# 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
With the second phase of the fire (full development) the main parameters of the fire (\(T_{f}\), \(v\), \(L_{bl}\)) stabilize; that is, they no longer vary with time. In addition, a numerical model using the Ventsim software 5.4 ([[https://ventsim.com/](https://ventsim.com/)]([https://ventsim.com/](https://ventsim.com/)), accessed on 1 March 2023) [29,30] of the studied case was developed. The only pair of fans present before the fire was inserted into the model at a distance of 135 m from the tunnel entrance. The degree of air friction on the tunnel surfaces was considered medium. A minimal dispersion of heat towards the lining and the surrounding rock was assumed in the calculation: this specific aspect is what most differentiates the developed numerical modeling from the proposed analytical procedure illustrated in the previous paragraphs. Figure 10: Trend of the distance reached by the counter-current fumes towards the entrance of the tunnel (\(L_{bl}\)), as the time from ignition varies. Figure 9: Trend of the temperature of the fumes \(T_{f}\) as the time from ignition varies. The red circle refers to the result of \(T_{f}\) obtained by the numerical calculation in a section downstream of the fire at a distance of 380 m from the tunnel entrance. The results of the calculation in terms of air speed and temperature over time from the start of the fire are represented through a colorimetric scale along the axis of the tunnel. Of particular interest is the analysis in a section upstream (340 m from the tunnel entrance) and downstream (380 m) of the fire. In the upstream section, an air speed of about 2.26 m/s is detected after 7 min from ignition (Figure 8). In the one downstream, an air temperature of about 425 K is detected after 4 min from the start of the fire (Figure 9). In both cases, numerical modeling produces slightly lower values than the proposed analytical calculation. The percentage difference between the values calculated with the proposed calculation procedure and numerical modeling is approximately 12% both for the air speed upstream of the fire (\(v\)) and for the flue fumes temperature downstream of the fire (\(T_{f}\)). The proposed simplified calculation procedure provides higher air speeds v and also higher fumes temperatures \(T_{f}\), compared to the more accurate numerical modeling. These differences can be traced back to some simplified hypotheses which are the basis of the developed calculation procedure: the lack of heat dispersion towards the rock surrounding the tunnel and the constancy of the mass flow rate of the air passing through the fire. The Effects of the Different Positioning of the Fire inside the Tunnel on the Safety Conditions during the Evacuation Phase  
To check the safety conditions of users with different fire positions inside the tunnel, the parameter \(\beta\) was varied from 0.05 to 0.95, using the proposed analytical approach illustrated in paragraph 3. From the study, it was possible to detect, for each possible position of the fire in the tunnel, the air velocity (upstream of the fire), the critical speed above which the back-layering phenomenon does not occur, the velocity and the maximum distance reached by the fumes in the opposite direction to the vehicular flow, the temperature of the fumes. The calculation results were able to show how a fire located at a distance from the entrance less than the distance of the first pair of jet fans can have dramatic effects on people in the tunnel. In particular, in the studied case, if the fire is located at a distance of less than 135 m from the entrance, one should assume the absence of mechanical ventilation during the fire and, therefore, the dispersion of the fumes of the fire in the two directions (upstream and downstream) in equal measure (Equations (2) and (3)). In this case, a large fire would produce a quantity of fumes capable of saturating the entire section of the tunnel upstream of the ignition point in less than 4 min, and the maximum speed of the fumes could reach 1.3 m/s.