Numerical Simulations of Turbulent Rayleigh-Bénard Convection with a Free Surface

Hay, William Andrew [UCL] Deledicque, Vincent [Bel V] Papalexandris, Miltiadis [UCL]

One fundamental conclusion from the OECD 2018 PIRT study on Loss of Cooling Accidents (LOCAs) in Spent Fuel Pools (SFPs) was the urgent need for improved understanding of single and two-phase natural convection phenomena during the pre-uncovery phase. Even before nucleation, complex thermal mixing and evaporation phenomena coexist. Further, the equilibrium temperature reached by a pool during a SFP-LOCA can exceed 80 ◦ C. The variation in fluid properties during heat-up, particularly that of density, means that the Boussinesq approximation cannot be utilized in numerical simulations for the problem in hand. The aforementioned dominant phenomena can still be captured when the upper pool volume is modeled as a cavity heated from below with a free surface, allowing for simultaneous study of turbulent natural convection and free-surface evaporation. The motivation of our research is therefore to carry out non-Boussinesq numerical simulations of turbulent Rayleigh-Bénard convection to better understand the impact of the free surface boundary condition. Our low-Mach-number solver is first validated against experimental data for a wall-bounded Rayleigh-Bénard convection flow. Subsequently, using water as the working fluid in a cubical geometry heated from below and with a free surface, we carry out Direct Numerical and wall- resolved Large-Eddy simulations up to Rayleigh numbers of 10 10 . Our studies confirm and quantitatively assess the influence of the free surface on the large-scale circulation and on the Nusselt number. Further, they contribute towards the ultimate goal of assessing the role of turbulence on free-surface evaporation during an SFP-LOCA.