## 3 Numerical Results
### Velocity of Airflow Throughout the Building Interior  
From the perspective of the airflow velocity analysis presented in Figures 9-11, it is observed that scenarios S1 and S2 exhibit lower velocity values compared with scenario S0,  
Figure 8: Temperature graph recorded above the fire source by a thermocouple positioned at X = 3.25 m, Y = 2.00 m, Z = 1.7 m.  
where smoke evacuation is achieved through organized natural ventilation. In scenario S0, the stack effect significantly increases air velocity within the stairwell, with the airflow direction being upward, as opposed to the downward direction observed in scenarios S1 and S2. Once the door to the fire compartment is opened, the pressure differential criterion is replaced by the velocity criterion. At this point, velocities of up to 2 m/s can be observed on the ground floor level of the stairwell, which are below the recommended thresholds established by the European standard SR EN 12101-6 [30]. The graphical representation of air velocity distribution (Figures 9-11) in three characteristic planes--two vertical (Y = 2 m and Y = 5.75 m) and two horizontal (Z = 1.3 m and Z = 26 m)--allows for a comprehensive spatial analysis of airflow behavior within the stairwell. This multi-sectional orthogonal approach provides a three-dimensional understanding of the interaction between ventilation dynamics, building geometry, and thermal buoyancy effects. The evaluation is carried out under the condition that the door to the fire compartment is open, in which case the relevant criterion becomes the airflow velocity through the opening, rather than the differential pressure. Scenario S0 (Figure 9) highlights an aerodynamic regime entirely dependent on thermal buoyancy, with no mechanical air supply. The recorded values (V2: 5.73 m/s; V3: 2.64 m/s; and V4: 2.51 m/s) suggest a rapid vertical propagation of hot gases within the stairwell, while the airflow remains undirected and uncontrolled, indicating the absence of effective smoke management. The airflow patterns in the horizontal Z planes reveal uneven smoke propagation both vertically and laterally, compromising evacuation safety. Additionally, the Y = 2 m slice shows high airflow velocities of up to 8.55 m/s at the fire compartment door leading into the stairwell, indicating a hazardous gas flow dynamic. Such conditions demand precise control through coordinated ventilation strategies and timed access  
Figure 9: Velocity distribution in the Y = 2 m and Y = 5.75 m slices for scenario S0. opening. In the absence of effective control, there is a high risk of secondary flashover or backdraft, endangering both structural integrity and human life. Therefore, scenario S0 proves to be ineffective, as high velocity alone does not equate to protection in the absence of controlled airflow. Scenario S1 (Figure 10) is the only case in which the fan configuration successfully generates a stable downward jet, precisely directed opposite to the smoke propagation. The recorded velocity in the lower area (V2: 2.01 m/s at T = 240 s) aligns exactly with the threshold set by the standard, forming an effective barrier against the flow of toxic gases. The vertical velocities (V3, V4: \(\sim\)1.6-1.7 m/s) are nearly uniform, ensuring consistent pressurization along the entire height of the stairwell. The horizontal planes (Z = 1.3 m and 26 m) confirm the absence of recirculating flows, supporting the validation of this scenario as the only strategy that meets both velocity requirements and flow stability criteria. Figure 11: Velocity distribution in the Y = 2 m and Y = 5.75 m slices for scenario S2. Figure 10: Velocity distribution in the Y = 2 m and Y = 5.75 m slices for scenario S1. In scenario S2 (Figure 11), although air is introduced both at the top and bottom of the stairwell, the resulting velocities are significantly lower: V2 = 0.85 m/s, V3 = 0.66 m/s, and V4 = 0.86 m/s.