## 3 Results and Discussions  
### Vertical Release Through the Leakages on the Floor  
#### 3.1.1 Iso-Surface of 4% Male Fraction  
In terms of the dispersion of hydrogen, the diffusion progress can be divided into two phases.  
1. The first phase: the hydrogen rises from the leakage position to the ceiling.
2. The second phase: the hydrogen extends along the ceiling after getting to the ceiling.  
The dispersion of hydrogen for Model 1, Model 2, and Model 3 can be seen in Figure 2. Hydrogen can be combusted at atmospheric pressure at concentrations from 4% to 74% by volume. The volume fraction of the substance is equal to the mole fraction for ideal gas mixture. Based on the assumptions for ideal gas, the mole fraction was used to investigate the diffusion progress of hydrogen in this study. The iso-surfaces show the extent of the flammable atmosphere (4% mole fraction). The time required for the iso-surface of 4% mole fraction to rise to the ceiling for Model 1, Model 2, and Model 3 was 250 s, 230 s, and 400 s,  
\begin{table}
\begin{tabular}{c} **Mole** \\ **Fraction** \\ \end{tabular} & & **Deviation** \\ \hline
113,812 & 0.030 & 3.82 & 2.35 & 5.18 & 0.97 & 5.11 & 0.19 & 3.35 & 2.98 \\ \hline
230,972 & 0.024 & 3.81 & 2.62 & 5.20 & 1.36 & 5.05 & 1.38 & 3.41 & 1.17 \\ \hline
369,015 & 0.018 & 3.91 & — & 5.13 & — & 5.12 &respectively, and the concentration envelope was of a cylindrical shape. It is evident that the time for Model 3 was the longest, while it was the shortest for Model 2.  
The velocity vectors in the median plane z = 0.37 of the envelope of the iso-surfaces of 4% mole fraction rising to the ceiling is illustrated in Figure 3. The distance between the leak position and the bottom vent in Model 2 is farther than that in Model 3, but closer than that in Model 1. From the results of the velocity vector shown in the circulation zone between the leakage position and the ceiling, the fastest vertical speed (approximately 0.33 m/s) was in Model 2. It contributed to the shortest time spent by Model 2 during the first phase. In the case of Model 3, the top vent is above the leakage position, and a large amount of hydrogen is discharged. Therefore, it takes a longer time for the iso-surface of 4% mole fraction to diffuse to the ceiling (Figure 3).  
Figure 2: Iso-surface of 4% hydrogen mole fraction for Model 1 (left), Model 2 (middle), and Model 3 (right).  
After getting to the ceiling, the envelope of iso-surfaces of 4% mole fraction extends along the ceiling and its shape is like a mushroom. The time required for the iso-surface of 4% mole fraction to extend along the ceiling for Model 1, Model 2, and Model 3 was 50 s, 30 s, and 50 s, respectively. It was found that the time required for Model 2 was about half of that for Model 1 and Model 3. This can be seen from the velocity vectors of the median plane z = 0.37 for the envelope of the iso-surfaces of 4% mole fraction diffusing horizontally along the ceiling (Figure 1). In the case of Model 1, Model 2, and Model 3, the horizontal diffusion velocity at the start and the end of the ceiling was about 0.15 m/s and 0.1 m/s, respectively, and the leakage position had little effect on the horizontal diffusion velocity of the iso-surface of 4% mole fraction at the ceiling.