Market strategy of European cars manufacturers has led to a high-speed direct injection Diesel engines market share of around 40%, provided that the barrier of contemporary reductions of NOx and particulates follow Euro IV and beyond emissions legislation.
The STYFF project was the last on-going activity within the DEXA-cluster. The focus on the cluster has been to follow a systematic, concurrent and simultaneous engineering approach focusing on:
- The component technology integration aspect within ART-DEXA – Advanced Regeneration Technologies for Diesel exhaust after-treatment.
- The system design aspect within SYLOC-DEXA - System Level Optimisation and Control Tools for Diesel Exhaust After-treatment
- The quality assessment and particulate measurement aspect within PSICO-DEXA – Particulate Size and Composition measurements for Diesel exhaust after-treatment.
- The systematic specification of foam filter material and analysis of filter efficiency on a microscopic level within STYFF-DEXA – Simulation Tool for Dynamic Flow in Foam Filters.
The past projects ART-DEXA, PSICO-DEXA and SYLOC-DEXA have been focusing on so called “cake filtration” (wall-flow monoliths) technology for Diesel particulate removal.
The objective of STYFF-DEXA was to develop a simulation tool to study “deep-bed filtration” (cellular structures such as fibres or foams) materials.
Foams material can exhibit significant structural and functional advantages. Particulates in the exhaust are collected essentially on the struts surface along the whole thickness of the device, according to the "deep-filtration mechanism" whereas the gaseous fraction can flow through the open pores.
The project was carried out through the following steps:
- Development of a systematic specification and reconstruction method for foam materials. The foam structure can be specified/generated given some of its low-order statistical properties such as porosity and two-point pore-pore correlation functions.
- Validation of the generation/reconstruction method by comparing and analysing microscopic images of real foam structures and artificially generated foam patterns.
- Extension of an existing single-phase fluid mechanics solver which is based on the Lattice-Boltzmann (LB) method to enable the simulation of particle laden flows inside porous structures on single and multiple processor computer systems.
- Development and integration of sub-models concerning particulate deposition, soot burn-off (thermal, catalytic or NO2-assisted) and ash accumulation into the LB solver.
- Comparison and analysis of predicted permeabilities with measured ones available from the SYLOC-DEXA database and assessment of the regeneration efficiency of filter material by correlating predicted soot conversion rates (based on the true surface area) with experimental data available from the SYLOC-DEXA project.
- Analysis of pressure wave attenuation behaviour and its effect on noise reduction inside foam filters of different porosities using the parallelised version of the LB solver (to overcome single processor performance constraints).
- Development of an efficient and user-friendly simulation package consisting of an artificial foam generator, LB flow solver and results analyser which can be used as part of the SYLOC-DEXA toolkit or stand alone and which enables the investigation of innovative foam filter concepts such as for example the combination of filter materials of different porosity and surface coating or "synthetic foams" based on prescribed regular pore structures and pore size distributions.
- Generation of a results database containing all relevant geometric information, fluid flow and chemical kinetics parameters for foam filter materials to enable the extraction of integral (macroscopic) properties (such as porosity and permeability) which can be used by conventional Computational Fluid Dynamics (CFD) codes.
In general, the results of the STYFF-DEXA project contribute to the strategy of the European industry. The automotive end-user, aided by the supplying companies according to their expertise, is now able to use the new software developed in this project in the reduction of time-to-market of emissions reduction systems for Diesel engines and in the development of new cars, in time for meeting the 2005 European emission requirements and beyond. As a consequence, the exploitation of the results of the project will be very rapid, pushed by important commercial interests, and carried out in close collaboration with marketing and purchasing departments. A huge potential for wider exploitation in the world market exists since legislation requiring particulate control is becoming a feature of worldwide legislation following the European lead.
The main specific results are as follows.
- During the course of the project, know-how concerning modelling of Diesel particulate deposition and soot burn-off has significantly increased. The final outcome, which is the DexaSIM software package, is considered to be the most advanced tool for state-of-the-art exhaust system layout, which can be used on a daily basis in research and development of exhaust gas after-treatment systems.
- The final STYFF-DEXA diesel particulate filter based onto foams stacks has shown quite interesting performances in terms of filtration efficiency and regeneration capability. Extensive testing campaigns have been planned for the future in order to better understand the foam stacks based filter behaviour and evaluate the feasibility to apply novel catalytic coating tailor-made for the ceramic foams.
- While the porous materials of established extruded ceramic type have been successfully modelled in a CFD context with effective medium theories, e.g. in the SYLOC-DEXA project, the prediction of foam material behaviour is much more sensitive to the microstructure. In STYFF-DEXA, much progress was made in establishing good algorithms for reconstruction of foam materials, but it is an area that will require refinements. There is an interest to continue or to adopt recent developments in X-ray tomography, not only for Diesel Particulate Filter (DPF) applications.
- The work made for a side-stream reconstruction method has led to a family of process-based approaches to foam reconstruction that, with on-going development, are providing useful results fo
A comparison between the two filtration technologies shows some important advantages of ceramic or metallic foams: due to their peculiar structure the particulate distribution along the device is quite uniform and, as a consequence, the pressure drop is not very sensitive to the particulate loading. During trap regeneration the thermal load is homogeneous, limiting thermal-mechanical stresses that the substrate has to withstand.
Moreover, the cellular structure assures a high thermal shock resistance; on the contrary, the wall flow monolith structure can suffer from cracks and flaws' formation due to the thermal peaks and mechanical vibrations. Finally, for the development of a passive system with a supported catalyst, open-pore ceramic foams allows a better contact between the catalyst and the particulate, since the "cake formation" is avoided.
In order to evaluate the real advantages of foam based traps, STYFF-DEXA focused on the following:
- Optimisation of foam material for low pressure drop, high separation and regeneration efficiency via computer modelling.
- Particulate loading of foam filters by different physical phenomena, such as interception phenomena, diffusion of particles, inertia effects (their relative importance can only be understood and assessed if a detailed analysis of the flow field inside the filter material can be made).
- Regeneration efficiency depends on the available true surface area for chemical reactions taking place inside the filter. However, today this is impossible to measure.
- Unsteady pulsating flows, as they are occurring in Diesel exhaust systems, are characterised by transient pressure waves and gas flow inertia effects. They also may cause transitional turbulence phenomena. Their influence on noise attenuation and filter efficiency so far is not well understood.
No mathematical model of the above mentioned fundamental mechanisms (i.e. Brownian diffusion, interception, inertial impaction) of deep particulate filtration nor for the combustion of collected soot in ceramic or metallic foam structures was known. The computational tool DexaSIM, developed within the STYFF-DEXA project, now can aid the design of the foam filter to fit specific applications and also the study and assessment of the regeneration behaviour of the filter. The code enabled the design of a full scale prototype DPF utilising a new slits concept.