# Experimental Study on Dynamic Exhaust Law for Ventilation System of Gas Tunnel
## 4 Results and Discussion
### Concentration Variation  
When the initial gas concentration in the model is 15%VOL and the equivalent diameter of the vent is 0.2 m, the variations of the average gas concentration in the tunnel with time under different exhaust velocities are shown in Figure 6. The exhaust velocity has a significant influence on the efficiency of gas discharging from the tunnel. The greater the ex \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Initial Gas Concentration** & **Equivalent Diameter of** & **Exhaust Velocity** \\  & **(\%VOL)** & **the Vent (m)** & **(m/s)** \\ \hline
1 & 15 & 0.2 & 1 \\
2 & 15 & 0.2 & 3 \\
3 & 15 & 0.2 & 5 \\
4 & 15 & 0.2 & 7 \\
5 & 15 & 0.2 & 9 \\
6 & 15 & 0.1 & 5 \\
7 & 15 & 0.15 & 5 \\
8 & 15 & 0.25 & 5 \\
9 & 15 & 0.3 & 5 \\
11 & 10 & 0.2 & 5 \\
12 & 20 & 0.2 & 5 \\
13 & 25 & 0.2 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Ventilation scenarios. Figure 5: Time–concentration curves at each position of the gas tunnel model. haust velocity, the faster the gas concentration in the tunnel decreases. Nevertheless, when the exhaust velocity exceeds 5 m/s, the decreasing rate of the average gas concentration does not increase significantly. Figure 7 shows the change curves of the average gas concentration under different ventilation sizes when the initial gas concentration in the model is 15%VOL and the exhaust velocity is constant at 5 m/s. It can be seen that the equivalent diameter of the vent also has a greater impact on the efficiency of gas discharging from the tunnel. The larger the equivalent diameter of the vent, the more quickly the gas concentration in the tunnel decreases. However, when the equivalent diameter of the vent exceeds 0.2 m, the increasing trend for the decreasing rate of the average gas concentration gradually slows down. When the exhaust velocity is 5 m/s and the equivalent diameter of the vent is 0.2 m, the variations of the average gas concentration in the model with time are shown in Figure 8. Obviously, the increase in the initial gas concentration increases the amount of gas discharged from the tunnel within a unit time, that is, the decreasing rate of the average gas concentration in the tunnel increases. But the higher the initial gas concentration, the longer the ventilation duration required. By observing all the change curves in Figures 6-8, it can be found that each curve is composed of a straight line and a curve, and the segmentation time points are different. In order to further study the relationship between the average gas concentration and time, the curves in Figures 6-8 are piecewise fitted, and the results are shown in Tables 2-4. Figure 6: Variation of average gas concentration under different exhaust velocities. Figure 7: Variation of average gas concentration under different equivalent vent diameters. It can be found, from Tables 2-4, that the relationship between the average gas concentration in the tunnel and time can be fitted into a linear formula and an exponential formula. In the exhaust state of the tunnel, the average gas concentration decreases linearly in the initial period of time, and the decreasing rate remains unchanged; after a certain period of time, the average gas concentration decreases exponentially, and the decreasing rate gradually decreases. The greater the exhaust velocity, equivalent diameter of the vent and initial gas concentration, the smaller the subsection time point of the curve.