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The design of advanced nuclear reactors [Generation IV (Gen IV)] involves an array of challenging fluid-flow issues that affect its safety and performance. Given that Gen IV designs have improved passive safety features, the downcomer plays a crucial role in loss-of-power scenarios. Fluid-flow behavior in the downcomer can involve forced to mixed to natural convection, and characterizing the heat transfer for these changing regimes is a daunting challenge. The creation of a high-resolution heat transfer numerical database can potentially support the development of precise and affordable reduced-resolution heat transfer models. These models can be designed based on a multiscale hierarchy developed as part of the recently U.S. Department of Energy–funded Center of Excellence for Thermal Fluids Applications in Nuclear Energy, which can help address industrial-driven issues associated with the heat transfer behavior of advanced reactors. In this paper, the downcomer is simplified to heated parallel plates, and high Prandtl number fluid (FLiBe) is considered for all simulations. The calculations are performed for a wide range of Richardson numbers from 0 to 400 at two different FLiBe Prandtl numbers (12 and 24), which result in 40 simulated cases in total. Time-averaged and time series statistics, as well as Nusselt number correlations, are investigated to illuminate mixed convection behavior. The calculated database will be instrumental in understanding flow behavior in the downcomer. Ultimately, we aim to evaluate existing heat transfer correlations, and some modifications are proposed. [ABSTRACT FROM AUTHOR]

Abstract Developing heat transfer correlations for buoyancy-driven flows and mixed convection is challenging, especially if the fluid’s Prandtl (Pr) number is not close to 1. For advanced nuclear reactor (Generation IV) designs, the downcomer plays a crucial role in normal operation and loss-of-power scenarios. The fluid-flow behavior in the downcomer can involve forced, mixed, or natural convection. Characterizing the heat transfer for these changing regimes is a serious challenge, especially in the heat transfer deterioration region. In this paper, the downcomer is simplified to heated parallel plates. The high–Pr number fluid FLiBe (a mixture of lithium fluoride and beryllium fluoride) is considered for all simulations. Direct numerical simulations using the graphics processing unit–based spectral element code NekRS are performed for a wide range of the Richardson number, from 0 to 400, at two different FLiBe Pr numbers (12 and 24). This results in an unprecedented 74 cases in total. Each case’s Nusselt number is calculated to evaluate existing heat transfer correlations.Moreover, we propose several new modifications for cases without satisfactory choice. As a result, several novel mixed-convection heat transfer correlations have been built for high–Pr number fluids. The correlations are expressed as a function of the buoyancy number, covering several mixed-convection regimes. The Pr number effect on the Nusselt number behavior is also analyzed in detail. We also propose a novel method to evaluate the heat transfer deterioration region. Modified Reynolds-Gnielinski forced-convection correlations are defined for the laminarization region, and a free-convection correlation is used for the natural-convection-dominated region. These correlations can describe well the trend in the heat transfer–deficient region. [ABSTRACT FROM AUTHOR]

Abstract An in-depth understanding of the flow physics in packed beds is critical for developing simulation tools for pebble bed reactors. Advances in computing power have now made the full-core pebble-resolved computational fluid dynamics simulation of these systems possible. This work presents validation of the velocity and pressure predictions made by the spectral element code NekRS followed by a study of the turbulent kinetic energy and turbulent heat flux budgets. Two cases with corresponding experiments are considered: a bed of 67 pebbles with Re = 1460 and a bed of 789 pebbles with 324 < Re < 1024. Velocity and pressure drop comparisons are performed with the two cases, respectively. Good agreement is found between the experiments and their respective NekRS simulations.The 67-pebble case was then used to perform a direct numerical simulation to extract the turbulent kinetic energy and turbulent heat flux budget terms. Analysis of the turbulent kinetic energy production revealed large areas of negative production near the bottom surfaces of the pebbles. Further investigation revealed a trend between the average amount of negative turbulent kinetic energy production and the local porosity. These results continue to suggest that inertial effects play a large role in differentiating near-wall flow from bed-interior flow. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation (DNS) and proper orthogonal decomposition (POD) of the flow in a randomly packed pebble bed. Nek5000, a spectral-element computational fluid dynamics code, was used to develop the DNS fluid flow data, including first- and second-order statistics for an experimental randomly packed pebble bed. Turbulence budgets were also produced. The flow domain consists of 147 pebbles enclosed by a bounding wall. In the present work, the Reynolds number is 1700 based on the hydraulic diameter and interstitial velocity. First- and second-order statistics were compared with the experimental data. The POD analysis was performed to identify dominant flow structures, especially in the wall channeling region. [ABSTRACT FROM AUTHOR]

We present results for a direct numerical simulation study of isothermal incompressible flow in a regularly packed pebble-bed domain with a bounding wall. We focus specifically on the near-wall behavior of the flow. Our simulation is carried out at a Reynolds number of 9308 to facilitate cross verification with available high-fidelity data. To reduce the required time to achieve statistically stationary results, we implemented an ensemble-averaging scheme that allowed for multiple simulation runs to be carried out concurrently. The close packing of the spheres in the domain causes significant acceleration effects in the domain, which result in boundary layer detachment and reattachment. Presented results include selected first- and second-order turbulence statistics, as well as selected terms of the turbulent kinetic energy transport equation. The acceleration effects in the near-wall region of the domain cause negative production of turbulent kinetic energy. The presented data may be useful for benchmarking Reynolds-averaged Navier-Stokes–based simulations of pebble beds. [ABSTRACT FROM AUTHOR]

At the beginning of the last decade, Petascale supercomputers (i.e., computers capable of more than 1 petaFLOP) emerged. Now, at the dawn of exascale supercomputing, we provide a review of recent landmark simulations of portions of reactor components with turbulence-resolving techniques that this computational power has made possible. In fact, these simulations have provided invaluable insight into flow dynamics, which is difficult or often impossible to obtain with experiments alone. We focus on simulations performed with the spectral element method, as this method has emerged as a powerful tool to deliver massively parallel calculations at high fidelity by using large eddy simulation or direct numerical simulation. We also limit this paper to constant-property incompressible flow of a Newtonian fluid in the absence of other body or external forces, although the method is by no means limited to this class of flows. We briefly review the fundamentals of the method and the reasons it is compelling for the simulation of nuclear engineering flows. We review in detail a series of Petascale simulations, including the simulations of helical coil steam generators, fuel assemblies, and pebble beds. Even with Petascale computing, however, limitations for nuclear modeling and simulation tools remain. In particular, the size and scope of turbulence-resolving simulations are still limited by computing power and resolution requirements, which scale with the Reynolds number. In the final part of this paper, we discuss the future of the field, including recent advancements in emerging architectures such as GPU-based supercomputers, which are expected to power the next generation of high-performance computers. [ABSTRACT FROM AUTHOR]

AbstractDensity stratification in a large enclosure is a crucial phenomenon to heat transfer and sustainable passive heat removal of a sodium fast reactor during reactivity transients. However, engineering turbulence models were identified to have unsatisfactory performance in predicting propagation of a stratified front. Yet, the scarcity of high-resolution data for stratification hampers the development of models. To explor e applications of leveraging direct numerical simulation (DNS) data to support turbulence model development, this work conducted DNS using NekRS to study a long stratification transient in the High-Resolution Jet (HiRJET) experimental facility. This work considers an experiment run where light fluid is injected into a tank containing a denser fluid with a relative density difference of 1.5%. Formation of the stratified layer is identified as impingement of the buoyant jet promoting mixing of the two fluids. Based on the transient statistics, transport of the concentration can be characterized by regions with dominating effects of turbulent mixing, buoyant dissipation, and molecular diffusion, respectively, as moving away from the elevation of jet impingement. Concentration near the stratified front also exhibits oscillation at Brunt-Väisälä frequency. Preliminary validation of the simulation showed encouraging agreement of the concentration distribution with the reference experiment. [ABSTRACT FROM AUTHOR]

• First DNS for turbulent flow in a 5x5 rod bundle geometry in open literature. • Anisotropy invariant analysis demonstrates shift to two-component turbulence unique to the wall gaps. • Turbulent kinetic energy budgets in different subchannels are provided. • Multiple analysis methods suggest presence of gap vortices in edge subchannels. Characterization of parallel flow through rod bundles is of key importance in assessing the performance and safety of several engineering systems, including a majority of nuclear reactor concepts. Inhomogeneities in the bundle cross-section can present complex flow phenomena, including varying local conditions of turbulence. With the ever-increasing capabilities of high-performance computing, Direct Numerical Simulation (DNS) of turbulent flows is becoming more feasible. Through resolving all scales of turbulence, DNS can serve as a "numerical experiment," and can provide substantial insight into flow physics, but at considerable computational cost. Thus to date, the DNS in open literature for rod bundle flows is relatively scarce, and largely limited to unit-cell domains. Since wall effects are important in rod bundle flows, a multiple-pin DNS study can expand understanding of rod bundle flows while providing valuable reference data for evaluating reduced-resolution techniques. In this work, DNS of a 5x5 square bare rod bundle representative of typical light water reactor fuel dimensions was performed using the spectral element code Nek5000. Turbulent microscales based on an advanced Reynolds-Averaged Navier–Stokes model were used to establish the required DNS resolution. Velocity and Reynolds stress fields are analyzed in detail, and invariant analysis is used for further investigation into flow physics. The results show stark changes in the structure of turbulence in the edge gaps, suggesting the presence of gap vortices in these regions. In addition, turbulent kinetic energy budgets are presented to more fully illustrate the various turbulent processes. These data can prove useful for rigorous evaluation of lower-fidelity turbulence modeling approaches. [ABSTRACT FROM AUTHOR]

Abstract Mixed convection of low and unitary Prandtl fluids in a vertical passage is fundamental to passive heat removal in liquid metal and gas-cooled advanced reactor designs. Capturing the influence of buoyancy in flow and heat transfer in engineering analysis is hence a cornerstone to the safety of the next-generation reactor. However, accurate prediction of the mixed convection phenomenon has eluded current turbulence and heat transfer modeling approaches, yet further development and validation of modeling methods is limited by a scarcity of high-fidelity data pertaining to reactor heat transfer. In this work, a series of direct numerical simulations was conducted to investigate the influence of buoyancy on descending flow of liquid sodium, lead, and unitary Prandtl fluid in a differentially heated channel that represents the reactor downcomer region. From time-averaged statistics, flow-opposing/aiding buoyant plumes near the heated/cooled wall distort the mean velocity distribution, which gives rise to promotion/suppression of turbulence intensity and modification of turbulent shear stress and heat flux distribution. Frequency analysis of time series also suggests the existence of large-scale convective and thermal structures rising from the heated wall. As a general trend, fluids of lower Prandtl number were found to be more susceptible to the buoyancy effect due to stronger differential buoyancy across the channel. On the other hand, the effectiveness of convective heat transfer of the three studied fluids showed a distinct trend against the influence of buoyancy. Physical reasoning on observation of the Nusselt number trend is also discussed. [ABSTRACT FROM AUTHOR]

• Applying tet-to-hex meshing method to SEM CFD code Nek5000 for complex nuclear-related geometries. • Boundary layers were created to obtain good near wall resolution. • Several example cases, including fuel assemblies, helical coil steam generator, random packed pebble bed, are simulated with Large Eddy Simulation, showing the capability of tet-to-hex meshing method of handling complicated geometry. • In most of the presented cases, experimental data was presented along with numerical data. In general, a good agreement was obtained, validating the accuracy of the tet-to-hex meshing method. • Compare to an ill-designed block mesh, tet-to-hex mesh is more robust and numerical efficient. The spectral element code Nek5000 is an open-source, higher-order computational fluid dynamics code developed at Argonne National Laboratory. It is designed to solve incompressible Navier-Stokes equations, but it also has a low-Mach-number approximation feature available. Large eddy simulation is approached by explicit filtering of the velocity field (and other fields) to mimic the effect of dissipation due to the unresolved scale. The computational domain is decomposed into second-order hexahedral elements that conform to the boundaries. However, generating a high-quality pure-hexahedral mesh can be challenging for some problems. For simple geometries, traditional blocking methods can be used to decompose the domain into smaller blocks to generate a so-called structural mesh. A structural mesh can maintain good orthogonality but can have a highly skewed mesh to conform to the geometry, as well as unnecessary refinement in the far field. Moreover, for geometries with relative complexity, blocking the geometry becomes impossible. To address these issues, we adopted a tet-to-hex strategy to generate a pure hexahedral mesh for Nek5000. First, we generate a pure tetrahedral mesh for the geometry; then we divide one tetrahedral element into four hexahedral elements. A pure tetrahedral mesh could be easily generated for complex geometries by using many current meshing codes. In this paper, we use the commercial codes ANSYS meshing and ANSYS ICEM to generate the pure tetrahedral mesh and then convert it to a pure hexahedral mesh. Boundary layers are extruded in ANSYSICEM to maintain near-wall resolution. [ABSTRACT FROM AUTHOR]