## 2 Modeling and Experimental Validation  
### Numerical Method  
A full-scale geometric model was established based on an actual Tianjin Metro Line 1 subway station, which portrays a typical subway station with smoke extraction systems. Figure 1 shows the three-dimensional computation domain used to simulate the outside flow field, with dimensions of 324 m \(\times\) 324 m \(\times\) 10 m. The subway station hall was regarded as the center of the outside flow field, and each side of the outdoor flow field was three times the station's maximum diameter [20]. The height of the outdoor flow field was a standard height [21], and the blockage ratio of the windward part at the entrance and exit was less than 3% [22]. The passageways connected the station hall with the external environment, with passageways A and C facing south and D facing east, respectively. The dimensions are listed in Table 1. The computational domain of this study included the subway station hall and the outdoor flow field.  
The ventilation system of the station hall consisted of two exhaust ducts and two air supply ducts. There were 40 air outlets with a diameter of 0.4 m for each air duct, of which the magenta lines represent the exhaust ducts and the cyan lines represent the air supply ducts, as shown in Figure 2a. During the normal operation of the station hall, a form of air distribution consisting of top air supply and return was employed. When fire occurred in the station hall, the air exhaust ducts were used as the smoke exhaust ducts. To discharge  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Composition** & **Length (m)** & **Width (m)** & **Height (m)** & **Direction** \\ \hline Outdoor flow field & 324.0 & 324.0 & 10.0 & - \\ Station hall & 86.6 & 9.1 & 3.5 & - \\ Supply outlet & 0.4 & 0.4 & - & - \\ Exhaust outlet & 0.4 & 0.4 & - & - \\ Passageway A & 82.6 & 8.0 & 2.6 & South \\ Passageway C & 85.0 & 6.2 & 2.6 & South \\ Passageway D & 74.0 & 8.0 & 2.6 & East \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dimensions of the station hall.  
Figure 1: Geometrical model of the subway station.  
smoke, the air handling unit was turned off and the air exhaust fan was replaced with a smoke extraction fan.  
The side of the station hall corresponding to passageways A and D was dubbed A-end, while the side corresponding to passageway C was dubbed B-end. With the fire source as the center, nine monitoring points were equidistantly arranged on both sides of A-end and B-end to monitor the variation in CO\({}_{2}\) concentration in real time, as illustrated in Figure 2b. The CO\({}_{2}\) concentration monitoring points are represented by C\({}_{\rm An}\) or C\({}_{\rm Bn}\).  
In this study, all analyses were performed using Fluent 2020 R2 version, and the finite volume method was utilized to discretize the governing equations. Specifically, a pressure-based solver was selected for the numerical method, and the SIMPLE scheme was used to couple velocity and pressure. The second-order upwind scheme was applied to solve the pressure, the momentum equation, the energy equation, and the species mass-conservation equation. Similarly, the first-order upwind scheme was applied to solve turbulent kinetic energy and turbulent dissipation rate. The residual of the governing equations should be less than 10\({}^{-5}\). In particular, the residual of the energy equation should be smaller than 10\({}^{-6}\).  
To investigate the sensitivity of time steps, the average velocity at the entrances and exits was simulated using three time step sizes of 0.4 s, 0.2 s, and 0.1 s. The results demonstrate that there was minimal change between the wind speeds of 0.2 s and 0.1 s. However, the calculation time would grow by 2.8 times when the time step size was reduced from 0.2 s to 0.1 s. As a result, the time step size of 0.2 s was used throughout the entire simulation.