## 6 Analysis and Discussion of the Results
#### 6.1.1 HF Amount  
Figure 6 shows that the burning of the BEB causes significantly higher HF concentrations downstream of the bus in flames than the ICEB fire, in both cases of ventilation investigated. It is to be noted that although the input data on HF were given to the FDS code in g/s, the software automatically provides as output the temporal and spatial profiles of the HF levels in terms of ppm, which is a common unit of measurement used to express the concentration of a given substance in a mixture and to make a comparison with the tenability limits for human health.  
Figure 5: Arrangement of the queued equivalent cars on both sides of the bus in flames, with a view of the escape routes and emergency exit.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Burning Vehicle**} & \multirow{2}{*}{**HRR\({}_{\text{max}}\) [MW]**} & \multirow{2}{*}{**HF [g/s]**} & **Natural Ventilation** & **Mechanical Ventilation** \\ \cline{3-4}  & & & **Distance Downstream of the Fire Center of the Last User Escaping** \\  & & & **toward Portal \(B\) after \(t_{max}\) = 9 min from the Fire Start (Their Average** \\  & & & **Effective Walking Speed):** \\ \hline BEB & 34 & 3 & 150 m (0.417 m/s) & 63 m (0.175 m/s) \\ ICEB & 30 & 1.5 & 157 m (0.436 m/s) & 69 m (0.192 m/s) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Distance downstream of the fire center of the last user escaping toward Portal \(B\) after \(t_{max}\) = 9 min from the Fire Start computed with reference to the burning of both the BEB and the ICEB, in both cases of ventilation investigated. The average effective walking speed of this user is reported in brackets.  
By considering the scenario of the naturally ventilated tunnel, from Figure 6, it can be seen that during the BEB fire, assumed to be characterized by an HRR\({}_{\text{max}}\) of 34 MW and an HF release rate of 3 g/s, the spatial profile (after \(t_{max}\) = 9 min) of the HF level starts to be above the tenability limit of 30 ppm [7] at a distance of about 80 m from the fire center; then, it remains above the mentioned limit up to around 400 m from the fire center, and it decreases at Portal \(B\) where there are conditions of fresh air, with a peak value of approximately 60 ppm measured at about 150 m from the electric bus in flames. In contrast, in the case of the burning ICEB, assumed to be characterized by an HRR\({}_{\text{max}}\) of 30 MW and an HF release rate of 1.5 g/s, the spatial profile of the HF concentration is always below 30 ppm, reaching a peak value of about 28 ppm at approximately 205 m downstream of the fire center. With reference to the last user escaping toward Portal \(B\), it is also possible to note that, under the condition of natural ventilation, they are exposed to HF levels above the threshold value only in the case of the BEB fire. This means that the burning BEB may cause a reduction in the safety level of tunnel users.  
Regarding the case of longitudinal mechanical ventilation, Figure 6 shows that with reference to the BEB fire, the spatial profile of the HF level exceeds 30 ppm only in the proximity of the burning bus, with a peak value of almost 80 ppm measured at a distance of about 20 m away from the fire center; then, it decreases, achieving a value below 30 ppm at a distance of about 30 m downstream of the fire center; thus, it remains below the mentioned limit downstream of the fire up to Portal \(B\). This shows the efficiency of the longitudinal mechanical ventilation, which, by pushing toxic substances out of the tunnel through Portal \(B\), also reduces the HF level inside the structure. In the event of the ICEB fire, the spatial profile of the HF level is always below both that of the burning BEB and the value of 30 ppm, with a peak value of only 34 ppm at about 20 m from the fire center. From Figure 6, it can also be noted that the last user exiting the tunnel through Portal \(B\) is exposed to HF levels below the tenability limit during the fire of both types of buses.