Direct numerical simulations of natural convection in a pool containing an internally heated porous medium

Hamtiaux, Victoria [UCL] Ruyer, Pierre [Institut de Radioprotection et de Sûreté Nucléaire (IRSN)] Bartosiewicz, Yann [UCL]

We present a numerical study of convection in a cavity comprising an internally heated porous medium surrounded by a pure-fluid region. Convection in such domains arises in a variety of systems in geophysics and engineering such as loss-of-cooling accidents in spent-fuel pools. In these pools, metallic racks containing fuel assemblies are placed away from the sidewalls near the bottom surface. As fuel rods still generate heat, buoyant plumes and complex convective motions establish in the pool. In this study, this arrangement is modelled through a porous medium approach with volumetric heat generation. Numerical simulations are carried out using a single-domain approach, which introduces the porosity as a field variable and enables the formulation of a single set of governing equations that are valid over the entire domain. To account for the heat transfer between the metallic structures and the surrounding water, we assume thermal non-equilibrium between the solid and the fluid phase inside the porous medium and solve energy balance equations for both solid and fluid parts. By placing the porous medium at a small distance from the bottom wall, we observe flow recirculation at the bottom of the pool and through the base of the porous medium. This leads to a rather specific configuration for the natural convection in the pure-fluid region. We examine the flow dynamics and heat fluxes through the porous matrix as well as the flow patterns that develop in the surrounding pure-fluid region. Then, we contrast these results with a case in which bottom flow recirculation is prevented by removing the space between the base of the porous medium and the bottom wall. This has a significant impact on the distribution of temperature and velocity in the porous matrix, which in turn affects the Nusselt number reflecting the heat transfer process across the entire domain.