## 2 Literature Review  
Risk analysis based on CFD models is the most used tool for assessing the safety level of a tunnel in the event of a fire. In this respect, several studies have been carried out to evaluate the effects of some fire mitigation systems such as, in particular, ventilation within the tunnel. [PERSON] et al. [18] evaluated the effects of the longitudinal ventilation provided by jet-fans in a curved bidirectional road tunnel. Using the fire simulation ANSYS CFX code coupled with the STEPS (Simulation of Transient Evacuation and Pedestrian movementS) software for the evacuation process, they showed the effects of temperature, smoke, and toxic substances concentration on occupants during their escape from the tunnel. Due to the combined effect of mechanical ventilation and tunnel geometry, the major risk was found to be when the burning vehicle is in the middle of the tunnel. [PERSON] et al. [19] have subsequently extended their previous study to several fire scenarios due to different types of burning vehicles within the tunnel. The major risk level was found to be when the petrol tanker burns, and the tunnel is full of vehicles. [PERSON] et al. [20], by using the results of the FDS, proposed a methodology for estimating the number of people at risk due to toxic substances caused by fire in road tunnels. The tunnel was assumed to be mechanically ventilated within 2 min after the start of the fire. [PERSON] et al. [21], by means of the FDS tool for simulating fire in tunnel, investigated the smoke flow layering length as a function of the longitudinal ventilation and point extraction. [PERSON] and [PERSON] [22] used the FDS+Evac for simulating fire scenarios and user evacuation from a short two-lane road tunnel. The emergency mechanical ventilation system was assumed to be activated within 1 min after the start of the fire. They found that when the number of people trapped increased, most of the occupants used the exit portal rather than the emergency exit. [PERSON] et al. [23] also used the FDS+Evac for simulating fire and user escape from a tunnel, which is equipped with jet-fans for mechanical ventilation. In particular, they focused on the factors that affect the evacuation process, emergency exit visibility, effects of smoke, and presence of obstacles. [PERSON] et al. [24], by using FDS, carried out several numerical simulations to study the critical velocity of longitudinal ventilation required for preventing the back-layering phenomenon as a function of the HRR both in horizontal and tilted tunnels. [PERSON] and [PERSON] [25], by basing on the use of FDS+Evac, modeled fire scenarios in a unidirectional road tunnel equipped with longitudinal ventilation; showing, in particular, the movement of smoke during counterwind (i.e., wind in opposite direction to jet-fans). The aim was to underline the uncertainties in fire modeling process, which were due to input data and fire scenario design. [PERSON] et al. [26], using FDS+Evac, investigated the influence of smoke distribution, queue formation, and tunnel geometry on the evacuation process in the event of a fire. The results were compared to the evacuation experiments carried out in a road tunnel, and a good agreement was obtained. They found that the evacuation time depends closely on the time needed for each vehicle to stop, which can be estimated by using an appropriate queue formation model. [PERSON] et al. [27], in keeping with their aforementioned studies and by continuing to use ANSYS CFX+STEPS, individualized the optimal distance from the emergency exits in a bidirectional tunnel equipped with jet-fans. [PERSON] et al. [28], by means of FDS, studied the effects of different velocities of longitudinal ventilation, as well as different tunnel widths and HRRs, on the propagation of smoke in tunnels. [PERSON] and [PERSON] [29], using FDS, assessed the consequences on the evacuation process due to two different fires in a road tunnel. They accounted for different types of ventilation systems, delay in fire detection system, and traffic conditions.