• Nuclear Engineering and Technology
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• v.44 no.3
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• pp.283-296
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• 2012

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body in the center is studied. Three different Prandtl numbers (0.01, 0.7 and 7) are considered for the investigation of the effect of the Prandtl number on natural convection. Adiabatic boundary conditions are employed for the side walls. A two-dimensional solution for unsteady natural convection is obtained, using an accurate and efficient Chebyshev spectral methodology for different Rayleigh numbers varying over the range of $10_3$ to $10_6$. It had been experimentally reported that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. It was found that when Ra=$10_6$, only the case for intermediate Pr (=0.7) reached a non-changing steady state and the low and high Pr number cases (Pr=0.01 and 7) showed a periodically oscillatory fashion hydrodynamically and thermally. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers are presented to show the overall heat transfer characteristics in the system. Further, the isotherms and streamline distributions are presented in detail to compare the physics related to their thermal behavior.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.9
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• pp.722-728
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• 2006

Natural convection flows in a cubical air-filled slanted cavity that has one pair of opposing faces isothermal at different temperatures, $T_h\;and\;T_c$, respectively, the remaining four faces having a linear variation from $T_c\;toT_h$ are numerically simulated by a solution code (PowerCFD) using unstructured cell-centered method. Special attention is paid to three-dimensional flow and thermal characteristics according to a new orientation (diamond type) for the cubical-cavity benchmark problem in natural convection. Comparisons of the average Nusselt number at the cold face are made with experimental benchmark solutions found in the literature. It is found that the code is capable of producing accurately the nature of the laminar convection in a cubical air-filled slanted cavity with differentially heated walls.

• Journal of computational fluids engineering
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• v.16 no.3
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• pp.29-36
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• 2011

The natural convection in a horizontal enclosure heated from the bottom wall, cooled at the top wall, and having a square adiabatic body at its centered area was studied. Three different Prandtl numbers (0.01, 0.7 and 7) were considered for an effect of the Prandtl number on natural convection. A two-dimensional solution for unsteady natural convection was obtained, using Chebyshev spectral methodology for different Rayleigh numbers varying over the range of $10^4$ to $10^6$. It had been experimentally and numerically reported [1,2] that the heat transfer mode becomes oscillatory when Pr is out of a specific Pr band beyond the critical Ra. In this study, we reproduced this phenomenon numerically. The variation of time- and surface-averaged Nusselt numbers on the hot and cold walls for different Rayleigh numbers and Prandtl numbers was presented to show the overall heat transfer characteristics in the system. And also, the isotherms and streamline distributions were presented in detail to compare the physics related to their thermal behavior.

• Nuclear Engineering and Technology
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• v.47 no.5
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• pp.534-545
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• 2015

In the very high temperature reactor (VHTR), which is a next generation nuclear reactor system, ceramics are used as a fuel coating material and graphite is used as a core structural material. Even if a depressurization accident occurs and the reactor power goes up instantly, the temperature of the core will change only slowly. This is because the thermal capacity of the core is so high. Therefore, the VHTR system can passively remove the decay heat of the core by natural convection and radiation from the surface of the reactor pressure vessel. The objectives of this study are to investigate the heat transfer characteristics of natural convection of a one-side heated vertical channel with inserted porous materials of high porosity and also to develop the passive cooling system for the VHTR. An experiment was carried out using a one-side heated vertical rectangular channel. To obtain the heat transfer and fluid flow characteristics of the vertical channel with inserted porous material, we have also carried out a numerical analysis using a commercial Computational Fluid Dynamics (CFD) code. This paper describes the thermal performances of the one-side heated vertical rectangular channel with an inserted copper wire of high porosity.

• Journal of Korean Society of Water and Wastewater
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• v.18 no.2
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• pp.165-173
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• 2004

Behavior of hazardous organic compounds including bisphenol A, phtalic acid, and phosphoric acid in low pressure nanofiltration process were investigated. In the case of NTR729HF, rejection of all target organic compounds except 2-H-Benzothiazol and 2-isopropyl phenol was more than 90%. The lowest rejection for 2-H-Benzothiazol was observed in another membranes. The UTC60 and UTC20 showed similar rejection characteristics of hazardous organic compounds. Although the rejection of Bisphenol A, n-buthyl benzenesulfoneamide, N-ethyl-p-toluensulfonamide, 2-H-benzothiazol, p-t-butylphenol and 2-isopropyl phenol was less than 30%, the rejection of tributyl phosphate, triethyl phosphate, camphor, 2,2,4 trimethyl 1,3 pentandiol and diphenyl amine was more than 90% in the case of UTC60 and UTC20. The rejection characteristics of various hazardous organic compounds were converted into one parameter Ks, which was proposed in the diffusion-convection model. The Ks of hazardous organic compounds were discussed by comparing with their solute size represented by Stokes radius. The diffusion convection model considering Ks was successful to interpret rejection characteristics of hazardous organic compounds by low-pressure nanofiltration membranes.

• The KSFM Journal of Fluid Machinery
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• v.19 no.4
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• pp.19-28
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• 2016

Objective of this study is to compare the inherent characteristics of natural convection flow inside the canister of spent fuel dry storage system with different backfill gases by utilizing computational fluid dynamics (CFD) code. Four working fluids were selected for comparison study. Helium currently used backfill gas for canister, air, nitrogen, and argon are frequently used as coolant in many heat transfer applications. The results indicate that helium has very distinct conductive behavior and show very weak natural convective flow compared to the others. Argon showed the strongest natural convective flow but also the worst coolability. Air and nitrogen showed similar characteristics to each other. However, due to difference in Prandtl number, nitrogen showed more effective natural convective flow. These results suggest that experimental validation for the nitrogen is needed to investigate the potential coolability other than currently commercially used helium.

• Atmosphere
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• v.26 no.2
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• pp.321-336
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• 2016

Damages caused by torrential rain occur every year in Korea and summer time convection can cause strong thunderstorms to develop which bring dangerous weather such as torrential rain, gusts, and flash flooding. On 6 August 2013 a sudden torrential rain concentrated over the inland of Southern Korean Peninsula occurred. This was an event characterized as a mesoscale multicellular convection. The purpose of this study is to analyze the conditions of the multicellular convection and the synoptic and mesoscale nature of the system development. To this end, dynamical and thermodynamic analyses of surface and upper-level weather charts, satellite images, soundings, reanalysis data and WRF model simulations are performed. At the beginning stage there was a cool, dry air intrusion in the upper-level of the Korean Peninsula, and a warm humid air flow from the southwest in the lower-level creating atmospheric instability. This produced a single cell cumulonimbus cloud in the vicinity of Baengnyeongdo, and due to baroclinic instability, shear and cyclonic vorticity the cloud further developed into a multicellular convection. The cloud system moved southeast towards Seoul metropolitan area accompanied by lightning, heavy precipitation and strong wind gusts. In addition, atmospheric instability due to daytime insolation caused new convective cells to develop in the upstream part of the Sobaek Mountain which merged with existing multicellular convection creating a larger system. This case was unusual because the system was affected little by the upper-level jet stream which is typical in Korea. The development and propagation of the multicellular convection showed strong mesoscale characteristics and was not governed by large synoptic-scale dynamics. In particular, the system moved southeast crossing the Peninsula diagonally from northwest to southeast and did not follow the upper-level westerly pattern. The analysis result shows that the movement of the system can be determined by the vertical wind shear.

• Journal of the Korean Magnetics Society
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• v.12 no.2
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• pp.73-79
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• 2002

Compared with Newtonial fluids, magnetic fluids have effects on magnetic force. In this study, the purpose is to research the heat transfer characteristic of magnetic fluids which have metalic and fluid characteristics as the external pipe is being cooled and internal pipe is heated. This study found the experimental results from the study of the variety of natural convection for magnetic fluids and the characteristics of the heat transfer by using numerical analysis according to the strength and direction of the magnetic fields from being imposed from the outside. Natural convection of magnetic fluids was controlled by the impressed magnetic fields, and the result of mean nusselt number was calculated. If the impressed magnetic field is in the direction of gravity or the strength of impressed magnetic field is more than -14 mT in the opposite direction, the heat transfer is more than that without the impressed magnetic field. If the strength of impressed magnetic field is less than -14 mT in the opposite direction, it is smaller than that without the impressed magnetic field. Especially, when the strength of the magnetic field is -14 mT, the heat transfer was at the minimum.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.11
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• pp.923-930
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• 2002

Cooling characteristics of a parallel channel with protruding heat sources using convection and conduction heat transfer are studied numerically. A two-dimensional model has been developed for numerical prediction of transient, compressible, viscous, laminar flow, and conjugate heat transfer between parallel plates with uniform block heat sources. The finite volume method is used to solve the problem. The assembly consists of two channels formed by two covers and one printed circuit board which has three uniform heat source blocks. Six different cooling methods are considered to find out the most efficient cooling method in a given geometry and heat sources. The velocity and temperature fields of cooling medium, the temperature distribution along the block surface, and the maximum temperature in each block are obtained. The results are compared to examine the cooling characteristics of the different cooling methods.

• Transactions of the Korean Society of Mechanical Engineers
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• v.5 no.3
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• pp.238-243
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• 1981

An empirical formula for the mass flow rate in natural convection flume above a spherical heated body which is submerged in a finite fluid contained in a tank was determined. The ratio of depth of submergence to diameter of the sphere for which the sphere sensed as infinite medium was found to be bigger than 7. A dimensionless heat transfer correlation for the natural convection from a sphere was determined for Gr.Prq etween 7x10$\^$7/ and 2x10$\^$8/ also.