# Effects of the Ventilation System by Using Jet Fans during a Fire in Road Tunnels
## 4 The Studied Case of a Road Tunnel in Northern Italy
If, on the other hand, the fire is located at a distance from the entrance greater than that of the first pair of jet fans (\(\beta>0.17\)), there would be the contribution of mechanical ventilation, capable of directing fumes mainly along the direction of travel of the vehicles, safeguarding the escape route upstream. The back-layering phenomenon would occur only in the presence of a single pair of active fans (\(\beta\) between 0.17 and 0.5); with two pairs or three pairs of active fans (\(\beta>0.5\)), the speed \(v\) of the air is such as to avoid back-layering and therefore allow all the fumes to follow the direction of the vehicular flow. Figure 11 (studied real case) shows the time (from the start of the fire) of the beginning of the back-layering, depending on the parameter \(\beta\). This time is zero for \(\beta<0.17\) and remains around 5 min when \(\beta\) is between 0.17 and 0.5; however, it is well above the average time assumed for the start of the escape on foot (wake-up time) by tunnel users that are present upstream of the fire inside their vehicle. For large fires such as the one hypothesized (\(HRR_{\rm max}=300\) MW), the wake-up time is, in fact, estimated at 0.5-1.5 min [11]. The speed of the counter-current fumes towards the entrance of the tunnel also plays a fundamental role in assessing the safety conditions during a fire. From Figure 12, it is possible to evaluate this maximum speed as the position of the fire (\(\beta\)) varies, only in cases where there is a time flow in the opposite direction to the vehicular one. In actual conditions of the studied case, the maximum speed reaches 1.30 m/s for \(\beta\) < 0.17 and is equal to about 0.34 m/s for the other values of \(\beta\) (for \(\beta\) > 0.5, the back-layering phenomenon does not occur). Considering that the average escape speed is generally estimated at 0.5-1 m/s, it is therefore particularly interesting to analyze the case of a fire located at a distance from the entrance of 135 m (\(\beta\) = 0.17), with no fan assumed to be active because they are all positioned beyond the fire. In particular, Figure 13 shows the trend of the distance reached by the fumes towards the entrance of the tunnel (\(L_{bl}\)), as the time from the fire ignition varies. Figure 11: Starting time of the back-layering phenomenon from the fire ignition for the studied real case and in the presence of the proposed modifications to the ventilation system. In the real case, the back-layering occurs for \(\beta\) between 0.17 and 0.5; for values of \(\beta\) lower than 0.17, the fumes immediately begin to flow towards the entrance to the tunnel; for \(\beta\) > 0.5, the air velocity is such as to avoid the back-layering phenomenon. In the presence of modifications to the ventilation system, the back-layering phenomenon occurs only for \(\beta\)\(\leq\) 0.25. Figure 12: Maximum speed reached by the fire fumes in the counter-current direction towards the entrance (back-layering speed), during the first phase of thermal power growth (\(HRR\)), for the studied real case and in the presence of the proposed modifications to the ventilation system. It is possible to see from Figure 13 how for a reduced escape speed of 0.5 m/s there could be a risk that users are reached by the fumes in the last meters of the escape route towards the entrance of the tunnel. To eliminate this risk, a different arrangement of the three pairs of jet fans was hypothesized, concentrating them in the initial part of the tunnel, with the following distances from the entrance (Figure 7b): 0 m, 200 m, 400 m.  
With the new arrangement of the jet fans, a back-layering time of approximately 5 min is obtained, even for fires located near the entrance to the tunnel (case of \"with the suggested modifications\" in Figure 11); the propagation speed of the counter-current fumes, moreover, never exceeds 0.34 m/s in all the cases in which the occurrence of the back-layering phenomenon is expected (\(\beta\leq 0.25\)) (Figure 12).