This work presents an analysis of the turbulent natural convection in a volumetrically hemisphere cavity, representing a lower plenum of a Light Water Reactor (LWR) vessel. Considering isothermal top and bottom walls at temperature of 298.5 K and a fluid with Prandtl number (Pr) equal to 8.52, the experiment performed by Asfia et al. [1] was reproduced, so that their experimental data could be used for the validation of the numerical data provided by the present work. Ranging the internal Rayleigh numbers (Rai) from 108 to 3.03 × 1013, numerical simulations were performed using the open-source Computational Fluid Dynamics (CFD) code OpenFOAM (Open Field Operation and Manipulation), making use of the “code-friendly” modification of the turbulence model V2-f, a Reynolds Averaged Navier-Stokes (RANS) based equation model. The calculation of the turbulent heat fluxes was done by the Simple Gradient Diffusion Hypothesis (SGDH), which proved to be adequate for the case considered. Bearing in mind the complexity of the phenomena inside a molten core after a severe accident in a nuclear power plant, some assumptions were done such as the fluid was considered Newtonian, with no-phase change and homogeneous. Numerical correlations of Nusselt number (Nu¯) over the bottom and top walls as function of the Rai were obtained and a good agreement with the experimental data provided by Asfia et al. [1] was reached for the local heat transfer with respect to the angle for the cooled top wall.

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