Modelling of diesel particulate filtration in wall-flow traps

The present work addresses the modelling aspects of soot deposition inside diesel particulate filters (DPFs). These wall-flow monoliths are commonly employed in the automotive sector to reduce particulate emissions of Diesel engines. Simulations were carried out using computational fluid dynamics under different operating conditions by varying the exhaust-gas inlet velocity, the filter permeability and the soot particle size. The numerical results show that the flow field inside the filter is highly responsible for the soot distribution along the axial coordinate: the velocity of the soot laden gas through the porous wall of the bare filter is not constant along the channel, which in turn promotes an uneven deposition of particles inside the filter itself. Moreover, it has been shown that the fraction of the particles that impact the wall surface and are trapped by the porous media is sensitive to the particle size: the collection efficiency was estimated, and the most penetrating particles were found to range from 200 nm to 500 nm. The accumulation of soot particles in the porous media (depth filtration) and the subsequent formation of a soot particles layer on the top of the filter wall (cake filtration) generates an additional pressure drop. Hence, the gas through-wall velocity evolves towards an “iso-permeability” profile characterized by an almost constant soot layer thickness, since its permeability is two orders of magnitude higher than that of the bare filter. Preliminary experimental results have confirmed this behaviour. In addition, it has been shown that the velocity profile at the inlet of the filter is highly responsible for different soot loadings of the inlet filter channels.