# Analysis on the Exhaust Air Recirculation of the Ventilation System in Multi-Story Building
## 3 Results of CFD Analysis
### Distribution of Exhaust Air Region  
As mentioned in Section 2.2 of this study, the recirculation of exhaust air was calculated using the mass fraction boundary condition. Each ID is assigned to the air exhausted from the five floors and the outdoor air to evaluate the effect of exhaust air individually. By assigning one to the mass fraction value of the outdoor and exhausted air from the exhaust port of each floor, the proportion of exhaust air in the intake air can be calculated using CFD analysis. In other words, since the mass fraction value of the exhaust air stays between zero and one, it is possible to visualize the region's distribution affected by the exhaust air through CFD post-processing. Additionally, since the sum of the mass fraction values of the air entering the intake port is always one, it is possible to calculate the recirculation rate by subtracting the mass fraction of the outdoor air from one. Since the mass fraction value of the exhaust air is set to one, the normalized concentration (NC) and the mass fraction value are the same. Figure 8 presents the NC distribution of Cases 1, 4, and 7, meaning that the closer the value is to one, the greater the effect of exhaust air. The NC of exhaust air in Case 1 shows that the emission direction of the exhaust air faces downward because of the exhaust cap; therefore, on the first and second floors, it flows more strongly downward against external airflow, and on the third and fourth floors, the downdraft is formed near the exhaust cap because of the stronger effect of the emission direction.  
On the fifth floor, because of the external strong updraft, the exhaust air near the exhaust cap moves upward. However, the direction of external wind direction does not push the exhaust air to the intake port. In Case 4, because of the direction of external air, the exhaust air was shown to move toward the intake port. Centered on the third floor, upper floors form updraft and lower floors form downdraft, and the external wind pushes the air from the exhaust port to the intake port. As the exhaust port on the same floor is located lower than the intake port, the exhaust air from the ventilation system on the fourth and fifth floors move closer to the intake port. In Case 7, an updraft forms on the exhaust and intake ports on all floors, but the power of the updraft is so small that, compared to the other cases, the air becomes more stagnant, and the exhaust air stays near the exhaust port. In Case 10, the exhaust air could flow closer to the wall compared with Case 4 as there was no guide panel, and similarly, based on the third floor, upper floors formed an updraft and lower floors formed a downdraft.  
Figure 9 shows an iso-surface in which the NC value of the exhaust air is less than 0.05, and the diffusion area of the exhaust air can be confirmed in 3D. Consequently, there was no case where the exhaust air of each floor is recirculated through the intake ports of the other floors. Thus, when the exhaust and intake ports are placed vertically on the outer wall of the building, cross-contamination between floors can be prevented by simply changing the direction of the exhaust and intake ports.