# 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  
The studied tunnel is a one-way straight road tunnel with two traffic lanes, approximately 800 m long. Two identical tunnels are dedicated to the two driving directions of the vehicles. The tunnel section is rectangular with a height of 6.7 m with a cross-section area of approximately 60 m\({}^{2}\) (hydraulic diameter \(D_{hyd}\) = 7.67 m) (Figure 6). Six jet fans are placed on the roof of the tunnel in pairs. Their characteristics are as follows:  
Diameter: 710 mm; thrust: 735 N; nominal power: 22 kW; efficiency: 0.6. The pairs of fans are evenly distributed along the tunnel; they are positioned at progressive 135 m, 400 m, and 665 m from the tunnel entrance, i.e., at 1/6, 1/2, and 5/6 of the tunnel length, respectively (Figure 7a). Figure 6: Representative diagram of the section of the one-way studied road tunnel, with two different tunnels for each vehicle direction. As shown below, the ventilation is longitudinal and has the same direction as the vehicular flow. The side walls of the tunnel are made of concrete with a relatively low resistance to air movement (\(\lambda\) assumed to be approximately 0.025, also considering the presence of other obstacles along the walls). To verify the effects of a large fire in the tunnel caused by a heavy vehicle transporting highly flammable materials (\(HRR_{\text{max}}\) = 300 MW), the analytical formulations illustrated in paragraph 3 (Equations (5)-(7), (9)-(12) and (14)-(22)) were used; different fire positions (\(L_{\text{fire}}\) = \(\beta\cdot L_{tot}\)) were adopted. The following precautionary assumptions were considered:  
* The initial evolution of the fire is represented with a parabola on the \(HRR\) time diagram, having a vertex on the origin and duration of the growth phase equal to 8 min;
* The following standard characteristics were considered for air: specific mass \(\rho\) = 1.2 kg/m\({}^{3}\); external temperature \(T\) = 293 K; specific heat \(Cp\) = 1009 J/(kg\(\cdot\)K);
* The fans downstream of the fire, affected by fumes at high temperatures, capable of putting them out of use shortly after ignition, were not considered active;
* The active fans work at their maximum power, i.e., when the fire is detected, they are activated at the maximum available power;
* The heat dispersion towards the ground/rock surrounding the tunnel is not considered, nor is the reduction in the temperature of the fumes during the flow downstream of the fire. For the particular case of a fire positioned at progressive 360 m (\(\beta\) = \(L_{\text{fire}}/L_{tot}\) = 0.45), for example, the results in Figures 8-10 are obtained. Figure 8 shows the trend of the air speed \(v\) (upstream of the fire) and of the critical speed \(v_{c}\), as the time t from ignition varies. In Figure 9 the temperature of the fumes \(T_{f}\) over time, and in Figure 10, the back-layering distance reached by the fumes in the opposite direction to the flow of air and vehicles over time. In brief, the obtained results indicate that during the initial phase of increase in the thermal power of the fire, back-layering does not occur in the first 5.5 min from the ignition, and only subsequently do the fumes begin to move against the current towards the entrance to the tunnel, with an approximately constant speed equal to 0.30-0.35 m/s. The maximum distance reached by the fumes at the end of the initial phase (8 min from ignition) is approximately equal to 47 m.  
Figure 8: Trend of the speed v upstream of the fire and of the critical speed \(v_{c}\) as the time from ignition varies. The red circle refers to the result of \(v\) obtained by the numerical calculation in a section upstream of the fire at a distance of 340 m from the tunnel entrance. After the first 2 min, the critical velocity remains constant and is equal to about 3.5 m/s; the air velocity upstream of the fire reduces over time from about 5.4 m/s (at ignition) to about 2.4 m/s at the end of the initial phase.