High-fidelity simulation of molten salt natural circulation loops using the spectral element method

This study presents high-fidelity Direct Numerical Simulations (DNS) of natural circulation flow of molten salt in a benchmark loop geometry using the GPU-accelerated spectral element code NekRS. The simulations focus on Test 5 from the University of Wisconsin-Madison FLiBe Natural Circulation Loop (UW-FNCL), using a computational model that matches the experimental setup in geometry, boundary conditions, and operational parameters. A low-Mach number formulation is employed to capture the strong temperature-dependent property variations inherent to FLiBe, a high-Prandtl-number molten salt. Validation against experimental data shows good agreement in temperature profiles and Nusselt numbers across a range of Reynolds numbers, demonstrating NekRS's capability to accurately and efficiently simulate buoyancy-driven flows with thermally varying fluid properties. Additionally, the DNS results provide novel insights into the three-dimensional flow and heat transfer characteristics that are challenging to obtain experimentally. Detailed flow analysis reveals pronounced buoyancy-induced velocity asymmetries in the bottom-heated leg, jet-driven shear instabilities in the reservoir, and localized unsteady phenomena near sharp bends. Proper Orthogonal Decomposition (POD) analysis identifies dominant energetic modes, highlighting a four-vortex Dean-like structure at the 90° elbow that deviates from classical two-vortex predictions, attributed to buoyancy-driven thermal stratification and pre-conditioned velocity profiles.