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石垣 将宏*; 廣瀬 意育; 安部 諭; 永井 亨*; 渡辺 正*
Fluids (Internet), 7(7), p.237_1 - 237_18, 2022/07
To estimate thermal flow in a nuclear reactor during an accident, it is important to improve the accuracy of computational fluid dynamics simulation. Temperature and flow velocity are not homogeneous and have large variations in a reactor containment vessel because of its very large volume. In addition, Kelm et al (2016) pointed out that the influence of variations of initial and boundary conditions was important. Therefore, it is necessary to set the initial and boundary conditions taking into account the variations of these physical quantities. However, it is a difficult subject to set such complicated initial and boundary conditions. Then, we can obtain realistic initial and boundary conditions by the data assimilation technique, and we can improve the accuracy of the simulation result. In this study, we applied the data assimilation by local ensemble transform Kalman filter (Hunt et al., 2007) to the simulation of natural convection behavior in density stratification, and we performed a twin model experiment. We succeeded in the estimation of the flow fields and improving the simulation accuracy by the data assimilation, even if we applied the boundary condition with error for the true condition.
石垣 将宏*; 安部 諭; Hamdani, A.; 廣瀬 意育
Annals of Nuclear Energy, 168, p.108867_1 - 108867_20, 2022/04
被引用回数：3 パーセンタイル：77.29(Nuclear Science & Technology)It is essential to improve computational fluid dynamics (CFD) analysis accuracy to estimate thermal flow in a containment vessel during a severe accident. Previous studies pointed out the importance of the influence of initial and boundary conditions on the CFD analysis. The purpose of this study is to evaluate the influence of initial and boundary conditions by numerical analysis of natural convection experiments in a large containment vessel test facility CIGMA(Containment InteGral effects Measurement Apparatus). A density stratification layer was initially formed in the vessel using helium and air, and external cooling of the vessel surface-induced natural convection. In this study, we carried out numerical simulations of the density stratification erosion driven by the natural convection using the RANS (Reynolds averaged Navier-Stokes) model. As a result, the temperature boundary condition of the small internal structure in the vessel had a significant influence on the fluid temperature distribution in the vessel. The erosion velocity of the density stratification layer changed depending on the initial gas concentration distribution. Then, appropriate settings of the temperature and gas concentration conditions are necessary for accurate analysis.
廣瀬 意育; 久木田 豊; 柴本 泰照; 佐川 淳*
Proceedings of 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19) (Internet), 12 Pages, 2022/03
Electric impedance tomography (EIT) is a non-invasive and radiation-free imaging method applicable to gas/liquid two-phase flow measurements. It determines the electrical resistivity distribution of an object from measurements of boundary potentials in response to current injection. Due to the severely ill posed nature of the problem, the quality of reconstructed image depends much on the quality and amount of information available from potential measurements. We have proposed a DC pulse-driven EIT system design equipped with countermeasures for the influences of electrode polarization on potential measurements (Hirose et al., in preparation). The usefulness of EIT in two-phase flow measurement is however restricted by the intrinsically limited spatial resolution. Due to the diffusive nature of electricity, the spatial resolution degrades quickly with the distance from the boundary. In this study, we attempt to improve the spatial resolution by adding thin electrodes inserted into the flow field away from the boundary. Although this means that non-invasiveness is traded off, the influence of invasive electrodes on flow field could be estimated and limited on the basis of experiences gained with other intrusive methods, e.g., needle probes for measurement of interfacial area. The benefit taken by the addition of invasive electrodes, on the other hand, would depend on two-phase flow regime and other flow parameters. In the present paper we consider dispersed bubbly flow and simulate the bubbles with thin cylindrical insulators. The results obtained with and without invasive electrodes are compared to discuss the effectiveness and limitations in measurement of two-phase flow.
Marchetto, C.*; Ha, K. S*; Herranz, L. E.*; 廣瀬 意育; Jankowski, T.*; Lee, Y.*; Nowack, H.*; Pellegrini, M.*; Sun, X.*
Proceedings of 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19) (Internet), 17 Pages, 2022/03
After the Fukushima Daiichi accident of March 2011, one of the main concerns of the nuclear industry has been the research works for improving atmospheric radioactive release mitigation systems. Pool scrubbing is an important process in reactors that mitigates radioactive release. It is based on the injection of gases containing fission products through a water pool. Bubble hydrodynamics, as a result of gas injection and the associated water pool thermal-hydraulics, is an important aspect of the process since the bubble size, shape, velocity, etc. influence the fission product trapping at the bubble interface with the water. Computer codes dedicated to the pool scrubbing have been mainly developed in the 90's last century and modelling drawbacks have been identified in particular for bubble hydrodynamics. One of IPRESCA project objectives is to improve the pool scrubbing modelling. In order to highlight the main modelling issues, a benchmark exercise has been performed focusing on the bubble hydrodynamics. This benchmark, performed by nine organisations coming from six countries, aims at simulating a basic configuration, a single upward injector in ambient conditions, experimentally characterized in the RSE tests carried out in the European PASSAM project. In this paper, a short description of the code modelling and a comparison between the code results and the experimental data are presented and discussed. Then, outcomes from the benchmark result analysis and proposals of improvements are emphasized.
廣瀬 意育; 佐川 淳*; 柴本 泰照; 久木田 豊
Flow Measurement and Instrumentation, 81, p.102006_1 - 102006_9, 2021/10
被引用回数：0 パーセンタイル：0(Engineering, Mechanical)An electrical impedance tomography (EIT) system design is proposed for imaging of phase distribution in gas-water two-phase flow from boundary measurement of electrical potentials in response to direct current (DC) injection. DC injection simplifies substantially the system design, but introduces problems due to polarization of injection electrodes. Electrode polarization means charge accumulation on the electrode-water interface causing a drift in the interfacial potential difference. The polarization problems are coped with by using dedicated electrodes for injection and potential measurement, and using a current source unaffected by the polarization of current-carrying electrodes (CCEs). Furthermore, the polarization of CCEs is controlled, to lessen the possible influence on the sensing electrodes (SEs), by using a short (milliseconds in width) pulse for injection with a charge balanced injection strategy. The impact of electrode polarization and the effectiveness of countermeasures introduced in the present design are discussed through comparisons of measured boundary potentials and of images reconstructed for a simple object simulating large bubbles in water.
孫 昊旻; 柴本 泰照; 廣瀬 意育; 久木田 豊
Journal of Nuclear Science and Technology, 58(9), p.1048 - 1057, 2021/09
被引用回数：3 パーセンタイル：51.72(Nuclear Science & Technology)以前のわれわれのプールスクラビング実験において、入口粒子数密度の上昇に伴って除染係数(DF)が減少する結果が得られており、その理由については過去の研究を含めて明らかにされていなかった。本研究では、現象の要因を粒子表面での水蒸気の凝縮による粒子成長であると仮定し、上昇気泡内の質量とエネルギーのバランスに基づく簡易評価モデルを構築し、同モデルを用いて評価した。粒子表面での水蒸気凝縮は、凝縮による気泡内の水蒸気濃度を低下させると同時に、凝縮潜熱放出による温度上昇効果をもたらし、気泡内蒸気過飽和度を減少させ、凝縮を抑制する。本効果は粒子数濃度に依存する。評価モデルでは、気泡の上昇距離の関数として粒子の成長と慣性DFを計算し、その結果、実験で観察されたDFの傾向を再現した。
岡垣 百合亜; 与能本 泰介; 石垣 将宏; 廣瀬 意育
Fluids (Internet), 6(2), p.80_1 - 80_17, 2021/02
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.
廣瀬 意育; 石垣 将宏; 安部 諭; 柴本 泰照
Proceedings of International Topical Meeting on Advances in Thermal Hydraulics (ATH '20) (Internet), p.757 - 767, 2020/10
This paper describes an application of an immersed boundary method (IBM) to internal structure in a containment vessel as a means to reduce the modeling cost for accident analysis. A brinkman penalization method, that is one of the IBMs, is selected and implemented in OpenFOAM code. The calculation is performed for a grating-type structure in a rectangular vessel and an upward jet flow impinging on the grating is analyzed. The IBM is an appealing approach for solving flow passing complex geometry, whereas a very complicated body fitted mesh with fine resolution might be required in a conventional CFD approach. The results were compared with experimental data of flow velocity distribution through the grating measured by Particle Image Velocimetry technique. The flow immediately downstream of the grating is characterized by multimodal profile with intervals determined by a lattice width of the grating. This multimodal flow merges into one with unimodal shape in the downstream far from the grating. The present analysis reproduced the distinctive flow structure observed in the experiment. The model predicted a serrated profile consistent with the experimental results and this could be reproduced even with a relatively regular computational mesh compared to the traditional method.
岡垣 百合亜; 与能本 泰介; 石垣 将宏; 廣瀬 意育
no journal, ,
As part of the improvement in numerical methods of two-phase flow, JAEA is underway to investigate CFD methods based on the VOF (Volume of Fluid) method. In the present work, we simulated the instability of the gas-liquid interphases between incompressible plane gas sheet and quiescent liquid showing an unstable mode of sinuous or varicose, using a VOF method solver available in the open source CFD code "OpenFOAM". The calculated results were found to agree reasonably well with the linear instability theory by Li. et al.. Furthermore, the effects of interface compression parameter and the number of mesh cells were investigated.
岡垣 百合亜; 与能本 泰介; 石垣 将宏; 廣瀬 意育
no journal, ,
オープンソースCFDコードOpenFOAMの等温・非圧縮の二相流を対象としたVOF法ソルバinterFoamは、界面追跡モデルを用いる一般的なVOF法とは異なり、界面法線方向にVOF値の分布を許容し、界面の数値拡散を制限するため、移流方程式に補正項を加えている。本研究では、静止液相内におけるガスシートの不安定性についてinterFoamを用いて解析を行い、線形理論解析との比較による妥当性評価から、補正項が拡散挙動に影響を及ぼす可能性や使用していく上での適切な格子数、界面圧縮パラメータの設定値について検討した。
和田 裕貴; 佐川 淳*; 広瀬 意育*; 佐藤 聡; 柴本 泰照; 与能本 泰介
no journal, ,
本研究では、パルスエコー法を利用した測定可能液膜厚さをより小さくするために中心周波数15MHzの圧電素子と独自の駆動回路を用いた超音波液膜厚さ測定装置を製作した。本装置では先行研究よりも使用する周波数が高いことから、0.2mm程度の液膜厚さまでは液膜内多重反射波が干渉することなく明確に分離して観測可能であり、それ以下の液膜厚さにおいても高い精度での液膜厚さ測定の可能性が示された。
孫 昊旻; 町田 真一*; 広瀬 意育*; 柴本 泰照; 岡垣 百合亜; 与能本 泰介
no journal, ,
プールスクラビングによるエアロゾル除去効率において支配的とされているガス注入領域に着目し、常温常圧の水-空気条件に対する上向きガス注入体系において、0.3m水位一定のプールスクラビング実験を実施した結果について報告する。
石垣 将宏*; 廣瀬 意育; 安部 諭; 永井 亨*; 渡辺 正*
no journal, ,
To estimate thermal flow in a nuclear reactor during an accident, it is important to improve the accuracy of computational fluid dynamics (CFD) analysis. Temperature and flow velocity are not homogeneous and have large variations in a reactor containment vessel because of its very large volume. In addition, the importance of the influence of variations of initial and boundary conditions is pointed out by Kelm, et al(2016). Therefore, it is necessary to set the initial and boundary conditions that take into account the variations of these physical quantities. However, it is a difficult subject to set such complicated initial and boundary conditions. Then, we can obtain realistic initial and boundary conditions by data assimilation, and we can improve the accuracy of the simulation result. We applied the data assimilation by ensemble Kalman filter for natural convection simulation, and we discuss the improvement of the simulation result. By the data assimilation using only the temperature as the observed data, we succeeded in calculating the temperature and velocity distributions accurately.
廣瀬 意育
no journal, ,
熱水力安全研究グループで取り組んでいる二相流計測機器研究開発の現状と課題について紹介する。電気インピーダンストモグラフィ法による気相分布計測、超音波による液膜厚さ計測、マルチプローブによる気泡速度計測の三つに関して、最近の成果を報告する。
石垣 将宏*; 廣瀬 意育; 安部 諭; 永井 亨*; 渡辺 正*
no journal, ,
過酷事故時の格納容器内熱流動挙動の安全評価においては、容器内ガス挙動を正確に評価することが重要である。格納容器内では3次元的な流動が支配的となるため、数値流体力学(CFD)解析の高度化が必要となる。格納容器内の熱流動挙動を解析する際、格納容器は非常に大きな体積を有するため、その内部の温度や流速は均質とならず、大きなばらつきが生じうる。そのため、高精度な解析のためには、これらの物理量のばらつきを考慮した初期条件・境界条件の設定が必要となる。そこで、観測データ(実験データ)とシミュレーションデータの双方を用いたデータ同化を適用することで、現実に則した初期条件・境界条件が得られ、高精度の解析の実現が期待できる。本研究では、原子力機構が有する大型格納容器実験装置CIGAM内の熱流動挙動のCFD解析に対し、データ同化を適用した。データ同化に対する観測データ配置およびデータ点数に関する感度解析を行い、高精度な解析を実現するために必要となる観測データに関する条件について検討する。
石垣 将宏*; 廣瀬 意育; 永井 亨*; 渡辺 正*
no journal, ,
自然対流挙動の数値流体力学(CFD)解析に対し、局所アンサンブルカルマンフィルターを適用し解析結果の高精度化について検討する。局所アンサンブルカルマンフィルターのパラメータが解析精度に与える影響を検討し、局所化が解析精度に大きく影響することを明らかにした。
