## 4 Results and Discussion
### FDS vs. FLUENT  
Figures 13-18 show comparisons between temperature, velocity magnitude and CO\({}_{2}\) mass fraction contours obtained with FDS and FLUENT at several vertical and horizontal planes and two different instants. It can be observed from the figures that the overall degree of agreement is very good. The most noticeable differences between FDS and FLUENT results are systematically observed in the distribution of the CO\({}_{2}\) concentration in the vicinity of the ceiling, as can be observed from Figures 17 and 18, and pictures at the right column of Figures 13 and 14. Figure 16: Temperature contours (left column) and velocity magnitude contours (right column) at \(t=240\) s, obtained as in Figure 13. (**a**) FDS; (**b**) FLUENT. Figure 17: Contours of CO\({}_{2}\) mass fraction (\(\gamma_{\text{CO}_{2}}\times 10^{4}\)) at \(t=90\) s and heights \(z=2.95\) m (left column) and \(z=2.05\) m (right column), obtained as in Figure 13. (**a**) FDS; (**b**) FLUENT. Figure 15: Temperature contours (left column) and velocity magnitude contours (right column) at \(t=90\) s, on the horizontal plane at \(z=2.95\) m, obtained as in Figure 13. (**a**) FDS; (**b**) FLUENT. As in previous sections, the most sensitive results to the use of FDS or FLUENT codes are related to the flow rates through the ventilation systems, and, therefore, we will discuss in the following the main differences found in the results for these quantities when using the two codes. Figure 19a,b compare the results obtained with FDS and FLUENT for the evolution of the CO\({}_{2}\) mass and heat flow rates, respectively, through the open vents and forced extraction grilles, using the simplified fire model with \(T_{\text{in}}=1500\) K (Case G-4). The degree of agreement between the FDS and FLUENT results is very good, especially for the heat flow rates. Note that FLUENT predicts values for the CO\({}_{2}\) mass flow rates extracted through the forced extraction grilles and through the west vent that are higher and lower, respectively, than those predicted by FDS, which can be attributed to the higher CO\({}_{2}\) concentrations at the ceiling in the vicinity of the forced extraction grilles, as shown in Figures 17 and 18. However, despite the differences in the values predicted by FLUENT and FDS for the mass of CO\({}_{2}\) extracted over 750 s through the forced (83.27 vs. 73.20 kg) and natural (134.5 vs. 143.6 kg) ventilation systems, the difference in the total CO\({}_{2}\) mass extracted over that time period is less than 1%. The reasons for these relatively low discrepancies are not completely clear. Note that the higher Schmidt number used by default in FLUENT would produce a lower diffusion and therefore a higher CO\({}_{2}\) concentration near the ceiling, resulting in a higher CO\({}_{2}\) mass flow rate through the forced ventilation grilles and a lower extraction rate through the west vent. Also note that the slightly lower outward mass flow rate through the west vent predicted by FLUENT, as will be shown below, may also contribute to explain the differences. Figure 19c compares the FDS and FLUENT results for the net mass flow rates through the west and east vents, and the net mass flow rate leaving the compartment, obtained with the simplified fire model with \(T_{\text{in}}=1500\) K (Case G-4). The agreement between both types of results for the net mass flow rate leaving the compartment is very good. Note that approximately 12 minutes after the beginning of the fire the net mass flow rate leaving the compartment vanishes, and the flow pattern in the compartment begins to oscillate around a steady state. On the other hand, the results obtained with FDS show larger outward and inward mass flow rates through the west and east vents, respectively, with greater oscillation amplitudes. Both codes predict that the mass flow rates through both natural vents oscillate with a frequency of about 0.03 Hz for \(\text{HRR}=2.5\) MW.