Abstract: The two-dimensional numerical simulation of turbulent natural convection in a model of BALI test section with internal heat source was presented. The simulation was carried out at Rayleigh number of 10¹²-10¹⁵. The turbulent flow of the fluid was captured with a large-eddy simulation (LES) model. Through area-weighted integration of local heat flux on lower boundary, the three-dimensional heat flow of lower boundary was calculated. The results show that the Rayleigh number has a significant effect on flow field, temperature field and distribution of local heat flux on lower boundary. The calculated Ra-Nu relation on both upper and lower boundaries is in good agreement with experimental results. The heat flux distribution on lower boundary caused by different physical mechanisms has also been analyzed.