Thermal exchanges at very high Rayleigh numbers (Ra) exist on geophysical scale, at civil engineering scale and increasingly in industrial applications and here particularly in the energy sector. At this point, we mention the cooling of solar panels or the heat removal from nuclear power plants under accidental conditions. In fact, the passive safety concept of Small Modular Reactors (SMR) is based on the transfer of residual heat from the reactor to a water pool in which the reactor is placed. Since the outer reactor vessel is very high, heat exchange occurs by natural convection at Rayleigh numbers (Ra) between 1010 and 1016. Reliable heat transfer correlations exist to date only up to about Ra < 1012 with very high uncertainties in the extrapolation to higher Ra. Understanding the heat transfer at very high Ra is thus of fundamental and practical interest. The associated challenges are twofold:
• Numerical challenges: CFD codes cannot model turbulent heat transfer at very high Ra with sufficient accuracy and appropriate calculation time. Improved physical and numerical models are required, which use high performance computing (HPC) capabilities.
• Experimental challenges: Detailed experiments are essential for code validation. Since experiments in water require impractical huge dimensions, cryogenic experiments with helium are planned at CEA, based on the interesting physical properties of this fluid in the range of 5 K (high thermal expansion associated to low viscosity and thermal conduction).