• 한국수소및신에너지학회논문집
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• 제27권1호
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• pp.42-48
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• 2016

This study presents an experiment investigation on natural convection heat transfer of air-cooling Proton exchange membrane fuel cells (PEMFCs) in a enclosure system for unmanned aerial vehicles (UAVs). Considered are replacing fuel cell stack with Aluminum block for heat generating inside a enclosure chamber. The volume ratio of fuel cell stack and chamber for simulation to the actual size of aerial vehicle is 1 to 15. The parameters considered for experimental study are the environmental temperature range from $25^{\circ}C$ to $-60^{\circ}C$ and the block heat input of 10 W, 20 W and 30 W. Effect of the thermal conductivity of the block and power level on heat transfer in the chamber are investigated. Experimental results illustrate the temperature rise at various locations inside the chamber as dependent upon heat input of fuel cell stack and environmental temperature. From the results, dimensionless correlation in natural convection was proposed with Nusselt number and Rayleigh number for designing air-cooling PEMFC powered high altitude long endurance (HALE) UAV.

• 대한기계학회논문집
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• 제11권2호
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• pp.279-286
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• 1987

본 논문에서는 수평관 내부의 체적평균온도를 가진유체의 난류강제대류에서 열전달이 일어나는 경우에 대하여 핀이 없는 수평전도관, 하향 핀이 부착된 수평전도 관 그리고 상향 핀이 부착된 수평전도관의 열전달 특성을 수치계산으로 연구하고 Mach -Zehnder 간섭계를 이용하여 실험하였고 수치해석과 실험결과를 비교하여 타당성을 확 인하였다.

• 한국전산유체공학회지
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• 제18권2호
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• pp.35-40
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• 2013

Natural convection of air with Pr=0.7 between two horizontal plates with small magnitude non-uniform temperature distribution[${\in}{\Delta}Tsin({\kappa}X/H)$, H : gap width, X : horizontal coordinate] in the lower plate is numerically(${\in}=0.01$) investigated. In the conduction-dominated regime with $Ra{\leq}1700$, two upright cells are formed over one wave length($2{\pi}/{\kappa}$). For small wave number, the flow becomes unstable with increase of Rayleigh number, and multicellular convection occurs above a critical Rayleigh number. The flow patterns are classified by the number of eddies over one wave length. When ${\kappa}=1$, a transition of $2{\rightarrow}4{\rightarrow}6$ eddy flow occurs with increase of Rayleigh number, and no hysteresis phenomenon is observed. Dual and triple solutions are found for ${\kappa}=1$, and transitions of $10{\rightarrow}8$, $8{\rightarrow}6$, $6{\rightarrow}4{\rightarrow}2$ eddy flow occur with decrease of Rayleigh number.

• 자연과학논문집
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• 제12권1호
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• pp.1-9
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• 2002

연결 그래프의 꼭지점에 자갈이 분포되어 있다고 하자. 한 꼭지점에서 두 개의 자갈을 취하여 한 개의 자갈만을 인접한 꼭지점에 보내는 이동을 할 때, 자갈이 분포될 수 있는 모든 경우에서 임의의 꼭지점에 한 개의 자갈을 보내기 위해 필요한 최소의 자갈의 수를 그 그래프의 pebbling number 라고 한다. 이 논문에서 Petersen Graph의 pebbling 수를 계산하였고 complete bipartite 그래프 $K_{m,n}$과 꼭지점의 수 h가 4개 이상인 complete 그래프의 categorical product 의 pebbling number가 (m+n)h 이 됨을 보였다.

• 대한수의학회지
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• 제27권2호
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• pp.185-189
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• 1987

The number of splenic lymphocyte, serum prostaglandin $E_2$ level and natural killer cell activity were assayed after single whole body irradiation of a sublethal dose of $^{60}Co-{\gamma}$ ray to C57BL/6J mice. With a view to knowing the relationships between radiation induced prostaglandin $E_2$ level and the normal natural killer cell activity after natural killer cell-target cell conjugation, The change of normal natural killer cell activity were measured by administration of prostaglandin $E_2$ containing serum from irradiated mice. The results were summarized as follows; 1. The total number of splenic lymphocyte was significantly decreased by irradiation and the number was not affected by indometacin, prostaglandin synthesis inhibitor, treatment. 2. Serum prostaglandin $E_2$ level was increased in irradiated mice, but indometacin treated mice group showed low level of prostaglandin $E_2$. 3. In the case of irradiated mice, natural killer cell activity was not shown any difference between irradiated group and indometacin combined group. But when natural killer cell-target cell conjugations were exposed to the serum of each group during cytotoxic activity assay, whereas the normal natural killer cell activity was significantly decreased by treatment of serum from irradiated mice, the activity was not changed by treatment of indometacin pretreated mice serum. This result indicated that the prostaglandin $E_2$ induced by the radiation inhibited the post-target binding cytolytic process of natural killer activity.

• 한국수학교육학회지시리즈B:순수및응용수학
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• 제20권2호
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• pp.129-136
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• 2013

We research properties of ternary numbers with values in ${\Lambda}(2)$. Also, we represent dual ternary numbers in the sense of Clifford algebras of real six dimensional spaces. We give generation theorems in dual ternary number systems in view of Clifford analysis, and obtain Cauchy theorems with respect to dual ternary numbers.

• 대한기계학회논문집
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• 제15권2호
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• pp.654-660
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• 1991

Natural convection heat transfer in a horizontal annulus from an inner tube with two vertical fins has been studied for the effects of dimensionless fin length and Rayleigh number. The maximum local Nusselt number of inner tube was obtained at .theta. = 145.deg. and that of outer cylinder at .theta. = 0.deg. for the case of $l_{F}$=0.3 Local Nusselt number distributions for the lower fins show higher values than that of the upper fins. The mean Nusselt number of inner tube was increased with the values of dimensionless fin length. The mean Nusselt number can be represented in an exponential function of Grashof number at various fin lengths. As compared with experimental and numerical results, isotherms and local Nusselt number show good agreement.t.

• 대한기계학회논문집B
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• 제20권11호
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• pp.3686-3694
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• 1996

A numerical investigation of natural convection flow along irregular vertical surfaces is reported. A transformation method is applied to the problem of natural convection under the assumption of a large Grashof number. A vertical wavy surface is used as an example to demonstrate the advantages of the transformation method, and to show the heat transfer mechanism near such surfaces. Surface non-uniformities on the boundary layer flow induced by a constant was temperature, semi-infinite surface are investigated. Also the effects of Prandtl number, flow index, and surface amplitude in Non-Newtonian fluids are discussed. When possible, the comparison of the numerical results shows a good agreement. The amplitude is proportional to the amplitude of a wavy surface. The results demonstrate that the local heat flux along a wavy surface is smaller than that of a flat surface. The frequency of the wavy surface is half that of the local heat transfer rate. The amplitude of the local Nusselt number gradually decreases downstream where the natural convection boundary layer grows thick.

• 한국전산유체공학회지
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• 제16권3호
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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.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2744-2749
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• 2008

Numerical calculations are carried out for the natural convection induced by temperature difference between a cold outer square cylinder and a hot inner circular cylinder for Rayleigh number of $Ra=10^7$. This study investigates the effect of the inner cylinder location on the heat transfer and fluid flow. The location of inner circular cylinder ($\delta$) is changed vertically along the center-line of square enclosure. The natural convection bifurcates from unsteady to steady state according to $\delta$. Two critical positions of ${\delta}_{C,L}$ and ${\delta}_{C,U}$ as a lower bound and an upper bound are ${\delta}_{C,L}=0.05$ and ${\delta}_{C,U}=0.18$, respectively. Within the defined bounds, the thermal and flow fields are steady state. When the inner cylinder locates at ${\delta}{\geq}{\delta}_{C,U}$, the space between the upper surface of inner cylinder and the top surface of the enclosure forms a relatively shallow layer where the natural convection characterized as the pure Rayleigh-Benard convection forms alternately the upwelling and downwelling plums, as a result that a series of cells known as Benard cells is derived.