## 3 Numerical Results
### Differential Pressure Inside the Building  
The distribution of differential pressure in the three analyzed scenarios (Figures 15-17) highlights the direct influence of the ventilation strategy on the system's ability to maintain an effective pressure barrier within the stairwell. Scenario S0 (Figure 15) illustrates the typical behavior of a building exposed to the thermal stack effect generated by a fire. Due to intense heating near the fire source and the absence of a controlled air supply, a pronounced negative differential pressure develops within the stairwell, with values reaching as low as \(-32.84\) Pa, caused by the upward movement of hot gases. This negative pressure is physically explained by the temperature and density differences between indoor and outdoor air, and the analytical estimation correlates well with the simulation results. In the absence of a pressure barrier, the stairwell becomes an open channel for  
Figure 14: Smoke distribution in front view for the scenarios corresponding to times T = 240 s and T = 360 s.  
smoke contamination, leading to a rapid and irreversible deterioration of visibility and evacuation conditions. In scenario S1 (Figure 16), by introducing air solely on the upper level, the system generates a positive and stable vertical pressure gradient. At T = 360 s, the pressure values range between 12.5 Pa (top) and 3.9 Pa (bottom). This distribution ensures a stable downward airflow and effective protection against smoke infiltration, even when the access door to the fire compartment remains open. The system effectively compensates for leakage losses. In scenario S2 (Figure 17), although the pressure values remain positive (18.6 Pa at the top, 3.9 Pa at the bottom at T = 360 s), the dual fan configuration introduces interference between the ascending and descending airflow streams. The presence of the lower fan,  
Figure 16: Differential pressure distribution in the sectional plane Y = 5.75 m for scenario S1 at different time moments. Figure 15: Differential pressure distribution in the sectional plane Y = 5.75 m for scenario S0 at different time moments. placed near the fire source with the door open, actively supplies air to the fire zone, reducing the local pressure differential and, consequently, the effectiveness of the aerodynamic barrier. Instead of supporting pressure maintenance in the stairwell, the lower fan may indirectly feed combustion or facilitate smoke propagation into intermediate volumes, negatively affecting pressure control in these areas. A key moment is recorded at t = 55 s, when a pressure of approximately 183 Pa is observed in the stairwell, compared with 110 Pa in the adjacent spaces in scenario S1, and 136 Pa versus 80 Pa in scenario S2. The pressure values in the adjacent spaces result from air infiltration from the stairwell due to its pressurization. The resulting pressure differentials--73 Pa in S1 and 56 Pa in S2--fall within the optimal range recommended by European standards (50 Pa \(\pm\) 10%), allowing doors to be opened without additional effort and ensuring safe evacuation conditions [30]. The door opening at 120 s in scenario S1 and at 60 s in scenario S2 produces a controlled decrease in differential pressure, with a balanced redistribution along the vertical axis over time. Although the pressure drops compared with the initial moment, the values remain functional for protecting evacuation routes, particularly in the upper zones. However, maintaining control on the lower levels is essential, where the pressure falls below the optimal threshold. According to the NFPA 92 standard [51], maintaining a minimum differential pressure of 25 Pa (for compartments without sprinklers) is essential to prevent smoke migration between compartments.