Heat Transfer LES Simulations in Application to Wire-Wrapped Fuel Pins

Abstract

We present a novel development of a domain decomposition method for the NavierStokes equations in a spectral element formulation which enables to investigate turbulent ∞ows in complex geometries. The current paper applies domain decomposition method to simulate turbulent heat transfer in a wire-wrapped fuel pin assembly. The method is flrst validated on a benchmark problem of a turbulent swept ∞ow over a single wire in a channel. Potential application of the method to simulation of a realistic wire-wrapped assemblies is the discussed. Wire-wrapped fuel pin assemblies represent a coolant system for next generation advanced burner fast reactors. The liquid coolant ∞ow passes through subassemblies of hexagonally arrayed fuel pins. Fuel pins are supported by a wire wrap spacer which wounds helically along the pin axes. In addition to providing a structural support, wire wrap also serves to promote a turbulent mixing of the coolant ∞ow in order to increase the heat transfer coe‐cient and enhance cooling of the fuel pin walls. Analysis of the ∞ow and heat transfer properties in the described conflguration presents signiflcant challenges, due to both geometrical complexity and high Reynolds number of the coolant ∞ow (Reh » 40000 i 65000 based on hydraulic diameter of the subchannels). Geometrical complexity manifested by the presence of multiple contact points and lines due to the wire wrap makes it practically impossible to create body-fltted structured hexagonal mesh of the exact geometry required by the computational solver. Thus, previous computational investigations of wire-wrapped fuel pins performed at Argonne National Laboratory 1{3 used approximated rather than exact geometry of a subassembly, where the singularity line representing a line of contact between the wire and the pin was artiflcially smoothed out in order to enable mesh generation. Unfortunately, this singularity line is the most critical place in terms of cooling considerations, since a hot spot can be generated there due to stagnation ∞ow conditions. Special computational methodology was developed in order to facilitate mesh generation procedure and allow for the exact geometrical representation of the given problem. This methodology, consisting of assembling overlapping grids and performing unsteady interface conditions exchange was implemented in a stable and accurate manner in a spectral element solver NEK5000. With this methodology, an accurate meshing of the wire-wrapped fuel pin with two overlapping subdomains becomes possible, so that the computations of heat transfer in a wire-wrap conflguration can be performed. After brief description of essential features of NEK5000 in Section II, computational methodology of the domain decomposition procedure is presented in Section III, the validation of the procedure on a benchmark problem of a turbulent swept ∞ow over a single wire in a channel is described in Section IV, and potential application of the method to simulation of wire-wrapped fuel pin assemblies is discussed in Section V.