Natural circulation in a short-enclosed rod bundle

Large Eddy Simulation (LES) is used to study natural circulation flow in a sealed rod bundle with a Rayleigh number (based on height) of approximately and a fluid Prandtl number of 0.74. Natural convection boundary layer development is observed on both the heating rods and cooling containment surfaces. The development of the boundary layers is key to the flow and heat transfer characteristics of the system. As the flow descends on the containment surface, the boundary layer gradually develops and transitions from laminar to turbulent flow, with transition occurring close to the bottom-end. Laminar flow at the containment is well represented by a flat plate similarity solution for buoyant flow and the Nusselt number correlation is akin to that for a flat isolated vertical surface. This indicates the boundary layer development on this surface is similar to an isolated vertical surface. On the rods boundary layer development occurs much earlier and is affected by turbulence convected from the containment surface. Laminar flow here compares well against a slender cylinder similarity solution. In the developed region, the Nusselt number correlation has a Rayleigh number dependency similar to that for a rectangular cavity.