Simulation of a jet flow rectified by a grating-type structure using immersed boundary methods

廣瀬 意育; 安部 諭; 石垣 将宏*; 柴本 泰照; 日引 俊*

Progress in Nuclear Energy, 169, p.105085_1 - 105085_13, 2024/04

https://doi.org/10.1016/j.pnucene.2024.105085

Immersed boundary methods (IBMs) have been developed as complementary methods for computational fluid dynamics (CFD). They allow a flow simulation in a mock-up model that includes complex-shaped inner structures and/or boundaries with a non-body conformal mesh. Such a model might force us to create a complicated body-fitted mesh with a high cost in the conventional CFD (CCFD) approach. We focus on the Brinkman penalization (BP) method and its extended version, which we call here the extended Brinkman penalization method (EBP), among the different types of IBMs, aiming to apply them to the phenomena that occur during severe accidents in a nuclear reactor containment vessel and explore the possibility that the methods can partially replace the CCFD. In this paper, as a preliminary step to validate the applicability of these methods, we measure the jet flow rectified by a grating-type structure used for the validation of numerical techniques and apply them to simulate the behavior of an upward jet rectified by a horizontally placed grating-type structure modeled as an immersed body. This type of structure is generally used in reactor buildings, and it is crucial to evaluate their influence on gaseous flows because the behaviors of hydrogen produced during severe accidents may be influenced by them. The structure is selected as our subject because it has moderate complexity, enabling us to examine the effects of the IBMs and compare them with CCFD. We investigate whether these methods can reproduce a result of corresponding CCFD in which the grating is modeled as body-conformal mesh and show that the former can produce the latter with equivalent accuracy. All these results are also compared with the experimental data on the flow velocity distributions downstream of the grating measured using particle image velocimetry.

Boundary layer measurements for validating CFD condensation model and analysis based on heat and mass transfer analogy in laminar flow condition

相馬 秀; 石垣 将宏*; 安部 諭; 柴本 泰照

Nuclear Engineering and Technology, 10 Pages, 2024/00

https://doi.org/10.1016/j.net.2024.02.011

When analyzing containment thermal-hydraulics, computational fluid dynamics (CFD) is a powerful tool because multi-dimensional and local analysis is required for some accident scenarios. According to the previous study, neglecting steam bulk condensation in the CFD analysis leads to a significant error in boundary layer profiles. Validating the condensation model requires the experimental data near the condensing surface, however, available boundary layer data is quite limited. It is also important to confirm whether the heat and mass transfer analogy (HMTA) is still valid in the presence of bulk condensation. In this study, the boundary layer measurements on the vertical condensing surface in the presence of air were performed with the rectangular channel facility WINCS, which was designed to measure the velocity, temperature, and concentration boundary layers. We set the laminar flow condition and varied the Richardson number (1.0-23) and the steam volume fraction (0.35-0.57). The experimental results were used to validate CFD analysis and HMTA models. For the former, we implemented a bulk condensation model assuming local thermal equilibrium into the CFD code and confirmed its validity. For the latter, we validated the HMTA-based correlations, confirming that the mixed convection correlation reasonably predicted the sum of wall and bulk condensation rates.

Measurement of velocity and temperature profiles in boundary layer with steam condensation

相馬 秀; 安部 諭; 柴本 泰照; 石垣 将宏*

Proceedings of 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19) (Internet), 13 Pages, 2022/03

The experimental data of boundary layer profiles are necessary to validate condensation models applied in numerical simulation of CFD codes and also to develop wall treatment models for heat and mass transfer in the presence of significant buoyancy and suction. The available data for velocity, temperature, and concentration boundary layer, however, is quite limited. In this study, we present experimental results of measuring boundary layer profiles by our experimental facility WINCS (WINd tunnel for Condensation of Steam and air mixture). WINCS is a once-through type of wind tunnel having a 1.5m-long measuring part of rectangular duct. The velocity and temperature profiles were obtained with laser doppler velocimetry and thermocouple, respectively. The temperature data was then used to calculate the steam concentration boundary layer by assuming local thermodynamic equilibrium. The laminar boundary layer profiles were obtained in the present condition. The dropwise condensation and fimwise condensation are available in this apparatus. We also conducted numerical simulations with CFD codes and compared the experimental and numerical results of boundary layer profiles and heat fluxes. The wall condensation model based on Stefan flow and bulk condensation model were used in the numerical analysis. The comparison, in general, shows good agreement between the experimental and numerical results.

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

安部 諭; Studer, E.*; 石垣 将宏; 柴本 泰照; 与能本 泰介

Nuclear Engineering and Design, 368, p.110785_1 - 110785_14, 2020/11

https://doi.org/10.1016/j.nucengdes.2020.110785

The 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. We implemented the dynamic modeling for 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. 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 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 on the slower erosive process. This study concludes that the dynamic modeling for is a useful and practical approach to improve the prediction accuracy.

Experimental investigation of density stratification behavior during outer surface cooling of a containment vessel with the CIGMA facility

石垣 将宏; 安部 諭; 柴本 泰照; 与能本 泰介

Nuclear Engineering and Design, 367, p.110790_1 - 110790_15, 2020/10

https://doi.org/10.1016/j.nucengdes.2020.110790

シビアアクシデント時の格納容器(CV)内の流体や構造物を冷却するための有効なアクシデントマネジメントとして、CVの外面冷却が期待されている。一方、以下のような可能性も考えられる。第一に、シビアアクシデント時に水-ジルコニウム反応により水素ガスが発生し、外表面冷却により水素を含む非凝縮性ガスが蓄積し、密度成層が形成される可能性がある。第二に、非凝縮性ガスの蓄積は熱伝達を低下させ、CVの冷却を阻害する可能性がある。これらの課題については、これまで多くの研究が行われてきた。しかし、外表面冷却によって生じる密度成層挙動や成層崩壊の条件に着目した体系的な検討は十分に行われていない。また、水素の蓄積による伝熱劣化を定量的に評価していない。そこで、実験設備CIGMA(Containment InteGral effects Measurement Apparatus)を構築し、CIGMA設備を用いて容器外面冷却時の格納容器熱流動挙動の実験的研究を行った。さらに、安定な密度成層が維持できる条件を考慮することで、自然対流が密度成層化挙動に与える影響を議論した。

Unsteady natural convection in a cylindrical containment vessel (CIGMA) with external wall cooling; Numerical CFD simulation

Hamdani, A.; 安部 諭; 石垣 将宏; 柴本 泰照; 与能本 泰介

Energies (Internet), 13(14), p.3652_1 - 3652_22, 2020/07

https://doi.org/10.3390/en13143652

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). 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-, standard k-, k- shear stress transport (SST), and low-Reynolds-k- Launder-Sharma. The results of the four turbulence models were compared versus the results of experimental data.

Experiment on density stratification behavior by containment venting using CIGMA Facility

石垣 将宏; 安部 諭; 柴本 泰照; 与能本 泰介

Proceedings of 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-18) (USB Flash Drive), p.5927 - 5940, 2019/08

格納容器ベントシステムはシビアアクシデント時における格納容器の過圧を防止するための手段として適用される。ベント時に発生する現象の理解と最適な運用という観点から、格納容器ベント時の水素挙動に関する研究は十分ではない。そこで本研究では大型格納容器実験装置CIGMAを用いて格納容器ベントに関する基礎的な実験を実施した。特に、ベントによる水素輸送に対するベント流量及び初期ガス分布の影響について調査を行った。水素の代替ガスであるヘリウム,蒸気,空気を用いて、初期に容器内に成層を形成し、下部ベントラインからベントを実施した。ベントにより初期に容器下部に存在する蒸気・空気のみが排出され、ヘリウム成層が下方へと拡大していく挙動が観察された。この間、上部のヘリウム濃度は一定であった。最終的には容器内のガス濃度分布は均質化した。今回実施した実験条件では、ベントによる容器内のガスの顕著な混合は観察されなかった。また異なる初期濃度分布に対してもヘリウム成層の挙動に大きな差は見られなかった。

Influence of grating type obstacle on stratification breakup by a vertical jet

安部 諭; 石垣 将宏; 柴本 泰照; 与能本 泰介

Proceedings of 12th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety (NUTHOS-12) (USB Flash Drive), 9 Pages, 2018/10

At Japan Atomic Energy Agency (JAEA), small scale experiment, named VIMES (VIsualization and MEasurement system on stratification behavior) experiment, has been performed since 2014. In this paper, we introduce the influence of grating type obstacle to the VIMES experiment. Two types of grating obstacle were constructed based on the aperture area ratio. The obstacles were placed at the intermediate position between the jet nozzle exit and bottom of the initial stratification. Experimental results showed that the vertical jet was strongly affected by the grating obstacle. Due to the rectifying effect, the radial spreading was suppressed and the velocity magnitude on the jet center line became larger than that in case without the grating obstacle. Meanwhile, due to the resistance effect, the integral momentum flux of the vertical jet was decayed with decrease of the aperture area ratio. It means that in case with the grating obstacle the integral jet penetration strength was decayed, although the local jet penetration to the stratification was stronger than that in case without the grating obstacle. Also, the slower stratification breakup could be observed with decrease of the aperture area ratio, indicating that stratification breakup rate to be discussed in detail considering every possible effect of a jet penetration.

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

安部 諭; Studer, E.*; 石垣 将宏; 柴本 泰照; 与能本 泰介

Proceedings of 17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-17) (USB Flash Drive), 13 Pages, 2017/09

Density stratification and its break-up are important phenomena for discussing the containment hydrogen behavior. Turbulence transport phenomenon 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 of easiness for detailed measurement. Thus, small scale experiments have been carried out in our research project named "ROSA-SA". A rectangular vessel apparatus named VIMES (VIsualization and MEasurement system on Stratification behavior) is one of such facilities, which has a vessel made of acrylic plates for visualizing flow field with the PIV measurement, whose volume is 4.05m (1.5m(L)1.5m(W)1.8m(H)). In this paper, we focus on Computational Fluid Dynamics (CFD) analysis on a density stratification erosion with a vertical buoyant jet observed in the VIMES experiments. Comparative study between Large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) are also performed to validate the dynamic turbulence Schmidt number () formulation. The results have indicated that the dynamic Sct model is advantageous to predict the observed stratification erosion behavior. This research is a collaboration activity between CEA and JAEA.

Influence of mesh non-orthogonality on numerical simulation of buoyant jet flows

石垣 将宏; 安部 諭; 柴本 泰照; 与能本 泰介

Nuclear Engineering and Design, 314, p.326 - 337, 2017/04

AA2016-0575.pdf:1.57MB

https://doi.org/10.1016/j.nucengdes.2017.02.010

本研究では浮力が支配的なジェット流れに対するメッシュの非直交性の影響を調査した。解析には三角形要素のメッシュと四角形要素のメッシュを用い、CFDコードOpenFOAMを適用した。非直交性補正を適用しない場合、三角形要素メッシュを用いて解析したジェットでは四角形要素メッシュを用いた場合よりも流れの不安定性が過大評価されることが分かった。一般に非直交補正によりメッシュの非直交性の影響は緩和されるが、計算時間が増大する。そこで、非直交補正を用いずにメッシュの非直交性の影響を緩和するための改良型ソルバを提案した。