## 4 Results and Discussion
### Combustion Model vs. Simplified Fire Models  
Figure 7 shows the HRR as a function of time in FDS simulations that use the simplified fire model (cases G-1 to G-4) and the EDC combustion model. In the first case, the energy enters the domain at a constant rate, whereas in the second one the HRR exhibits high frequency fluctuations, with an amplitude that approximately varies in the range 1.0 to 1.5 \(\text{MW}\). As will be shown below, these fluctuations produce high-frequency fluctuations of small amplitude in the heat and mass flow rates through the ventilation systems, especially through the west vent. Figure 8 shows the temperature and CO\({}_{2}\) mass fraction contours at instant \(t=90\) s and vertical planes at \(x=11,17,23,29\) and 36 m, obtained with the combustion model and the simplified fire model (Case G-1). Figure 9 shows the temperature and velocity magnitude contours at \(t=90\) s, on the horizontal plane at \(z=2.95\) m, obtained with FDS using the EDC combustion model and the simplified fire model (Case G-1). Figure 10 shows the CO\({}_{2}\) mass fraction contours at \(t=90\) s, on the horizontal planes at \(z=2.95\) m (left column) and \(z=2.05\) m (right column), obtained as in Figure 9. It can be observed from the figures the very good overall agreement between the three types of results obtained with the combustion and simplified models. Figure 8: Temperature contours (“C) (left column) and CO\({}_{2}\) mass fraction contours (\(Y_{\text{CO}_{2}}\times 10^{4}\)) (right column) at instant \(t=90\) s and \(x=11,17,23,29\) and 36 m, obtained with FDS using a grid with \(L/\delta\approx 10\). **(a)** Combustion model with RADIATIVE FRACTION \(=0\) and **(b)** simplified fire model with \(T_{\text{in}}=2436\) K (Case G-1). Figure 7: Heat release rate (HRR) in the simulations of cases G-1 to G-4 (Table 1), compared with that of the EDC combustion model. Figure 11 shows the results for the mass of CO\({}_{2}\) and heat flow rates through the ventilation systems obtained with FDS using the simplified fire model (Case G-1) and the combustion model. Note that the combustion model predicts mass of CO\({}_{2}\) flow rates through the forced ventilation grilles slightly larger than those obtained with the simplified fire model, a behavior that could be explained by the higher CO\({}_{2}\) concentrations at the ceiling in the vicinity of the forced extraction grilles, as shown in Figure 10. Also note the very good degree of agreement between the results for the flow rates through the west vent obtained with FDS and the two fire models. This can be seen with more detail in Figure 12, which compares results similar to those presented in Figure 5b, obtained with FDS using the combustion model and the simplified fire model. Note that the outward mass flow rate through the west vent predicted with the combustion model oscillates with a slightly smaller amplitude than when using the simplified fire model, an effect that is also observed in the net mass flow rate through the west vent and, as a consequence, in the inward mass flow rate through the east vent. Figure 10: Contours of CO\({}_{2}\) mass fraction (\(Y_{\text{CO}_{2}}\times 10^{4}\)) at \(t=90\) s and heights \(z=2.95\) m (left column) and \(z=2.05\) m (right column), obtained as in Figure 8. (**a**) Combustion model with RADIATIVE \(\text{FRACTION}=0\) and (**b**) simplified fire model with \(T_{\text{in}}=2436\) K (Case G-1). Figure 9: 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 8. (**a**) Combustion model with RADIATIVE \(\text{FRACTION}=0\) and (**b**) simplified fire model with \(T_{\text{in}}=2436\) K (Case G-1). Table 4 shows the relative CPU time consumed by FDS using the EDC combustion model and the simplified fire model. Note that the combustion model consumes a CPU time around 40% larger than that consumed when the simplified fire model is used, with no appreciable change in the results, as shown above.