This work is devoted to the security assessment of systems using hydrogen as energy carriers, where the problematic is mainly with the hydrogen leakage which is not only dangerous, but can be destructive when mixed with air. The french Atomic Energy Commission (CEA) is showing a great interest to the presented problem and have been participating in several concerned projects that focus on reducing the risks in such systems.

 We present a numerical Large Eddy Simulation (LES) study to mimic the experiment held at CEA-Saclay, where the above problem is modelled by injecting Helium gas (replacing the Hydrogen in the real problem) through a cylindrical pipe that enters a two-vented parallelepiped cavity filled initially with air. Understanding the mixing/dispersion of the binary fluids at the transient and quasi-steady state is one of the goals that we target on. In spite of the fact that the experiments had been held over a wide range of varied injection flow-rates, we restrict our study to the case of a 5 Nl/min injection resulting in a formation of a buoyant jet that is mainly influenced by momentum in the near source and buoyancy dominant upstream. The dimensions of the cavity had been chosen so that the interesting physical phenomena of the problem is captured; basically the transition from a laminar regime to a turbulent one and the self-similarity of the jet horizontal profiles.

 In this paper, we present some of our LES results obtained by making use of the CEA code TRUST and the Smagoranski model. We point out basically to the influence of the exterior domain, if it is/isn't considered in the computational domain, on the flow inside the cavity. We demonstrate that imposing classical fixed pressure boundary conditions directly at the vents positions, when compared with the experimental Particle Image Velocitimetry (PIV) measurements, tends to under-estimate the air flow in the cavity and thus leading to over-estimate the maximum Helium concentration.

Different simulations had been achieved by varying the size of the exterior domain which is opened from all directions not in contact with the plexi-glass of the cavity. Following a space and a temporal convergence study, our statistical results seems to be satisfactory when compared to the experimental data.

This work is a first step as we look on developing new outlet boundary conditions which can be directly applied at the vents positions for a perfect Direct Numerical Simulation (DNS), at the Kolmogorov scales, with a better computational cost.