## 1 Introduction
### Use of Gaseous Fire Extinguishing Systems  
FES-gaseous systems operate by filling the protected space with fire-extinguishing gas, stored under pressure in reserve (cylinders), connected with a pipeline ending with nozzles. The gas outflow is automatic, once the fire detectors detect a fire. The outlet nozzles are selected and arranged to ensure uniform distribution of the fire-extinguishing gas in the whole protected space. FES-gaseous systems tend to be used to reduce the risk of property loss and fire spreading around the facility, but they are also regarded as an essential solution from the safety of humans, rescue teams and building [11]. Before the gas discharge (Figure 1A), the flow field in the compartment is dominated by the turbulent motion created by buoyant smoke plume and flames. During discharge (Figure 1B) the flame is quenched, and any earlier flow-field overpowered by highly turbulent, supersonic gas discharge. At the end of the discharge phase, the pressure inside and outside of the compartment is evened out by a pressure relief damper, and the resulting gas concentration is uniform in the space. After the discharge (Figure 1C), the flow inside the compartment settles down, and the concentration of gas is uniform and should be equal to the designed concentration. The turbulent gas exchange takes place at the leakages, and near heat sources, ventilation elements. Stage (Figure 1C) is taken as the initial conditions for simulations in this paper. The extinguishing gas concentration (and the Oxygen concentration as a result) during, and immediately after, the discharge is variable in the compartment volume. However, as observed during the experiment [11], these concentrations level out in a short time after the discharge in the whole protected volume. Further to that, in the standardized approach (described further in Section 1.3) the \"retention time\" is measured not from the discharge, but from the moment when Oxygen concentration in the compartment reaches the design value. Thus, taking intoaccount the complexity of discharge modelling, the simplification of the CFD approach to initialize the simulation with the \"design conditions\" and not the discharge seem justified. Successful firefighting with fire-extinguishing gas entails the need to maintain the designed fire-extinguishing gas concentration in the protected space for the required period, which is called the retention time. It stands for the time necessary to suppress the flame and take actions by appropriately trained staff. A longer retention time increases the probability of effective rescue operations and extinguishing the fire in its initial phase. Consequently, the longer the retention time, the higher the fire safety, while too short retention time may result in the fire recurrence and spreading. The retention-time optimisation method, by altering the FES-gaseous mixture density, was presented and described in depth in [11]. In this paper, we attempt to model the FES-gaseous retention process with the use of a Computational Fluid Dynamics (CFD) model--ANSYS(r) Fluent(r) (version 14.5.0 [18])--based on the previously published experimental study [11; 19]. Significant simplifications in the initial conditions of the simulation were introduced to limit the complexity of numerical modelling of FES-gaseous protection. It was observed in the bench-scale experiments [11] that after initial gas discharge, the compartment is uniformly filled with the gas, and there were no significant differences in the gas concentration in space. This is mainly due to pressure relief mechanisms used in protected areas, that act as a fail-safe to reduce the damage caused by the gas release. Modelling the super-sonic discharge through a minuscule nozzle along with the pressure relief mechanism could be considered as a significant challenge.