| Summary: | Active fire protection systems are critical elements of good process safety. Among them, gaseous extinguishing systems provide quick, clean suppression and prolonged protection due to long retention process of the gas. Standard design methods do not provide sufficient tools for optimisation of the retention process, thus the necessity for development and validation of new tools and methods—such as Computational Fluid Dynamics (CFD) simulations. This paper presents a simplified approach to CFD modelling, by the omission of the discharge phase of the gas system. As the flow field after discharge is stable and driven mainly by the hydrostatic pressure difference, buoyancy and diffusion, this simplified approach appears as an efficient and cost-effective approach. This hypothesis was tested through performing CFD simulations, and their comparison against experimental measurements in a bench scale in a small compartment (0.72 m<sup>3</sup>), for six mixtures that differ in their density. Modelling the retention of the standard IG55 mixture was very close to the experiment. Modelling of mixtures with a density close to the density of ambient air has proven to be a challenge. However, the obtained results had sufficient accuracy (in most cases relative error <10%). This study shows the viability of the simplified approach in modelling the retention process, and indicates additional benefits of the numerical analyses in the determination of the fire safety of protected premises.
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