## 2 Literature Review  
In the last few years, several studies have been made on the evaluation of the effectiveness of fire-fighting safety measures based on the use of water mist systems (i.e., a protection system that controls fire by projecting ultrafine droplets) in road tunnels. [PERSON] et al. [3], for example, developed an analytical model to predict the trajectories of water mist droplets in tunnels. They found that, unlike the conventional sprinklers, water mist systems are highly affected by the ventilation velocity inside the tunnel due to the much smaller size of droplets. Under current ventilation in the event of a fire, with the nozzles located at the ceiling, the droplets may reach the fire location covering a distance even longer than 50 m, while a very higher ventilation velocity might carry the droplets beyond the fire location. The results suggested that additional research concerning the interaction between the water mist system and tunnel ventilation was required. [PERSON] et al. [4] conducted a series of small-scale experiments for investigating the effectiveness of water mist systems in the event of a fire in a tunnel under different longitudinal ventilation velocities. According to the results obtained, a suitable design regarding both the water mist system and longitudinal ventilation in the tunnel might significantly increase the success of fire suppression. [PERSON] et al. [5] developed a numerical model for assessing the capability of a water mist system in mitigating thermoplastic fires in a tunnel. They carried out a parametric study by varying the water flow rate under longitudinal ventilation velocities inside the tunnel between 1.5 and 2 m/s. The results showed that the efficiency of the water mist system in mitigating the fire in the tunnel depends on fire characteristics, ventilation velocity, nozzle location, water flow rate, and droplet size. [PERSON] and [PERSON] [6] reported some experimental data on water mist systems in suppressing fires in confined spaces. High-pressure water mist systems extinguished fires more effectively: in particular, the time required to extinguish a 1.6 MW diesel oil spray fire was found to be 40 s for an operating pressure of 6 MPa. [PERSON] et al. [7], by performing CFD simulations, reproduced a fire test with the scope to show the interaction between a water mist system and hot gases in a tunnel with longitudinal ventilation. A water mist system consisting of six nozzles with an operating pressure of 90 bars and a water flow rate of 5.5 litres/min was implemented. The CFD code accurately predicted the reduction of gas temperatures and heat fluxes due to the water mist system activation. [PERSON] [8] developed a theoretical model for assessing the effectiveness of a water mist system on major fire spread in a longitudinally ventilated tunnel. The author showed that the efficiency of the suppression system depends on several factors, such as nozzle design and activation method, water discharge rate density, the spatial distribution of droplets, and ventilation velocity within the tunnel. However, he stressed that if a fire is relatively small when the water mist system is activated, it may, maybe, be extinguished or never reach a critical value of the Heat Release Rate (HRR). [PERSON] et al. [9] proposed a system that combines water mist screens and transverse ventilation in the event of a tunnel fire. The water mist screens are used, in particular, to confine smoke from spreading widely. The effectiveness of the proposed system was evaluated using the Fire Dynamics Simulator (FDS). The results showed that the hot smoke due to a fire can be effectively confined in the region between the water mist screens, and as a result, a safer environment for user evacuation is expected. [PERSON] et al. [10] conducted a series of reduced-scale tunnel fire experiments for investigating the influence of the water mist particle size, in the event of a fire in a tunnel, on smoke layer, temperature, and CO concentration under natural or mechanical ventilation conditions.