Natural circulation in an enclosed rod bundle of large aspect ratio

Highly turbulent natural circulation in an enclosed tall rod bundle is investigated using Large Eddy Simulation (LES). The rod bundle consists of 36 heated rods and an insulated central tubeenclosed by a cooled containment wall. This geometry is directly relevant to nuclear fuel bundles and generic heat exchangers. For nuclear fuel bundles, natural circulation is of importance in accident scenarios. A 60° azimuthally periodic sector of the bundle is modelled. The Rayleigh number based on the height is 7.6 x 10 12 and 1.23 x 10 13 for the two cases studied. The flow, turbulence, and thermal characteristics of the system show strong distinct features in the top, middle, and bottom regions (about 0.2, 0.6, and 0.2 splits), which are strongly influenced by the vertically developing buoyancy-driven boundary layer on the containment surface. The flow in the top region is largely stagnant with strong thermal stratification and weak cross flow. A laminar boundary layer is however formed on the containment wall which later transitions to turbulence. This turns out to be the dominant flow feature in the rod bundle. The velocity profile and heat transfer in this top region are well represented, respectively, by a similarity solution for laminar flow and a well-established Nusselt correlation for an unconfined flat plate. In the middle-height region, the flow can be naturally split into an outer flow and a central flow zone. The former resembles the flow in a simple, rectangular, asymmetrically heated/cooled cavity, and the heat transfer to the containment is well represented by a turbulent natural convection correlation. The central flow zone largely resembles mixed convection in a heated upwards pipe/channel flow, though the entrainment of fluid from the outer zone means that the flow accelerates axially until the middle-height. Irrespective of the differences in the two zones, the near-wall behaviour of the flow and thermal fields on the rods and containment wall shows a strong similarity at different heights as well as between the two zones. They are also shown to be consistent with buoyancy-driven flows in simple geometries. In the bottom region, the flow is dominated by an impinging jet from the containment wall and a resultant cross flow from the outer zone to the inner one.