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Katsumi, Toshiyuki*; Zemba, Atsushi*; Kogishi, Makoto*; Iwanaga, Kohei*; Abe, Satoshi; Di
vart, P.*; Chaumeix, N.*; Kadowaki, Satoshi*
Journal of Thermal Science and Technology (Internet), 20(1), p.25-00103_1 - 25-00103_13, 2025/06
Times Cited Count：0 Percentile：0.00(Thermodynamics)In the serious accident at Fukushima Daiichi Nuclear Power Station, the presence of steam together with flammable organic compounds affected the hydrogen explosion. To investigate the effects of addition of inert gas, i.e. steam or nitrogen, on the explosion characteristics, we conducted the experiments of spherically expanding hydrogen-methane-air premixed flames in closed combustion vessels. Two types of vessels were used, and expanding flames were caught by Schlieren method. The flame propagation velocity depending on the flame radius was obtained by analyzing Schlieren images. When the flame radius was sufficiently small, smooth surface was found. The addition of inert gas to hydrogen-methane-air premixtures caused the decrease of propagation velocity of unstretched flame. When the flame radius was large, on the other hand, cellular surface generated by intrinsic instability was found. In this range, the flame acceleration was confirmed, which was induced by the evolution of cellular surface. We obtained the parameters of flame acceleration model and predicted the flame propagation velocity depending on the flame radius. The increment coefficient normalized by the propagation velocity of unstretched flame became larger at low equivalence ratios, which was due to stronger diffusive-thermal instability. Under the same equivalence ratio, the inert gas addition caused the increase of normalized increment coefficient. This denoted that the inert gas addition promoted the instability of premixed flames, which was due to the reduction of the effective Lewis number. The maximum pressure in a combustion vessel became lower in the case of inert gas addition. Moreover, the maximum pressure of experiments was lower than that of calculations under the adiabatic conditions, because of heat loss during premixed combustion. The obtained results were valuable knowledge to elucidate the hydrogen explosion at Fukushima Daiichi Nuclear Power Station.
Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Takagi, Yasuhiko*; Nakamura, Tomoki*; Hiroi, Takahiro*; Matsuoka, Moe*; et al.
Nature Astronomy (Internet), 5(3), p.246 - 250, 2021/03
Times Cited Count：60 Percentile：95.74(Astronomy & Astrophysics)Here we report observations of Ryugu's subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. However, the strength and shape of the OH feature still suggests that the subsurface material experienced heating above 300 
C, similar to the surface. In contrast, thermophysical modeling indicates that radiative heating does not increase the temperature above 200 C at the estimated excavation depth of 1 m, even if the semimajor axis is reduced to 0.344 au. This supports the hypothesis that primary thermal alteration occurred due to radiogenic and/or impact heating on Ryugu's parent body.
Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Arai, Takehiko*; Nakauchi, Yusuke*; Nakamura, Tomoki*; Matsuoka, Moe*; et al.
Science, 364(6437), p.272 - 275, 2019/04
Times Cited Count：302 Percentile：99.64(Multidisciplinary Sciences)The near-Earth asteroid 162173 Ryugu, the target of Hayabusa2 sample return mission, is believed to be a primitive carbonaceous object. The Near Infrared Spectrometer (NIRS3) on Hayabusa2 acquired reflectance spectra of Ryugu's surface to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micron was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally- and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.
Saigusa, Mikio*; Atsumi, Kohei*; Yamaguchi, Tomoki*; Oda, Yasuhisa; Sakamoto, Keishi
Fusion Engineering and Design, 88(6-8), p.964 - 969, 2013/10
Times Cited Count：2 Percentile：17.16(Nuclear Science & Technology)Atsumi, Kohei*; Yamaguchi, Tomoki*; Nagashima, Koji*; Saigusa, Mikio*; Fukunari, Masafumi*; Oda, Yasuhisa; Sakamoto, Keishi
Plasma and Fusion Research (Internet), 8(Sp.1), p.2405077_1 - 2405077_4, 2013/06
Saigusa, Mikio*; Atsumi, Kohei*; Yamaguchi, Tomoki*; Nagashima, Koji*; Fukunari, Masafumi*; Oda, Yasuhisa; Sakamoto, Keishi
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Yamaguchi, Tomoki*; Atsumi, Kohei*; Nagashima, Koji*; Saigusa, Mikio*; Oda, Yasuhisa; Fukunari, Masafumi*; Sakamoto, Keishi
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no abstracts in English
Oda, Yasuhisa; Atsumi, Kohei*; Yamaguchi, Tomoki*; Nagashima, Koji*; Saigusa, Mikio*; Fukunari, Masafumi*; Sakamoto, Keishi
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no abstracts in English
Atsumi, Kohei*; Sugawara, Shuhei*; Yamaguchi, Tomoki*; Saigusa, Mikio*; Oda, Yasuhisa; Takahashi, Koji; Kajiwara, Ken; Sakamoto, Keishi
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no abstracts in English
Takir, D.*; Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Arai, Takehiko*; Nakauchi, Yusuke*; Nakamura, Tomoki*; et al.
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JAXA spacecraft and sample return mission Hayabusa2 has arrived at the near-Earth asteroid 162173 Ryugu, which is classified a primitive carbonaceous object. Here we report recent results of near-infrared spectrometer (NIRS3) on the Hayabusa2 spacecraft. The observations provide direct measurements of the surface composition of Ryugu and context for the returned samples. NIRS3 has detected a weak and narrow absorption feature centered at 2.72 micrometer across entire observed surface. This absorption feature is attributed to the presence of OH-bearing minerals. The NIRS3 observations also revealed that Ryugu is the darkest object to be observed up-close by a visiting spacecraft. The intensity of the OH feature and low albedo are consistent with thermally-and/or shock-metamorphosed, and/or carbon-rich space-weathered primitive and hydrated carbonaceous chondrites.
Saigusa, Mikio*; Sugawara, Shuhei*; Atsumi, Kohei*; Oda, Yasuhisa; Yamaguchi, Tomoki*; Sakamoto, Keishi
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Sugawara, Shuhei*; Atsumi, Kohei*; Yamaguchi, Tomoki*; Saigusa, Mikio*; Oda, Yasuhisa; Takahashi, Koji; Kajiwara, Ken; Sakamoto, Keishi
no journal, ,
Oda, Yasuhisa; Sugawara, Shuhei*; Atsumi, Kohei*; Yamaguchi, Tomoki*; Saigusa, Mikio*; Sakamoto, Keishi
no journal, ,
no abstracts in English
Atsumi, Kohei*; Yamaguchi, Tomoki*; Nagashima, Koji*; Saigusa, Mikio*; Fukunari, Masafumi*; Oda, Yasuhisa; Sakamoto, Keishi
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