Large eddy simulation of a randomly stacked nuclear pebble bed

This paper reports large eddy simulation results for a randomly stacked bed of spherical pebbles, using a second-order accurate, cell-centered finite volume method on an unstructured polyhedral mesh. The selected flow configuration represents the core of a high temperature reactor, in which nuclear fuel is embedded in the pebbles. The geometrical arrangement consists of approximately 30 pebbles, which are randomly stacked and in contact with each other. Extensive analyses of flow and thermal fields are performed to derive valuable insights on the flow characteristics. The predicted flow-field is fairly complex and exhibits highly unsteady and three-dimensional turbulent behavior with strong rotational and cross flow regions. The flow, while moving over the pebbles shows attenuation and enhancement in the turbulence levels, and eventually yields to flow separation and its subsequent reattachment. Consequently, a wide range of temperatures over the pebbles is observed; this includes the appearance of hot-spots especially around contact areas. The occurrence of high- and low-velocity streak regions, and the corresponding temperature fields are examined and quantified. The reported analyses can be used for validation of low-order turbulence modeling applications to complex pebble flow configurations.