DEM-CFD Coupled Simulation for Determination of Character of Heat and Mass Transfer and Purge Gas Flow in Li4SiO4 Pebble Bed

The helium cold ceramic blanket is one of the three candidate blanket concepts of China Fusion Engineering Test Reactor (CFETR), and the tritium breeding pebble bed is the core component of the blanket, Li4SiO4 is selected as the tritium breeding material. The structure of the pebble bed plays an important role in the transport behavior, flow and heat transfer of tritium in the pebble bed. In this paper, a randomly packed pebble bed was generated based on the discrete element method (DEM), the equivalent diffusion coefficient of tritium in the pebble bed and the flow parameters of the purge gas were calculated by computational fluid dynamics (CFD) method, including velocity distribution, pressure distribution and inlet/outlet pressure drop. The equivalent thermal conductivity coefficient of pebble bed was simulated under two conditions: external heat flux condition and internal heat source condition. The calculation results show that the equivalent diffusion coefficient of tritium in the purge gas under the pebble bed structure is 40% of the binary gas diffusion coefficient. Flow hysteresis zone in the pebble bed can be observed while the flow of purge gas is accelerated at the near wall region. The viscous drag coefficients of the Ergun equation are obtained, of which C1 equals 87. The equivalent thermal conductivity coefficient of pebble bed under internal heat source condition is lower than that under external heat flux condition.