The paper reports computational simulation of natural convection of a molten core during a hypothetical severe accident in the lower head of a typical PWR pressure vessel. Numerical simulations were carried out for a two-dimensional semi-circular slice geometry which represents the lower head of a PWR pressure vessel. Transient turbulent natural convection heat transfer of a fluid with uniformly distributed volumetric heat generation rate is simulated by using a commercial computational fluid dynamics (CFD) software ANSYS CFX 12.0. The bottom circular surface and top flat surface are both kept at a same constant temperature. The Boussinesq model is used for the buoyancy effect generated by the internal heat source in the flow field. The two-equation k-ω based SST (shear stress transport) turbulent model is used to model the turbulent eddy viscosity. Two Prandtl numbers are considered, 6.13 and 7.0. Five Rayleigh numbers were simulated for each Prandtl number: 109, 1010, 1011, 1012, and 1013. The average Nusselt numbers on the bottom surface calculated by the computational simulations are in excellent agreement with Mayinger et al. (1976) correlation, with the average Nusselt number on the top flat surface calculated by the computational simulation is in excellent agreement with Mayinger et al. (1976) and Kulacki and Emara (1975) correlations only at Ra = 109. Numerical results of the velocity and temperature fields show the α-phenomena at lower Rayleigh number and ν-phenomena at lower Rayleigh number.

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