Multi-sphere Unit Cell model to calculate the effective thermal conductivity in packed pebble beds of mono-sized spheres

This paper presents a new approach to the calculation of the effective thermal conductivity in packed pebble bed reactors, namely the Multi-sphere Unit Cell model. The model specifically accounts for the porous structure, which is characterised using the radial variation in porosity, coordination number and contact angles between adjacent spheres. It also accounts for solid and gas thermal conduction, contact area, surface roughness as well as the thermal radiation for pebble temperatures up to 1200 °C. This more rigorous approach to characterising the porous structure enables improved prediction of the effective thermal conductivity in the near-wall and wall regions, resulting in better prediction of the temperatures at the reflector interface. Results obtained with the Multi-sphere Unit Cell model are compared with existing correlations and experimental data including those from the SANA-I experimental test facility.