transformations. In the development of more detailed and accurate chemical kinetics for the reactions of PM in filters, it will be necessary to also predict the time‐resolved properties of the particles collected in the filter (e.g., reactivity, amount of adsorbed HCs). It is shown in this work how the data necessary to construct such models can be obtained in situ with the aid of the conceptual model and PM measurements over an inert open substrate.

The Knudsen paradox—the non-monotonous variation of mass-flow rate with the Knudsen number—is a unique and well-established signature of micro-channel rarefied flows. A particle which is not of insignificant size in relation to the duct geometry can significantly alter the flow behavior when introduced in such a system. In this work, we investigate the effects of a stationary particle on a micro-channel Poiseuille flow, from continuum to free-molecular conditions, using the direct simulation Monte-Carlo (DSMC) method. We establish a hydrodynamic basis for such an investigation by evaluating the flow around the particle and study the blockage effect on the Knudsen paradox. Our results show that with the presence of a particle this paradoxical behavior is altered. The effect is more significant as the particle becomes large and results from a shift towards relatively more ballistic molecular motion at shorter geometrical distances. The need to account for combinations of local and non-local transport effects in modeling reactive gas–solid flows in confined geometries at the nano-scale and in nanofabrication of model pore systems is discussed in relation to these results.

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