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Particulate matter trapping and regeneration processes in wall-flow diesel particulate filters (DPFs) without catalysts were investigated through micro-and macroscopic visualization experiment. The vertical walls of a small DPF were polished using a lapping process to create a mirror-like surface on each ceramic particle grain. Using an all-in-focus optical microscope, micro-scale flow pores inside the DPF wall could be clearly observed from the polished surface to a depth of 100 mm. Furthermore, a real-time video with a speed of 30 frames per second could be sharply recorded. Through the microscopic cross-sectional view, transition from depth filtration to surface filtration could be observed clearly. Only surface pores opened on the wall surface were related to the filtration depth, i.e. the penetration depth. During regeneration of the DPF without catalyst, after a particulate (soot) cake was burnt out, the particulates trapped inside the surface pores were oxidized. On the other hand, using a halfcylindrical-shaped, wall-flow DPF, the overall trapping phenomena and regeneration process were clarified through a long-distance focusing lens camera. Diesel particulates were trapped almost uniformly over the entire surface of an inflow channel of the DPF in the direction of the channel flow, while the trapped particulates were not necessarily oxidized uniformly since there was a large temperature difference between the inlet and the outlet of the flow channel. The regeneration patterns were strongly dependent on the initial particulate mass and the inlet temperature of the working gas, including the microscopic phenomena in each location. Consequently, microscopic surface pores played a significant role in the regeneration process as well as in the beginning of trapping. Furthermore, at a macroscopic level, a uniform temperature and wall-flow distributions were found to be significant for quick regeneration.

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