A series of numerical simulations were performed using the Fire Dynamics Simulator (FDS) a low-Mach number, large eddy simulation (LES), computational fluid dynamics (CFD) code, to model an experiment conducted by INERIS to study the release of small amounts of hydrogen in a confined space. The experimental setup consisted of a 1 g/s vertical hydrogen release for 240 s from an orifice of 20 mm diameter in a rectangular room (garage) of dimensions 3.78 x 7.2 x 2.88 m. Two small openings (50 mm diameter) at the front and bottom side of the room assured constant pressure conditions. Hydrogen concentrations were measured as a function of time during both the release phase and the subsequent diffusion phase (5160 s) at various heights in the garage.

Although the simulations were conducted after the experiments had been completed, the simulations were performed blind so as to fully evaluate the capability of the FDS software. Since the dimensions of the orifice through which hydrogen releases and that of the two openings were small compared to the garage dimensions, a grid resolution study was completed to fully resolve the important physical processes. Sensitivity studies were also completed to understand the effect of orifice diameter and inlet jet velocity. Results indicate that resolving the flow field around the orifice to capture the air entrainment into the hydrogen jet was critical. Resolving the flow field through the two small openings was only necessary to model the pressure increase accurately and had little effect on the predicted hydrogen concentration. The predicted hydrogen concentrations compared favorably with the measurements in the plume region as well as in the upper layers of the buoyancy driven flow field. However, the concentrations were over-predicted in the diffusion dominated lower layer. The numerical simulations are compared with experimental data and other calculations performed as part of the INERIS study, to understand the short and long term mixing and distributions obtained during small releases of hydrogen in a large confined space.