Improved delayed detached eddy simulation of a randomly stacked nuclear pebble bed

The high temperature reactor (HTR) design concept exhibits excellent safety features due to the low power density and the large amount of graphite present in the core which gives a large thermal inertia in the event of an accident such as loss of coolant. However, the possible appearance of hot spots in a pebble bed core design may affect the integrity of the pebbles and the fuel. This has drawn the attention of several scientists to understand this highly three-dimensional complex phenomenon. To obtain accurate predictions based on techniques such as DNS and LES, for a realistic (even for a small size) pebble bed flow, is still computationally too expensive and not foreseeable in the near future. On the other hand, the prediction capabilities of turbulence modelling approaches such as RANS and hybrid RANS-LES methods for such complex flow regime have not yet been rigorously evaluated. In this paper, numerical simulations of a limited sized randomly stacked bed of spherical pebbles are performed by using improved delayed detached eddy simulation (IDDES). The pebble bed configuration analysed consists of approximately 30 pebbles, which are randomly stacked to represent the core of an HTR. The obtained results are compared (qualitatively and quantitatively) with the available reference LES for validation purposes. The results show that the selected IDDES based on the SST k-ω model is able to reproduce the overall flow topology. The mean flow and thermal fields are found to be in good agreement with the LES. However, the second order statistics have shown significant disagreement. Nevertheless, these results are explicitly discussed in detail to understand the flow and thermal fields appearing in this complex flow configuration.