LES of natural convection along a side-heated vertical wall in a water cavity applied for the scale of Small Modular Reactor (Ra ≈ 1015)

The passive safety concept of Small Modular Reactors (SMR) is based on the transfer of residual heat from the reactor to a water pool. Since the height of the reactor vessel reaches approximately 15 m, this leads to a strong heat exchange between the wall and the water by the way of natural convection. The maximum Rayleigh number (Ra) reaches 10(14) or 10(15). Reliable heat transfer correlations exist only up to Ra R:: 1012, still along with un-certainties in the extrapolation to higher Rayleigh number values. To improve the understanding of natural convection at high Rayleigh number values, the turbulent natural convection boundary layer (TNCBL) along a 15 m high, side-heated vertical wall in a water tank is simulated by employing the Large Eddy Simulation method with the CEA in-house code Triocfd (Angeli et al., 2015). The numerical results in terms of heat transfer cor-relation, mean temperature and mean velocity, second moment variables show a reasonable agreement with available experimental data in the range of Ra < 10(12). It is observed in the simulation that the powers of the heat transfer correlations in the laminar (Nuy = 0.56(Gry center dot Pr)0.25) and the turbulent regimes (Nuy = 0.114(Gry center dot Pr)1/3) correspond well to the classical regimes at Ra < 2 x 1012. A slight deviation from the classical exponent (1/3) is found at higher Rayleigh number (2 x 10(12) < Ra< 1.8 x 10(15)), namely Nuy = 0.114(Gry center dot Pr)0.33 . Through the analysis of turbulent kinetic energy budget, it is found that turbulent contri-bution from shear stress increases continuously with the increase of Rayleigh number. However, the turbulence contribution from buoyancy keeps increasing until Ra R:: 6.6 x 1013, then decreases gradually at higher Rayleigh number. The behavior of the maximum buoyant force shifts slowly from the inner boundary layer to the outer boundary layer may indicate the potential transition of the buoyancy-dominated regime to the shear stress dominated regime, thus confirming previous theoretical prediction (Wells Andrew and Worster, 2008). The increasing scale of the streak-like turbulent structures with the increase of Rayleigh number are clearly demonstrated through visualization process.