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• Assessment of different, linear and non-linear, RANS models is performed for an infinite wire-wrapped fuel assembly. • The obtained results, for both flow field and the thermal fields, are compared with the reference q-DNS data. • The considered non-linear models, RSM-EB and SST k - ω cubic, have shown good agreement. Assessment of different Reynolds Averaged Navier Stokes (RANS) based modelling approaches is performed for an infinite wire-wrapped fuel assembly configuration of a Liquid Metal Fast Reactor (LMFR). The geometric fuel assembly dimensions refer to the present MYRRHA design. In total, four turbulence models are considered, i.e. linear k - ω SST, realizable k - ε , k - ω SST cubic and Elliptic Blending model of the Reynolds Stress Model (RSM-EB). For this configuration, a high fidelity reference is available and is used for numerical comparisons. Following best practice guidelines, a thorough mesh sensitivity, as well as a sensitivity analysis on different turbulence models, are performed. Both qualitative and quantitative analysis are presented in terms of velocity, temperature and turbulent kinetic energy profiles. Of all models tested, RSM-EB, along with k - ω SST cubic, provide best performance, especially for the flow and the thermal fields, consistent with their capability of modelling anisotropic effects in turbulent flows characterised by highly three-dimensional cross flow and complex flow channel structures, such as the one studied in this work. [ABSTRACT FROM AUTHOR]

Highlights • An extension of the AFHM-NRG model to an advanced low-Reynolds differential Reynolds stress model is proposed. • This new version of the model is validated against different planar channel flows at unity and low-Prandtl number. • The model is then applied to two relatively complex flows, i.e. a planar impinging jet and a bare rod bundle configuration. • The AHFM-NRG model with an advanced closure for the Reynolds stresses gives encouraging results. Abstract In the numerical simulation of turbulent heat transfer, a reliable representation of the turbulent flow field is a prerequisite in order to obtain an accurate prediction of the thermal field. In the framework of RANS modelling approach, classical two-equation eddy-viscosity models present intrinsic limitations for applications characterised by complex flow fields where significant turbulence anisotropy can be expected. In this respect, differential Reynolds stress models represent the natural choice for dealing with such highly anisotropic flows. On the other hand, the modelling of the turbulent heat flux term almost universally relies on the so-called Reynolds analogy, due to its simplicity and robustness. Nevertheless, this approach presents several well-known limitations, especially when dealing with low-Prandtl fluids. In this context, algebraic heat flux models are regarded as a promising approach to overcome the shortcomings of the Reynolds analogy. In particular, an implicit algebraic heat flux closure, called as AHFM-NRG, has been proposed for applications involving low-Prandtl fluids. In its original formulation, the algebraic turbulent heat flux closure was coupled with a low-Reynolds linear k - ε model for the closure of the Reynolds stress tensor. In the present work, the applicability of the AHFM-NRG model is extended to an advanced low-Reynolds differential Reynolds stress model. This new version of the model is validated against different planar channel flows at unity and low-Prandtl number; successively, the model is applied to two relatively complex flows, i.e. a planar impinging jet and a bare rod bundle configuration. It is shown that the coupling of the AHFM-NRG with an advanced closure for the Reynolds stresses gives encouraging results for these cases, where an accurate prediction of the anisotropy in the flow field results in a noticeable improvement in the prediction of the turbulent heat transfer. [ABSTRACT FROM AUTHOR]

The aim of the present research work is to design a numerical experiment for a closely-spaced bare rod bundle in order to perform a DNS, which will serve as a reference for the verification purpose. The considered geometric design is based on the well-known Hooper experiment, which contains a bare rod bundle with pitch-to-diameter ratio of P/D = 1.107. However, performing a DNS computation corresponding to the Hooper experiment requires a huge amount of computational power. Hence, in this article, a wide range of unsteady RANS study has been performed to calibrate and optimize the Hooper case for the targeted DNS study. As a first step, the Reynolds number of the original Hooper case is scaled down in such a way that the overall phenomenology of the flow field remains the same, i.e. the very existence of the axial flow pulsations. Afterwards, the calibration of the computational domain with the respective boundary conditions is performed to obtain an optimized Hooper case, which is feasible for the available computational resources. In addition to the flow field, a parametric study for four different passive scalars is performed to take into account the heat transfer analysis, which was not included in the original Hooper case. These passive scalars correspond to the Prandtl numbers of three different working fluids, i.e. air, water and liquid metal fluids. The heat transfer of these three fluids has been studied in combination with two different boundary conditions at the walls, i.e. a constant temperature and a constant heat flux. Accordingly, the final DNS will yield in an extensive verification database for flow and the thermal fields representing different reactor coolants. [ABSTRACT FROM AUTHOR]

The knowledge of heat transfer in corium pools is one of the important issues for corium retention, as it defines the safety margin for vessel integrity. In this regard, in the 90’s the BALI experimental program was performed at the CEA, France. The principal idea was to create a database regarding the heat transfer distribution at corium pool boundaries for in-vessel and ex-vessel configurations at high internal Rayleigh number (10 15 to 10 17 ). One of the tasks within the ongoing IVMR project, part of the HORIZON 2020 program, is to assess the up-to-date CFD turbulence models over a wide range of Rayleigh number for the homogenous pool tests of the BALI experiments. In the present study, the assessment of three different turbulence models is performed for two BALI test cases. These turbulence models include a linear k-ε model, a non-linear Reynolds stress model and an advanced turbulent heat flux model known as the AHFM-NRG. After an extensive and careful assessment it has been found that none of the previous models is able to correctly predict the complex heat transfer phenomena appearing in natural convection flow regimes at such high Rayleigh numbers. Hence, a new model is proposed to deal with a wide range of Rayleigh number flow regimes. Accordingly, the results obtained using this new approach are found to be in a very good agreement with the available experimental data. [ABSTRACT FROM AUTHOR]

This article reports the CFD benchmark to validate different turbulent modelling approaches for transient heat transfer in the context of high-cycle thermal fatigue due to mixing of hot and cold flow in a T-junction including the wall. This validation exercise has been carried out within the MOTHER project. In the framework of the project, new experiments were performed with a novel measurement sensor allowing the measurements of the fluctuating wall temperature inside the solid in the mixing region between hot and cold water after a T-junction. The tests were performed for two different Reynolds numbers 40,000 and 60,000 and for two different T-junction geometries; a sharp corner and a round corner. The CFD validation study has been performed using five different CFD softwares, namely STAR-CCM+, Code_Saturne, LESOCC2, Fluent and OpenFOAM. In addition, seven different turbulence models i.e. wall resolved LES, wall function LES, VLES, DES, PRNS, URANS, RANS and AHFM (RANS), were used to perform the CFD computations. The validation exercise has shown that LES shows the best agreement with the experimental data followed by hybrid (LES/RANS), URANS and RANS models, respectively. The velocity and the thermal fields in the fluid region are correctly predicted by the proper use of the LES modelling. Whereas, the accurate prediction of the thermal field in the solid requires very long sampling time to achieve statistically converged solution, which of course requires an enormous computational power. Therefore, the statistical convergence of the thermal field in the solid has been found to be one bottleneck in order to accurately predict the temperature fluctuations in the wall and the transient heat transfer. However, measuring the small amplitude temperature fluctuations is also associated with an uncertainty so the disagreement between CFD and measurements (of the order of 10%) can also be attributed, in part, to uncertainties in the measurements. [ABSTRACT FROM AUTHOR]

• Liquid Metal thermal-hydraulics in forced and mixed circulation conditions is considered. • The capabilities of some selected turbulence models are assessed against DNS data. • The AHFM-SC model is found to represent a solid approach for the calculation of Turbulent Heat Flux. The present paper investigates the capabilities of some selected Reynolds-Averaged Navier Stokes (RANS) based turbulence models in reproducing liquid metals thermal hydraulics Direct Numerical Simulation (DNS) data. For this purpose, forced and mixed convection conditions, both addressing buoyancy-aided and buoyancy-opposed flow, are considered. The paper mainly focuses on velocity and temperature fields estimation, providing a comparison between the RANS and DNS computations. The capabilities of the turbulence models are discussed with the aim to highlight which ones provide the best predictions. In particular, attention is paid to the approach adopted for the calculation of the turbulent heat flux contributions. Together with models assuming the commonly adopted Simple Gradient Diffusion Hypothesis (SGDH) approach and the Reynolds analogy, a model including the Algebraic Heat Flux Model (AHFM) approach is considered. While being a practical and robust approach to deal with turbulent heat fluxes, the SGDH approach shows intrinsic limitation in dealing with liquid metal thermal hydraulics, mainly because of their low-Prandtl number. The adoption of a more advanced AHFM method may instead relevantly improve the quality of the obtained predictions. The obtained results show that the selected model adopting the AHFM method provides definitively better predictions of the addressed phenomena with respect to the ones considering the SGDH approach. While some discrepancies are still observed for the velocity fields, the temperature fields are captured very well, suggesting a clear superiority of the AHFM model. The present paper thus provides further validation and supports the use of AHFM as a valuable tool to predict turbulent heat fluxes. [ABSTRACT FROM AUTHOR]

• The calibration and optimization procedure is adopted to design a numerical experiment for a closely-spaced bare rod bundle in order to perform a DNS. • Reference database (DNS) is generated for Re = 9800 and three different Pr number fluids (Pr = 0.025, 1 & 2) combined with two thermal boundary conditions: constant temperature and constant heat flux. • DNS will serve as a reference for validation purposes. • An overview of 6 pragmatic RANS model results is given in order to assess the prediction capabilities of a linear and non-liner models. The understanding of flow behaviour and temperature distribution in rod bundles during operation is of great importance for the safety and economical design of existing nuclear reactors and as well as new nuclear technologies. In order to study the thermal-hydraulics phenomena in the fuel assembly geometries, researchers all over the world are increasingly using Computational Fluid Dynamics (CFD) as a reliable research tool. Nevertheless, the prediction capabilities of the most commonly used Reynolds Average Navier-Stokes (RANS) models must be assessed. In this regard, in the present research work, an extensive effort has been put forward and is described in three main steps. As a first step, a wide range of RANS and unsteady RANS computations have been performed to design a numerical experiment for a closely-spaced bare rod bundle in order to perform a direct numerical simulation (DNS), which will serve as a reference for the validation purpose. As a second step, the DNS of this bare rod bundle has been performed for three different Prandtl number (Pr) fluids, i.e. Pr = 2, 1 and 0.025. This DNS has been performed using a higher-order spectral element code and consists of 660 million grid points. Accordingly, an extensive database has been generated for validation purposes. Finally, this database is used to assess the prediction capabilities of different linear and non-linear RANS models. This article will provide a good overview of all three steps and will also highlight the main lessons learned from this research. [ABSTRACT FROM AUTHOR]

• An overview of the AHFM-NRG formulations has been presented. • These formulations are based on an implicit algebraic heat flux model. • The AHFM-NRG formulations are well calibrated to predict flow and heat transfer in low-Prandtl flows. Turbulent heat transfer is a complex phenomenon, which is ubiquitous in engineering applications and has challenged turbulence modellers for several decades. In an attempt to simplify the problem it is often assumed that turbulent heat transfer can be inferred from the knowledge of the turbulent momentum transport, in what is known as the Reynolds analogy. This approach presents well-known drawbacks that limit its applicability to low-Prandtl fluids such as liquid metals. In an effort to overcome such limitations, an implicit Algebraic Heat Flux Model named AHFM-NRG has been recently proposed by the Nuclear Research and Consultancy Group (NRG). In the framework of the EU THINS project, this model was initially tested for a limited number of academic test cases in all three flow regimes (i.e.: natural, mixed and forced convection) and showed encouraging results. Further assessment and development of this or any other turbulent heat flux model with low-Prandtl fluids was hampered by the lack of accurate and reliable reference data. Thanks to the extensive reference database generated within the subsequent EU SESAME and MYRTE projects, the AHFM-NRG formulation has been further tested and developed. This article reports the development of the AHFM-NRG approach and its assessment against some representative test cases in all three flow regimes. It is shown that the AHFM-NRG formulations result in significant improvement with respect to the classical Reynolds analogy in all considered flow configurations. [ABSTRACT FROM AUTHOR]

Today computational fluid dynamics (CFD) is widely used in industrial companies, research institutes and technical safety organizations to supplement the design and analysis of diverse technical components and large systems. Such numerical programs are applied to better understand complex fluid flow and heat transfer phenomena. In the last decades there is an increasing interest in the nuclear community to utilize such advanced programs for the evaluation of different nuclear reactor safety issues, where traditional analysis tools show deficiencies. Within the FP7 European project THINS (Thermal Hydraulics of Innovative Nuclear Systems), CFD and coupled 1D-3D thermal–hydraulic simulations are being carried out. These are dedicated to the analysis of the thermal–hydraulics of gas, liquid metal and supercritical water cooled reactors. Such concepts utilize innovative fluids, which have different properties from the ones used in the current nuclear reactors. In order to improve the thermal–hydraulic predictions of their behavior, CFD development, application and validation activities are performed within THINS. This overview paper highlights some of the CFD related work within the European project. [ABSTRACT FROM AUTHOR]

This article reports new experiments performed with the purpose of generating novel data of the fluctuating temperature inside the solid in the mixing region between hot and cold water in a T-junction. This data has been measured using a novel sensor (coefh) developed at the Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA) in Cadarache, France. These experiments are performed within the framework of the MOTHER project. The main objective of the MOTHER project is to validate various CFD approaches (such as LES, Hybrid i.e. RANS/LES and RANS) for transient heat transfer in a T-junction configuration including the pipe wall. Hence, the performed experiments have focused on accurately measuring and documenting the boundary conditions to be able to have a well-defined database for CFD validation. The tests are performed for two different Reynolds numbers 40000 and 60000 and for two different T-junction geometries; a sharp corner and a round corner. [ABSTRACT FROM AUTHOR]