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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：0.01(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：1 Percentile：20.8(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：0 Percentile：0.01(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：4 Percentile：30.26(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：5 Percentile：36.79(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：0.01(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.; Hino, Ryutaro; Kadowaki, Satoshi*
no journal, ,
-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.
Thwe Thwe, A.; Terada, Atsuhiko; Hino, Ryutaro
no journal, ,
Since hydrogen is continuously generated and releases inside of high-level radioactive waste vessels, the awareness must be taken on the risk of hydrogen combustion and explosion. In hydrogen safety management, besides the experimental investigations, CFD approaches in predictions of flame propagation phenomena are of important role. As an approach to analysis of hydrogen combustion inside of vessels, we used the open source software, OpenFOAM and performed the simulations for propagation of H-air premixed flame. A new laminar flame speed model deduced from H-air explosion experiments by T. Katsumi et al. [Nagaoka Univ. Tech.] was implemented in the XiFoam solver and reproduced the propagation of H-air flame. Flame radius obtained by simulation agreed with the experimental results within 0.005s. Wrinkle flame formation was observed when the flame propagated outwardly as in 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.
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.; 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.
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.
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; Furukawahara, Ryo; Ito, Tatsuya
no journal, ,
For hydrogen safety in the storage of wet radioactive materials under the decommissioning process of nuclear power stations including 1F, not only the experimental but also analytical studies on hydrogen generation from water radiolysis are important. Until now, we have conducted those closely related to the 1F decommissioning by the experiments for the effects of seawater salts, liquid depth and temperature, and by the calculations for the effect of liquid flow. In this study, we calculated the liquid-depth effect on hydrogen generation by using a one-dimensional reaction model in the vertical direction based on the open-source software CANTERA. The division of reaction cells was found to be important. As the division number increased, the slope of the increase in the amount of hydrogen generated with respect to the absorbed dose became smaller, approaching to the experimental results. However, when the number exceeded 100, the calculation results were underestimated under the current experimental and calculation conditions.
