Soot particle deposition efficiency of diesel pm-catalyst structures - The influence of structure geometry and transient temperature inhomogeneities

The soot particle deposition in novel diesel particulate matter (DPM) catalyst structures was investigated on a lab-scale at a model test gas bench and in the exhaust system of a heavy duty diesel vehicle engine. Three different design approaches are compared. Under stationary conditions particle deposition is found to be caused by diffusional deposition, interception as well as particle transport to the structures' wall induced by exhaust flow accelerated around the corrugations. Diffusion leads to a pronounced deposition of small particles with mobility diameters smaller than 60 nm. The measured size-resolved filtration efficiency can be described by a phenomenological model derived from foam filtration. The influence of thermophoresis on soot particle deposition during transient engine operation was investigated applying a known mathematic description for thermophoretic deposition combined with a thermophoretic coefficient K_th = 0.55, that has been derived experimentally for soot agglomerates exhibiting a reduced intra-particular heat conductivity. Thermophoresis contributes significantly to diesel soot particle deposition under transient conditions, especially during acceleration periods when hot exhaust gas with a high particulate mass fraction enters the colder exhaust line. The PM catalyst approach based on specially corrugated stainless steel foils with a microparticle coating is found to be a promising approach for continuous deposition and oxidation of diesel soot for retrofitting as well as novel HDV exhaust systems leading to a reduction of particulate mass emission by up to 52 %.