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

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

https://doi.org/10.3327/taesj.J11.047

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.

CFD Simulation of trasient phenomena on gas entrainment

Nishimura, Motohiko*; Nonaka, Yoshiharu*; Maekawa, Isamu*

JNC TJ9400 2005-002, 55 Pages, 2005/07

JNC-TJ9400-2005-002.pdf:74.29MB

Sodium cooled FBR reactor in a feasibility study on commercialized fast reactor system has been designed so compact that flow velocity is much higher in the reactor than in the reactors so far. This design requires an evaluation method for gas entrainment from reactor free surface and effective countermeasures. Focusing on the establishment of gas entrainment evaluation method based on CFD, CFD simulation has been carried out with analyses of an existing gas entrainment experiment. In the analyses, four turbulence models such as standard k-e, RNG k-e, non-linear k-e and k-w have been applied and evaluated for their analytical capability in addition to laminar model. Main results have been obtained as follows;1) Laminar model shows faster vortices behavior accompanied with strong flow and pressure fluctuations including random variations than any other turbulence models. RNG model shows the solutions more similar to the laminar model than the other turbulence models.2) k-w, non-linear k-e and Standard k-e models show milder vortices behavior in this order due to excess turbulence viscosity.3) Main characteristics of major turbulence models have been obtained when they are applied to gas entrainment analysis.

Analytical study on thermal-hydraulics of gas cooled fast reactors

Ohshima, Hiroyuki; Nishimura, Motohiko*

JNC TN9400 2002-072, 97 Pages, 2002/08

JNC-TN9400-2002-072.pdf:3.85MB

A feasibHity study has been carried out at JNC to construct new design concepts of commercialized fast reactors. This report describes two kinds of numerical investigations related to thermal-hydraulics of gas-cooled fast reactors of which design studies are being performed as part of the feasibility study. A series of thermal hydraulic analyses was carried out using multi-dimensional analysis program AQUA in order to confirm the heat removal capability of the helium-gas-cooled fast reactor with a coated-particle-type fuel assembly design under a rated power operation, a low power/low flow and an accident conditions. The calculation results indicates that the lateral gas flow which is indispensable for normal heat removal in the fuel particle region is kept under each calculation condition and the maximum temperature does not exceed the tentative design limitation as far as the inlet surface permeability is uniform. Only in the case of high pressure and very low flow rate, a possibility that local high temperature region exceeding the design limitation appears may not be denied due to upward flow driven by buoyancy force. Improvement on knowledge of gas property functions in high temperature and pressure drop correlations are required for more accurate analysis. Natural circulation decay heat removal characteristics of the CO gas cooled fast reactor are examined using a one-dimensional nuclear-thermal-hydraulics network analysis code, MR-X equipped with correlations for the core thermal-hydraulics of gas cooled fast reactors. Simulation parameters are the shutdown time of steam generators (SGs), restart time of gravitational water feed to the SGs, and flow rate of the feed water. It was predicted that the reactor satisfied limitation of the maximum cladding temperature under the realistic operation condition of SGs with the shutdown time of 30 s and the restart time of 20 min. Moreover, the cladding temperature still satisfied the limitation, even ...

Study on Convective Mixing for Thermal Striping Phenomena -Experimental Analyses on Mixing Process in Parallel Triple-Jet and Comparisons between Numerical Methods.

Kimura, Nobuyuki; ;

CD-ROM,No.4, (4), 0 Pages, 2001/00

None

Study on convective mixing for thermal striping phenomena; Thermal-Hydraulic analyses on mixing process in parallel triple-jet and comparisons between numerical methods

Kimura, Nobuyuki; Miyake, Yasuhiro*; ; Kamide, Hideki

JNC TN9400 2000-099, 71 Pages, 2000/03

JNC-TN9400-2000-099.pdf:3.79MB

A quantitative evaluation on thermal striping, in which temperature fluctuation due to convective mixing among jets imposes thermal fatigue on structural components, js of importance for reactor safety. ln the present study, a water experiment was performed on parallel triple-jet: cold jet at the center and hot jets in both sides. Three kinds of numerical analyses based on the finite difference method were carried out to compare the similarity with the experiment by use of respective different handling of turbulence such as a k- two equation turbulence model (k- Mode1), a low Reynolds number stress and heat flux equation model (LRSFM) and a direct numerical simulation (DNS). ln the experiment, the jets were mainly mixed due to the coherent oscillation. The numerical result using k- Model could not reproduce the coherent oscillating motion of jets due to rolling-up fluid. The oscillations of the jets predicted by LRSFM and DNS were in good agreements with the experiment. The comparison between the coherent and random components in experimental temperature fluctuation obtained by using the phase-averaging shows that k- Model and LRSFM overestimated the random component and the coherent component respectively. The ratios of coherent to random components in total temperature fluctuation obtained from DNS were in good agreements with the experiment. The numerical analysis using DNS can reproduce the coherent oscillation of the jets and the coherent / random components in temperature fluctuation. The analysis using LRSFM could simulate the mixing process of the jets with the low frequency.

None

Muramatsu, Toshiharu;

Nihon Genshiryoku Gakkai-Shi, 42(3), p.1242 - 1259, 2000/00

None

Numerical Study on Mixing of Oscillating Quasi-planar Jets with Low Reynolds number Turbulent Stress and Flux Equation Models

; Tokuhiro, Akira; Kimura, Nobuyuki;

Nuclear Engineering and Design, 202(1), p.77 - 95, 2000/00

https://doi.org/10.1016/S0029-5493(00)00287-9

None

Post processing system for multi-dimensionaI thermal-hydraulic analyses

Miyake, Yasuhiro*; *; ; Kimura, Nobuyuki

JNC TN9400 2000-016, 40 Pages, 1999/12

JNC-TN9400-2000-016.pdf:3.71MB

ln the conventional visualization system for the computational results, only Japanese (Nihongo) Line Printer (NLP) was available to print two dimensional cross sectional plots of vector and scalar fields. To evaluate the phenomena, an analyst had to print many plots on the NLP. This task makes difficult to check the computational results immediately after the calculation. Recently, as the visualization tools, we introduced Micro AVS and Field View which are utilized widely in the scientific and the industrial fields. ln order to show the numerical results on the visualization software, we constructed a post processing system which convert the results of the numerical code to "lntermediate files" which can be read by the visualization tools. As using this system, the examination of the numerical results can be executed on the display of the personal computer. Furthermore, the persuasive report and paper with high quality can be produced due to the color printing. As for the transient calculation, the change of the phenomena can be visually evaluated by using the animation function.

Development of flow velocity measurement techniques in visible images; Improvement of particle image velocimetry techniques on image process

Kimura, Nobuyuki; Miyake, Yasuhiro*; ; Kamide, Hideki; Hishida, Koichi

JNC TN9400 99-078, 44 Pages, 1999/10

JNC-TN9400-99-078.pdf:1.91MB

Noise reduction system was developed to improve applicability of Particle Image Velocimetly (PIV) to complicated configure bounded flows. For fast reactor safety and thermal hydraulic studies, experiments are performed in scale models which usually have rather complicated geometry and structures such as fuel subassemblies, heat exchangers, etc. The structures and stuck dusts on the view window of the models obscure the particle image. Thus the image cxcept the moving particles can be regarded as a noise. In the present study, two noise reduction techniques are proposed. The one is the Time-averaged Light Intensity Subtraction method (TIS) which subtracts the the-averaged light intensity of each pixel in the sequential images from the each corresponding pixel. The other one is the Minimum Light Intensity Subtraction method (MIS) which subtracts the minimum light intensity of each pixel in the sequential images from the each corresponding pixel. Both methods are examined on their capabilities of noise reduction. As for the original "bench mark" image, the image made from Large Eddy Simulation was used. To the bench mark image, noises are added which are referred as sample images, Both methods reduce the rate of vector with the error of more than one pixel from 90% to less than 5%. Also, more than 50% of the vectors have the error of less than 0.2 pixel. The analysis of uncertainty shows that these methods enhances the accuracy of vector measurement 312 times if the image with noise were processed, and the TIS method has 1.1 2.1 times accuracy compared to the TIS. Thus the present noise reduction methods are quite efficient to enhance the accuracy of flow velocity fields measured with particle images including structures and deposits on the view window.

Flow visualization on a natural circulation inter-wrapper row; Experimental and numerical results under a geometric condition of button type spacer pads

Yasuda, Akihiro; ; Hayashi, Kenji; ; Kamide, Hideki; Hishida, Koichi

JNC TN9400 99-072, 70 Pages, 1999/04

JNC-TN9400-99-072.pdf:7.29MB

Investigations on the inter-wrapper flow (IWF) in a liquid metal cooled fast breeder reactor core have been carried out. The IWF is a natural circulation flow between wrapper tubes in the core barrel where cold fluid is coming from a direct heat exchanger (DHX) in the upper plenum. It was shown by the sodium experiment using 7-subassembly core model that the IWF can cool the subassemblies. To clarify thermal-hydraulic characteristics of the IWF in the core, the water experiment was performed using the flow visualization technique. The test rig for IWF (TRIF) has the core simulating the fuel subassemblies and radial reflectors. The subassemblies are constructed featuring transparent heater to enable both Joule heating and flow visualization. The transparent heater was made of glass with thin conductor film coating of tin oxide, and the glass heater was embedded on the wall of modeled wrapper tube made of acrylic plexiglass. In the present experiment, influences of peripheral geometric parameters such as flow holes of core formers on the thermal-hydraulic field were investigated with the button type spacer pads of the wrapper tube. Through the water tests, flow patterns of the IWF were revealed and velocity fields were quantitatively measured with a particle image velocimetry (PIV). Also, no substantial influence of peripheral geometry was found on the temperature field of the IWF, as far as the button type spacer pad was applied. Numerical simulation was applied to the experimental analysis of IWF by using multi-dimensional code with porous body model. The numerical results reproduced the flow patterns within TRIF and agreed well to experimental temperature distributions, showing capability of predicting IWF with porous body model.