## 3 Results and Discussion
### Distribution of the Flow Field and Pollutant Concentration along the Tunnel  
Figure 5 shows the variations in air velocity at the four analysis positions in the tunnel. The curve in Figure 5 indicates that there is a significant difference in air velocity at different positions of the tunnel section. Node A\({}_{\text{III}}\), located near the ventilation duct, shows a significant change in air velocity near the locations of air leakage: the air velocity increases sharply, and the direction of flow is opposite to the airflow of the tunnel. The velocities corresponding to A\({}_{\text{III}}\) at the five hole locations are \(-\)0.4, \(-\)3.24, \(-\)5.24, \(-\)7.36, and \(-\)7.92 m/s. The range of air velocity changes gradually reduced as the distance to the working face decreased. The velocity distribution of node A\({}_{\text{I}}\) was similar to that of node A\({}_{\text{III}}\), but the degree of the increase was smaller. The corresponding velocity values were found to be 1.04, 0.97, 0.37, \(-\)0.19, and \(-\)0.19 m/s. The change in air velocity is smaller at the node positions of A\({}_{\text{II}}\) and A\({}_{\text{IV}}\). The air velocity increased slightly near the locations of air leakage, and the direction of flow was the same as that of the average flow velocity. This indicates that the areas on both sides of the tunnel section are less affected by ventilation duct leakage. Figure 6 shows the process of CO gas gradually discharging from the working face to the tunnel outlet. The figure shows the distribution of CO concentration in the tunnel at different times. Due to the leakage of fresh gas, the CO concentration near the ventilation duct hole has diluted. The diluted concentration area is mainly distributed in the area  
Figure 5: Air velocity with increasing distance from the working face at the four analysis nodes in the tunnel. below the pores, consistent with the direction of the leaked gas. The difference in the gas flow rate of duct leakage results in different distributions of CO concentration fields. According to the results of the duct model simulation, as the distance from the leakage point to the air inlet decreases, the amount of air flowing from the duct into the tunnel increases. Therefore, the hole L1 with the highest leakage gas flow rate has the greatest impact on the CO distribution inside the tunnel. The leaked gas has a significant dilution effect on the CO concentration inside the tunnel. Air leakage near the working face has a smaller effect on the concentration-field distribution in the tunnel. The leaked gas only slightly reduces the concentration around the ventilation duct and has little impact at the bottom of the tunnel. In order to observe the mechanism of the influence of air leakage in the ventilation duct at different positions of the tunnel section, Figure 7 shows the variation curve of CO concentration values at different positions of the tunnel section. The variation pattern of CO concentration at different positions of the tunnel is generally consistent. The curve shows a parabolic distribution at different times, indicating that there is always a peak in the CO concentration distribution inside the tunnel. The peak value of the curve gradually decreases with ventilation time, but the span of the curve gradually increases. Therefore, the emission process of CO gas can be divided into the movement process and diffusion process. The movement process moves harmful gases from the working face towards the tunnel outlet, while the diffusion process gradually expands the distribution range of harmful gases during the movement, causing the concentration to gradually dilute. This pattern is consistent with the mass transfer process of turbulent airflow. The discharge of harmful gases has both the transport effect of the main airflow and the diffusion effect of the turbulent airflow. Figure 6: Distribution of CO concentration in the tunnel over a 10 min ventilation period.