## 3 Results and Discussion
### CO Diffusion Law
#### 3.2.3 Effect of \(\theta\) on the CO Diffusion  
Figure 14 indicates the CO concentration at five different positions along the tunnel under the influence of \(\theta\). The CO concentration profiles exhibit varying peak values and appearance times under the influence of \(\theta\). Vertically along the axes, the CO concentration exhibits a difference in peak height (Figure 14a,c,d). This shows that the greater the \(\theta\) value, the lower the peak height. Horizontally on the axes, there is a time difference in the appearance of the CO concentration curves, and it is evident that the CO concentration increase can be detected earlier at the same measurement point for small \(\theta\) values (Figure 14c,d). For more intuition, the integrated numerical simulation approach was used for further qualitative discussion. Ultra-extreme cases with \(\theta\) values of 12%, 16%, and 20% were considered. The software Ansys Fluent 20.2 [32] was used and the basic parameters were set as shown in Table 4. Figure 15 shows the simulation results at \(t=80\) s, while Figure 16 validates them. Figure 14: CO concentration curves of each measurement point under different \(\theta\) values: (**a**) test point 1; (**b**) test point 2; (**c**) test point 3; (**d**) test point 4; (**e**) test point 5. Figure 15: Concentration distribution of CO within the tunnel model at different \(\theta\) values (\(t=80\) s). As shown in Figure 15, the diffusion distance is longer on the side near the top and shorter on the side near the ground, and the CO concentration at the cross-section is higher near the top and lower near the ground, as indicated by the red box. The results demonstrate that the distribution of CO exhibits an inverted trapezoidal shape with high concentrations in the upper part and low concentrations in the lower part. As \(\theta\) increases, the area of CO contamination along the tunnel generally increases, and the vertical diffusion \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter Type** & **Parameter** & **Parameter Setting** \\ \hline Mesh & Quantity and quality & 3.2 \(\times\) 10\({}^{5}\), 0.35-0.99 \\ General & Time & Transient \\  & Solver type & Pressure-based \\  & Gravity & X: 0, Y: \(-\)9.81, Z: 0 (m/s\({}^{2}\)) \\ Model & Viscous model & Standard \(k-\epsilon\) (2 eqn) \\  & & Mixture template (CO, air) \\  & Species (Species transport model) & Density: volume–weight mixing law \\  & & Viscosity: constant \\ Materials & CO (fluid) & Density: constant, 1.1233 kg/m\({}^{3}\) \\  & Air (fluid) & Density: constant, 1.225 kg/m\({}^{3}\) \\ Boundary & Velocity inlet & 6 m/s \\  & CO inlet (mass-flow inlet) & 0.00752 kg/s (release for 25 s) \\  & Tunnel model and duct (wall) & No slip wall \\  & Open end (pressure outlet) & 101.325 kPa, Turbulence intensity 5\% \\ Solution & Pressure–velocity coupling scheme & Simple \\  & Gradient & Least-squares cell-based \\  & Pressure & Second-order \\  & Momentum & Second-order upwind \\  & Turbulent kinetic energy & First-order upwind \\  & Turbulent dissipation rate & First-order upwind \\  & CO & Second-order upwind \\  & Solution controls & Pressure 0.3, Density 1, CO 1 \\ Initialization & Initialization methods & Standard \\  & Initial values & CO: 0 \\ Calculation & Type & Fixed \\  & Time steps (s) & 1 \\  & Max iterations & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Basic parameter settings for numerical simulation. Figure 16: Comparison between experimental results and simulation results. area also increases (red box in Figure 15). The lowest concentration of CO at the cross-section increases gradually, while the highest concentration decreases, and the vertical concentration gradient becomes smaller. As shown by the red dots in Figure 15, the position of the pollution front is further forward for \(\theta\) values of 4% and 8% than for \(\theta\) values of 12%, 16%, and 20%, and this pattern is not consistent, as it is observed that \(\theta=8\)% is earlier than \(\theta=4\)%.