# Flow Analysis of PM/NOX Aftertreatment System for Emergency Generator
## 4 Conclusions  
In this study, flow analysis of PM/NO\({}_{\chi}\) reduction system for large emergency generators was performed. CFD analysis was performed to understand the change in back pressure and flow uniformity according to the cross-sectional shape of the system, urea injector locations, the angle of the selective catalytic resolution (SCR) diffuser, and the porosity. The conclusions are summarized as follows.  
* The rectangle-type substrate has lower flow velocity and flow uniformity compared to the circular type, but the difference decreases as the engine load increases.
* As the distance between the urea injector and the mixer increases, the flow uniformity decreases. If the urea injection direction is arranged in the reverse direction to the exhaust flow, the back pressure increases, but the flow uniformity increases significantly as the static pressure acting in the vertical direction increases.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Circle-Type Back Pressure**} \\ \hline CFD result (mbar) & 51.5 \\ \hline Experimental result (mbar) & 53.0 \\ \hline Deviation (\%) & 2.83 \\ \hline \hline \end{tabular}
\end{table}
Table 4: CFD and experimental back pressure test results.  
Figure 14: Back pressure test rig: circle-type system.  
* When the diffuser angle is within 30\({}^{\circ}\), when the angle rises, the flow uniformity increases because the increase in static pressure has a greater influence than the decrease in the flow rate. However, if the angle of the magnifying tube is greater than 30\({}^{\circ}\), the flow phenomenon due to the increase in the angle is large, and the flow uniformity decreases as the flow stagnation phenomenon due to flow recirculation intensifies.
* As the porosity of a filter increases, the back pressure decreases, but the flow distribution characteristic degrades around the dead zone, and the flow uniformity decreases. When the porosity increases by more than 90%, the back pressure increases as the flow stagnation intensifies in the dead zone.  
Conceptualization, H.B. and S.L.; methodology, G.P. and H.B.; investigation, H.B., G.P. and S.L.; supervision, S.L.; writing--original draft, G.P. and H.B.; writing--review and editing, H.B. and S.L. All authors have read and agreed to the published version of the manuscript.  
This research was supported partly by the Korea Environment Industry & Technology Institute (KEITI), through the Reduction Management Program of Fine Dust Blind-Spots grant funded by the Korea government (MOE; Ministry of Environment) (No. 2020003070001), through BK21 Program grant funded by the Korea government (MOE; Ministry of Education) (No. 5199990814084).  
Not applicable.  
All necessary data have been reported in this article and there are no other data to share.  
The authors thank the researchers at the Korea Automotive Technology Institute (Katech) for their assistance in this study.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
DPF & Diesel particulate filter  
DOC & Diesel oxidation catalyst  
SCR & Selective catalyst reduction  
PM & Particulate matter