*4.2. Filtration Efficiency and Regeneration*

Experimental data confirmed that the use of pre-DPF water injection does not affect the filtration efficiency of the DPF [36], which was between 99.35% and 99.85% (number-based filtration efficiency) during the soot loading process shown in Figure 2. According to the theoretical and visualization results shown in Section 4.1, this behavior can be justified based on the lack of variation of the soot penetration into the porous wall, keeping a saturated portion that acts as a barrier filter of high collection efficiency. It ensures high filtration efficiency in the entrance region before the onset of the particulate layer. In addition, the sparse thin particulate layer along the inlet channels and its concentration towards the end region also contribute to ensure the filtration performance of the DPF. Thus, the theoretical mass-based filtration efficiency is over 99.95% for all considered particulate layer characteristics, as shown in Figure 13 for operating conditions after the first and 13th water injections.

**Figure 13.** Filtration efficiency as a function of the particulate layer porosity and the soot mass distribution onset after different injection events.

With respect to the regeneration behavior, previous works showed the lack of relevant influence of pre-DPF water injection on passive and active regeneration processes based on pressure drop and outlet gas temperature evolution [12]. In this work, an active regeneration process have been modeled. Figure 14 shows in (a) the experimental and modeled pressure drop variation along the active regeneration, while (b) is devoted to the temperature fashion. In both tests (baseline and applying consecutive water injections), the DPF was previously loaded up to 60 g. After the soot loading, two consecutive NEDCs were performed before active regeneration. Details on the soot loading can be consulted in [12]. At the beginning of the regeneration, lower pressure drop in the case of the soot loading applying pre-DPF water injections can be clearly observed. This is even obtained with higher temperature along the monolith (higher outlet gas temperature), whose effect is the pressure drop increase (lower gas density). The interest for these tests is in fact such a difference in outlet temperature at the beginning of the regeneration. The outlet gas temperature is higher in the case of the DPF sample subjected to water injections. This is due to the fact that a longer thermal stabilization period took place in this case before the regeneration started. Being that the inlet gas temperature is equal during the two regenerations and the outlet one higher in the pre-DPF water injection sample, a faster regeneration process is expected for this last case. This result is also deduced from the pressure drop dynamics, which is properly predicted by the model for both tests. The temperature peak at the DPF outlet is almost equal in both tests, obtaining also consistent modeled results. The gas temperature within the monolith, which is shown in Figure 15, is also very similar in both regenerations. The temperature increase is faster in the pre-DPF water injection case mainly due to higher initial temperature. Nevertheless, this situation does not promote hot spots' appearance keeping the maximum temperature in the same order of magnitude as the baseline regeneration.

**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.

Nevertheless, a more detailed analysis of the modeling results provides interesting insights. Figure 16a shows the evolution of the DPF soot mass during the regeneration process. The soot mass depletion rate is higher in the case of the pre-DPF water injection during the first seconds of the regeneration, but it gets progressively slower. In fact, the amount of soot still accumulated into the DPF in the case of the pre-DPF water injection is clearly higher from 200 s on. 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 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 O2 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.

**Figure 16.** Comparison of the evolution of (**a**) soot mass; (**b**) particulater layer thickness; (**c**) soot depletion rate; (**d**) outlet O2 mass fraction and (**e**) filtration velocity during active regeneration: baseline vs. pre-DPF water injection.

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 O2 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

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 O2 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.
