Scanning Electron Microscopic Visualization of Bridge Formation inside the Porous Channels of Diesel Particulate Filters

Time-lapse images of particulate matter (PM) deposition on diesel particulate filters (DPFs) at the PM-particle scale were obtained via field-emission scanning electron microscopy (FE-SEM). This particle scale time-series visualization showed the detailed processes of PM accumulation inside the DPF. First, PM introduced into a micro-pore of the DPF wall was deposited onto the surface of SiC grains composing the DPF, where it formed dendritic structures. The dendrite structures were locally grown at the contracted flow area between the SiC grains by accumulation of PM, ultimately constructing a bridge and closing the porous channel. To investigate the dominant parameters governing bridge formation, the filtration efficiency by Brownian diffusion and by interception obtained using theoretical filtration efficiency analysis of a spherical collector model were compared with the visualization results. The initial deposition of PM on the SiC grains showed good agreement with theoretical observations, where Brownian diffusion played a principle role in PM trapping; however, once the bridge formation commenced, another model was required to explain the locally and intensively growing dendritic structure inside the porous channel because the filtration efficiency by pre-deposited PM on SiC exceeded that of the SiC. Since the bridge formation was accelerated by the physical extent of particles due to size reduction of the flow channel via particle deposition, the flow conditions did not significantly affect the position of bridge formation; however, they did strongly affect the time duration of the bridge formation and also the PM accumulation within the DPF wall.