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Terada, Atsuhiko; Thwe Thwe, A.; Hino, Ryutaro*
JAEA-Review 2024-049, 400 Pages, 2025/03
JAEA-Review-2024-049.pdf:13.94MB
In the aftermath of the Fukushima Daiichi Nuclear Power Station accident, safety measures against hydrogen in severe accident has been recognized as a serious technical problem in Japan. As one of efforts to form a common knowledge base between nuclear engineers and experts on combustion and explosion, we issued the "Handbook of Advanced Nuclear Hydrogen Safety (1st edition)" in 2017. For improvement of the rational advancement of hydrogen safety measures and further reliability of hydrogen safety evaluation, a CFD analysis is highly expected to produce more precisely and quantitative results. We have been developing an integrated CFD analysis code system which can analyze hydrogen diffusion, explosion-combustion and structural integrity at the severe accident especially for pressurized water reactors (PWRs). We organized the role of LP and the CFD analyses and their utilization examples of hydrogen safety validation. Based on these results, we made the "Handbook of Advanced Nuclear Hydrogen Safety (2nd volume)". The analysis results of real scale PWR described in 2nd volume are confirmed by cross-analysis models and existing data obtained through representative small, medium and large-scale tests.
Thwe Thwe, A.; Kadowaki, Satoshi; Nagaishi, Ryuji
Journal of Nuclear Science and Technology, 60(6), p.731 - 742, 2023/06
Times Cited Count：0 Percentile：0.00(Nuclear Science & Technology)In this study, we performed numerical calculations of unsteady reaction flow considering detailed chemical reactions, investigated the unstable behavior of hydrogen-air dilute premixed flame due to intrinsic instability, and clarified the effects of unburned gas temperature and pressure. I made it. The unstable behavior of the flame in a wide space was simulated, and the burning rate of the cellular flame was obtained. Then, the effects of heat loss and flame scale on flame unstable behavior were investigated. The burning velocity of a planar flame increases as the unburned-gas temperature increases and it decreases as the unburned-gas pressure and heat loss increase. The normalized burning velocity increases when the pressure increases and heat loss becomes large, and it decreases when the temperature increases. This is because the high unburned-gas pressure and heat loss promote the unstable behavior and instability of flame.
Katsumi, Toshiyuki; Thwe Thwe, A.; Kadowaki, Satoshi
Journal of Visualization, 25(5), p.1075 - 1083, 2022/10
Times Cited Count：2 Percentile：18.78(Computer Science, Interdisciplinary Applications)Lean combustion and inert-gas addition are useful to control the burning velocity of hydrogen premixed flames, and it is well known that the cellular structure forms on the front of lean hydrogen flames owing to intrinsic instability. However, the influences of inert-gas addition on the instability phenomena of lean hydrogen premixed flames are not understood fully, and then it is needed to be investigated the flame instability experimentally. In the experiments, the cellular structure and fluctuation of H
/O/inert gases (Ar, N,CO) premixed flames on a flat burner were obtained using direct observation, laser diagnostics and light emission intensity to elucidate the characteristics of instability phenomena. As the results, the correlation of inert-gas addition, equivalence ratio and total flow rate with the characteristics of cellular flames was revealed, and the influences of these parameters on flame instability were discussed.
Thwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro; Nagaishi, Ryuji; Kadowaki, Satoshi
Journal of Nuclear Science and Technology, 59(5), p.573 - 579, 2022/05
Times Cited Count：1 Percentile：0.00(Nuclear Science & Technology)The simulations of the combustion of self-propagating hydrogen-air premixed flame are performed by an open-source CFD code. The flame propagation behavior, flame radius, temperature and pressure are analyzed by varying the initial laminar flame speed and grid size. When the initial laminar speed increases, the thermal expansion effects become strong which leads the increase of flame radius along with the increase of flame surface area, flame temperature and pressure. A new laminar flame speed model derived previously from the results of experiment is also introduced to the code and the obtained flame radii are compared with those from the experiments. The formation of cellular flame fronts is captured by simulation and the cell separation on the flame surface vividly appears when the gird resolution becomes sufficiently higher. The propagation behavior of cellular flame front and the flame radius obtained from the simulations have the reasonable agreement with the previous experiments.
Furuyama, Taisei*; Thwe Thwe, A.; Katsumi, Toshiyuki; Kobayashi, Hideaki*; Kadowaki, Satoshi
Nihon Kikai Gakkai Rombunshu (Internet), 87(898), p.21-00107_1 - 21-00107_12, 2021/06
The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions were investigated by numerical calculations. Adopting a detailed chemical reaction mechanism of hydrogen-oxyfuel combustion modeled by 17 reversible reactions of 8 active species and diluents, a two-dimensional unsteady reaction flow was treated based on the compressible Navier-Stokes equation. As the steam addition and heat loss increased, the burning velocity of a planar flame decreased and the normalized burning velocity increased. The addition of water vapor promotes the unstable behavior of the hydrogen-air lean premixed flame. This is because the thermal diffusivity of the gas decreases and the diffusion-thermal instability increases. The effect of adding water vapor on the instability of hydrogen premixed flames is a new finding, and it is expected to connect with hydrogen explosion-prevention measures as in NPP.
Kadowaki, Satoshi; Thwe Thwe, A.; Furuyama, Taisei*; Kawata, Kazumasa*; Katsumi, Toshiyuki; Kobayashi, Hideaki*
Journal of Thermal Science and Technology (Internet), 16(2), p.20-00491_1 - 20-00491_12, 2021/00
Times Cited Count：9 Percentile：41.71(Thermodynamics)Effects of pressure and heat loss on the unstable motion of cellular-flame fronts in hydrogen-air lean premixed flames were numerically investigated. The reaction mechanism for hydrogen-oxygen combustion was modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, and the compressibility, viscosity, heat conduction, molecular diffusion and heat loss were taken into account. As the pressure became higher, the maximum growth rate increased and the unstable range widened. These were due mainly to the decrease of flame thickness. The burning velocity of a cellular flame normalized by that of a planar flame increased as the pressure became higher and the heat loss became larger. This indicated that the pressure and heat loss affected strongly the unstable motion of cellular-flame fronts. In addition, the fractal dimension became larger, which denoted that the flame shape became more complicated.
Katsumi, Toshiyuki; Yoshida, Yasuhito*; Nakagawa, Ryo*; Yazawa, Shinya*; Kumada, Masashi*; Sato, Daisuke*; Thwe Thwe, A.; Chaumeix, N.*; Kadowaki, Satoshi
Journal of Thermal Science and Technology (Internet), 16(2), p.21-00044_1 - 21-00044_13, 2021/00
Times Cited Count：8 Percentile：38.13(Thermodynamics)The effects of addition of CO and water vapor on characteristics of dynamic behavior of hydrogen/air premixed flames were elucidated experimentally. By Schlieren photography, wrinkles on the flame surface were clearly observed in low equivalence ratios. The propagation velocity increased monotonically as the flame radius became larger and flame acceleration was found. Increasing the addition of inert gas, the propagation velocity decreased, especially in the case of CO addition. Moreover, the Markstein length and the wrinkling factor decreased. This indicated that the addition of Co or HO promoted the unstable motion of hydrogen flames, which could be due to the enhancement of the diffusive-thermal effect. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters used in the mathematical model on propagation velocity including flame acceleration was obtained, and then the flame propagation velocity under various conditions was predicted.
Kadowaki, Satoshi; Nogami, Masato*; Thwe Thwe, A.; Katsumi, Toshiyuki*; Yamazaki, Wataru*; Kobayashi, Hideaki*
Nihon Kikai Gakkai Rombunshu (Internet), 85(879), p.19-00274_1 - 19-00274_13, 2019/11
We dealt with three-dimensional cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities to elucidate the effects of unburned-gas temperature and heat loss by adopting the three-dimensional compressible Navier-Stokes equation. As the unburned-gas temperature became lower and the heat loss became larger, the growth rate decreased and the unstable range narrowed. With a decrease of unburned-gas temperature, the normalized growth rate increased and the normalized unstable range widened, which was because the temperature ratio of burned and unburned gases became larger. The obtained hexagonal cellular fronts were qualitatively consistent with the experimental results. As the heat loss became larger, the burning velocity of a cellular flame normalized by that of a planar flame increased. This was because diffusive-thermal effects became stronger owing to the increase of apparent Zeldovich number caused by the decrease of flame temperature.
Thwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro
JAEA-Technology 2018-012, 45 Pages, 2019/01
JAEA-Technology-2018-012.pdf:4.34MB
Under long-term storage of nuclear wastes including low- and high-level wastes, hydrogen can be spontaneously generated from corrosion of metal wastes and container wall itself, and from radiolysis of water in the waste. For the sake of hydrogen safety and the risk reduction of environmental contamination, we have started to investigate the behavior and characteristics of hydrogen combustion and explosion in waste vessel. In this report, we performed numerical simulation to investigate the characteristics of methane combustion by applying OpenFOAM. For combustion scenario, FireFoam solver with LES frame was used. As the results, the average temperature increased when the container height and inlet size increased. The simulation of gas diffusion by FireFoam results showed that helium diffused faster than hydrogen and methane. By XiFoame solver, the simulation was performed to obtain flame propagation radius for hydrogen-air premixed flame.
Thwe Thwe, A.; Kadowaki, Satoshi; Hino, Ryutaro
Journal of Thermal Science and Technology (Internet), 13(2), p.18-00457_1 - 18-00457_12, 2018/12
Times Cited Count：0 Percentile：0.00(Thermodynamics)Two dimensional unsteady calculations of reactive flows were performed in large domain to investigate the unstable behaviors of cellular premixed flames at low Lewis numbers based on the diffusive-thermal (D-T) model and compressible Navier-Stokes (N-S) equations. The growth rates obtained by the compressible N-S equations were large and the unstable ranges were wide compared with those obtained by the D-T model equations. When the length of computational domain increased, the number of small cells separated from large cells of the cellular flame increased drastically. The stronger unstable behaviors and the larger average burning velocities were observed especially in the numerical results based on the compressible N-S equations. In addition, the fractal dimension obtained by the compressible N-S equations was larger than that by the D-T model equations. Moreover, we confirmed that the radiative heat loss promoted the instability of premixed flames at low Lewis numbers.
Thwe Thwe, A.; Terada, Atsuhiko
no journal, ,
The Japan Atomic Energy Agency is developing simulation technology to predict the behavior of hydrogen combustion, explosion, diffusion, and distribution, in order to support the evaluation of hydrogen safety management technology (disaster mitigation and disaster prevention systems) including not only nuclear reactors but also decommissioning measures and waste management. In this presentation, we will introduce our research activities on hydrogen combustion behavior, which is particularly important from a practical perspective, and compare the results of simulations (STAR-CCM+) with experiments.
Terada, Atsuhiko; Nagaishi, Ryuji; Thwe Thwe, A.; Ito, Tatsuya
no journal, ,
In the Development of Technologies for Containing, Transportation and Storage of Fuel Debris, efforts have been made to develop methods for evaluating the amount of hydrogen (H) generated from lump-shaped debris, and we are now developing methods for powdered debris. As a new attempt, we started computational fluid dynamics (CFD) analysis to visually capture the state of powdered debris over time, such as movement and changes in a mixture of powdered debris and water. As this movement or change, we are currently conducting numerical analysis of two-phase systems, focusing on the growth and behavior of H bubbles in viscoelastic fluids (liquid and gas phases), as well as the structural changes (im-homogenization) over time within the mixture (liquid and solid phases) in relation to H bubbles (H pools) that are predicted to be generated during the radiolysis of water and retained in the mixture. In this report, oxides such as ceria and zirconia were used as debris simulants, and we will introduce the results of an analysis of the time-dependent im-homogenization of oxide powder inside water suspensions, as well as future challenges and developments.
Nagaishi, Ryuji; Terada, Atsuhiko; Ito, Tatsuya; Thwe Thwe, A.
no journal, ,
In the "Development of Technologies for Containing, Transportation and Storage of Fuel Debris", efforts have been made to develop methods for evaluating the amount of hydrogen (H) generated from lump-shaped debris, and we are now developing methods for powdered debris. Using oxides of ceria and zirconia as debris simulants, experiments on H generation accompanying radiolysis of water in suspensions of these powders are being carried out. The viscosity of this suspension depends on the particle size of oxide powder and its concentration in the water, but is not determined solely by these. In this study, we discussed the effects of the charge (pH dependence) and the adsorption of water molecules on the oxide surface on the viscosity properties of the suspension, based on the data from physical property analysis and irradiation experiments.
-air premixed flame inside of a vessel; Numerical simulations by using XiFoamThwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro; Kadowaki, Satoshi
no journal, ,
For the risk reduction on hydrogen combustion and explosion due to hydrogen generated by radiolysis of water inside of high-level radioactive waste vessels, understanding the phenomena and characteristics of hydrogen combustion is necessary, and CFD approaches are of important role. In this numerical simulation, XiFoam solver was modified by adding a new laminar flame speed model deduced from experiment by Katsumi et al. (Nagaoka University of Technology). It reproduced the propagation of H-air premixed flame and we clarified the effects of mesh size (2 mm to 0.625 mm) on wrinkle behavior. The results showed that the mesh size should be equal to or less than 1.0mm to observe the wrinkle behavior of flame in which the flame temperature and radius for each mesh size were almost same at 0.003s to 0.006s, and then the former increased and the later became large due to intrinsic instabilities. The wrinkle flame shape from the simulation was similar to that from the experiment when the mesh size became small.
Terada, Atsuhiko; Thwe Thwe, A.; Hino, Ryutaro; Nagaishi, Ryuji
no journal, ,
To improve a hydrogen safety management technology for long term storage of radioactive wastes such as fuel debris retrieved in the decommissioning of Fukushima Daiichi Nuclear Power Station, a hydrogen behavior simulation code for analyzing its dispersion and combustion has been developing with considering inert gas atmosphere containing steam assumed in a storage container. Trial simulation results of hydrogen dispersion and combustion are reported to understand its steam effects.
-air premixed flameThwe Thwe, A.; Hino, Ryutaro; Terada, Atsuhiko; Kadowaki, Satoshi
no journal, ,
We performed the two- and three-dimensional simulations to investigate the behavior of spherically expanding H-air premixed flame by using XiFoam. The equivalence ratio was set to unity. The ignition started from the center of domain, and the flame expanded spherically and became wrinkle. The obtained flame radius has the agreement with the experimental result.
Terada, Atsuhiko; Nagaishi, Ryuji; Thwe Thwe, A.
no journal, ,
With a storage containing wet radioactive waste such as fuel debris, it is important to reduce the risk of explosion and combustion due to hydrogen generated by radiolysis of water. This study, as a part of the development of simulation technologies for verification of reduction countermeasures and proposal of new countermeasures, conducted simulation of the hydrogen flow analysis and combustion behavior. It was found that the hydrogen safety risk reduction effects and issues affected by the natural ventilation and PAR, inert gas conditions in the container and facility.
Thwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro; Nagaishi, Ryuji; Kadowaki, Satoshi
no journal, ,
For long-term safe storage of fuel debris from hydrogen (H) explosion, the development of simulation codes for H behaviors is important. In this simulation, propagation of H-air premixed flames was reproduced by using a public open source code OpenFOAM. Not only default solver but also modified one based on a new laminar flame speed model derived from the experiments at Nagaoka University of Technology were applied. Three-dimensional 1/8 of cubic shaped vessels with two sizes of 0.21m and 0.13m in length were modeled with mesh sizes of 0.625mm and 0.5mm, respectively. H-air mixture with a unity equivalence ratio, temperature 298K and pressure 101,325Pa was used. The flames propagated spherically and the propagation phenomena had good agreement with the experimental results within 0.005s. The wrinkle flame shape appeared in the smaller model was more likely with that observed in the experiments. This is because mesh size has influence on the wrinkle shape formation in the simulations.
Terada, Atsuhiko; Thwe Thwe, A.; Nagaishi, Ryuji
no journal, ,
Under the long-term storage of wet radioactive waste materials, hydrogen (H) is generated by radiolysis of water, and the reduction of the risk of explosion and combustion plays an important role. Since water vapor condenses on the walls of storage vessel, the behavior of H is mainly in H-air-steam (water vapor) atmosphere inside of storage vessel. Therefore, it is important to understand the condensation process of water vapor accurately and grasp the change of H concentration. In this study, CFD simulation method for estimating the amount of condensation of water vapor based on the heat-mass transfer analogy was used when the introduced gas was set to helium (He). An evaluation based on the comparison between the analytical results and the experimental values was carried out. As a result, it was found that the reproducibility of the experimental results was improved by using the condensation model considering the simple liquid film behavior.
Thwe Thwe, A.; Nagaishi, Ryuji; Ito, Tatsuya; Terada, Atsuhiko
no journal, ,
Concerning with the Development of Technologies for Containing, Transportation and Storage of Fuel Debris, we are now developing methods for powdered debris. Research on oxide powders such as uranium dioxide has suggested that if water is adsorbed onto powdered debris and this is irradiated with low-LET gamma-rays, the water on the debris will not only be directly given with the energy of the gamma rays but will also be indirectly transferred with the energy given to the debris. Therefore, in this case, there is a concern that the amount of hydrogen (H) generated by water radiolysis would increase significantly. However, the total energy given to the water in this case has not been evaluated, and no clear evaluation or prediction of the increase in H generation has been made. In this study, we attempted to evaluate the absorbed dose of water adsorbed on oxide powders, as reported, by using a radiation transport analysis (PHITS) developed by JAEA and other organizations.
