## 4 Results and Discussion
### Formation and Dissipation of Reactants and Reaction Products
The uneven wall and floor changes the incident angle of the shock wave and the propagation direction of the reflected wave. Subsequently, the superposition of the shock wave changes. Therefore, the overpressure distribution in an approximate real tunnel is asymmetric. At 5-10 ms, the complete consumption of methane reactants causes the shock wave to lose its energy replenishment, thereby reducing the shock wave overpressure. The orange-red high-pressure area in the approximate real tunnel inclines to the right straight tunnel with severe concave and convex fluctuations. The overpressure distribution of the fluid in the tunnel with smooth walls maintains symmetry. The propagation distance of the shock wave in two different tunnel models during this time period is the same. After 15 ms, the shock wave propagating to the left has reached the intersection of the straight tunnel and the branch tunnel whereas the shock wave on the right has entered the bend tunnel. Within 15 ms to 20 ms, the propagation range and overpressure distribution of the shock wave in the left of the approximate real tunnel with only slight fluctuations are almost the same as those in the tunnel with smooth walls. The propagation distance of shock waves in the bend tunnel of the approximately real tunnel is lower than that in a smooth wall tunnel. At the same time, the area of the orange high-pressure zone in the bend tunnel of the approximately real tunnel gradually declines. In 35 ms, the shock waves have reached the tunnel exit and the maximum overpressure is 220.5 kPa. Figure 8: CO and CO\({}_{2}\) concentration time-variation curves at different positions after methane explosion. The changes in overpressure over time in the A-Q section of the tunnel were monitored to further explore the reasons for the different distributions of overpressure in the two different tunnel models. At the same time, the changes in shock wave velocity field and vorticity in straight tunnels, bend tunnels, and T-shaped tunnels in the two models were analyzed separately, as shown in Figure 10.  
When the shock wave propagates in the straight tunnel, the velocity direction of the fluid near the right tunnel in the approximate real tunnel changes significantly, increasing the local resistance of the fluid and energy dissipation. Ultimately, it leads to a concentric distribution of shock wave velocity in the right straight channel. After 11 ms, compared with the left straight lane, the shock wave velocity and overpressure attenuation amplitude increase. As the distance between the overpressure monitoring points and the center of the tunnel increases, the difference between the overpressure time curves of P2, P4, P6, and P8 on the right side of the approximate real tunnel and those of P1, P3, P5, and P7 on the left side gradually increases. The peak overpressure at points P2, P4, P6, and P8 is low, and considerable time is required to reach the peak overpressure. For the smooth wall tunnel model, the change in the velocity direction of the fluid near the wall is unremarkable, the local resistance inside the fluid is small, and the shock wave velocity is distributed in a layered manner. At 11 ms, the velocity and overpressure attenuation amplitude of the shock wave in the  
Figure 9: Propagation process of shock waves in approximate real tunnel and smooth wall tunnel. straight tunnels on both sides are relatively small. The overpressure time curves of monitoring points P1, P3, P5, P7 and P2, P4, and P6 and P8 are close to overlapping. When the shock wave enters the bend, the direction of fluid velocity deviates significantly. The approximate real tunnel has higher vorticity near the wall and at the turning of the tunnel. In the smooth wall tunnel, only the fluid at the turning point of the tunnel has high vorticity. The propagation speed of shock waves in the approximate real tunnel is only half of that in the smooth wall tunnel.