## 5 Conclusions  
The prolonged suppression effect of gaseous extinguishing systems, achieved by maintaining a high concentration of the extinguishing gas in the post-discharge phase, is critical for the safety of protected premises. High gas concentration prevents fire recurrence and spreading and allows longer rescue operation time. In order to design a gaseous extinguishing system that can maximise this retention time, a simplified CFD approach was proposed. The simplifications were in the omission of the complicated discharge phase, based on the assumption that after discharge, the flow field within the protected domain is stable and the gas mixing is driven mainly by turbulent flow near leakages, hydrostatic pressure and buoyant forces in the layer and gas dispersion. To verify this concept, we performed CFD calculations of extinguishing gases retention processes, referring to a previous experimental study [11]. Both physical and numerical experiments were performed in an idealised test array, that represented a difficult case of protected casing: high (slender), narrow and with a large leakage to volume ratio. The experiments can be considered representative for industrial casings of servers, 3D printers and other small-sized equipment. The test compartment can also be partially representative of larger compartments, because of the large leakage to volume, and leakage to floor area ratios. In typical compartments, the leakage-volume ratios are lower, and the distribution of the leakage in the height of the room is uneven. For more generalised conclusions, tests on a larger scale must be performed. This study did not include any heat or airflow sources (fans, radiators, power supply units, etc.) that can be sources of flow disturbances inside the rig. However, contrary to existing analytical models, it can be expected that the CFD solver should be able to model the thermal plumes and forced ventilation along the retention of the extinguishing gas. The verification of initial conditions on the results of numerical modelling was performed based on a comparative analysis of (1) the values of gas concentration distribution obtained by computer simulations with the results of physical experiments and (2) the value of retention time based on the oxygen concentration on the most onerous sensor. The completed tests helped to identify the validity of the CFD methods, simplified about the gaseous discharge, to describe the fire-extinguishing gas outflow from the protected compartment. Application of the CFD methods does not mean the elimination of the theoretical and experimental approaches, but can be used complementary to them. For high \(\Delta\rho\) gas mixtures, the retention time estimated through CFD was close to the measured in the experiment or estimated with standardised methods. For low \(\Delta\rho\) gas mixtures, high discrepancies were observed, due to the difficulties in predicting the moment of time in which the value of 13% oxygen concentration in volume was exceeded (the measured value was close to 13% for a long period). The inability to precisely estimate the retention time value for low \(\Delta\rho\) gas mixtures does not pose a problem in the practical use of CFD for the design of gaseous systems. In this series, the turbulent model did not influence the results in a significant way--all of the tested closure approaches gave similar results, regarding the retention time and oxygen concentrations. This could mean that simplified models (standard and Realizable k-\(\varepsilon\)) are viable and recommended from the cost-efficiency point of view. However, a significant effect of the coarse mesh resolution was observed. Thus, the numerical mesh for the retention modelling should be chosen after a mesh sensitivity study, which proves that further mesh adjustment does not change the quantitative results in a significant way. In future, the use of computational fluid dynamics (CFD) can become a new important engineering tool in studies covering the protection of premises with fire-fighting gases.