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Journal Articles

Pulsed pressure induced cavitation erosion in mercury narrow channel under flowing conditions

Naoe, Takashi; Kogawa, Hiroyuki; Tanaka, Nobuatsu*; Futakawa, Masatoshi

Advanced Experimental Mechanics, 4, p.17 - 21, 2019/08

We have introduced the following two techniques to mitigate the pressure wave-induced cavitation damage in the mercury target. One is the gas microbubble injection into the flowing mercury, and the other is the double-walled structure with a narrow gap channel at the proton beam entrance portion of the mercury vessel. The latter is expected to mitigate the cavitation damage due to the high-speed liquid flow ($$sim$$ 4 m/s) and the narrow gap boundary (2 mm). To quantitatively investigate the effect of double-walled structure on cavitation damage, cavitation damage tests were conducted by parametrically changing mercury flow velocity and gap width of the channel wall. The results showed that the damage evaluated as a surface roughness was reduced by increasing the flow velocity. By contrast, the effect of gap width on cavitation damage was hardly observed under flowing conditions.

Journal Articles

Evaluation growing and collapsing behaviors of cavitation bubbles under flowing condition

Kawamura, Shunsuke; Naoe, Takashi; Ikeda, Tsubasa*; Tanaka, Nobuatsu*; Futakawa, Masatoshi

Advanced Experimental Mechanics, 4, p.33 - 37, 2019/08

A mercury enclosure vessel made of stainless steel is used as a spallation target in the pulsed spallation neutron source at J-PARC. It is severely damaged by the cavitation induced with pressure waves in association with the pulsed proton beam injection. A double-walled structure with a narrow mercury channel was adopted in the front end of the target vessel to reduce the cavitation damage. It has been experimentally demonstrated that the cavitation damage could be mitigated in the narrow channel but its mechanism has been unclarified yet. In this study, we investigated the cavitation from growing to collapsing through visualizing the spark-induced cavitation bubbles under flow field using a high-speed video camera. Furthermore, we measured the wall vibration due to the cavitation bubble collapse with changing flow velocity parametrically. It was found that the microjet collided perpendicular to the wall in the stagnant flow condition while it collided with an inclined angle from the perpendicular direction, suggesting that the collision pressure on the wall was reduced by flowing.

Journal Articles

Development on laser cutting technique to suppress spatter particles aiming at disposal of radio-active waste

Naoe, Takashi; Teshigawara, Makoto; Futakawa, Masatoshi; Mizutani, Haruki; Muramatsu, Toshiharu; Yamada, Tomonori; Ushitsuka, Yuji*; Tanaka, Nobuatsu*; Yamasaki, Kazuhiko*

Proceedings of 8th International Congress on Laser Advanced Materials Processing (LAMP 2019) (Internet), 5 Pages, 2019/05

Laser cutting is one of the options in the disposal of radio-active waste, such as spallation neutron target vessel in J-PARC, etc. Due to unique characteristic of laser, such as non-contact system, it is more easily to provide remote-controlled system in comparison with conventional one, such as mechanical cutting machine, etc. However, a demerit of laser cutting is the sputter and fume caused by laser cutting, resulting in contamination with radio-active materials its surroundings. Recently it was developed that the spatter suppression technique by controlling laser beam profile in laser welding process. In order to apply this suppression technique to laser cutting, first of all, we attempted to observe the phenomenon at melting area during laser cutting using a high-speed video camera in order to make the physical model. The result showed that the appearance of fume and sputter were independently confirmed in the time evolution.

Journal Articles

Standard for prevention of gas entrainment phenomena in fast reactors, 1; Validations of CFD methods for reproducibilities of gas entrainment phenomena

Ohshima, Hiroyuki; Tanaka, Nobuatsu*; Eguchi, Yuzuru*; Nishimura, Motohiko*; Kunugi, Tomoaki*; Uchibori, Akihiro; Ito, Kei; Sakai, Takaaki

Nippon Genshiryoku Gakkai Wabun Rombunshi, 11(4), p.316 - 328, 2012/12

It is of importance for stable operations of sodium-cooled fast reactors (SFRs) to prevent gas entrainment (GE) phenomena due to free surface vortices. Therefore, the authors have been developing an evaluation method for GE based on computational fluid dynamics (CFD) methods. In this study, we determine the suitable CFD method for GE phenomena from several candidates through some numerical benchmarks. As the results, we obtain the following guideline for the vortex-induced gas entrainment. Free vortex flow around the vortex core can be correctly evaluated by using the appropriate numerical models such as enough mesh resolution, suitable advection solver, suitable turbulence and free surface modeling.

Journal Articles

Cavitation in high-power pulsed spallation neutron sources

Futakawa, Masatoshi; Tanaka, Nobuatsu*

Konsoryu, 24(2), p.138 - 145, 2010/06

High-power pulsed spallation neutron sources are being developed in the world. A mercury target is installed at MLF (Materials and Life science experimental Facility) in J-PARC (Japan Proton Accelerator Research Complex), which will promote innovative science. At the moment the proton beams bombard the mercury target, pressure waves will be generated in mercury by thermally shocked heat deposition. Cavitation will be induced through the pressure wave propagation in mercury and eroded the vessel inner surface contacting with mercury, i.e. pitting damage. The eroded vessel wall is damaged by cyclic fatigue because pulsed proton beams strike the target repeatedly. In the paper, the effects of pitting damage, cyclic fatigue damage and mercury environment on the lifetime of the mercury target, and mitigation techniques for pressure waves and cavitation damage are described.

Journal Articles

Bubble flow simulations in target vessel

Haga, Katsuhiro; Kogawa, Hiroyuki; Wakui, Takashi; Naoe, Takashi; Futakawa, Masatoshi; Yamazaki, Shogo*; Tanaka, Nobuatsu*

Nuclear Instruments and Methods in Physics Research A, 600(1), p.64 - 67, 2009/02

 Times Cited Count:2 Percentile:77.37(Instruments & Instrumentation)

Pitting damage on the target vessel wall caused by the pressure wave is one of the crucial issues to realize a pulsed high-power mercury target for spallation neutron sources. Micro-bubbles injection into mercury is one of prospective technologies to mitigate the pressure waves. As one of the studies for bubbling system design for pitting damage mitigation, preliminary simulation of bubble flow field in the actual target vessel was carried out and the optimal condition of bubble injection was investigated. The simulations were carried out with FLUENT. In two dimensional simulations, the bubbles of 50 $$mu$$m diameter could be carried more than 2000 mm, which corresponds to the length of the target vessel, with small rising height. Then three dimensional simulations were carried out for the case of the bubble diameter of 50 $$mu$$m and 500 $$mu$$m. As results, we could have prospect that bubbles are distributed to the desirable regions when they are injected in proper position according to the bubble diameter.

Journal Articles

Mitigation technologies for damage induced by pressure waves in high-power mercury spallation neutron sources, 2; Bubbling effect to reduce pressure wave

Futakawa, Masatoshi; Kogawa, Hiroyuki; Hasegawa, Shoichi; Naoe, Takashi; Ida, Masato; Haga, Katsuhiro; Wakui, Takashi; Tanaka, Nobuatsu*; Matsumoto, Yoichiro*; Ikeda, Yujiro

Journal of Nuclear Science and Technology, 45(10), p.1041 - 1048, 2008/10

 Times Cited Count:26 Percentile:12.89(Nuclear Science & Technology)

The life time of mercury target will be reduced by the cavitation damage induced by pressure waves. Micro-bubble injection into mercury is one of prospective technologies to mitigate the pressure waves. We have carried out damage tests using mercury loop with an impact generator to evaluate the bubbling effect on the pressure response and cavitation damage. The impulsive pressure was hardly changed but the damage was reduced remarkably by the micro-bubble injection. Numerical simulation was performed to investigate the bubbling effects. It was understandable that micro bubbles suppressed cavitation inception.

JAEA Reports

Description of "Design guideline for gas entrainment prevention using CFD method" (Cooperative research)

Ohshima, Hiroyuki; Sakai, Takaaki; Kamide, Hideki; Kimura, Nobuyuki; Ezure, Toshiki; Uchibori, Akihiro; Ito, Kei; Kunugi, Tomoaki*; Okamoto, Koji*; Tanaka, Nobuatsu*; et al.

JAEA-Research 2008-049, 44 Pages, 2008/06

JAEA-Research-2008-049.pdf:42.3MB

Japan Atomic Energy Agency has conducted a conceptional design study of a sodium-cooled fast reactor in a frame work of the FBR feasibility study. The plant system concept for a commercial step is intended to minimize a vessel diameter to achieve an economical competitiveness. Therefore, the coolant in the vessel has relatively higher velocity and gas entrainment (GE) prevention from a liquid surface in the reactor vessel becomes one of important issues for the thermal-hydraulic design. In order to establish a design criteria for the GE prevention, the GE from vortex dimples at the liquid surface was investigated by a working group. The 1st proposal of "Design Guideline for Gas Entrainment Prevention Using CFD Method" was established based on the knowledge gained from the working group activities. This report introduces each study in the working group to clarify the basis of the design guideline.

Journal Articles

Cavitation damage prediction for spallation target vessels by assessment of acoustic vibration

Futakawa, Masatoshi; Kogawa, Hiroyuki; Hasegawa, Shoichi; Ikeda, Yujiro; Riemer, B.*; Wendel, M.*; Haines, J.*; Bauer, G.*; Naoe, Takashi; Okita, Kohei*; et al.

Journal of Nuclear Materials, 377(1), p.182 - 188, 2008/06

 Times Cited Count:27 Percentile:12.89(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

R&D on pressure-wave mitigation technology for pulsed high-power mercury targets; Bubble flow simulations

Haga, Katsuhiro; Futakawa, Masatoshi; Kogawa, Hiroyuki; Wakui, Takashi; Naoe, Takashi; Yamazaki, Shogo*; Tanaka, Nobuatsu*

PSI-Proceedings 07-01, p.67 - 70, 2008/01

Pressure wave, which is caused by the thermal shock in mercury at the moment the high intense proton beams bombard mercury, is a crucial issue to realize a pulsed high-power mercury target for spallation neutron sources. R&D on pressure wave mitigation technologies is carried out in JSNS. Micro-bubbles injection into mercury is one of prospective technologies to mitigate the pressure waves. The CFD simulation was carried out to investigate the injected bubble distribution in our mercury target that is influenced by bubble size and bubbling location. Also, the micro-bubbles effect was experimentally investigated from the viewpoint of pitting damage in the mercury loop with MIMTM (electro-Magnetic IMpact Testing Machine) and examined numerically from the viewpoint of bubble dynamics. As results, we confirmed that the micro-bubble is very effective to reduce the pressure response and pitting damage.

Journal Articles

Applicability of numerical simulation to gas entrainment phenomena

Ito, Kei; Sakai, Takaaki; Ohshima, Hiroyuki; Tanaka, Nobuatsu*

Proceedings of 5th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS-5), p.398 - 405, 2006/11

Gas entrainment (GE) phenomena in a basic unsteady GE experiments were numerically surveyed to make clear an applicability of numerical simulations to a GE evaluation. To establish realistic evaluation methods, the numerical simulations were conducted on rather coarse mesh partition to reduce computational costs and the GE behaviors were evaluated by GE related parameters. As the results, it was confirmed that the numerical methods could reproduce the velocity fluctuation data and the GE related parameters even with the coarse mesh partition. In addition, the sensitivities of a turbulent modeling and a mesh partition on the simulation accuracy were discussed. These two factors could not lead significant improvements of the GE related parameters for the GE evaluation in this study. These results showed that the proposed numerical methods were appropriate to evaluate the GE phenomena.

Journal Articles

Numerical analysis of unsteady gas entrainment phenomena using dynamic SGS model

Sakai, Masayuki*; Tanaka, Nobuatsu*; Ohshima, Hiroyuki

Proceedings of 5th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS-5), p.301 - 304, 2006/11

The flowing gas or liquid in the industrial machines, it is important that improving in quality. We analyzed about systems of unsteady gas entrainment with the analysis code for turbulence eddy flow, which based on local inter-scale equilibrium assumption (LISEA). And we speculated about adequacy of LES application and affect of mesh division on unsteady gas entrainment phenomenon. In the result, we obtain the following knowledge about unsteady eddy problems as object. (1) LES affected by mesh division more than DNS. So appropriate result needs to give consideration to mesh division. (2) DNS affected lightly by mesh mutation in the purview of this analysis. (3) In the case of using LES, it needs to set proper parameters carefully.

JAEA Reports

Numerical study of gas entrainment phenomena, II

Tanaka, Nobuatsu*; Ohshima, Hiroyuki

JNC-TY9400 2005-011, 41 Pages, 2005/07

JNC-TY9400-2005-011.pdf:2.66MB

In this study, we carried out some numerical benchmark in order to survey effectiveness of some LES methods for the phenomena of gas entrainment. Considering the complicated geometry problems, we chose a collocated variable system combined with the high order scheme proposed by Inagaki. In numerical analysis of the Moriya's experiment, we focused on the effect of Smagorinsky model (SM) and Dynamic Smagorinsky Model (DSM) with LISEA (Local Inter-Scale Equilibrium Assumption of Energy), propsed by Murota, and in numerical analysis of the Okamoto's experiment, we focused on the effectiveness of LISEA.(1) Numerical analysis of Moriya's experimentWe compared the results obtained by the LISEA with those of the SM. In such a cylindrical geometry, the empirical parameter of SM is not known, so we performed two cases of simulations with Cs=0.1 and 0.2. Comparing the velocity profiles, clear difference between SM (Cs=0.1) and LISEA was not observed. In the case of Cs=0.2, the results are worse than the results of the other two cases. The eddy viscosity distributions of SM and LISEA are quite different. While the results by the SM are depends on the definition of empirical parameter, the LISEA do not need any empirical parameters or dumping function. Consequently, the LISEA is superior to the SM in the case of such a complicated problems. (2) Numerical analysis of Okamoto's experimentWe surveyed effects of turbulence models through comparison of numerical results of DNS and LISEA models. From viewpoint of some distinguishing physical quantities, there is little difference between the results of DNS and those of LISEA. Since the reversed flow around the outlet boundaries, which is not observed in the LISEA simulation, was observed in the DNS simulation, the LISEA makes an effect of numerical viscosity and stabilize the numerical solution.

Oral presentation

Evaluation of gas entrainment phenomena from free surface in sodium-cooled fast reactor

Ohshima, Hiroyuki; Sakai, Takaaki; Kamide, Hideki; Kunugi, Tomoaki*; Okamoto, Koji*; Tanaka, Nobuatsu*; Eguchi, Yuzuru*

no journal, , 

no abstracts in English

Oral presentation

Study on gas entrainment in sodium cooled fast reactors, 2; Examination of appropriate numerical method

Ito, Kei; Sakai, Takaaki; Ohshima, Hiroyuki; Tanaka, Nobuatsu*

no journal, , 

Based on numerical simulations, a gas entarainment (GE) evaluation method have been developed to estimate GE occurence in a sodium-cooled fast reactor. In this research, several numerical simulations were conducted on a basic GE experiment and the GE phenomena in the experiments were evaluated by the developing GE evaluation method using the velocity data obtained from simulation results. From the results of the GE evaluation, sensitivities of mesh partition or turbulent modeling on the simulation accuracy were examined.

Oral presentation

Experimental evaluation of effect flowing condition on cavitation bubble growing and collapsing behaviors

Kawamura, Shunsuke; Naoe, Takashi; Ikeda, Tsubasa; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

A target vessel enclosing mercury made of stainless steel is used for the J-PARC spallation neutron source. It is severely damaged by the pressure-wave-induced cavitation with injecting intense proton beam. The front end of the target vessel has a double-walled structure with a narrow channel was adopted to the vessel for expecting to reduce cavitation damage. Effect of cavitation damage mitigation in narrow channel has been experimentally demonstrated. However, damage mitigation mechanism is not clarified yet. As a first step of studies to understand the mechanism of cavitation damage mitigation in narrow channel, growth and collapse behaviors of the spark-induced cavitation bubbles under flow condition were observed by using a high-speed video camera. Furthermore, the wall vibration by cavitation bubble collapse was measured by parametrically changing the flow velocity. The experimental results showed that the ejection angle of the microjet ejected by bubble collapsing leaned towards flowing direction as the flow velocity increases. The wall vibration was reduced with increasing flow velocity.

Oral presentation

Experimental evaluation of characteristics of the microbubble generator using swirl flow

Ikeda, Tsubasa; Kogawa, Hiroyuki; Naoe, Takashi; Kawamura, Shunsuke; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

In a mercury target used for the pulsed spallation neutron source at J-PARC, pressure waves are generated by the rapid thermal expansion of mercury due to the high-intensity pulsed-proton beam bombardment. They induces cavitation, causing severe erosion damage on the mercury enclosure vessel made of stainless steel. Gas microbubbles injection into mercury is one of effective techniques to suppress the pressure. At J-PARC, a swirl-flow bubble-generator has been developed and installed in the mercury target. Increasing the gas void fraction is effective to enhance the suppression effect. In this study, dependencies of the vane angle and reduction rate of the Venturi were parametrically investigated through a water experiment in order to optimize the swirl-flow bubble-generator for decreasing the aspiration pressure without increasing pressure drop. The result showed that the gas aspiration rate of the swirl-flow bubble-generator increased as the reduction rate at the Venturi increased.

Oral presentation

Visualization of cavitation growing and collapsing behaviors in narrow channel

Kawamura, Shunsuke; Naoe, Takashi; Ikeda, Tsubasa; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

A mercury enclosure vessel made of stainless steel is used as a spallation target in the pulsed spallation neutron source at J-PARC. It is severely damaged by the cavitation induced with pressure waves in association with the pulsed proton beam injection. A double-walled structure with a narrow mercury channel was adopted in the front end of the target vessel to reduce the cavitation damage. It has been experimentally demonstrated that the cavitation damage could be mitigated in the narrow channel but its mechanism has been unclarified yet. In this study, we investigated the cavitation from growing to collapsing through visualizing the spark-induced cavitation bubbles under flow field using a high-speed video camera. Furthermore, we measured the wall vibration due to the cavitation bubble collapse with changing flow velocity parametrically. It was found that the microjet collided perpendicular to the wall in the stagnant flow condition while it collided with an inclined angle from the perpendicular direction, suggesting that the collision pressure on the wall was reduced by flowing.

Oral presentation

Optimization of the swirl gas microbubble generator for a liquid mercury target vessel

Ikeda, Tsubasa; Kogawa, Hiroyuki; Naoe, Takashi; Kawamura, Shunsuke; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

In the mercury target of the pulsed spallation neutron source at J-PARC, pressure waves are generated by the high-intensity pulsed-proton beam bombardment, resulting in inducing cavitation. Because the cavitation causes severe erosion damages on the mercury enclosure vessel made of stainless steel, suppressing the pressure waves and the cavitation are crucial issues to realize stable target operations at rated proton beam power of 1 MW. Gas microbubbles injection into flowing mercury is one of the prospective techniques to suppress pressure waves. At the J-PARC, a swirl-flow bubble-generator has been developed to generate microbubbles and installed in the mercury target. In order to improve the performance of the pressure wave suppression by increasing the amount of microbubbles, effects of the vane angle and throttling ratio of the Venturi on the amount of microbubbles were parametrically investigated through water experiments. The experimental results showed that the amount of the microbubbles was increased with decreasing the throttling ratio of the Venturi.

Oral presentation

Flow effect on cavitation bubble behaviors in narrow channel

Kawamura, Shunsuke; Naoe, Takashi; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

In the mercury target for the pulsed spallation neutron source at J-PARC, cavitation damage at the beam window of the mercury target vessel is a key factor to decide lifetime of target because the damage degrade the vessel structural integrity. A double-walled structure with a narrow channel was adopted to the vessel for expecting to reduce cavitation damage. In this study, the cavitation bubble behaviors of the growth and collapse under water flow field were investigated to determine the effective factor for mitigating cavitation damage in narrow channel. We measured the equivalent diameter and wall vibration due to the cavitation bubble collapse with parametrically changing flow velocity. It was found that the maximum equivalent diameter of the cavitation bubble and the response vibrational acceleration of the wall are decreased with the increasing velocity. As the results, it was found that cavitation bubble collapse pressure was affected by flowing condition in the narrow channel.

Oral presentation

Effect of wall boundary on growing and collapsing behaviors of cavitation bubble

Kawamura, Shunsuke; Naoe, Takashi; Tanaka, Nobuatsu*; Futakawa, Masatoshi

no journal, , 

A mercury target for spallation neutron source has been in operation at the J-PARC. When the high-intense proton beams hits the mercury to produce spallation neutrons, pressure waves are generated due to the abrupt heat deposition of mercury. Mercury enclosure vessel made of stainless steel with a thin wall thickness of 3 mm is severely damaged by pressure wave-induced cavitation erosion. Recently a double-walled structure with narrow gap channel at the front part of the vessel has been developed to mitigate the cavitation damage. In this study, we observed experimentally the growth and collapse behavior of cavitation bubbles in the narrow channel by using a technique of spark discharge in water and a high-speed video camera. Furthermore, the effects of flow velocity and gap width on collapsing pressure were investigated with focusing on the ratio of the projection radius to the gap width. The relationship between the narrow gap and the collapsing pressure will be discussed.

21 (Records 1-20 displayed on this page)