SDPF Performance – A Modeling Assessment of Diffusive Transport Phenomena
Johann Wurzenberger, AVL List GmbH
Wall flow Particulate Filters (PF), both for diesel and gasoline engines, are a well-established technology which have been developed over more than three decades. The steadily tightening emission certification limits significantly influenced the development of modern filters. The requirement of short cold-start times and low temperature PM conversion altered PFs from a pure filter device to a ‘multi-functional reactor/separator’ as classified by Konstandopoulos. Modern PFs typically feature different types of catalytic coatings supporting PM conversion or other types of conversion reactions such as selective catalytic reduction (SCR). The overall performance of catalytically supported filters is defined by several key phenomena. Advective transport takes place through the inlet/outlet channel and the porous wall in-between. Species and enthalpy are transferred between inlet/outlet channel and the wall through convection. Within the porous wall (and the PM cake) diffusive species transport dominantly overlays advection. Chemical reactions take place in vicinity of the porous wall and the PM cake involving PM, various gas species and catalysts.
Modeling and simulation of PFs have always been a central part of filter development. With the goal to investigate certain phenomena or operating conditions, rate determining steps can be found and reasonable model simplifications can be made. However, this is not possible with real engine outlet conditions. Different exhaust mass flows (engine start, idle, full load) change the dominating transport mechanisms from advection to diffusion controlled regimes. Different exhaust temperature and species compositions change the dominance in individual reactions and significantly influence local species concentrations and further diffusive transport.
The aim of this paper is to elaborate the impact of diffusive transport phenomena on the PM and NOx conversion performance on an SDPF. A comprehensive 1D+1D particulate filter model is presented and essential modeling phenomena are discussed in detail. Diffusion within the PM cake and wall is modeled using the mixed pore model, and the reaction chemistries of the SCR and passive PM conversion are taken from literature. The model is validated with the help of analytically available reference solutions of 1D reaction diffusion systems. The impact of diffusive transport from the wall to the PM cake is investigated by qualitative comparison simulations. Considered are different diffusive transfer resistances (ranging from no-diffusion to pure gas phase diffusion) applied to an SDPF operated at different typical engine outlet conditions. A variation simulation of different spatial coating zones is performed and preferred coating positions to reach best PM and NOx conversion performance are elaborated.