Numerical simulations of a pebble bed configuration using hybrid (RANS-LES) methods

A High Temperature Reactor (HTR) is envisaged to be one of the reactor designs to play a role in nuclear power generation including process heat applications. The 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 the pebble bed cores of HTR may affect the integrity of the pebbles. 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 pebble bed flow, is still computationally too expensive and not foreseeable in the near future. On the other hand the prediction capabilities of turbulence modeling approaches such as RANS and hybrid RANS-LES methods for such complex flow regime have not yet been rigorously evaluated. In the present study, numerical simulations of a single cubic pebble bed configuration using hybrid RANS-LES methods are presented. Various delayed detached eddy simulation (DDES) methods based on Spalart-Allmaras and k-ω SST models are used for the calculations. Moreover, an extensive qualitative and quantitative comparison has been performed between the obtained results and available quasi-DNS data. Results are found to be in good agreement and support the use of hybrid (RANS/LES) for such type of complex flow configurations.