# On the Impact of Particulate Matter Distribution on Pressure Drop of Wall-Flow Particulate Filters
## 4 Discussion of the Results
### Filtration Efficiency and Regeneration
Therefore, the fast reduction in pressure drop during this test can be only explained by non-uniformity in the soot depletion rate along the monolith. Figure 16b shows the variation in particulate layer thickness during the regeneration process in different channels' locations. In the baseline regeneration, the change in particulate layer  
Figure 14: Caption can be rewritten as: Comparison between experimental and modeled (**a**) DPF pressure drop and (**b**) gas temperature during active regeneration: baseline vs. pre-DPF water injection. Figure 15: Gas temperature evolution during the regeneration process in different locations of the monolith. thickness follows the same dynamics at all distances observing just some delay towards the monolith rear end governed by the thermal transient. This is confirmed by the soot depletion rate and the O\({}_{2}\) outlet mass fraction shown in (c) and (d) of Figure 16, respectively. This behavior is governed by the filtration velocity, which is shown in Figure 16e. It is very homogenous along the inlet channels, thus leading to similar gas mass flow and dwell time. Nevertheless, the different initial particulate layer thickness in pre-DPF water injection case conditions its subsequent reduction dynamics along the regeneration process. Figure 16b confirms how the particulate layer reaches a thin thickness and disappears very fast up to the intermediate inlet channel region. It explains the fast pressure drop reduction inducing to conclude that the regeneration is close to the end. However, the oxidation of soot in the rear end region, where most of the soot has been dragged, is very slow. It is interesting to note that according to Figure 16c, the depletion rate is very similar in the inlet and rear end region, being maximum in the middle region. This is explained by the filtration velocity. It is very high at 6 cm from the very beginning because of the thin particulate layer. Consequently, the soot depletion rate is determined by a reduced dwell time, but high gas temperature and O\({}_{2}\) concentration (outlet mass fraction shown in Figure 16d). As a result, the soot depletion rate is high enough to quickly remove the particulate layer. As the particulate layer is progressively removed along the inlet channels, the filtration velocities tend to coincide, as deduced  
Figure 16: Comparison of the evolution of (**a**) soot mass; (**b**) particular layer thickness; (**c**) soot depletion rate; (**d**) outlet O2 mass fraction and (**e**) filtration velocity during active regeneration: baseline vs. pre-DPF water injection. from the analysis of the filtration velocity at 6 cm and 12 cm. Compared to these distances, the soot depletion rate in the rear end (18 cm), where the particulate layer is thicker, is as high as at 6 cm. However, the filtration velocity is very small. It provides high dwell time, favoring soot oxidation, but the small total amount of O\({}_{2}\) mass (despite high inlet concentration) produced is all consumed, thus limiting the soot depletion rate. This behavior is more penalized as soot is oxidized in the inlet and middle channel regions because the flow tends to go across the porous wall in these sections. Consequently, the filtration velocity is further reduced in the channel rear end, slowing down the final regeneration phase.