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論文

Opposing mixed convection heat transfer for turbulent single-phase flows

茂木 孝介; 柴本 泰照; 日引 俊詞*; 塚本 直史*; 金子 順一*

International Journal of Energy Research, 2024, p.6029412_1 - 6029412_22, 2024/01

自然対流熱伝達と強制対流熱伝達が共存する流れを複合対流と呼ぶ。特に強制対流が下降流の場合をopposing flow複合対流と呼ぶ。既往研究において様々な単相opposing flow複合対流の熱伝達相関式が提案されているが、それらは様々な試験装置流路形状、作動流体、熱流動パラメータの範囲で実施された実験結果に基づいている。無次元支配因子の定義や実験的に確認された適用範囲も相関式ごとに異なるため、使用に際してその適用範囲や外挿性を踏まえた上でどの相関式を選択すべきかを整理しておくことは重要である。本稿では既存のopposing flow複合対流の熱伝達相関式と、熱水力システムコードに実装されている単相流壁面熱伝達相関式についてレビューした。また、複数の既往実験データと各相関式との比較を行い、相関式の予測性能を評価した。その結果、Jackson and Fewster相関式、Churchill相関式、Swanson and Catton (IJHMT)相関式は全ての実験データを精度よく予測可能であった。また、乱流複合対流では等温・等熱流束の熱的境界条件による熱伝達率への影響は顕著ではなく、既存の相関式は熱伝達率予測に適用可能であった。さらに、代表長さに水力学相当直径を用いることにより試験装置流路形状の違いに関わらず相関式が適用可能であり、支配パラメータの無次元化により作動流体によらず相関式が適用可能であることを確認した。幅広い無次元数範囲に対して相関式の外挿性を調査した所、Jackson and Fewster相関式、Churchill相関式、Aicher and Martin相関式は自然対流熱伝達、強制対流熱伝達への優れた外挿性を有しており、実験で妥当性が確認されたパラメータ範囲を超えて相関式が適用できることを示した。

論文

Scalability of inertial particle deposition in bubbles with internal circulation

茂木 孝介; 柴本 泰照; 久木田 豊

Annals of Nuclear Energy, 184, p.109679_1 - 109679_10, 2023/05

 被引用回数:0 パーセンタイル:0.01(Nuclear Science & Technology)

Inertial deposition of small (less than a few $$mu$$ m in diameter) aerosol particles in mm-scale bubbles is an old but unsettled issue in modeling of pool scrubbing phenomenon. Whereas existing practical models give no specific consideration to the bubble-internal transport, some studies have shown that inertial transport affects considerably the particle deposition rate. We show, on the basis of Lagrangian simulations of particles advected by steady internal circulation in a spherical bubble, that particle centrifugal velocity becomes scale invariant for low- Stokes numbers (St $$le$$ $$10^{-2}$$) when the characteristic timescale is chosen to be that for transversal particle motion at the Stokes terminal velocity corresponding to the local fluid acceleration. Because a scaling law can be derived by running simulations with a small number of particles, it can provide a practical tool for considering the influence of inertial particle transport within the bubble on the decontamination factor.

論文

Nonuniform particle distribution and interference between removal mechanisms during unsteady aerosol deposition from a rising spherical bubble

茂木 孝介; 柴本 泰照; 久木田 豊

Journal of Nuclear Science and Technology, 59(8), p.1037 - 1046, 2022/08

 被引用回数:3 パーセンタイル:71.05(Nuclear Science & Technology)

Lagrangian particle tracking simulations are performed on aerosol particle removal from a spherical bubble rising in a water pool. The bubble internal circulation is assumed to be given by the Hill's vortex stream function. Monodispersed particles in the 0.1-1.0 um radius range are simulated. The results are discussed with a focus on the development of particle concentration profile and its influence on removal rates, with the aid of comparisons with Eulerian models including the lumped-parameter model by Fuchs. The results are discussed with a focus on the development of particle concentration profile and its influence on removal rates, with the aid of comparisons with Eulerian models including the lumped-parameter model by Fuchs. It is shown that a quick growth of bubble-surface diffusion layer causes a transition of dominant removal mechanisms from unsteady Brownian diffusion to inertial migration. Fuchs's model, not designed to consider such transition, overpredicts the removal rates for small particles. It is also shown that inertial migration produces a surface-peaked concentration profile in the bubble, resulting in higher removal rates than predicted by Fuchs's lumped-parameter model.

論文

ETSON-SAMHYCO-NET benchmark on simulations of upward flame propagation experiment in representative hydrogen-air-steam mixtures of severe accidents containments atmosphere

Bentaib, A.*; Chaumeix, N.*; Nyrenstedt, G.*; Bleyer, A.*; Maas, L.*; Gastaldo, L.*; Kljenak, I.*; Dovizio, D.*; Kudriakov, S.*; Schramm, B.*; et al.

Proceedings of 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19) (Internet), 11 Pages, 2022/03

In case of a core melt-down accident in a light water nuclear reactor, hydrogen is produced during reactor core degradation and released into the reactor building. In case of failure of in-vessel corium retention, a large amount of carbon monoxide (CO) in addition to H$$_{2}$$ and other gases may be produced during molten core concrete interaction (MCCI). This subsequently creates a combustion hazard. A local ignition of the combustible mixture may generate standing flames or initially slow propagating flames. Depending on geometry, mixture composition and turbulence level, the flame can accelerate or be quenched after a certain distance. The pressure and temperature loads generated by the combustion process may threaten the integrity of the containment building and safety equipment. The evaluation of such loads requires validated codes which can be used with a high level of confidence. Currently, turbulence and steam effect on flame propagation mechanisms are not well reproduced by combustion models usually implemented in safety tools and further model enhancement and validation are still needed. For this purpose and at the initiative of the SAMHYCO-NET project consortium and of the European Technical Safety Organization Network (ETSON), a benchmark on hydrogen combustion was organized with the goal to identify the current level of the computational tools in the area of hydrogen combustion simulation under conditions typical for safety considerations in a Nuclear Power Plant (NPP). This benchmark is composed of four main steps with increasing difficulty starting from flame propagation in homogenous dry atmosphere and finishing with more representative conditions with (H$$_{2}$$/H$$_{2}$$O/O$$_{2}$$/N$$_{2}$$) stratified mixtures. In this paper, only experiments related to flame propagation in homogenous atmosphere are considered.

論文

Computational study on the spherical laminar flame speed of hydrogen-air mixtures

Trianti, N.; 茂木 孝介; 杉山 智之; 丸山 結

Proceedings of 2020 International Conference on Nuclear Engineering (ICONE 2020) (Internet), 9 Pages, 2020/08

The computational fluid dynamics (CFD) have been developed to analyze the correlation equation for laminar flame speed of hydrogen-air mixtures. This analysis was carried out on the combustion of hydrogen-air mixtures performed at the spherical bomb experiment facility consists of a spherical vessel equipped (563 mm internal diameter). The facility has been designed and built at CNRS-ICARE laboratory. The simulation was carried out using the reactingFoam solver, one of a transient chemical reaction solver in OpenFOAM 5.0. The LaunderSharmaKE model was applied for turbulent flow. The interaction of the chemical reaction with the turbulent flow was taken into account using PaSR (Partial Stirred Reactor) model with 19 elementary reactions for the hydrogen combustion. The initial condition of spherical flame analysis was set so as to be consistent with those of the experiment. The position of the flame front was detected by the steep drop of hydrogen mass fraction in the spherical radii, and the flame propagation velocity was estimated from the time-position relationship. The analysis result showed the characteristic of spherical flame acceleration was qualitatively reproduced even though it has a discrepancy with the experiment. After validating the calculation of spherical experiments, a laminar burning velocity correlation is presented using the same boundary conditions with the variation of hydrogen concentration, temperature, and pressure. The calculation of laminar flame speed of hydrogen-air mixtures by reactingFoam use reference temperature T$$_{rm ref}$$ = 293 K and reference pressure P$$_{rm ref}$$ = 1 atm with validated in the range of hydrogen concentration 6-20%; range of temperature 293-493 K; and range of pressure 1-3 atm.

論文

CFD analysis of hydrogen flame acceleration with burning velocity models

茂木 孝介; Trianti, N.; 松本 俊慶; 杉山 智之; 丸山 結

Proceedings of 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-18) (USB Flash Drive), p.4324 - 4335, 2019/08

Hydrogen managements under severe accidents are one of the most crucial problems and have attracted a great deal of attention after the occurrence of hydrogen explosions in the accident at Fukushima Daiichi Nuclear Power Plant in March 2011. The primary purpose of our research is improvements in computational fluid dynamics techniques to simulate hydrogen combustion. Our target of analysis is ENACCEF2 hydrogen combustion benchmark test conducted in the framework of ETOSON-MITHYGENE project. Flame acceleration experiments of hydrogen premixed turbulent combustions were simulated by the Turbulent Flame Closure (TFC) model. We implemented several laminar flame speed correlations and turbulent flame speed models on XiFoam solver of OpenFOAM and compared the results to investigate the applicability of these correlation and model equations. We found that all the laminar flame speed correlations could predict qualitative behavior of the flame acceleration, but Ravi & Petersen laminar flame speed correlation that is originally implemented in OpenFOAM underestimated the maximum flame speed for the lean hydrogen concentration. Zimont model and G$"u$lder model of the turbulent flame speed could reasonably simulate the flame acceleration behavior and maximum pressure peaks. The flame velocities calculated with G$"u$lder model tend to be faster than that calculated with Zimont model.

口頭

燃焼速度モデルを用いた水素火炎伝播のCFD解析

茂木 孝介; Trianti, N.; 松本 俊慶; 杉山 智之; 丸山 結

no journal, , 

原子炉シビアアクシデント時には、高温になったジルコニウムと水蒸気の化学反応により、大量の水素が発生する。事故時の水素安全対策の向上は重要な課題であり、福島第一原子力発電所事故における水素爆発の発生以降、さらなる関心を集めている。数値流体力学による水素燃焼解析は事故シナリオ予測, 現象解明, 評価モデル構築を詳細に行うツールとして期待されているが、信頼性のある定量予測は現時点でも困難な課題である。本研究ではMITHYGENEプロジェクトで実施されたENACCEF2実験を対象とし、燃焼速度モデルの比較検討を試みた。燃焼の初期段階は基準状態における層流燃焼速度に大きく影響を受ける。一方で燃焼の後半は圧力依存性の重要度が増す。実験を再現するためには、層流燃焼速度に圧力依存性を適切に取り込む必要があることが分かった。計算した火炎伝播は、実験結果を定性的に良く再現した。

口頭

単一球形気泡からのエアロゾル除去のスケーリング則

茂木 孝介; 久木田 豊; 柴本 泰照

no journal, , 

プールスクラビングによるエアロゾル除去は、原子炉シビアアクシデント時に放射性物質の環境放出を抑制させる有力な手段のひとつとして、事故対策に適用されている。既存のスクラビングコードに採用されたモデルには、気泡内の粒子濃度分布を均一として扱うことで気泡内部の非定常な粒子運動の解析を省略し、粒子除去率を定常解として求めるという過度な単純化の課題がある。本項では気泡内気体流動場にHillの球形渦を用い、気体中を移行するエアロゾルをLagrangian粒子追跡することで、粒子数濃度分布を非定常で解析し除去率を求めた。その結果、気泡内を運動する低Stokes数の慣性粒子軌道に対してスケーリング則が成立することを明らかにした。これを用いて、単一粒子の軌道から気泡内の粒子不均一性を考慮した除染係数の計算方法を提案し、既存モデルと比較する。この様にして得られ除染係数にも同様なスケーリング則が成立することを示す。

口頭

JAEAにおけるプールスクラビング研究の現状

孫 昊旻; 茂木 孝介; 柴本 泰照

no journal, , 

シビアアクシデント時のソースターム評価において、エアロゾル除染の有力な手段とされるプールスクラビングに関して、原子力機構の最近の研究成果を紹介する。本発表では特に、プールスクラビングの最終段である気泡群ステージを対象として、気泡内におけるエアロゾル粒子の沈着機構(慣性衝突,重力,拡散)、気泡内気相の質量・熱バランス、それらによって生じる粒子成長などの物理現象を実験データ及び数値解析に基づいて検討し、粒子運動機構の解明や既存評価モデルの不確かさと限界を検討すると共に、それらに基づいて考案した新たな評価モデルやスケーリング則を提案する。

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