Wall-flow filters are applied in the exhaust treatment of internal combustion engines, where they are used for the removal of emitted particulate matter (PM).
Such filters consist of oppositely arranged inlet and outlet channels separated by porous substrate walls.
When traversing the filter, the entering exhaust gas is forced to pass through those walls while being stripped from the majority of the contained particles.
This way, the introduced solid material forms a permeable deposition layer on the filter substrate with continuously increasing thickness.
Over time, the pressure drop over the filter increases, respectively, leading to the necessity of the filter's regeneration.
During such a regeneration process, the coherent layer can break up into individual layer fragments, which potentially rearrange inside single filter channels.
This can, eventually, lead to the formation of specific deposition patterns, which affect a filter's pressure drop, its ash storage capacity and the overall separation efficiency.
The rearrangement process is governed by the interaction of hydrodynamic and adhesive forces, which can only be accurately modelled by taking the surfaces of individual layer fragments into account.
Despite its well-known significance on the filtration process, the formation of specific deposition patterns can still not consistently be attributed to particular influence factors and appropriate calculation models that enable a quantification of respective factors do not exist.
This present work, therefore, approaches a fundamental investigation of particle-layer rearrangement events in ceramic wall-flow filters by means of surface resolved particle simulations with lattice Boltzmann methods (LBM).
The main goal is the determination of the sensitivity and the quantification of the impact of engineering-related influence factors on the transient rearrangement process in wall-flow filters.
A versatile and easily extendable implementation of the lattice Boltzmann method is provided by the open source software OpenLB.
It is therefore used and extended accordingly in the present work.
This way, the conservation of the developed methodology is ensured in a publicly available resource that enables full reproducibility of the obtained results.
The development and application of an appropriate simulation model is considered in four consecutive solution steps, which are based on four separate works.
The first step is dedicated to the model development by providing a simulation approach, which is capable of capturing the time dependent development of the gaseous flow field inside a wall-flow filter during its regeneration.
Its validity and applicability is then assessed by conducting grid convergence studies and comparing it to a reference solution.
A static fragmented particle layer is examined regarding the spatial distribution of the acting hydrodynamic forces and the impact of fragment clusters at different channel positions.
Afterwards, some first transient simulations of fragment detachment are conducted in a low velocity regime and the impact of assorted layer properties is examined.
A second step is intended to provide closer insights into the behaviour of individual particle layer fragments during the filter regeneration.
In this context, the temporal evolution of the hydrodynamic forces acting on a fragment's surface are examined in detail for different fragment densities and detachment positions.
Relevant key quantities, such as the impact velocity at the channel's back wall and the stopping distance, are defined, determined and interpreted with respect to predictions regarding the resulting deposition patterns.
At first, a physically sensible substrate permeability is determined by performing aligned experimental and numerical pressure drop studies.
Afterwards, the transient behaviour of single particle layer fragments with different densities are examined in detail with respect to all acting forces.
The fragment's pneumatic transport through the channel is then investigated for different starting positions, substrate permeabilities, inflow velocities and average densities of the introduced particulate matter.
Eventually, the general movement characteristics are assessed with respect to their qualitative accordance with experimental observations.
In a third step, the developed LBM approach is applied to an extended velocity range that covers as much as possible of the physically relevant domain, which includes averaged channel inflow velocities of up to 80 m/s.
After an adaption of the boundary conditions, three different scenarios are investigated, by considering the particle-free flow only, a single layer fragment attached to one of the substrate walls and a fragmented particle layer.
At first, a comprehensive quantification of the stability and accuracy of both particle-free and particle-including flows, considering static, impermeable deposition-layer fragments, is conducted.
Afterwards, the local effects of a fragmented particle layer are investigated for different static situations along the channel.
In order to derive predictions on the detachment likelihood of individual layer fragments and their mutual influence, the spatial distribution of hydrodynamic forces is evaluated additionally.
A final step is specifically dedicated to the investigation of the process of plug formation, by additionally considering the fragments' dynamic behaviour following the layer fragmentation.
This includes the detachment and transport of the fragments along the channel, as well as the subsequent formation of a channel-plug.
In order to accurately account for fragment contact during their transport, the model is extended by a previously developed discrete contact method.
With the updated model, the influence of the fragmented layer topology, the fragment properties and the operating conditions is evaluated and relevant key quantities are determined.
The results attained with those four steps represent a comprehensive quantification of the impact of relevant influence factors on various aspects of the transient rearrangement process in wall-flow filters.
In that way, the present work represents a significant contribution to the persisting research gaps in this field.
It additionally, provides potential opportunities for modifications of the filtration process, which could lead to an optimization of engine performance, fuel consumption and service life of wall-flow filters.
