Numerical simulation of heat transfer in packed pebble beds: CFD-DEM coupled with particle thermal radiation

Particle thermal radiation plays a significant role in many heat transport devices, such as pebble-bed high temperature gas-cooled nuclear reactors (HTGR). The effect of particle thermal radiation in packed pebble beds has been rarely investigated at particle scale. In this work, a complete CFD-DEM method coupled with particle-scale radiation is discussed for packed pebble beds, considering particle motion, fluid flow, particle-fluid interactions and heat convection, conduction and particle radiation. It is shown that, compared to the case without particle radiation, heat transfer is enhanced greatly by particle radiation at high temperatures. A particle radiation factor (PRF) is proposed as an independent non-dimensional parameter to qualify the effect of particle-scale radiation in packed pebble beds. The PRF increases significantly with temperature and decreases gradually as the heat storage capacity or the thermal conductivity of the fluid increases. A demonstrative utilization of the present model is performed for a benchmark problem based on the HTR-10 nuclear reactor, and the results in general are in agreement with the results predicted by other empirical codes. When the nuclear reactor is above full power, the power required by the fan will increase significantly. In the decay heat removal process, particle radiation is essential to keep the bed temperature below the allowable limit, which is significant for nuclear reactor safety.