| dc.description.abstract | The premise of the work presented here is to use a common analytical tool,
Computational Fluid dynamics (CFD), along with a difference turbulence models. Eddy
viscosity models as well as state-of-the-art Large Eddy Simulation (LES) were used to
study the flow past bluff bodies. A suitable CFD code (CFX5.6b) was selected and
implemented.
Simulation of turbulent transport for the gas through the gaps of the randomly
distributed spherical fuel elements (pebbles) was performed. Although there are a
number of numerical studies () on flows around spherical bodies, none of them use the
necessary turbulence models that are required to simulate flow where strong separation
exists. With the development of high performance computers built for applications that
require high CPU time and memory; numerical simulation becomes one of the more
effective approaches for such investigations and LES type of turbulence models can be
used more effectively.
Since there are objects that are touching each other in the present study, a special
approach was applied at the stage of building computational domain. This is supposed to
be a considerable improvement for CFD applications. Zero thickness was achieved
between the pebbles in which fission reaction takes place.
Since there is a strong pressure gradient as a result of high Reynolds Number on
the computational domain, which strongly affects the boundary layer behavior, heat
transfer in both laminar and turbulent flows varies noticeably. Therefore, noncircular
curved flows as in the pebble-bed situatio n, in detailed local sense, is interesting to be
investigated.
Since a compromise is needed between accuracy of results and time/cost of effort
in acquiring the results numerically, selection of turbulence model should be done
carefully. Resolving all the scales of a turbulent flow is too costly, while employing
highly empirical turbulence models to complex problems could give inaccurate
simulation results. The Large Eddy Simulation (LES) method would achieve the
requirements to obtain a reasonable result. In LES, the large scales in the flow are solved
and the small scales are modeled.
Eddy viscosity and Reynolds stress models were also be used to investigate the
applicability of these models for this kind of flow past bluff bodies at high Re numbers. | en |
