# Analysis on the Exhaust Air Recirculation of the Ventilation System in Multi-Story Building
## 4 Discussion  
This study conducted CFD analysis based on wind directions to quantitatively analyze the recirculation rate of pollutants from a mechanical ventilation system, which was installed in an actual kindergarten building located in Seoul, and analyze the effect of the installed guide panel on the prevention of recirculation of exhaust air. It produced the following results. The recirculation rate of exhaust air was 7.7% at its maximum, and the recirculation rate was highest with the southwesterly wind (7.7%), followed by the northerly (4.6%) and southerly wind (<0.1%). The diffusion of pollutants is identical to the air draft pattern, formed by the external wind tunnel conditions and characteristics of the building shape, and recirculation rate is in inverse proportion to the external wind speed, i.e., with increas \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Variable**} & \multicolumn{1}{c}{**1**} & \multicolumn{1}{c}{**2**} & \multicolumn{1}{c}{**3**} & \multicolumn{1}{c}{**4**} & \multicolumn{1}{c}{**5**} & \multicolumn{1}{c}{**6**} & \multicolumn{1}{c}{**7**} & \multicolumn{1}{c}{**8**} & \multicolumn{1}{c}{**9**} & \multicolumn{1}{c}{**10**} \\ \hline \multicolumn{1}{c}{Ventilation rate (m\({}^{3}\)/h)} & 150 & 250 & 400 & 150 & 250 & 400 & 150 & 250 & 400 & 150 \\ \hline \multicolumn{1}{c}{Wind direction} & \multicolumn{1}{c}{S, \(\theta\) = 180\({}^{\circ}\)} & \multicolumn{5}{c}{SW, \(\theta\) = 225\({}^{\circ}\)} & \multicolumn{5}{c}{N, \(\theta\) = 360\({}^{\circ}\)} & SW, \(\theta\) = 225\({}^{\circ}\) \\ \hline \multicolumn{1}{c}{Installation status of the guide panel} & \multicolumn{1}{c}{Installed} & \multicolumn{1}{c}{Not installed} \\ \hline \multirow{4}{*}{Recirculation rate (\%)} & 1 F & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & 1.0 & 1.0 & \textless{}0.1 \\ \cline{2-11}  & 2 F & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & 2.6 & 2.1 & 1.9 & \textless{}0.1 \\ \cline{2-11}  & 3 F & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & 0.1 & 0.1 & 0.1 & 4.6 & 2.7 & 1.7 & 0.1 \\ \cline{2-11}  & 4 F & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & 6.1 & 1.6 & 0.6 & 2.5 & 2.3 & 0.9 & 3.5 \\ \cline{2-11}  & 5 F & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & 7.7 & 3.7 & 3.1 & 1.9 & 0.7 & 0.3 & 20.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Exhaust air recirculation rate on each floor by case. Figure 9: The scope of the exhaust air diffusion (iso-surface): (**a**) Case \(1\); (**b**) Case 4; (**c**) Case 7; (**d**) Case 10.  
ing wind speed, the recirculation rate becomes lower. In addition, the recirculation rate on each floor differed by the direction of wind. The analysis results in this study show that there was a high recirculation rate on the upper floors in case of the southwesterly wind, and the recirculation rate on the middle floor was higher with a northerly external wind because of the stagnant area with a lower wind speed on the middle floor. As the characteristics of the recirculation rate of exhaust air can differ due to the deviation of the recirculation rate based on the vertical height as well as the direction of the exhaust cap and the height difference between intake and exhaust caps, these factors should be considered in designing a ventilation system. The analysis results on the recirculation rate based on the existence of a guide panel, which is installed between the intake and exhaust ports, along with the exhaust cap to prevent recirculation of exhaust air, show that the maximum recirculation rate in the case with the installation of a guide panel was 7.7%. On the contrary, in the case with the identical conditions except with the guide panel removed, the maximum recirculation rate on the same floor increased up to 20.0%, and it decreased by some degree on the other four floors. Therefore, it is determined that the installation of a guide panel has a huge impact on the recirculation of exhaust air.