## 4 Development of the Numerical Model
### Domain and Mesh  
For conducting simulations on the testing room, a simulation domain with dimensions exceeding those of the building was considered. This approach allows for the definition of varied climatic conditions and the observation of airflow phenomena both inside and outside the building. The domain was defined with dimensions of 6 \(\times\) 6 \(\times\) 6 m as shown in Figure 9.  
The subdomains where the burner is located has dimensions of 0.6 \(\times\) 0.6 \(\times\) 1 m, situated at the lower part of the testing room on the ground floor, as shown in Figure 9.  
In the calibration of the FDS model, extensive testing was conducted on various cell sizes--0.025 m, 0.05 m, 0.1 m, and 0.25 m--to determine the most effective mesh resolution and ensure a detailed reproduction of phenomena, similar to other studies [15; 36; 37]. This investigation revealed key insights, as the 0.25 m cell size was insufficiently refined to initiate a combustion reaction, yielding HRR values lower than those achieved with smaller cell sizes. Conversely, the 0.1 m cell size offered improved fidelity in heat transfer and smoke flow in regions adjacent to the fire source, yet it still fell short of matching experimental HRR values. The 0.5 m cell size demonstrated adequate precision for both adjacent domains and reaction initiation, though it required an eightfold increase in processing time and resources compared to the 0.1 m cell.  
The evaluation of the 0.025 m cell size, despite providing a high accuracy of 1-2% relative to experimental data, was deemed impractical due to significantly higher resource and time costs--eight times that of the 0.05 m cell size. Consequently, a dual-mesh strategy was selected: utilizing a coarse 0.1 m cell for peripheral zones and a refined 0.05 m cell for the combustion focal area. This approach was calibrated to deliver results from a single simulation with an average relative error below 10%, efficiently completed in under 100 h. This analysis required eight trial simulations with reduced runtimes between 100 and 300 s.  
An optimal mesh resolution necessitated compliance with the appropriate \(D*/dx\) criterion, as identified in the literature, to ensure feasible results. We adhered to this framework, acknowledging that values ranging from 16.7 to 33.4 offer adequate precision. A \(D*/dx\) value of 16.7 corresponds to a coarse mesh (0.1 m), while 33.4 represents a fine mesh (0.05 m), as shown in Figure 9.  
Sensitivity analyses were conducted to evaluate the impact of mesh size on the convergence of heat flux and species concentration fields. While finer meshes improved field stability, they also increased computational demands [38; 39; 40]. In contrast, overly coarse meshes, as noted in [41], risk omitting critical details necessary for accurately capturing  
Figure 9: Simulation domain (**left side**) and burner subdomains (**right side**).  
transient fire dynamics. Our investigation of mesh resolutions, using \(D*/dx\) ratios between 16.7 and 33.4 [41; 42; 43], highlighted the influence of cell size on simulation accuracy, especially given the geometric complexity of the domain and the physical scale of the wooden chips.  
Mesh selection thus required balancing computational efficiency and predictive accuracy. Analysis of the Mean Relative Error (MRE), detailed in the calibration chapter, showed diminishing returns in accuracy when refining the mesh beyond a certain threshold--consistent with previous findings [41]. By adhering to validated \(D*/dx\) values and performing systematic sensitivity testing, the FDS model was robustly calibrated, successfully replicating experimental conditions and reliably predicting fire behavior in open-plan office environments.  
In conclusion, the number of cells in the burner subdomains was 25,920, while the cumulative number of cells of the domain left was 212,760. In total, the number of cells in the entire simulation domain was 238,680.