## 2 Modeling and Parameter Adjustment
### Geometrical Model and Ventilation Setup
Additional research [33] shows that the pressure can reach hazardous levels, obstructing evacuation and rescue operations, highlighting that ventilation settings are essential for accurate simulations. * Given that in PyroSim software version 6.7.9, introducing a pressure relief vent condition causes a significant drop in overpressure, such a condition was deliberately omitted from the model in order to preserve the accuracy of the simulated pressure behavior. * Infiltration openings measuring 0.10 m \(\times\) 0.10 m were taken into account, representing the cumulative area of leakage through windows and doors of compartments aligned with the external windows [10]. * The airflow rate was determined based on the total building volume, the required air exchange rate, and the effective leakage through each flow path. The resulting analysis established an air supply rate of 1 m\({}^{3}\)/s on the upper level and 1 m\({}^{3}\)/s on the lower level. Furthermore, when the stairwell door is opened at t = 120 s (scenario S1) and t = 60 s (scenario S2), the differential pressure criterion is replaced with a velocity-based criterion [31], which requires a minimum airflow velocity of 2 m/s at the doorway leading to the fire compartment. Ventilation performance is governed by two opposing constraints: ensuring stairwell velocities exceed 2 m/s upon opening the door to the fire-affected area, and limiting pressure differentials between the stairwell and adjacent compartments to below 50 Pa. In the context of our analyzed building, characterized by specific infiltration rates outlined in the preceding chapter, these conditions cannot be simultaneously achieved. Consequently, a flow rate of 2 cubic meters per second (m\({}^{3}\)/s) is selected as an optimal compromise. The ventilation scenarios, S1 and S2, utilize similar airflow rates, distinguished by their points of introduction; scenario S1 introduces air exclusively from the top, whereas scenario S2 introduces air from both the bottom and the top of the structure. In this study, PyroSim 2023.3.1312 is employed to perform targeted simulations aimed at analyzing fire behavior and assessing the influence of ventilation on smoke propagation. By utilizing computational fluid dynamics (CFD)-based modeling, the results contribute to a deeper understanding of associated risks and the optimization of protective measures within the evaluated scenarios. PyroSim [34] is an intuitive software platform that supports fire and smoke spread simulations, offering a graphical interface for the Fire Dynamics Simulator (FDS). Ventilation modeling plays a critical role in fire simulations, as it directly affects both smoke movement and the performance of HVAC systems. While PyroSim allows a detailed configuration of fan parameters, conventional models such as the quadratic fan model have limited accuracy in capturing dynamic pressure variations, which may compromise the reliability of simulation outputs. Recent studies [35; 36] have proposed a novel approach based on the operating point method, where fan flow rates are adjusted according to system curves. This approach yields more precise and realistic results, particularly in dynamic fire conditions. Considering that the focus of this research is the improvement of evacuation conditions via stairwells in existing buildings, where the installation of air supply ductwork is not feasible, a pressure build up strategy was adopted using the supply function and strategically placed fans (Figure 2). This method allows for air to be introduced directly into a specific zone and is significantly more practical and time efficient compared with implementing a full-scale HVAC system. A supply vent enables the localized control of airflow and positive pressure generation, making it ideal for preventing smoke spread into critical areas without the complexity and infrastructure requirements of a full HVAC system, which typically involves extensive duct networks and multiple fans.