## 1 Introduction  
Fires in tunnels can cause considerable material and environmental damage and loss of life and health of tunnel users. Road tunnels belong to such capital-intensive construction projects, which often contribute to solving acute traffic problems in the region, protecting the environment and reducing the number of road accidents. They are considered as ecological structures protecting nature and original habitats, as they do not negatively affect the migration of animals. Tunnels are often part of supra-regional or transnational transport corridors and national critical infrastructures. A fire in a tunnel is considered to be one of the most destructive events that can cause significant secondary damage with a transnational impact due to a long-term interruption, or operation disruptions during tunnel closures. Therefore, special attention is paid to fire prevention, safe operation and resilience of the tunnel, as well as to readiness for firefighting and rescue work. The tunnel is considered as an intelligent structure that is equipped with many safety installations, such as traffic control system, emergency ventilation, camera monitoring system, emergency lighting, emergency power supply, measurement of physical quantities, etc. These installations are used by the central control system (CCS) for autonomous fire detection, ensuring conditions inside the tunnel suitable for safe self-rescue and evacuation, to prevent entry into the affected tunnel tube and to provide the necessary information for the escape of people until the arrival of firefighters and rescuers. This CCS effort is supported by tunnel control operators who continuously monitor the situation inside the tunnel and can also participate in fire detection and localization using fire detection and camera monitoring systems. One of the key tasks of the CCS in the event of a fire is to monitor the airflowvelocity inside the tunnel and to regulate it using emergency ventilation to create and to maintain conditions suitable for smoke stratification in the upper part of the tunnel. The proper functioning of all tunnel safety systems and the readiness of tunnel control operators, firefighters and rescuers is critical for mitigating damage and preventing the loss of lives. For various possible fire scenarios, for each tunnel, there are pre-prepared and tested automatic responses of the tunnel to fire, which include various emergency ventilation operation strategies. These ventilation tests are conducted in the tunnel even before it is put into operation. Computer simulation of fires is, at present, considered to be an effective means of preventing fires, increasing preparedness to deal with them and mitigating their consequences. Considerable efforts towards more accurate and reliable mathematical and physical fire models have led to the development of advanced simulation systems for modeling fires in a variety of important environments. Experimental investigation of fire behavior outside the laboratory is expensive and allows only a limited number of fire scenarios to be investigated. Such an experiment often leads to the destruction of the tested structure. The goal is therefore to develop such a software environment that will enable to reliably simulate the behavior of a real fire in a given structure using 3D visualization. Such a simulation makes it possible to flexibly change the parameters of the fire scenario according to the user's requirements at present, and to test different extinguishing strategies and their effectiveness. Computational Fluid Dynamics (CFD) techniques are at present often used to design and test ventilation systems and to simulate fire and smoke. The two most popular software tools are Fire Dynamics Simulator (FDS) and ANSYS Fluent. FDS is primarily used to simulate fire development [1; 2] and ANSYS Fluent is a multi-purpose package that can be used for any fluid and heat flow problem [3].