## 3 Results  
### Overview of the Results of Numerical Modelling  
CFD simulations were performed for standard fire-extinguishing gases composed of Ar = 50% v/v and N\({}_{2}\) = 50% v/v (type IG 55), and Ar = 0% v/v and N\({}_{2}\) = 100% v/v (type IG 100), and for four other fire-extinguishing mixtures with densities similar to the density of air. The additional mixtures were based on the work presented in [11], and normalised to the retention concentration of the gas of 13% in volume. The Oxygen concentration was measured at five points, as shown on Figure 2.  
The retention time values identified numerically based on the obtained changes in the distribution of the fire-extinguishing gas concentration ranged from 50 to 869 s (Table 2). In order to compare the accuracy of the models, a relative error at the retention time identification for each kind of fire-extinguishing gas respectively was calculated. The retention time is not a physical value that would characterize the fire safety of a premise, but purely a measurement of the time that passes from the release and the achievement of extinguishing concentration of the gas within the compartment, to the moment when the oxygen concentration (v/v) does exceed a 13% value in one of the measurement points. This means that this value of time is not explicitly calculated by the CFD, but can be estimated based on the local concentration of extinguishing gas or oxygen. The differences in the assessed retention time for gasses with a small value of \(\Delta\rho\) (difference of density between the extinguishing gas and the ambient air) are large. However, the CFD model presents values closer to the experiment than the standard model, Figure 9. Moreover, as can be noticed in the presented further time-plots of the oxygen concentration, this is not only a difference in the estimation of the time, but a significant difference in the estimation of the concentration or flow within the room. As shown in Figure 10, even though the retention time criterion is passed before the 1200 th second in all of the simulations, for scenarios 2-6, a significant part of the premise is still protected with an effective concentration of extinguishing gas. The differences between different gasses may be a result of different flow rate through leakages, due to the differences in hydrostatic pressure. Figure 11 presents the maximum flow velocity measured at the leakages for different values of \(\Delta\rho\). It can be noted that for large \(\Delta\rho\) = 0.192 kg/m\({}^{3}\), the velocity is close to 0.8 m/s, while for the gasses with density very close to the density of air this velocity is below 0.1 m/s. With such low air flow velocity, a small change in the velocity may cause significant change in the retention time--which may be an explanation to the discrepancies found between CFD and experiment for gasses with small value of \(\Delta\rho\). The user should be aware that in this scenario, the CFD model is very sensitive to the initial conditions. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**Ar [5\%/v]**} & \multirow{2}{*}{**N2 [5\%/v]**} & \multirow{2}{*}{**\(\Delta\rho\) = d\({}_{m}\) = d\({}_{o}\)**} & \multirow{2}{*}{**Retention Time**} & \multirow{2}{*}{**Retention Time**} & \multirow{2}{*}{**Retention Time**} \\  & & & & **(Experiment) T\({}_{R}\) [s]** & & & **(Stand. Model) t\({}_{R}\) [s]** & \\ \cline{5-8}  & & & & & & **Standard Model** & **CFD** \\ \hline \(1\) & 50 & 50 & (+) 0.192 & 57 & 60.8 & 50 & \(-\)6.67 & 12.28 \\ \(2\) & 0  
Figure 10: Oxygen concentration distribution in the test chamber in chosen time points for all evaluated scenarios (Table 1). In cases 2 and 4 the gas escapes the protected chamber through top leakage. Prolonged protective effect of gas mixtures 3–6 may be noted. Figure 9: Comparison of the retention time estimate in experiment [11], standard model calculations and CFD analysis for different values of \(\Delta\rho\).