• Journal of Korea Water Resources Association
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• v.53 no.1
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• pp.29-43
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• 2020

The purpose of this study is to propose the design criteria and detailed design model by reviewing the issues related to geometry, formation process, destruction process, hydraulic function, restoration and ecological function of the step-pool from the existing research results, to apply the step-pool sequences to river restoration. Based on the analysis and review results, the design criteria for the structure and size of the step-pool are presented as ratio of the step spacing and the channel width, ratio of the unit step slope and channel slope, and ratio of step height and the particle size. To ensure structural stability of the step, stability analysis method of overturning based on the keystone theory was proposed as a design criterion. As a detailed design concept, a layout model was proposed by applying the imbricating structure of keystones and arch stones to the planar, longitudinal and transverse configurations of the step-pool.

• Nuclear Engineering and Technology
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• v.48 no.2
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• pp.457-469
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• 2016

During the late phase of severe accidents of light water reactors, a porous debris bed is expected to develop on the bottom of the flooded reactor cavity after breakup of the melt in water. The geometrical configuration, i.e., internal and external characteristics, of the debris bed is significant for the adequate assessment of the coolability of the relocated corium. The internal structure of a debris bed was investigated experimentally using the DAVINCI (Debris bed research Apparatus for Validation of the bubble-Induced Natural Convection effect Issue) test facility. Particle sedimentation under the influence of a two-phase natural convection flow due to the decay heat in the debris bed was simulated by dropping various sizes of particles into a water vessel with air bubble injection from the bottom. Settled particles were collected and sieved to obtain the particle mass, size distribution in the radial and axial positions, and the bed porosity and permeability. The experimental results showed that the center part of the particle bed tended to have larger particles than the peripheral area. For the axial distribution, the lower layer had a higher fraction of larger particles. As the sedimentation progressed, the size distribution in the upper layers can shift to larger sizes because of the higher vapor generation rate and stronger flow intensity.

• Water for future
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• v.27 no.1
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• pp.123-131
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• 1994

A plunge pool is often employed as an energy-dissipating device at the end of a spillway or a pipe culvert. A jet from spillways or pipes frequently generates a scour hole which threatens the stability of the hydraulic structure. Existing scour prediction formulas of plunge pool of spillways or pipe culverts give a wide range of scour depths, and it is, therefore, difficult to accurately predict those scour depths. In this study, a new experimental method and new scour prediction formulas under submerged circular jet for large bed materials with shallow tailwater depths were developed. A major variale, which was not used in previous scour prediction equations, was the ratio of jet size to bed material size. In this study, jet momentum acting on a bed particle and jet diffustion theory were employed to derive scour prediction formulas. Four theoretical formulas were suggested for the two regions of jet diffusion, i.e., the region of flow establishment and the region of established flow. The semi-theoretically developed scour prediction formulas showed close agreement with laboratory experiments performed on a movable bed made of large spherical particles.

• Journal of the Korean Solar Energy Society
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• v.35 no.2
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• pp.53-62
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• 2015

Boiling heat transfer characteristic is very important in the various industries such as solar thermal system, power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors. Therefore, in this study, boiling heat transfer characteristics such as critical heat flux (CHF) and heat transfer coefficient under the pool boiling state were tested using graphene nanofluids. Graphene used in this study, which have the same thermal conductivity but with different sizes. The experimental results showed that the highest the CHF and boiling heat transfer coefficient increase ratio for graphene nanofluids was at the 0.01 vol.%. At the present juncture, the CHF and boiling heat transfer coefficient increase ratio of the small-sized graphene nanofluids was higher than the large-sized graphene nanofluids.

• Journal of Welding and Joining
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• v.15 no.5
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• pp.134-143
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• 1997

The formation of thick alloyed layer with high Si content have been investigated on the surface of Al alloy (A5083) plate by PTA process with Si powder. Hardening characteristics and wear resistance of alloyed layer was examined in relation to the microstructure of alloyed layer. Thick hardened layer in mm-order thickness on the surface of A5083 plate can be formed by PTA process with wide range of process condition by using Si powder as alloying element because of eutectic reaction of Al-Si binary alloy. High temperature and rapid solidification rate of molten pool, which are features of PTA process, enable the formation of high Si content alloyed layer with uniform distribution of fine primary Si paticle. High plasma arc current was beneficial to make the alloyed layer with smooth surface appearance in wide range of powder feeding rate, because enough volume of molten pool was necessary make alloyed layer. Uniform dispersion of fine primary Si particle with about 30${\mu}{\textrm}{m}$ in particle size can be obtained in layer with Si content ranging from 30 to 50 mass %. Hardness of alloyed layer increased with increasing Si content, but increasing rate of hardness differed with macrostructure of alloyed layer. Wear resistance of alloyed layer depended on $V_{si}$(volume fraction of primary Si) and was remarkably improved to two times of base metal at 20-30% $V_{si}$ without cracking, but no more improvement was obtained at larger $V_{si}$.

• Journal of Powder Materials
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• v.27 no.2
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• pp.103-110
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• 2020

Aluminum (Al) - based powders have attracted attention as key materials for 3D printing because of their excellent specific mechanical strength, formability, and durability. Although many studies on the fabrication of 3D-printed Al-based alloys have been reported, the influence of the size of raw powder materials on the bulk samples processed by selective laser melting (SLM) has not been fully investigated. In this study, AlSi10Mg powders of 65 ㎛ in average particle size, prepared by a gas atomizing process, are additively manufactured by using an SLM process. AlSi10Mg powders of 45 ㎛ average size are also fabricated into bulk samples in order to compare their properties. The processing parameters of laser power and scan speed are optimized to achieve densified AlSi10Mg alloys. The Vickers hardness value of the bulk sample prepared from 45 ㎛-sized powders is somewhat higher than that of the 65 ㎛m-sized powder. Such differences in hardness are analyzed because the reduction in melt pool size stems from the rapid melting and solidification of small powders, compared to those of coarse powders, during the SLM process. These results show that the size of the powder should be considered in order to achieve optimization of the SLM process.

• Nuclear Engineering and Technology
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• v.45 no.3
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• pp.323-334
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• 2013

During a hypothetical core-disruptive accident (CDA) in a sodium-cooled fast reactor (SFR), degraded core materials can form roughly conically-shaped debris beds over the core-support structure and/or in the lower inlet plenum of the reactor vessel from rapid quenching and fragmentation of the core material pool. However, coolant boiling may ultimately lead to leveling of the debris bed, which is crucial to the relocation of the molten core and heat-removal capability of the debris bed. To clarify the mechanisms underlying this self-leveling behavior, a large number of experiments were performed within a variety of conditions in recent years, under the constructive collaboration between the Japan Atomic Energy Agency (JAEA) and Kyushu University (Japan). The present contribution synthesizes and gives detailed comparative analyses of those experiments. Effects of various experimental parameters that may have potential influence on the leveling process, such as boiling mode, particle size, particle density, particle shape, bubbling rate, water depth and column geometry, were investigated, thus giving a large palette of favorable data for the better understanding of CDAs, and improved verifications of computer models developed in advanced fast reactor safety analysis codes.

• Nuclear Engineering and Technology
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• v.52 no.9
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• pp.1939-1944
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• 2020

Pool boiling heat transfer of a copper microporous coating was experimentally studied in borated water with a concentration of boric acid from 0.0 to 5.0 vol percent (vol%) to determine the effect of boric acid on boiling heat transfer in water. A high-temperature, thermally conductive microporous coating (HTCMC) was created by sintering copper powder with an average particle size of 67 ㎛ onto a 1 cm × 1 cm plain copper surface with a coating thickness of ~300 ㎛ within a furnace in a vacuum environment. The tests showed that the nucleate boiling heat transfer coefficient (NBHT) of HTCMC became slightly less enhanced as the concentration of boric acid increased but the NBHT coefficient values were still significantly higher than those of the plain surface. The critical heat flux (CHF) values from 0 to 1.0 vol% were maintained at ~2,000 kW/㎡, and then, they gradually decreased down to ~1,700 kW/㎡ as the concentration increased further to 5.0 vol%. It is believed that the micro-scale pores of the HTCMC were partially blocked by the high boric acid concentration during the nucleate boiling such that the small bubbles were not effectively created using the HTCMC reentrant cavities as the boric acid concentration increased.

• Journal of Korean Society of Forest Science
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• v.100 no.2
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• pp.202-211
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• 2011

The geometric characteristics of step-pool structures and how they are influenced by channel characteristics were investigated in a steep mountain stream in the Experimental Forests of Kangwon National University in Chuncheon, Gangwon-do. Average values of steps for the study reaches were as follows: step spacing, 4.69 m; step height, 0.47 m; step drop, 0.71 m; step-forming particle sizes, 0.68 m; number, 21steps/ 100 m; the ratio of step spacing to channel width, 0.5; and step steepness, 0.13. Relationships between spacing and height of steps and channel gradient showed a negative- and positive correlation, respectively, whereas all geometric variables of steps manifested poor correlation with channel width. Therefore, step steepness, expressed as the ratio of step height to step spacing, increased as channel gradient increased. The ratio of step steepness to channel gradient representing the criterion of maximum flow resistance was 1.2, indicating the channel bed's stable condition. In particular, the relationship between the ratio of step drop to step height and channel gradient showed a significant negative correlation, suggesting the influence of step-pool geometry in trapping sediment and providing an aquatic habitat. Positive correlations also exist between spacing and drop of steps and step particles. Our findings suggest that the dynamics of step-pool structures may strongly control physical and ecological environments in steep mountain streams, so understanding them is essential for stream management.

• Journal of the Korean Society of Safety
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• v.27 no.6
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• pp.59-63
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• 2012

The use of dry chemical powder has been increased as it can be stored for a long period and sustain in stable condition compared to gas or liquid phase extinguishing agents. A new type of dry chemical powder using Zeolite was produced in the research. Chemical powder was adsorbed into Zeolite 13X, a porous material appearing negative catalytic effect, to create extinguishing powder obtaining core shell structure and measured physical properties and run a small scale fire extinguishment. SEM, XRD, TA analysis was also executed, and extinguishing characteristics were measured by fire extinguishing experiment on oil pool fire. The experiment showed that the average particle size of Zeolite 13X was equivalent, indicating about $3{\pm}1{\mu}m$ and thermal analysis result illustrated that Zeolite 13X showed exothermic reaction peaks at $900^{\circ}C$ due to solid-state transformation. Extinguishing characteristics on oil fire of $NaHCO_3$/Zeolite 13X and $NH_4H_2PO_4$/Zeolite were improved, influenced by adsorbed extinguishing powders on Zeolite 13X and Zeolite 13X that contains high phase transition temperature.