Passive Oxidation Case

PO-B20-13 is a passive oxidation experiment carried out at a CPF inlet temperature of 403 ^◦C and an inlet N O₂/N O_x ratio of 64/204 for a duration of 42 minutes and an inlet O₂ concentration of 7.3 % by volume as shown in Table4.3. The following sub-sections discuss the relevant results obtained from the model calibration to this experiment using filtration parameters as shown in Table I.1, PM kinetics for fuel type B20 as shown in Table5.3and catalytic reaction kinetics as shown in Table5.2.

Pressure Drop

The overall pressure drop across the CPF was calibrated to within (-0.34/+0.23 kPa) of the experimental values of CPF pressure drop measured during the passive oxidation and active regeneration experiments as shown in TableJ.1. Figure5.5 shows the experimental and simulated pressure drop from PO-B20-13. The largest differences observed between experimental and simulated pressure drops were -0.12/+0.19 kPa. The total simulated CPF pressure drop is a resultant of 3 different components: cake, wall and channels. These components can be analyzed from the CPF model outputs as shown in Figure 5.5. The highest contribution to the total pressure drop at all points in time for PO-B20-13 is by the substrate wall.

PM Mass

Figure5.6shows the simulated cumulative PM mass balance along with a comparison of experimental (blue circles) and simulated (green line) PM mass retained. A comparison of PM mass retained at the end of various stages for all passive oxidation and active regeneration experiments is shown in TableJ.2.

The total PM mass retained is the sum of PM mass retained in the PM cake layer and substrate wall. The results from the calibrated model are as shown in Figure 5.7 for PO-B20-13. The PM mass retained in the PM cake layer accounts for majority of the total PM mass retained in the CPF, whereas the PM mass retained in the substrate wall

Figure 5.5: Comparison of experimental and model total pressure drop across CPF and its components - PO-B20-13

Figure 5.6: Model cumulative PM mass balance and comparison of experimental and model PM mass retained - PO-B20-13 (Table in the inset shows a comparison of experimental and model PM

mass retained at the end of all stages)

increases linearly to ~1.5 g in the deep bed filtration region (time < 0.3 hrs) and thereafter decreases to ~0.5 g during the passive oxidation stage due to the PM oxidation mainly from the N O₂-assisted reaction.

Figure 5.7: Distribution of model total PM mass retained into PM cake and substrate wall -PO-B20-13

The PM mass oxidized in the CPF is the sum of that oxidized by two reactions: thermal (O₂) and N O₂-assisted. The calibrated CPF model outputs can be used to derive the distribution of the total PM mass oxidized by thermal and N O₂-assisted. A plot of this distribution is shown in Figure5.8. The top plot shows the instantaneous PM oxidation rates (expressed in g/s) and the bottom plot shows the cumulative PM mass oxidized (expressed in g) through the duration of the experiment. An analysis of the data shows that during this passive oxidation experiment, 95% of the total PM mass oxidized during passive oxidation (22.7 g) was by N O₂-assisted PM oxidation reaction.

PM mass oxidized in the filter is also the sum of PM mass oxidized in the PM cake layer and the substrate wall. The outputs from the model can also be used to obtain the distribution of the PM mass oxidized between the PM cake layer and substrate wall as shown in Figure 5.9. This shows that the majority of PM mass oxidation occurs in the PM cake layer which was 96% of the total PM mass oxidized during the passive oxidation experiment.

Figure 5.8: Distribution of total PM oxidation rate (top) and cumulative PM mass oxidized (bottom) into thermal (red) and N O2-assisted (blue) mechanisms

Figure 5.9: Distribution of total PM oxidation rate (top) and cumulative PM mass oxidized (bottom) into PM cake (purple) and substrate wall (green) - PO-B20-13

Filtration Efficiency

Figure5.10 shows the simulated total filtration efficiency of the CPF compared to the PM mass-based filtration efficiency measured (97% - shown as the black ‘x’) during stage-2 loading. This plot also shows the components of the efficiency namely cake and wall. The total filtration efficiency of the filter at initiation of the experiments comes entirely from the substrate wall (37%). Filtration efficiency of the substrate wall increases to about 47%

due to wall PM loading during deep-bed filtration, followed by PM cake layer formation.

Once PM cake layer starts to form, cake filtration efficiency increases to 95.8% and wall filtration efficiency remains constant at ~47%.

Figure 5.10: Comparison of experimental and model total filtration efficiency and distribution of filtration efficiency into PM cake and substrate wall - PO-B20-13

Exhaust Gas Temperature

The CPF model calculates the exhaust gas temperature as it passes through the inlet channel, substrate wall and outlet channel. Agreement with experimentally measured temperature data is critical in accurately predicting the rates of all reactions since they are strongly dependent on the temperature via the exponential term. Figure 5.11 shows a comparison of simulated and experimental CPF outlet temperatures for PO-B20-13, showing agreement within -2/+1^◦C of the experimental outlet temperature values as well as transient behavior (thermal response) of the CPF.

Figure 5.11: Comparison of experimental and model CPF outlet gas temperature with model input CPF inlet gas temperature - PO-B20-13 [Top left: comparison of temperatures for 12 minutes during the start of passive oxidation stage, Top right: comparison of temperatures for 12 minutes

during the end of passive oxidation stage, Bottom: comparison of temperatures for the entire duration of the experiment]

N O₂ Concentration

Figure5.12 shows CPF inlet N O₂ concentrations from measured experimental values as well as a comparison of model outlet N O2 concentrations (red line) to experimental CPF outlet N O₂ concentrations (red ‘x’), showing that the overall agreement of model N O₂ concentrations was within -15/+12 ppm of experimental values for PO-B20-13.

Figure 5.12: Comparison of experimental and model CPF outlet N O2concentrations with model input CPF inlet N O₂concentrations - PO-B20-13