## 2 Numerical Model
### Model Details  
The studied tunnel is made of rectangular cross-sections with dimensions typical for a double railway tunnel (y = 11 m, z = 7 m), and its length is 1.5 Km. In all cases, the center of the fire source was located in the middle section of the tunnel, as shown in Figure 1 at (x = 795.5 m, y = 2.25 m, z = 4 m), and the intensity was 20 MW, which corresponds to a traction engine fire. The fire's surface was 1 m\({}^{2}\) and it was located on the train's roof. The surface of the fire was modeled as a burner type and was inserted into the model as an open vent. The fire growth followed a typical \(at^{2}\) rule, where the growth rate \(a\) was 0.1876 kW/s\({}^{2}\) and achieved nominal HRR within 327 s. C\({}_{12}\)H\({}_{23}\) was used as the fuel, which is the standard composition of diesel, with a yield of combustion products of 0.1 for CO and 0.09 for soot. Combustion efficiency is defined as the amount produced per gram of fuel diffused along with the flow, compared to the diffusion of fire into the environment. The jet fans used in the simulations are 3 m long with a surface area of 1 m\({}^{2}\). Four pairs of them were placed every 300 m at the height of 5 m and 1 m off the side walls  
According to [PERSON] et al. [32], the non-dimensional expression \(D^{*}/\delta x\) was used to assess mesh resolution, where \(\delta x\) is the nominal size grid (in m), and the characteristic length of \(D^{*}\) is expressed as follows  
\[D^{*}=\left(\frac{\dot{Q}}{C_{p}T_{\infty}\rho_{\infty}\sqrt{g}}\right)^{2/5} \tag{7}\]  
Figure 1: Side view of the simulation tunnel, tunnel dimensions, jet fans, train, and fire source location. where \(Q\) is the fire heat release rate (HRR) (W), \(\rho_{\infty}\) is the ambient density (kg/m\({}^{3}\)), \(C_{p}\) is the specific heat capacity (J/kgK), \(T_{\infty}\) is the ambient temperature (K), and \(g\) is the gravitational acceleration constant (m/s\({}^{2}\)). In the NUREG-1824 Directive published by the United States Nuclear Energy Regulatory Commission (USNRC) and the Electricity Research Institute (EPRI), it defines that \(D^{*}\) values should be in a range between 4-16 for an optimal analysis in FDS Code [33]. Unlike the number of cells, the grid resolution does not depend on the volume of the building, so the corresponding standard has the advantage of universal application regardless of the size of the building. The grid independence test was conducted for three different meshes: coarse mesh with a mesh size of 0.75 m, medium mesh with a mesh size of 0.5 m, and fine mesh with a mesh resolution of 0.25 m. The number of cells for each type of mesh was 269,865, 924,000, and 4,254,149, respectively. As shown in Figure 2, the fine and medium meshes were compared. The grid errors using two computational grids were calculated using the following equation [34]  
\[GCI=\frac{f_{2}-f_{1}}{1-r^{p}} \tag{8}\]  
where \(f_{2}\) is the numerical solution that results from the medium computational grid, and the corresponding from the finer is \(f_{1}\), \(r\) is the refinement factor between the two computational grids, and \(p\) is the accuracy of the algorithm, which is 3 for the present study. The grid independence examined the U velocity and carbon monoxide concentration values. The values were extracted by a line, downwind of the fire source, in the symmetry plane of the tunnel with coordinates (x = 1100 m, y = 5.5 m, and z = from 0 to 7 m) with 28 sampling points. Figure 2: (**a**) Comparison of the grid convergence index (GCI) for the U mean velocity and (**b**) mean carbon monoxide concentration for the medium and fine mesh in the symmetry plane of the tunnel, with coordinates (x = 1100 m, y = 5.5 m, and z = from 0 to 7 m) and 28 sampling points. The results did not show great deviation, so a combination of the fine and medium mesh was used as the final mesh for the examination of the three main scenarios. The fine mesh was used for the field around the fire, while, for the rest of the tunnel, the medium mesh was used.