Highly resolved large eddy simulations of a binary mixture flow in a cavity with two vents: Influence of the computational domain

Abstract

In this article, numerical results from a highly resolved large eddy simulations (LES) of an airhelium buoyant jet developing in a two vented cavity are presented. The simulated configuration mimics the helium-release experiment carried out at CEA Saclay in the framework of security assessment of indoor used hydrogen-based systems. The height of the enclosure was chosen so that a laminar-turbulent transition occurs approximately at the middle of the upstream direction. An exterior region, of different spatial dimensions, has been modelled in the computational domain in order to move the boundary conditions away from the vent surface and to approach the natural inlet/outlet conditions. A sensitivity analysis regarding the size of the exterior region is presented to define the minimum horizontal extension so that the flow inside the cavity is not furthermore influenced by bigger computational domains. We observe mainly that applying an ambient equilibrium-hydrostatic pressure outlet condition directly on the surface of the vent reduces the volume of the air inflow, and thus predicts larger helium mass inside the cavity, in contrary with the cases where an exterior region is considered. A qualification analysis shows that the sub-grid scale model plays a small role in the calculations and thus implies that the LES predictions approach the direct numerical simulation (DNS) solution. Analysis carried out on the time-averaged helium field depict a stratified regime not situated in the framework of the theoretical model used in safety pre-calculations.

A well-resolved numerical study of a turbulent buoyant helium jet in a highly-confined two-vented enclosure

“…The experimental set-up has been developed at CEA Saclay in the framework of hydrogen security in accidental release scenarios [20]. It consists of a cavity with two vents whose dimensions in the horizontal, span-wise and vertical directions are W × L × H = 0.049 × 0.05 × 0.149 m 3 , respectively.…”

“…The numerical configuration mimics the CEA experiment [20]. Unless otherwise defined, the dimensions are identical to those of the experimental cavity.…”

“…An external region attached directly to the vent surfaces is used to push the system boundary conditions away from the vents. As shown in our previous work [20], and taking into account the physical parameters of the problem, a sufficiently large external domain, equivalent to three volumes of the region of interest, is chosen in order to obtain a flow and a mixture distribution independent of the computational domain.…”

“…3 profile along the buoyant jet axis. Our previous LES results [20] are added to emphasize the improvement obtained by performing DNS. Indeed, contrary to LES, the turbulence level of the DNS results is of the same order than the PIV data, and even falls within the PIV error bars.…”

“…This is indeed a very challenging task due to the fact that there is some evidence that one can find both inward and outward flow through a given vent, as is the case in a vertical chimney [19]. One way to circumvent this problem is to consider an exterior computational domain, as was done in Saikali et al [20]. This study was performed in order to determine, by trial and error, the size of the exterior domain and appropriate boundary conditions in order to obtain satisfactory comparisons with experimental data from particle image velocimetry (PIV) measurements carried out at CEA Saclay (LIEFT) [21,22].…”