A review of methods to predict the effective thermal conductivity of packed pebble beds, with emphasis on the near-wall region

The effective thermal conductivity is an important parameter that is representative of the overall heat transfer in a packed bed of spheres. It includes the effects of conduction through the solid material and contact areas between spheres, conduction through the stagnant gas phase as well as thermal radiation between the surfaces of the spheres. An accurate prediction of the effective thermal conductivity is necessary for the design and analysis of packed pebble bed gas-cooled and solid fuel molten salt-cooled generation IV reactors, especially when considering the safety case. This includes the near-wall region where the packing structure is altered significantly. The well-known ZBS correlation is widely applied to predict the effective thermal conductivity and it is often implicitly assumed that it is equally applicable in the near-wall region. This paper presents an analysis of the validity of the ZBS correlation in this regard. In addition to this, an up-to-date review of methods used to predict the effective thermal conductivity is presented and methods that specifically account for the near-wall region are identified. It is noted that the contributions of the various heat transfer mechanisms are not yet fully understood and therefore the ability to separate the different mechanisms can be useful for understanding the characteristics in the near-wall region.