| Summary: | Through the years different models have been proposed to calculate the total effective thermal
conductivity in packed beds. The purpose amongst others of these models is to calculate the
temperature distribution and heat flux in high temperature pebble bed reactors. Recently a new model
has been developed at the North–West University in South Africa and is called the Multi–Sphere Unit
Cell (MSUC) model. The unique contribution of this model is that it manages to also predict the
effective thermal conductivity in the near wall region by taking into account the local variation in the
porosity.
Within the MSUC model the thermal radiation has been separated into two components. The first
component is the thermal radiation exchange between spheres in contact with one another, which for
the purpose of this study is called the short range radiation. The second, which is defined as the longrange
radiation, is the thermal radiation between spheres further than one sphere diameter apart and
therefore not in contact with each other. Currently a few shortcomings exist in the modelling of the
long–range radiation heat transfer in the MSUC model. It was the purpose of this study to address
these shortcomings.
Recently, work has been done by Pitso (2011) where Computational Fluid Dynamics (CFD) was used
to characterise the long–range radiation in a packed bed. From this work the Spherical Unit
Nodalisation (SUN) model has been developed. This study introduces a method where the SUN
model has been modified in order to model the long–range radiation heat transfer in an annular reactor
packed with uniform spheres. The proposed solution has been named the Cylindrical Spherical Unit
Nodalisation (CSUN, pronounced see–sun) model.
For validation of the CSUN model, a computer program was written to simulate the bulk region of the
High Temperature Test Unit (HTTU). The simulated results were compared with the measured
temperatures and the associated heat flux of the HTTU experiments. The simulated results from the
CSUN model correlated well with these experimental values. Other thermal radiation models were
also used for comparison. When compared with the other radiation models, the CSUN model was
shown to predict results with comparable accuracy. Further research is however required by
comparing the new model to experimental values at high temperatures. Once the model has been
validated at high temperatures, it can be expanded to near wall regions where the packing is different
from that in the bulk region. === Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.
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