Characterisation of long range radiation heat transfer in packed pebble beds

Abstract

Due to its importance in the safety case of high temperature gas-cooled nuclear reactors, the effective thermal conductivity in packed pebble beds has been extensively studied by various researchers and several correlations have been developed. The correlations, mostly based on a unit cell approach, can model the total heat transfer through packed beds with reasonable accuracy. However, these correlations are typically highly empirical and do not discriminate between short and long-range radiation phenomena. It therefore does not specifically address the long-range radiation in detail at higher temperatures and in cases with large temperature gradients through the bed. It also does not discriminate between the pebble-to-pebble radiation in the bulk region and the pebble-to-reflector radiation in the near-wall regions. Long range radiation in such cases becomes important and it greatly influences the temperature distribution and heat flux. A need therefore arose to study long range radiation in packed pebble beds in more detail. Using the Computational Fluid Dynamics (CFD) program Star-CCM+, different unstructured and structured beds were studied in an effort to characterise the long range radiation phenomenon. Long range radiation in the bulk regions and near-wall regions of the beds was quantified through the use of view factors. Based on data from the characterisation of long range radiation, a new model is proposed in which long range radiation is simplified and predicted through a so-called Spherical Unit Nodalization (SUN) approach. This model was validated using CFD. The new model can now form the basis for the development of a specific term in the effective thermal conductivity which may be used in correlations to represent the effects of long range radiation in more detail.