Characterisation of the effective thermal conductivity in the near wall region of a packed pebble bed

Abstract

The effective thermal conductivity is an important parameter that is representative of the overall heat transfer in a packed bed of spheres. An accurate prediction of the effective thermal conductivity is necessary for the design and analysis of packed pebble bed gas-cooled reactors, especially when considering the safety case. The wall effect present in the near-wall region affects the conduction and radiation heat transfer in this region. Various correlations and models exist to predict the effective thermal conductivity. However, most of these are only applicable to the bulk region and fail to accurately predict the heat transfer in the near-wall region. The effective thermal conductivity trend is marked by a prominent reduction in the near-wall region followed by a characteristic “peaks-and-dip” trend which can be observed in the High Temperature Test Unit (HTTU) and SANA-I experimental results. The objective of this study is to develop a methodology which can be used to gain insight into the characteristics of the conduction and radiation heat transfer in the near-wall region of a packed pebble bed. The methodology entails experimental and numerical work to separate the conduction and radiation components of the heat transfer. This allows one to observe the interplay between the heat transfer phenomena and the resulting effect on the overall effective thermal conductivity trend. The Near-wall Thermal Conductivity Test Facility (NWTCTF) was used to gather the temperature and heat transfer distributions through a packed bed for different packing configurations namely Simple Cubic (SC), Body Centred Cubic (BCC) and random. From the experimental results the effective thermal conductivity was derived. A Computational Fluid Dynamics (CFD) model was set up for a numerically packed bed that is representative of the experimental packed bed. The numerical heat transfer results were obtained by using the CFD model. The CFD model can be calibrated by adjusting the contact resistance values at the solid-solid interfaces of the pebble-pebble and pebble-wall contact points in the packed bed. Results were obtained by applying the developed methodology and separating the conduction and radiation components of the effective thermal conductivity. From the results, one can observe the interplay between the contributing heat transfer phenomena and the resulting effect on the “peaks-and-dip” trend of the overall effective thermal conductivity.