# Flow Analysis of PM/NOX Aftertreatment System for Emergency Generator
## 3 Results  
### Difference according to the Shape of the Substrate  
As described in the introduction, when designing a single type of aftertreatment system for large generators, it is recommended to apply a system with a square substrate rather than a circular one for manufacturing reasons, since it is necessary to secure a cross-sectional diameter of 762 mm or more according to the specifications. Therefore, since it is expected to show different flow characteristics from the system to which the conventional circular substrate is applied, ammonia flow uniformity was compared at the front of the SCR according to the shape of each substrate to compare the difference in reduction efficiency between the rectangular and circular substrates. The flow area except the porous filter was designed in a circular shape to ensure maximum uniformity, and the urea mixer was designed in front of the injector to evenly mix ammonia in both systems. First, the distribution of the exhaust flow rate for the system of the square type during 75% load operation was analyzed, and Figure 3 visually shows the result. The flow velocity distribution is an index that can grasp the characteristics of exhaust flow throughout the system, so it is the first CFD analysis process to be preceded. As a result of analyzing the flow velocity distribution, the flow velocity increased to about 55 m/s in DOC, and DPF made of porous media decreased to about 27 m/s immediately after passing through the filter, and then turbulent flow was formed by the mixer, and the flow velocity in the reactor section rose again to about 37 m/s. After that, the exhaust flow passes through the SCR composed of 16 square substrates. At this time, the average flow rate is 30 m/s, and then it passes through the outlet without a large change in flow rate. Figure 4 is an analysis of the flow velocity distribution for the system based on a circular type having the same volume. The average exhaust flow rate is about 17% higher than that of the rectangle type, which is estimated to be due to the difference \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Item** & **Value** \\ \hline  & Type & V-12 \\ Engine & Displacement & 39.0 L \\ Specifications & Power output & 560 kW/1800 rpm \\  & Emission level & Tier-2 \\ \hline  & Inlet mass flow rate & 1.223 kg/s \\ Exhaust system & Outlet pressure & 0 \\  & Exhaust temp & 678 K \\ \hline  & Number of nozzles & 4 EA \\ Urea & Cone angle (Solid) & 39\({}^{\circ}\) \\ injection system & Hole diameter & 200 μm \\  & Liquid temp & 298.15 K \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Boundary conditions for CFD analysis [17]. Figure 3: Velocity magnitude schematic of rectangle-type PM/NOX aftertreatment system. in flow resistance due to the difference in cross-sectional shape, and analysis through flow uniformity analysis at the front of the SCR according to the cross-sectional shape is necessary. The exhaust flow rates at each part according to the cross-sectional shape of the system are summarized in Table 3.  
Figure 5 shows the flow distribution at the front of the SCR according to the load condition of the engine by comparing the shape of the substrate. In the case of the rectangular type, under low load, ammonia was not sufficiently distributed on the wall surface due to dead zones generated at the corners when the flow velocity was not sufficiently secured. However, as the load increases, the flow rate increases, and a relatively large amount of ammonia is distributed well to the wall surface. In the case of the circular type, there are no dead zones that are severely restricted by the flow such as with the rectangular type, so a relatively even distribution is shown, but when the flow velocity increases as the load increases, a phenomenon where a part of ammonia is locally concentrated on the wall surface appears. This can be seen as a result of relatively stagnant ammonia flow near the wall due to the result that the influence of friction acting on the wall increases with the increase of the flow velocity.