## 1 Introduction
Numerical modeling was used to verify the most critical conditions that can occur by varying the position and extent of a fire in the studied underground mine, such as taking into account the position of the workers inside it. At the end of this preliminary study, four specific cases were identified, for which it was possible to carry out further investigation; furthermore, the results of these cases will be illustrated and discussed in greater detail. Evaluation of the Environmental Conditions during a Fire by Numerical Modeling of the Ventilation System  
Fire dynamics are very complex and they significantly depend on the supply of oxygen and the air speed that reaches and passes through the fire. It implies a considerable increase in air temperatures and, therefore, an increase in the circuit resistances, as well as a change in the specific weight of the air/gases. This interaction between the fire and ventilation must also be carefully analyzed to verify the effects of the fire on the personnel present inside the mine and, in particular, to estimate the time available for a safe escape to internal or external locations, or for extinguishing the fire in its initial phase [9; 10; 11; 12]. Through the numerical modeling of the ventilation circuits, it is now possible to consider the presence of a localized fire inside the mine by considering all of the following main aspects:  
* The characteristics of the fire, i.e., the quantity and type of combustible substances present;
* The heat transmissibility in the rock, which affects the dispersion of energy toward the rock in the fire area;
* The characteristics of the ventilation circuit, such as the shape, section, length, and depth of the underground cavities, as well as the degree of roughness of the side surfaces and the presence of any obstructions or barriers inside the mine;
* The characteristics and operating modes of the main fans;
* The air temperature and pressure outside the mine (at the circuit entrance and exit);
* The temperature gradient of the rock with depth, which affects the transmission of heat to the air inside the mine. Through numerical modeling, one can consider, in the calculation, the different possible scenarios in order to analyze their effects on the fire dynamics, as well as on the type and quantity of fumes produced, and on the timing of their propagation within the mine; in particular, the following scenarios can be evaluated:  
* The different locations of the fire inside the mine;
* The different type of machine or plant on which the fire is triggered, with a pessimistic forecast of the quantity and type of combustible materials [13];
* The main fans during the fire that will be able to operate either in their standard conditions, with increased power, turn off when the fire is started after a certain period from the ignition, or reverse to change the direction of the airflow inside of the mine and in correspondence with the fire;
* The possible turning on of the auxiliary booster fans in certain areas of the mine to help the action of the main fans during a fire;
* The introduction or elimination of barriers in the ventilation circuit in order to simulate the closing or opening of doors inside the mine in emergency conditions. In this way, it is possible to simulate the intervention of safety personnel on the ventilation circuit to obtain better environmental conditions during the fire. From the calculation results, it is then possible to detect the best strategy to contain, mitigate, or even extinguish the fire, thus minimizing damage to people and property. More specifically, it is interesting to determine from the calculation the following values within the ventilation circuit [14]:  
* The airflow rates;
* The air composition and, therefore, the concentration of oxygen and toxic gases in the fumes (including CO and CO\({}_{2}\));
* The air temperatures. These values are helpful, particularly downstream of the fire (along escape routes that can be used by staff and worksites); their evolution over time is also fascinating.