## 1 Introduction  
After catastrophic events due to fires occurred in some road tunnels in Europe (i.e., Mont Blanc tunnel, Tauerm tunnel, and Saint Gotthard tunnel), the European Parliament and Council adopted the Directive 2004/54/EC [1]. This Directive introduced the minimum safety requirements, for tunnels belonging to the Trans-European Road Network with lengths of over 500 m, with the purpose of ensuring an appropriate safety level to tunnel users. The minimum safety requirements regard both structural measures and equipment which are applicable for both the design of new tunnels and the existing ones. In the case of tunnels already open to traffic, the compliance of the existing safety measures with the requirements of the Directive must be assessed, and in the event of a negative response, refurbishment works must be made for adapting the tunnel to the provisions mentioned that are mandatory or recommended. The Italian Ministry of Infrastructure and Transportsadopted the mentioned Directive in 2006 [2], and fixed, for the existing tunnels, the date of 30 April 2019 as the deadline within which the refurbishment works were to be completed. At the present time, due to the very high density of tunnels in Italy, most of these structures still present works in progress. For this reason, the Italian Tunnels Permanent Commission has recently imposed on the Tunnel Management Agencies (TMAs) the implementation of certain temporary safety measures for tunnels that have not yet achieved the minimum safety requirements. Among these temporary measures, a crucial role for people safety is played--more especially if hydrants and/or extinguishers inside the tunnel have not yet been installed--the presence of rescue teams stationed in the proximity of the tunnel portals for emergency services, operating 24 h a day and seven days a week. The emergency service team is foreseen to be trained by qualified personal for fighting a fire, and appropriately equipped for containing the fire growth. The intervention time of the rescue team for this purpose, for example in the case of a tunnel open to traffic with a length \(<\) 1000 m and without any emergency exit (except the entrance and exit portals of vehicular traffic), is required to be less than 10 min from the fire's start. Indeed, the effectiveness of the equipment generally used by the cited emergency staff for contrasting a fire in a tunnel is not hitherto clearly known. This represents a gap of knowledge that the present paper intends to fill. The emergency procedure involves, in general, the use of a four-wheeled vehicle (e.g., a pick-up van) equipped with a fire-extinguishing technology based on a micronized water system; moreover, this vehicle is often provided also with certain portable extinguishers. However, the main issues that appear still to be unsolved are: (i) fire size on which the mentioned micronized water system may be effective; (ii) time within which to act for contrasting the fire; (iii) consequent environmental conditions in the tunnel (i.e., temperatures, radiant heat fluxes, visibility distances, and CO and CO\({}_{2}\) concentrations); (iv) effects on tunnel users during their evacuation process in terms of toxic gases and heat exposure; (v) relationship between the annual cumulative frequency (F) and the number of potential fatalities (N) that might be expected; (vi) risk acceptability. The mentioned lacuna of knowledge is the major driving motivation for the authors of the present paper in investigating in greater depth the potentialities of using a micronized water system, as a temporary safety measure, for containing the fire growth phase in road tunnels before the arrival of the fire brigade. The article is organized as follows: the next section contains a literature review regarding studies on the use of water mist systems for fighting a fire. Then, the Computational Fluid Dynamics (CFD) modeling is presented and implemented.