[en] The present work presents the development of a Large Eddy Simulation (LES) methodology viable for complex geometries and suitable for the simulation of rod-bundles. The use of LES and Direct Numerical Simulation (DNS) allows for a deeper analysis of the flow field and the use of stochastical tools in order to obtain additional insight into rod-bundle hydrodynamics. Moreover, traditional steady-state CFD simulations fail to accurately predict distributions of velocity and temperature in rod bundles when the pitch (P) to diameter (D) ratio P/D is smaller than 1.1 for triangular lattices of cylindrical pins. This deficiency is considered to be due to the failure to predict large-scale coherent structures in the region of the gap. The main features of the code include multi-block capability and the use of the fractional step algorithm. As a Sub-Grid-Scale (SGS) model, a Dynamic Smagorinsky model has been used. The code has been tested on plane channel flow and the flow in annular ducts. The results are in excellent agreement with experiments and previous calculations

[en] The flow in tight pin bundles (pitch to diameter ratio below 1.1) has been the object of several investigations. The large difference in velocity within the cross section induced by the tight pitch creates the possibility of a Kelvin-Helmholtz instability and the generation of a vortex street in the gap. Recently, it has been proven that the presence of a grid spacer of the type usually encountered in SFRs (Sodium Fast Reactors) does not prevent this instability. Such SFR spacers (e.g. the PFR spacers) are usually comprised of a honeycomb array with dimples attached to surface to insure separation. Most investigations of this phenomenon conducted so far have been limited to single pins or wedges of real bundles. Grid spacers are usually ignored. In the present work the first large scale Large Eddy Simulation (LES) of the flow in a realistic 19 pin bundle has been conducted. The code used is the spectral element CFD code Nek5000. The focus will be in particular in analyzing in the interaction of coherent structures generating in different gap regions and the characterization of the vortex network in a realistic rod bundle. (author)

[en] Highlights: • LES of the flow in a channel and in a rod bundle at high Reynolds number are performed. • Accurate prediction of the local wall shear stress requires finer resolution for LES simulations. • We demonstrate the advantage of the use of higher order polynomials for such calculations. - Abstract: Resolving flow near walls is critical to reproducing the high rates of shear that generate turbulence in high Reynolds number, wall-bounded flows. In the present study, we examine the resolution requirements for correctly reproducing mean flow quantities and wall shear stress distribution in a large eddy simulation using the spectral element method. In this method, derivatives are only guaranteed in a weak sense, and the same is true of quantities composed of derivatives, such as the wall shear stress. We are interested in what is required to resolve the wall shear stress in problems that lack homogeneity in at least one direction. The problem of interest is that of parallel flow through a rod bundle configuration. Several meshes for this problem are systematically compared. In addition, we conduct a study of channel flow in order to examine the issues in a canonical flow that contains spanwise homogeneity missing in rod bundle flow. In the case of channel flow, we compare several meshes and subgrid scale models. We find that typical measures of accuracy, such as the law of the wall, are not sufficient for determining the resolution of quantities that vary along the wall. Spanwise variation of wall shear stress in underresolved flows is characterized by spikes—physical points without well-defined derivatives of the velocity—found at element boundaries. These spikes are not particular to any subgrid scale model and are the unavoidable consequence of underresolution. Accurately reproducing the wall shear stress distribution, while minimizing the computational costs, requires increasing the number of elements along the wall (local h-refinement) and using very high order (N=19) basis functions (p-refinement). We suggest that while these requirements are not easily generalized to grid spacing guidelines, one can apply a general process: construct a mesh that progressively increases elements along any walls, and increase the order of basis functions until the distribution of wall shear stress or any other quantity of interest is smooth

[en] LES (Large Eddy Simulation) numerical methodology has been used to fully reproduce the characteristics of the flow field in eccentric annular channels, to verify and characterize the pressure of large-scale coherent structures, to examine their behavior at different Reynolds numbers and to characterize the anisotropy associated to these structures. The numerical approach is based upon boundary fitted coordinates and a fractional step algorithm; a dynamical Sub Grid Scale (SGS) model suited for this numerical environment has been implemented and tested. Agreement with previous experimental and DNS results has been found good overall for the streamwise velocity, shear stress and the rms of the velocity components. The instantaneous flow field presented large scale coherent structures in the streamwise direction at low Reynolds numbers, while these are absent or less dominant at higher Reynolds. The effect of secondary flows on anisotropy is studied over an extensive velocity range through invariant analysis. In order to further validate the computational results, the use of POD (Proper Orthogonal Decomposition) of the flow field has been made and provided additional insights into the physics of turbulence in this geometry. (author)

[en] The study of supercritical CO₂ (s-CO₂) heat transfer has become popular in recent decades. The driving factor is the growing interest of using s-CO₂ in future power conversion cycles. The supercritical CO₂ Brayton cycle has gained the most focus for its high efficiency, compact design, economics, etc, . In this cycle, heat exchangers are used to heat up or cool down CO₂. Thus, the heat transfer characteristics of s-CO₂ need to be well understood for heat exchanger design. In this research, we use the open-source spectral-element code Nek5000, developed by Argonne National Laboratory, to perform a direct numerical simulation (DNS) for s-CO₂ flow through a straight heated tube. The pioneering paper by Bae et al. is used to validate the results from the Nek5000 simulation. (authors)

[en] Helical tubes have been used in a number of systems, including the energy and chemical processing industries. Helical-Coil Steam Generators (HCSGs) in particular have special application in systems for which spatial constraints are important. Variants of these designs are notably included in a number of integral-vessel small modular reactor (SMR) designs which are currently in development by multiple companies. These designs typically have the primary side located within the vessel, with the secondary side flowing upward through the helical tubes. One complexity in implementing an HCSG within the reactor pressure vessel is that any potential flow-induced vibration (FIV) can impact reactor safety. In a typical power plant, heat exchanger tube failures caused by FIV can be costly, but generally do not have immediate impact on the core safety due to their relative isolation from the core area. In an integral-vessel design, a tube rupture could eject material from the secondary side into the primary flow, potentially damaging the core itself or other crucial components. Thus a higher degree of confidence in predicting and/or preventing FIV should be attained for implementing the helical tubes in these designs. A significant campaign is currently underway to address the topic of FIV in helical tube arrays. The U.S. Department of Energy, under the Nuclear Energy Advanced Modeling and Simulation (NEAMS) initiative, has deemed this a 'high-impact problem'. This will include both experimental and computational contributions from a consortium consisting of multiple universities, industry partners, and national laboratories, including Argonne National Laboratory. The primary industrial partner is NuScale, which will see direct benefits from this initiative to the development of their SMR and its component design. The primary focus of this paper is to establish some of the computational efforts that have been made to this stage and the initial benchmarking of computational tools. (authors)

[en] Highlights: • CFD using large eddy simulation with variable density and viscosity is developed for the cold leg mixing benchmark. • Taylor length scales and integral time scales are calculated. • Sensitivity of initial conditions, Schmidt numbers, Atwood numbers, and fluid models are numerically studied. • Influences of nozzle geometry and presence of small perturbations on downcomer are presented. • Recommendations are made for similar buoyant mixing problems. -- Abstract: The influences of the Schmidt number and fluid models of the Boussinesq approximation and variable density (low-Mach formulation) are investigated using large eddy simulation (LES) where fluid naturally advects between two tanks connected by a pipe. The geometry resembles a reactor-like vessel which comprises of a cold leg-downcomer region. The closed system consists of two miscible fluids with different densities and viscosities separated by a valve. Analysis with Reynolds-Averaged Navier–Stokes (RANS) is used for calculating Taylor length scales to establish minimum grid resolution for LES. Using particle image velocimetry (PIV) data, validation is conducted for four different magnitudes of Schmidt numbers. Evidence indicates that capturing the effects from the highest Schmidt number is essential for properly predicting fluctuations and instabilities in the flow. Although downcomer comparisons with PIV consist of larger errors, they are explained through additional sensitivity analyses involving initial conditions, geometry, and residual energy. Lastly, the influence of larger density differences is demonstrated using simulations with a higher Atwood number, temporally scaled using the fluid front velocity. Recommendations are made regarding future experiments and simulations for design of similar buoyant mixing problems.

[en] In order to predict the water-vapor two-phase flow dynamics in a fuel bundle of an advanced light-water reactor, large-scale numerical simulations were performed using a highly parallel-vector supercomputer, the earth simulator. Although conventional analysis methods such as subchannel codes and system analysis codes need composition equations based on the experimental data, it is difficult to obtain high prediction accuracy when experimental data are nothing. Then, the present large-scale direct two-phase flow simulation method was proposed, and the void fraction distributions under the heated flow condition were analyzed. On the other hand, in order to estimate the turbulence characteristics in the fast breeder nuclear rector core, large eddy simulation and direct numerical simulation were carried out under the concentric and eccentric annular channels simulating the simplified bare fuel bundle. An objective of this work is to identify reliable and practical approaches for the simulation of unsteady flows in rod-bundles and related geometries. (author)

[en] A well-characterized validation program has been defined for the thermal-hydraulic CFD modeling of 61-pin hexagonal packed fuel assemblies with helically wire-wrapped pins for both non-deformed beginning-of-life geometries and deformed later-in-life geometries typical of sodium-cooled fast reactors. The motivation for this work is the need to validate CFD codes to accurately model the flow and thermal performance of wire-wrapped fuel assemblies, particularly the deformed later-in-life bundles. The effect of pin and duct deformation is increasingly important as reactor designs look toward longer assembly lifetimes. As a part of a larger collaborative project with TerraPower, Texas A and M University (TAMU), and AREVA, this validation work consists of CFD simulations of four con-current experiments. Pre-test numerical simulations corresponding to each of the experiments has been conducted and are planned to be followed by post-test simulations that will incorporate test information as it becomes available to account for any as-built differences in geometry of test section and its specifications including the differences in boundary conditions of the actual tests. This paper focuses on Nek5000 LES results for the isothermal/TAMU undeformed configuration as a first step toward and initial conditions for the deformed isothermal/TAMU and/or conjugate heat transfer/AREVA cases. (authors)

[en] Highlights: • We review the history of the International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH) conference series. • We provide a summary of progress in the thermal-hydraulic field in the last 4 decades. The year 2020 marks the 40th anniversary of the first International Topical Meeting on Nuclear Reactor Thermal-hydraulics (NURETH-1). Hosted by the thermal-hydraulics division (THD) of the American Nuclear Society, the NURETH series is the premier topical meeting exclusively dedicated to advances in nuclear reactor thermal-hydraulics. In this article, which opens a special issue dedicated to the 40th anniversary of NURETH-1, we provide a brief history of the NURETH series. We dedicate the bulk of the manuscript to a summary of the progress in thermal-hydraulics in the past 40 years, highlighting, key contributions presented in the NURETH series of conferences. We emphasize that, given the size and complexity of the field examined, this cannot be considered a comprehensive review. However, We hope the reader will find this article useful to reflect on the advances in the past 40 years and the current state of the art.