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[en] New needs and opportunities drive the development of novel computational methods for the design and safety analysis of light water reactors (LWRs). Some new methods are likely to be three dimensional. Coupling is expected between system codes, computational fluid dynamics (CFD) modules, and cascades of computations at scales ranging from the macro- or system scale to the micro- or turbulence scales, with the various levels continuously exchanging information back and forth. The ISP-42/PANDA and the international SETH project provide opportunities for testing applications of single-phase CFD methods to LWR safety problems. Although industrial single-phase CFD applications are commonplace, computational multifluid dynamics is still under development. However, first applications are appearing; the state of the art and its potential uses are discussed. The case study of condensation of steam/air mixtures injected from a downward-facing vent into a pool of water is a perfect illustration of a simulation cascade: At the top of the hierarchy of scales, system behavior can be modeled with a system code; at the central level, the volume-of-fluid method can be applied to predict large-scale bubbling behavior; at the bottom of the cascade, direct-contact condensation can be treated with direct numerical simulation, in which turbulent flow (in both the gas and the liquid), interfacial dynamics, and heat/mass transfer are directly simulated without resorting to models
[en] A LES analysis of turbulent convective boiling flow along the rods of a PWR sub-channel was performed using the code TransAT. The campaign is an extension of a former attempt in which the flow was predicted by means of DNS (or super resolved LES). The extension refers to accounting for wall-boiling heat transfer using the triple flux decomposition model, employed within the LES approach constructed on the basis of the filtered mixture equations. The results in the wall boiling configuration differ from the single-flow case in many ways, but more particularly: the flow seems to be portioned in three zones, (i) pre-boiling, (ii) transitional boiling, and (iii) post-boiling, where turbulence activity increases with distance from the onset of nucleate boiling location, affecting heavily the dynamic and thermal boundary layers. The boundary layers thin quite substantially in the post-boiling section, and the flow accelerates due to higher volumetric flow rate induced by phase change, notably in the gap region. The vorticity distribution between the single flow and boiling flow cases suggests that boiling causes a higher rate of vorticity generation. These results corroborate with the other finding, that is: the average frictional velocity in the boiling case increases by a factor of 1.5. Scrutinizing the power density spectra reveals that the boiling flow embodies about twice more energy than the single phase flow, with marked large-scale energetic eddies at the upper end of the rods, i.e. z/L > 0.8. A clear transitional behavior is observed in the boiling flow PSD, in that the amount of energy embodied in the flow increases with vapor production.
[en] The first time I met George was in March 1998 for dinner. And the last time I had dinner with him was in March of last year, or two decades later exactly. During the first evening, we discussed and confronted our strategies for the short period of time left then until his retirement in 2004; was it sufficient time to contribute to the discipline with something risky and innovative? During the last four-hour dinner, after having evoked life and death rather joyfully and serenely, we looked back and analysed what came out of our two decades of collaboration. This paper, which was prepared with the aim to portray George’s achievements in thermal-hydraulics during the last 20 years of our partnership (1998–2018), is written in the spirit of narrating our epilogue culinary-science-talk, while trying to be faithful to his thoughts and ideas about the developments of the discipline and its perspectives. The paper introduces first the cascade of computational tools and discusses trends related to reactor safety problems and developments needed, as well as the need for new kinds of refined experimental data. Although George was not directly involved in all the examples presented here, he felt so concerned about the success of each project/case presented in this paper that he was virtually part of it; and as he wrote in our last paper (Yadigaroglu and Lakehal, 2016): this is after all his near-home work. Finally, in memory of the two other great scientists who have left us recently, Geoff Hewitt and Sam Martin, the content of the paper includes two cases in which both of them had collaborated directly or indirectly.
[en] The paper centers on the use of the so-defined LEIS approach (Large-Eddy and Interface Simulation) for turbulent multi-fluid flows present in thermal hydraulics applications. Interfacial flows involving deformable, sheared fronts separating immiscible fluids are shown to be within reach of this new approach, featuring direct resolution of turbulence and sheared interface deformations within the Interface Tracking (ITM) framework, such as Level Sets and VOF. In this technique super-grid turbulence and interfacial scales are directly solved whereas the sub-grid (SGS) parts are modelled, at least the turbulence part of it. First results are shown (feasibility), and difficulties and open issues are discussed. The connection between these two particular scales will also be discussed, and potential modelling routes evoked, including combining two-fluid and ITM, local grid refinement, or combing particle tracking and ITM for sub-grid inclusions smaller than the grid size. (author)
[en] The paper centres on the use of the so-defined LEIS approach (Large-Eddy and Interface Simulation) for turbulent multifluid flows present in thermal-hydraulics applications. Interfacial flows involving deformable, sheared fronts separating immiscible fluids are shown to be within reach of this new approach, featuring direct resolution of turbulence and sheared interface deformations within the interface tracking (ITM) framework, such as level sets and VOF. In this technique supergrid turbulence and interfacial scales are directly solved whereas the sub-grid (SGS) parts are modelled, at least the turbulence part of it. First results are shown (feasibility), and difficulties and open issues are discussed. The connection between these two particular scales will also be discussed, and potential modelling routes evoked, including combining two-fluid and ITM, local grid refinement, or combing particle tracking and ITM for sub-grid inclusions smaller than the grid size.
[en] Highlights: • We describe a new paradigm for treating and exploiting simulation data, serving in the meantime as an alternative model evaluation workflow. • Instead of reporting simulations of base-case and specific variations scenarios, databases covering a wide spectrum of operational conditions are built by means of machine Learning, which is also used to assess the predictive performance of the models over a wider range of experimental conditions. • We also describe a method to assess the model fitness over a large set of operating conditions, provided that sufficient experimental data is available. This method can be used to identify the regions where a simulation model is expected to reproduce (with an acceptable margin of error) the experimental results, and the regions where it would fail. - Abstract: We introduce a new paradigm for treating and exploiting simulation data, serving in parallel as an alternative workflow for model evaluation and uncertainty quantification. Instead of reporting simulations of base-case and specific variations scenarios, databases covering a wide spectrum of operational conditions are built by means of machine-learning using sophisticated mathematical algorithms. While the approach works for all sorts of computer-aided engineering applications, the present contribution addresses the CFD/CMFD sub-branch, with application to a widely used benchmark of convective flow boiling. In addition to comparing simulation and experimental results on a case-by-case basis, machine-learning is used to create their respective (CFD and experiment) data-driven models (DDM), which will in a later stage serve for assessing the predictive performance of the CFD models over a wider range of experimental conditions, hence providing a high-level classification of their range of applicability.
[en] Highlights: → We model the flow across a tube bundle using V-LES. → We compare the results to RANS and LES data. → V-LES seems to represent a good compromise between accuracy and computational costs. → V-LES could safely be used for similar flows. - Abstract: A new turbulence modelling approach (Very-Large Eddy Simulation; V-LES) is developed and compared to conventional RANS and LES for a flow across a tube bundle. The method, which belongs to the large-scale simulation category, represents a good compromise between efficiency and precision, and may thus be used for industrial problems for which LES remains computationally expensive under high to very-high Reynolds number flow conditions. It can also be used for gas-liquid two-phase flows such as pressurized thermal shocks. The method is a sort of blend between U-RANS and LES, in that it resolves very large structures - way larger than the grid size - and models all subscale of turbulence using a two-equation model, by reference to RANS. The original model is shown here to share the same characteristics as the Detached Eddy Simulation (DES) approach, in that when the filter width is smaller than the wall-distance at which viscous effects are negligible (fμ = 1), the fixed filter width is replaced by the wall distance. First conclusions to be drawn from its extension here is that the flow must be resolved in three-dimensions, under transient conditions, with refined grids. Sensitivity to various computational parameters has been addressed: grid, filter width, domain size, and inflow conditions. This modelling strategy is proved to provide the flow unsteadiness in three-dimensions, while saving computational cost compared to LES. The method is computationally efficient (it can be applied using an implicit solver which permits a higher CFL than with LES; typically 1 versus 0.1), and numerically robust. The computational cost decreases with increasing filter width, though at the expenses of the quality of the results.
[en] An experiment related to steam discharge into sub-cooled water was carried out with a scaled down condensation pool test facility at Lappeenranta University of Technology. The vertical blowdown pipe was submerged by 1.81 m and thermally insulated. Condensation took place only at the steam-water interface near the pipe outlet. Since very low steam flow rates (1.0...1.5 g/s) were used, the steam-water interface remained steady close to the pipe outlet. Several quasi-steady intervals suitable for the validation of direct contact condensation models can be found from the experiment data. Simulations with the Hughes-Duffey based DCC model of the NEPTUNE CFD code indicated two orders of magnitude higher condensation rates than the experiment. This overestimation was reduced by one order of magnitude by decreasing the numerical truncation parameter and by disabling the residual droplet handling. By implementing the DNS-based model of Lakehal et al. (2008) the heat transfer coefficient reached the same order of magnitude as indicated by experiments. More stable transfer rate values were also attained. However, uncertainties prevail in the experimental and simulation results as the presence of non-condensables, which has a significant suppressing effect on condensation, has not been taken into account. The work was accomplished in the framework of the EU/NURESIM project. (authors)
[en] Highlights: • A numerical strategy coupling the Large Eddy Simulation and the level-set method was proposed to handle the highly turbulent multiphase flow. • The evolution of disturbance wave in vertical steam-water annulus was simulated. • The influence of disturbance wave on heat transfer in annular flow was investigated. • The inception criteria of disturbance wave were explored by adjusted the mass flux of saturated water inside the annulus system. - Abstract: A numerical method for forced convective boiling in an annulus needs to be developed in order to elucidate the reason for nucleation enhancement by disturbance waves. We first developed a numerical strategy to model the development of disturbance waves in annular flows where the highly turbulent gas core flow drives the laminar liquid flow upwards using advanced CFD tool TransAT. In which, the interface tracking method (e.g. Level-set) combined with a scale-resolving turbulence simulation technique (Large Eddy Simulation) was employed to capture dominant turbulence and interfacial scales. Then, the disturbance wave phenomenon in a vertical steam-water annulus system was investigated and analyzed. The finding reported in the present work provides insight into the evolution of disturbance wave and its influence on the heat transfer in annular flow. The modeling results revealed that locally hot ‘spots’ occurred upstream of disturbance wave. These locally overheated zones could play key roles in activating the nucleation boiling sites. In addition, the inception criteria of disturbance wave were explored by adjusting the mass flux of saturated water. And it was found no disturbance waves occurred at liquid film Reynolds number lower than the critical value, 225.
[en] A novel large-eddy simulation (LES) approach for computation of incompressible multi-fluid flows is presented and applied to a turbulent bubbling process driven by the downward injection of air into a water pool at Re pipe ∼ 17,000. Turbulence is found to assume its highest intensity in the bulk of the gas flow, and to decay as the interface of the growing bubble is approached. Shear flow prevails in the area of jetting from the pipe, buoyancy-driven flow prevails away from the jetting region, and a third region of vigorous bubble break-up lay O(10 )-O(101) pipe diameters above the tip. Cascading of turbulent kinetic energy is accompanied by an instability-induced linear cascading of interface length scales (i.e. azimuthal modes), transferring energy from the most unstable mode to the smallest interface deformation scales. The LES shows the out-scatter of energy from the large-scale gas-side vortices down to interface wrinkling scales, and statistics prove the existence of a strong correlation between turbulence and interface deformations. Surface curvature was found to constitute a source of small-scale vorticity, and therefore of dissipation of turbulent kinetic energy