## 3 Fire Scenario  
### Description of the Facility  
In a previous contribution we studied the bus maneuvering and parking zones of the underground transportation hub \"Avenida de America\", in Madrid, Spain. Here, we focus on a fire that starts in a commercial premises (one of twenty) situated on level \(-3\) (Figure 1a). The different levels are connected by escalators. The computational domain considered in all the simulations, shown in Figure 1b, only includes the commercial area at level \(-3\). The overall dimensions of the zone studied are \(66.66\times 39.86\) m, with a usable floor area of \(2085\) m\({}^{2}\). In the geometric models created in FDS and FLUENT, the \(x\) and \(y\) axes are horizontal, and the \(z\) axis is vertical. The origin of the coordinate system is located in the S-W corner of the commercial area. It is assumed that the floor and ceiling surfaces are flat and horizontal and that the height is uniformly \(3\) m. As mentioned above, the origin of the fire is a commercial premises in the N-W corner of the shopping area. The danger is greater at this site because of the proximity to the stairs and consequent risk of spreading vertically and because of the greater affluence of people, which would hinder evacuation. There are two escalators connecting level \(-3\) with level \(-2\), and \(11\)\(1200\times 600\) mm air extraction grilles installed in the ceiling, which form part of the forced ventilation system.  
In a previous work [26], upper levels of the station (Figure 1a) were included in the computational domain in order to study the smoke and heat propagated not only in the commercial area at level \(-3\), but also in the stairwell that connects levels from \(-3\) to \(0\) and through the natural ventilation outlets installed at the ceiling of level \(-1\). The evolution of the fire throughout level \(-3\) when the upper levels were included and when the stairwells of level \(-3\) were assumed to be open to the atmosphere did not show significant variations. Therefore, in order to better focus on the fire spread through level \(-3\) only, where the most relevant effects were observed, we will assume that the commercial area is directly open to the atmosphere through two outlet sections (see Figure 1b), located \(70\) cm above the ceiling of level \(-3\), with the same areas of \(81.37\) m\({}^{2}\) (west vent) and \(60.06\) m\({}^{2}\) (east vent) as the horizontal pass-through sections of the stairwells located on the W and E sides, respectively. As the aim of the  
Figure 1: **(a)** Schematic view of the transportation station building (excluding platforms). **(b)** Top view of the shopping area where the fire originates (level \(-3\)): natural and forced ventilation systems, fire focus and reference system.  
present work is to analyze the evolution of the fire using different codes and fire models rather than evaluating the effectiveness of particular measures to limit the spread of the fire to the top floors of the building, no smoke barriers were included around the outlets. Note that, since there are not any additional natural vents or other means for introducing fresh air from outside, the necessary makeup air will inevitably be drawn through one or both existing vents, as will be shown below.