Turbulence modeling in liquid metal free convection

Natural convection within an enclosed cavity filled by liquid sodium is reviewed and the transition to turbulence is investigated with a numerical model, to study heat removal in a reactor core where the forced circulation of the coolant fails. For side heated square cavities turbulence onset is detected at Grashof number (Gr) = 10 million, of the same order as the ones found for fluids having higher Prandtl numbers. However, due to the extremely high value of the turbulent Prandtl number heat transfer remains unaffected by the turbulence, until Gr becomes at least one order of magnitude higher. A laminar prediction model can be applied up to Gr = 100 million. For bottom heated square cavities transition to turbulence begins at relatively low values of Gr and high levels of turbulence are reached quite soon. Unsteady flow patterns develop and laminar solutions tend quickly to instability.