Numerical Study on an Interface Compression Method for the Volume of Fluid Approach

Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in the convection term of the volume fraction equation. Thus, in this study, we focused on an interface compression (IC) method for such a VOF approach; this technique prevents numerical diffusion issues and maintains boundedness and conservation with negative diffusion. First, on a sufficiently high mesh resolution and without the IC method, the validation process was considered by comparing the amplitude growth of the interfacial wave between a two-dimensional gas sheet and a quiescent liquid using the linear theory. The disturbance growth rates were consistent with the linear theory, and the validation process was considered appropriate. Then, this validation process confirmed the effects of the IC method on numerical diffusion, and we derived the optimum value of the IC coefficient, which is the parameter that controls the numerical diffusion

Density stratification breakup by a vertical jet: Experimental and numerical investigation on the effect of dynamic change of turbulent Schmidt number

International audienceThe hydrogen behavior in a nuclear containment vessel is one of the significant issues raised when discussing the potential of hydrogen combustion during a severe accident. Computational Fluid Dynamics (CFD) is a powerful tool for better understanding the turbulence transport behavior of a gas mixture, including hydrogen. Reynolds-averaged Navier–Stokes (RANS) is a practical-use approach for simulating the averaged gaseous behavior in a large and complicated geometry, such as a nuclear containment vessel; however, some improvements are required. In this paper, we focused on the turbulent Schmidt number Sct for improving the RANS accuracy. Some previous studies on ocean engineering mentioned that the value gradually increases with the increasing stratification strength. We implemented the dynamic modeling for Sct based on the previous studies into the OpenFOAM ver 2.3.1 package. The experimental data obtained by using a small scale test apparatus at Japan Atomic Energy Agency (JAEA) was used to validate the RANS methodology. In the experiment, we measured the velocity field around the interaction region between vertical jet and stratification by using the Particle Image Velocimetry (PIV) system and time transient of gas concentration by using the Quadrupole Mass Spectrometer (QMS) system. Moreover, Large-Eddy Simulation (LES) was performed to phenomenologically discuss the interaction behavior. The comparison study indicated that the turbulence production ratio by shear stress and buoyancy force predicted by the RANS with the dynamic modeling for Sct was a better agreement with the LES result, and the gradual decay of the turbulence fluctuation in the stratification was predicted accurately. The time transient of the helium molar fraction in the case with the dynamic modeling was very closed to the VIMES experimental data. The improvement on the RANS accuracy was produced by the accurate prediction of the turbulent mixing region, which was explained with the turbulent helium mass flux in the interaction region. Moreover, the parametric study on the jet velocity indicates the good performance of the RANS with the dynamic modeling for Sct on the slower erosive process. This study concludes that the dynamic modeling for Sct is a useful and practical approach to improve the prediction accuracy

Unsteady Natural Convection in a Cylindrical Containment Vessel (CIGMA) With External Wall Cooling: Numerical CFD Simulation

In the case of a severe accident, natural convection plays an important role in the atmosphere mixing of nuclear reactor containments. In this case, the natural convection might not in the steady-state condition. Hence, instead of steady-state simulation, the transient simulation should be performed to understand natural convection in the accident scenario within a nuclear reactor containment. The present study, therefore, was aimed at the transient 3-D numerical simulations of natural convection of air around a cylindrical containment with unsteady thermal boundary conditions (BCs) at the vessel wall. For this purpose, the experiment series was done in the CIGMA facility at Japan Atomic Energy Agency (JAEA). The upper vessel or both the upper vessel and the middle jacket was cooled by subcooled water, while the lower vessel was thermally insulated. A 3-D model was simulated with OpenFOAM®, applying the unsteady Reynolds-averaged Navier–Stokes equations (URANS) model. Different turbulence models were studied, such as the standard k-ε, standard k-ω, k-ω shear stress transport (SST), and low-Reynolds-k-ε Launder–Sharma. The results of the four turbulence models were compared versus the results of experimental data. The k-ω SST showed a better prediction compared to other turbulence models. Additionally, the accuracy of the predicted temperature and pressure were improved when the heat conduction on the internal structure, i.e., flat bar, was considered in the simulation. Otherwise, the predictions on both temperature and pressure were underestimated compared with the experimental results. Hence, the conjugate heat transfer in the internal structure inside the containment vessel must be modeled accurately

Stratification breakup by a diffuse buoyant jet: The MISTRA HM1-1 and 1-1bis experiments and their CFD analysis

International audienceDensity stratification and its breakup are important phenomena to consider in the analysis of the hydrogen distribution during a severe accident. Many research projects to understand the stratification behavior have been performed with large and small scale test facilities. Many previous experimental studies, using helium as mimic gas of hydrogen, focused on the stratification breakup by a vertical or horizontal jet. However, in a real containment vessel, the upward flow pattern can be considered “diffuse” and buoyant neither pure jet nor pure plume. HM1-1 and HM1-1bis tests in the MISTRA facility were performed to investigate such erosive flow pattern created from a horizontal hot air jet impinging on a vertical cylinder. The experimental results indicated that the jet flow was quickly mixed with the surrounding gas in the lower region of the initial stratification, and deaccelerated by buoyancy force therein. Consequently, the erosive process became slower at the upper region of the initial stratification. Those observed behavior was analyzed using the computational fluid dynamics (CFD) techniques focusing on models for turbulent Schmidt Sct and Prndtl Prt numbers. Some previous studies mentioned that these numbers significantly change in the stratified flow. The changes of Sct and Prt are very important factor to predict the stratification erosion process. The results have indicated that the simulation can be much improved by using appropriate dynamic models for those numbers. This research is a collaboration activity between CEA and JAEA

RANS analysis with a dynamic model for turbulent Schmidt number (SctSc_t) on density stratification erosion in a small rectangular vessel

International audienceHydrogen behavior in a reactor containment vessel is one of significant issues in discussing a potential ofhydrogen combustion during a severe accident. Density stratification and its break-up are importantphenomena for discussing the hydrogen behavior. Many research projects to understand the stratificationbehavior have been performed with large scale test facilities. Many experiments with helium as alternategas of hydrogen focus on the stratification erosion by a vertical or horizontal jet. Turbulence transportphenomenon is one of the important factor in the stratification erosion behavior.A small scale test is a useful approach to develop a physical model for the stratification erosion because ofeasiness for detailed measurement. Thus, small scale experiments have been carried out in the ROSA-SAproject conducted at JAEA for study on containment thermalhydraulics during a severe accident. Arectangular vessel apparatus named VIMES (VIsualization and MEasurement system on Stratificationbehavior) is one of such facilities, which has a vessel made of acrylic plates for visualizing flow field withthe PIV measurement, whose volume is 4.05 m$^3$ (1.5m(L) × 1.5m(W) × 1.8m(H)).In this paper, we focus on Computational Fluid Dynamics (CFD) analysis on a density stratification erosionwith a vertical buoyant jet observed in the VIMES experiments. Comparative study between Large-eddysimulation (LES) and Reynolds-averaged Navier-Stokes (RANS) are also performed to validate thedynamic turbulence Schmidt number ($Sc_t$) formulation. The results have indicated that the dynamic Sctmodel is advantageous to predict the observed stratification erosion over the conventional model using aconstant value of Sct. This research is a collaboration activity between CEA and JAEA