## 3 Analysis of Results and Discussion  
### Analysis of the Influence of the Gas Baffle on Gas Leakage Monitoring  
#### 3.1.1 Leak Response Time Effect Analysis  
Based on the nature of methane, its lower explosion limit can be considered to be 5% (volume fraction), corresponding to a mass fraction of 2.82%. The set value of the gas monitoring alarm concentration in the utility tunnel should not be greater than 20% of its lower explosive limit value. Therefore the monitor alarm concentration value can be set to 0.005 (mass fraction) [30]. In order to examine the longest alarm response time, the leak hole was located at the midpoint of the two gas monitors. One gas monitor was installed every 15 m in accordance with the Xu Wei utility tunnel. The gas baffle is set on the leeward side of the monitor, and the horizontal distance from the monitor is controlled at 1 m. The width of the gas baffle is set at 0.25 m, and the height is set at 0.5 m. The width of the gas baffle is set at the width of the gas cabin. (1) The effect of the gas baffle under different leakage volumes  
In order to study the effect of the gas baffle on the detection effect under different leakage volume (leakage velocity) conditions, the ventilation air velocity of 0 m/s and the leakage velocity of 50 m/s and 100 m/s, respectively, were selected for analysis. The variation of CH4 concentration with time at the detection point before and after the addition of the baffle is shown in Figure 4.  
In order to study the effect of installing gas baffles on the monitoring effect under different leakage volume (leakage speed) conditions, the calculated results of the working conditions with a ventilation wind speed of 0 m/s, a leakage speed of 50 m/s and 100 m/s respectively were selected for analysis. The methane concentration (mass fraction) at the monitoring points before and after the addition of the baffle is shown in Figure 4.  
According to the monitoring data corresponding to Figure 4, with the mass fraction of 0.005 as the alarm response concentration, the alarm response times were 4.6 s for no air baffle and 4.0 s for air baffle for a leak rate of 50 m/s; the alarm response times were 2.2 s for no air baffle and 1.9 s for air baffle for a leak rate of 100 m/s. At the same time, the mass fraction of CH\({}_{4}\) was 0.078 at the monitoring point with the gas baffle and 0.087 at the monitoring point without the gas baffle for a continuous leak of 10 s and a leak velocity of 50 m/s, compared to 0.103 and 0.136 for a leak velocity of 100 m/s, respectively. The results showed that the detection effect was influenced by the blocking effect of the detection zone formed between the baffles. The results show that, due to the blocking effect of the detection zone formed between the gas baffles, the detection effect was improved after the addition of the gas baffles for different leakage volumes, and the peak CH\({}_{4}\) concentration at the detection point increased, but the overall improvement effect was not significant. (2) Effect of the gas baffle under normal ventilation  
In order to study the effect of the gas baffle on the monitoring effect under normal ventilation conditions, the ventilation air velocity of 1.38 m/s (induced air volume 6 times/h) and the leakage velocity of 50 m/s and 100 m/s were selected to continue the calculation and analysis. The variation of CH\({}_{4}\) concentration at the monitoring point with time is shown in Figure 5.  
According to Figure 5 and the corresponding monitoring data, under normal ventilation conditions, the peak CH\({}_{4}\) concentration at the location of the monitoring point still increased after 20 s of continuous leakage with the addition of the gas baffle, but the alarm response time did not advance. Through several simulations, it was found that due to the blocking effect of the gas baffle, a vortex would be formed on the windward side of the gas baffle, and the location near the windward side of the gas baffle would be a safe concentration area for a period of time.