## 4 Results and Discussion
### Analysis of the Evolution of Flame Propagation Patterns  
When a weakly confined structure containing combustible gas is ignited somewhere within the structure, the explosive flame continues to propagate toward the vent, releasing a large amount of heat. The presence of obstacles, however, leads to changes in flame \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Case** & **Angle** & **Location** & **Case** & **Angle** & **Location** \\ \hline
1 & & 200 mm & 7 & 200 mm \\
2 & 30\({}^{\circ}\) & 500 mm & 8 & 60\({}^{\circ}\) & 500 mm \\
3 & & 800 mm & 9 & 800 mm \\
4 & & 200 mm & 10 & 200 mm \\
5 & 45\({}^{\circ}\) & 500 mm & 11 & 90\({}^{\circ}\) & 500 mm \\
6 & & 800 mm & 12 & 800 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Working condition layout. Figure 6: Comparison of experimental and simulated pressures. morphology [31] and turbulence in the flame wavefront surface, increasing the contact area between the flame and the fuel gas. The precursor shock heats and compresses the unburned mixture, which accelerates the explosive reaction and energy release. The flame transient inside the pipe was divided into four stages: laminar stretching stage, destructive deformation stage, explosion stage outside the pipe, and reflux combustion stage [32]. Observation of Figure 7 shows that the flame spreads outward in the form of a \"jellyfish\" after passing through an obstacle (200 mm) close to the ignition source, which was forced to accelerate in advance because the obstacle was closer to the ignition position. Due to the pipeline restrictions, the axial flame propagation speed is higher than the longitudinal direction, so the formation of the \"jellyfish\" flame head stretches longer. The presence of a localized turbulence zone between the obstacle and the inner wall surface of the pipe causes the edges of the flame branches to curl at the end. The space for flame development was limited at small angles in (a) and (b), and the curling of the tail of the branch was relatively obvious (as shown by the black circles in the figure). As the angle increases, the length of the branch at the end of the flame tail becomes shorter (shown by the black box in the figure). This may be because, as the angle increases, the length of the obstacle becomes shorter, the area of the flame in contact with the obstacle decreases, and the ability of the flame to transfer heat by thermal radiation and convection decreases, which affects the combustion and propagation of the flame [33]. Figure 7: Flame evolution at 200 mm for obstacles of different angles. Figure 8 shows that at t = 27 ms, after the flame passes through the obstacle (500 mm) located in the middle of the confined space, the shape of the flame became elongated, mainly due to the presence of the obstacle, resulting in a smaller cross-section of the pipe. The expansion of the combustion products encouraged the unburned material to pass through the gap, which lasted for a short period. During this period, the elongated flame attempted to develop branches, but the effect was not significant. Eventually, the flame's front face split and shattered. The flame to the left of the obstacle was also affected, taking on a tulip-shaped form and increasing in length as the angle of the obstacle increased (as indicated by the black border in the figure). Finally, the two flame fronts at the top and bottom gradually converged and merged toward the center axis of the pipe. Figure 9, around t = 33 ms, when the premixed flame reached the obstacle at the position of the pipe opening (800 mm), the flame was affected by the obstacle and the airflow. It rushed out from the obstacle gap and then arrived at the pipe opening in the shape of a long strip within the limited space. After that, the unburned gas formed a turbulent region with varying intensities in the area of the obstacle and the deflagration opening, and the flame transformed into broken turbulent combustion.