Development of Advanced Turbulent Flow Simulation Techniques for Use in Nuclear Reactor Safety Analysis, 20-R9680

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Principal Investigators
Kaushik Das
Debashis Basu
Scott Painter
Lane Howard
Steve Green

Inclusive Dates:  01/01/07 – 12/31/08

Background - Design, safety analyses and accident simulations for nuclear reactors rely primarily on one-dimensional system level codes. These system level codes use empirical correlations to resolve complex phenomena such as thermal mixing due to cross flow pulsation and turbulence that affect heat transfer fuel bundles. With rapid advancement in computational fluid dynamics (CFD) techniques, it is feasible to perform detailed analysis of reactor components by solving the full Navier-Stokes equations that provide a more detailed and accurate spatiotemporal description of the system. Turbulence plays a significant role in prediction of pulsation, mixing and heat transfer in reactor components. Hence, issues related to turbulence modeling need to be adequately resolved before models can be applied to produce results at reasonable cost. A team of SwRI engineers explored existing turbulence models and developed some advanced multiscale turbulence models that were applied to reactor core channel flow with unsteady inter sub-channel pulsation and secondary flows.

Approach - Existing commercial CFD solvers employ turbulence models ranging from traditional Reynolds averaged models like k-ε or k-ω to high fidelity large eddy simulation (LES) models. Previous studies have shown that the Reynolds averaged representations of the turbulence fail to capture the desired unsteadiness and other intricate flow phenomena, while high fidelity models like LES are too computationally expensive. Hybrid multiscale turbulence models such as the Detached Eddy Simulation (DES) are promising new approaches that strike a balance between the traditional RANS-type formulation and the LES techniques. These multiscale models provide a better representation of turbulence compared to the available models in commercial solvers and can be adapted by customizing and extending the already implemented models in the solvers.

Accomplishments - The present work has developed multiscale models to simulate flow regimes typically encountered in coolant flow through fuel rod bundles. It includes flow through the wall-rod gap in a single rod-channel configuration as well as pulsation and mixing between sub-channels for multi-rod channel configuration. The project team tested a number of well-established turbulence models for fuel rod configurations. The unsteady data obtained using the single-rod geometry was analyzed to understand the frequency contents of the signal. Flow over packed rod bundles resembling the lower plenum flow in a gas-cooled reactor was successfully simulated, and results are comparable to experimental observations. A number of multiscale DES models have been developed and benchmarked against standard flow configurations. These models have been used for subchannel flow analysis of two different 37 rod channel configurations: for a 60° sector in a cylindrical channel typically used in CANDU reactors and a hexagonal channel with pins arranged in a triangular array. Predicted results captured the flow unsteadiness and pulsation. This research resulted in three full-length conference papers, one professional conference presentation, and one grant proposal. Project staff members have prepared a number of brochures and promotional materials based on this research and are actively developing business and collaboration opportunities.

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