# Effects of the Ventilation System by Using Jet Fans during a Fire in Road Tunnels
## 1 Introduction
Tunnel fires differ from open fires for a few main reasons:  
* The effects of the heat produced are more important in a tunnel due to the space confinement (a vehicle that does not burn intensely outside can burn vigorously inside a tunnel); the maximum fire thermal power reached inside a tunnel can be 3-4 times larger for the same material than in the open space;
* Inside a tunnel, there may be a lack of oxygen compared to open fires;
* The fire in a tunnel interacts with the ventilation producing an aerodynamic disturbance in the airflow, that is not simple to evaluate preliminary. According to [PERSON] [9], the fire development is divided into separate phases (Figure 1): growth (with an upward concavity curve indicating a progressive increase in _HRR_ over time up to the flashover); full development of the fire (with a constant _HRR_ value for a certain interval of time); decay (with a progressive decrease in the _HRR_, until its complete extinction). Similar power curves for fires can be found in the literature [10] and can be used with computational fluid dynamics methods to accurately simulate a fire, particularly during its initial stages and the start of its complete development. To emphasize, the early stages of a fire are crucial because they allow for the possibility of saving those who are inside the tunnel before the fire reaches a hazardous (and challenging to control) power level. The following parabolic equations for the growth phase of a fire in a road tunnel (_HRR_ in MW, \(t\) in min) can be written based on the curves suggested by PIARC and NFPA [11]:  
\[HRR\cong a\cdot t^{2} \tag{1}\]  
\[HRR_{\max}=10\text{ MW: }a=0.4; t_{0}=5\text{ min}\] \[HRR_{\max}=30\text{ MW: }a=1.33; t_{0}=4.75\text{ min}\]The maximum power (\(HRR_{\max}\)) reached by a tunnel fire depends on the type of vehicle that catches fire [12]: a car (5 MW), a van (15 MW), a bus (30 MW), a loaded heavy vehicle (up to 100 MW), a heavy vehicle with dangerous goods (up to 300 MW). In the literature, it is also possible to have an estimate of the quantity of fumes \(Q\) (in m\({}^{3}\)/s) produced during a vehicle fire [12] with reference to the \(HRR\) value (in MW):  
\[\text{if }HRR\leq 100\text{ MW}:\;Q\cong 1.774\text{-}HRR+23.484 \tag{2}\]  
\[\text{if }HRR\;>\;100\text{ MW}:\;Q\cong 0.5\text{-}HRR+150 \tag{3}\]  
Due to the aforementioned factors, heavy vehicles transporting hazardous materials are a major source of safety worry. Additionally, heavy vehicles alone account for 50% of tunnel fire causes, so they must be carefully taken into account when evaluating the safety conditions of a road tunnel. Tests conducted at the Memorial Tunnel have shown how the fumes produced by a 20 MW fire can move in the tunnel (in both directions) with a speed of 2.5-3 m/s when there is no mechanical ventilation; fires of 50 MW showed smoke velocities of 4-4.5 m/s. In recent large fires in the Mont Blanc and Gotthard Tunnel, even higher speeds were detected, corresponding to the peak power reached by the fire (\(HRR_{\max}\)) [12]. The results of the studies on full-scale tests conducted as part of the mentioned Eureka Project have highlighted some interesting aspects of road tunnel fires:  
* The flashover is reached after 7-10 min from the start of the fire;
* The total duration of a fire involving a vehicle in a tunnel can vary from 30 min to several hours;
* Even small fires can emit a large quantity of fumes which can cause serious danger to the exposed persons in the tunnel;
* In some cases, the saturation with fumes (with a visibility of less than 1 m) of the entire tunnel during the fire was detected. The interaction of the fire with the ventilation system can lead to a very important phenomenon for the analysis of the risks for the users of the tunnel: the counter-current flow of hot gases and fumes (back-layering) from the fire towards the upstream entrance, against the air direction. In short, in one-way tunnels with longitudinal ventilation in the  
Figure 1: Simplified representation of fire development phases over time.