# Flow Analysis of PM/NOX Aftertreatment System for Emergency Generator
## 3 Results
### Difference according to the Injector Locations  
Ammonia distribution at the front of SCR, according to the location and direction of urea injection, was examined. The urea injection location is a variable that directly affects flow uniformity because it determines the residence time and ammonia production location of urea. Therefore, it is necessary to derive an optimal injection position capable of minimizing the pressure loss and minimizing the loss due to the dead zone occurring at the corner of the rectangle type. In the basic design, a 45-degree standard mixer is mounted on the front-SCR, which promotes mixing of Urea more uniformly. In this study, the change in ammonia distribution characteristics was analyzed by changing the location and direction of the urea injector.  
Four cases were interpreted: Case 1, where the injector is located as close as possible, Case 2, where the injector is located relatively far from the mixer, Case 3, where the positions of the mixer and the injector are reversed, and Case 4, where the injection direction is set as opposite to exhaust flow. Figure 7 summarizes the distribution of front-SCR ammonia by arrangement case of each urea injector for circular and rectangle systems.  
Figure 8 shows the comparison of flow uniformity and back pressure characteristics according to the urea injector position for circular and rectangle systems. As a result of Case 1, it is observed that ammonia is distributed unevenly overall. That is, although turbulence was formed by a mixer mounted on the front end of the injector, a distance between the expansion tube and the injector was relatively close; thus, even distribution was not generated, and ammonia distribution was locally rich. As in Case 2, when the injector is positioned farther from the mixer and closer to the SCR front end, the turbulence formation effect by the mixer is reduced, and the sprayed urea does not sufficiently reach the dead zone; thus, ammonia distribution is significantly concentrated in the center. As a result, the lowest flow uniformity value was shown. However, the low back pressure was recorded, relatively.  
In Case 3, where the injector is set to a reverse direction facing the exhaust flow direction, an aqueous element solution sprayed in the reverse direction meets turbulent exhaust flow formed by a mixer and is mixed fast, thereby improving in terms of ammonia distribution. Resistance is generated in the process of meeting the exhaust flow and the flow rate of the aqueous element solution in opposite directions, leading to a decrease in the flow rate and an increase in back pressure, but the flow uniformity is greatly improved.  
Figure 7: NH\({}_{3}\) distribution with injector locations.  
Lastly, Case 4, in which a mixer is positioned at the rear end of the injector, the sprayed urea-water mixture is directly mixed by the mixer, and efficient mixing occurs at the front end of the SCR. However, due to the stagnation of some ammonia in the dead zone, the back pressure was lower than Case 3, but the uniformity was relatively low. Therefore, considering the increase in flow uniformity, Case 3, which sprays urea in the reverse direction to exhaust flow, can be seen as the most efficient arrangement. If the urea injector is installed in the reverse direction, the back pressure may increase compared to the initial model, and thus the power may be partially degraded, but NH\({}_{3}\) and the exhaust gas are much more evenly mixed in the exhaust pipe, hence raising the reduction efficiency of NO\({}_{\mathrm{X}}\) to a high level.