Evaporation-driven turbulent convection in water pools heated from below

This thesis is a numerical study of turbulent thermal convection driven by free-surface evaporation in water pools. The studied physics is closely related to the flow occurring in the early stages of a spent-fuel pool (SFP) loss-of-cooling accident (LOCA), such as that which occurred at Fukushima 2011. Here, the important interdependent physical phenomena are free-surface evaporation and turbulent thermal convection. First, a configuration similar to turbulent Rayleigh-Bénard Convection (RBC) is investigated, but with a free-slip upper boundary condition approximating a free surface. Fixed temperature boundary conditions are applied above and below, and the role of the free-slip and non-Oberbeck-Boussinesq effects on the mean flow properties are investigated. The free-slip is shown to play an important role on heat transfer in the domain and on the homogeneity of the thermal boundary layer heights. Secondly, a representative evaporative heat flux is prescribed uniformly as the thermal boundary condition above. Time-averaged properties of the flow are shown to be influenced by the presence of the free surface and a novel Rayleigh number scaling of the Nusselt number is found. Finally, a novel dynamic thermal boundary condition based on evaporative and convective heat losses at a free surface is developed and implemented. An aspect ratio study shows that the large-scale circulation state switches from a single-roll, to a dual-roll state at small aspect ratios, changing both the temperature and velocity fields at the free surface. However, no discernible impact on the evaporation rate is found.