GENES4/ANP2003, Sep . 15-19, 2003, Kyoto, JAPAN Paper 1 204 LES Simulation in Pebble Bed Modular Reactor Core through Randomly Distributed Fuel Elements Gokhan YESILYURT * , Yassin A. HASSAN Department of Nuclear Engineering, Texas A&M University, College Station, TX, 77843-3133 USA The premise of the work presented here is to use a common analytical tool, Computational Fluid Dynamics (CFD) code, along with different turbulence models. The state-of-the-art turbulence model Large Eddy Simulation (LES) as well as Eddy viscosity models were used to study the flow past bluff bodies. A suitable and commercially available CFD code (CFX5.6b) was selected to resolve the flow field. Simulation of turbulent transport for the gas through the gaps of the randomly distributed spherical fuel elements (pebbles) was performed. With the development of high performance computers built for applications that require high CPU time and memory, numerical simulation became 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. Because of the 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 situation, 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 used to investigate the applicability of these models for this kind of flow past bluff bodies at high Re numbers. KEYWORDS: computational fluid dynamics, pebble bed modular reactor, large-eddy simulation I. Introduction Unsteady 3D fluid flows are very widespread phenomena in nature. The understanding of such flows is very important from both theoretical and practical point of view. The laboratory investigations of such flows are difficult and in some cases impossible. 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. In literature, several attempts of numerical simulation [1,2,3] for separated fluid flows around a sphere were undertaken. A large number of these numerical studies [4,5] have been devoted to the analysis of flow around a circular cylinder at low and moderate Reynolds numbers. Unfortunately, there are only few studies [6] which represent the flow around randomly distributed spheres as in Pebble Bed Modular Reactors (PBMR) under high Reynolds number flow conditions. Furthermore, most of the turbulence models that were used for these simulations are eddy viscosity models which doesn’t resolve the flow field appropriately where curved flows exist. The simulation of turbulent transport for the gas through the gaps of the randomly distributed spherical fuel elements (pebbles) was performed with the help of appropriate turbulence model where flow separation exists. This helped in understanding the highly three-dimensional, complex flow phenomena in pebble bed caused by flow curvature. In summary, the premise of the work is to use Computational Fluid Dynamics (CFD) tools along with different turbulence models including the state-of-the-art Large Eddy Simulation to study the flow around randomly distributed pebbles of High Temperature Gas Cooled Reactors (HTGR). Heat transfer in both laminar and turbulent flows varies noticeably around curved surfaces. Curved flows would be present in the presence of contiguous curved surfaces. In laminar flow condition and appreciable effect of thermo gravitational forces, the Nusselt (Nu) number depends significantly on the curvature shape of the surface. It changes with order of 10 times. The flow passages through the gap between the fuel balls have concave and convex * Corresponding author, Tel. 1-979-845-7090, Fax. 1-979-6443, E-mail: gokhany@tamu.edu