## 3 FDS Simulation Results  
Three different scenarios were examined (A, B, and C) with three different fan activation times after a fire accident, at +180 s, +300 s, and +480 s, respectively. For all the cases, all the fans were activated at the same time. The three different scenarios for the different jet fan activation times were set up with the Pyrosim interface provided by Thunderhead Engineering. The extracted results were temperature, flow velocity, the volume fraction of gases, and heat release rate (HRR) values.  
As shown in Figure 6, the input fire curve is in good agreement with the numerical results for the HRR curve.  
Figures 7 and 8 show that the longer the activation of the ventilation system is delayed, the higher the temperatures appear in the tunnel ceiling. This rapid rise in high temperature can result in spalling, which is the detachment of structural material, such as concrete, often happening in an explosive way. This poses a great threat for the population in the tunnel.  
Moreover, a time delay in the ventilation system activation creates a greater smoke dispersion, as shown in Figure 9, and thus a greater dispersion of the harmful combustion products. This can affect the evacuation process due to reduced visibility and soot inhalation.  
As shown in Figure 10, the temperatures at the ceiling above the fire source are in good agreement with the RABT ZTV fire curve in the first minutes of the simulation. Then, when the ventilation system was activated, the temperature dropped and reached a steady value of around 450 \({}^{\circ}\)C.  
Figure 11 shows that the development of ventilation flow started immediately after the activation of the jet fans. The air flow inside the tunnel needs about 300 s, after the jet fans achieve the nominal power, to reach the critical velocity value, but the thrust of the jet fans, even before they reach full power, is capable of pushing the smoke downwind of the fire source. Moreover, Scenario A showed some smoke layer entertainment, which was due to the pressure difference at the tunnel openings, which created a draft with a velocity around 0.5 m/s (Figure 12 at the 180 s time frame). As shown in Figure 12, the peak at 750 m (fire source) was due to the smoke layer and the presence of the train, which reduced the area of the tunnel in this section, resulting in higher velocities. This phenomenon can be seen in Figure 11, where, near the train location, a great volume of the flow had the critical velocity value. As depicted in Figure 13, the mean longitudinal velocity was in the range of 2.7 m/s and was achieved within 600 s from the jet fan activation time. Ventilation conditions are achieved as long as the jet fans reach their nominal output volume flow, but this does not mean the tunnel airflow velocity has reached the critical value.  
Figure 6: FDS input fire curve and FDS results fire curve.  
Figure 8: Tunnel ceiling (z = 7 m) temperatures for each scenario at the jet fan activation time frame: (**a**) Scenario A at +180 s, (**b**) Scenario B at +300 s, and (**c**) Scenario C at +480 s after the fire accident.  
Figure 7: Tunnel ceiling (z = 7 m) gas temperature above (x = 0 m) and downwind (x = 10, 30, 50, and 100 m) from the fire source (**a**) for Scenario A, (**b**) Scenario B, and (**c**) Scenario C.  
Figure 11: Development of the critical ventilation velocity of 2.7 m/s (isosurface with the green colour), at the train location, for (**a**) +90 s, (**b**) +180 s, and (**c**) +300 s after the activation of the jet fans. The jet fans reached their maximum output 90 s after their activation, while the development of the flow was reached approximately after 300 s.  
Figure 10: FDS ceiling temperatures above the fire source (x = 795 m, z = 7 m) for all three scenarios in comparison with the RABT ZTV fire curve.  
Figure 9: Smoke dispersion along the tunnel and backlayering at the jet fan activation time frame for (**a**) Scenario A at +180 s, (**b**) Scenario B at +300 s, and (**c**) Scenario C at +480 s.