Simulation of Diesel Particulate Filters

Optimization of Sinter Metal Filters with Ceramic Fiber Layers

Fraunhofer ITWM cooperates with Robert Bosch GmbH to develop innovative particle filtering systems for the exhaust gas system of diesel vehicles. Research mainly focusses on sinter metal filters with ceramic fiber layers. These materials have to be optimized regarding to backpressure and filter lifetime.
This requires generating the sinter substrate as a virtual geometry using ITWM's structure generator GeoDict.

Due to the large scale range, flow and particle simulations are performed on the micro scale and on the nano scale.

Simulation Dieselpartikelfilter — © Photo ITWM
Simulation of diesel particle filters

Geometry Generation

The first thing to do is generating a virtual geometry of the sinter substrate and fiber layer using ITWM's structure generator GeoDict. Modelling the substrate is done based on a virtual packing of cylinders and spheres, whose sizes are distributed similarly to real sinter grains.

Using morphological dilation and erosion, it is possible to obtain structures, whose properties like porosity or chord length distribution are similar to grinding surface patterns of real samples. The fiber layer is generated by the nonwoven-generator of GeoDict. Required input parameters like porosity or fiber orientation are being extracted from real grinding surface images.

Simulation of Diesel Particulate Filters - Simulation Process

Filtration siumlation in virtual materials is done using the GeoDict-module FilterDict. After specifying the soot particle distribution and various process parameters, flow field and particle transport are calculated.

The Lagrangian description of particle motion includes:

Particle-Fluid Friction
Particle Inertia
Brownian Motion

Deposited particles modify the geometry for the next iteration step.

Challenge of Scale Differences

One difficulty is the scale difference between filter material and soot particles. Typically, the filter has a resolution of a few micrometers, while the smallest particles have just a size of some nanometers. For that reason, deposited particles are modelled as porous structures.

They influence flow simulation in a Stokes-Brinkmann-model as permeability. It is worth mentioning that model parameters for porous structures in the micro simulation are being derived from high-resolution simulations of single fibers on the nano scale.

Simulation Partikelfilter — © Photo ITWM
Simulation of diesel particle filters

Vergleich zweier Filter mit und ohne Faserauflage — © Photo ITWM
Comparison of two filters with and without fiber layer

Generally, simulations are highly demanding and require multiple days on ITWM's cluster. In a simulative comparison of two filters with and without fiber layer, we have to ascertain the delay of the back pressure increase, i.e. the extended filter life time by using an additional fiber layer. This behavior can also be observed in experiments.

Project Partner

Robert Bosch GmbH

Fraunhofer Institute for Industrial Mathematics ITWM