• 설비공학논문집
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• 제8권1호
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• pp.1-12
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• 1996

In lots of application to heat exchanger systems, closed two-phase thermosyphons are tilted from a horizontal. If the tilt angle, especially, is less than 30$^{\circ}$, the operational performances of thermosyphon are highly dependent on tilt angle. The present study was conducted to better understand such operational behaviors as mech-anni는 of phase change, and flow patterns inside a tilted thermosyphon. For experiment, an ethanol thermosyphon with a 35% of fill charge rate was designed and manufactured, using a copper tube with a diameter 19mm and a length 1500mm. Through a series of test, the tilt angle was kept constant at each of 4 different values in the range 10~25deg. and the heat supply to the evaporator was stepwisely increased up to 30㎾/$m^2$. When a steady state was established to the thermosyphon for each step of thermal loads, the wall temperature distribution and vapor temperature at the condenser were measured. The wall temperature distributions demonstrated a formation of dry patch in the top end zone of the evaporator, with a values of temperature 20~4$0^{\circ}C$ higher than the wetted surface for a moderate heat flux q≒20㎾/$m^2$. Inspite of the presence of hot dry patch, however, the mean values of boiling heat transfer coefficient at the evaporator wall were still in a good agreement with those predicted by Rohsenow's formula, which was based on nucleate boiling. For the condenser, the wall temperatures were practically uniform, and the measured values of condensation heat transfer coefficient were 1.7 times higher than the predicted values obtained from Nusselt's film condensation theory on tilted plate. Using those two expressions, a correlation was formulated as a function of heat flux and tilt angle, to determine the total thermal resistance of a tilted thermosyphon. The correlation formula showed a good agreement with the experimental data within 20%.

• 대한기계학회논문집B
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• 제34권6호
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• pp.655-663
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• 2010

표면 젖음성은 비등 상황에서 주요 인자인 임계열유속과 비등열전달 모두에 영향을 미치는 중요한 표면인자이다. 지금까지 표면 젖음성을 이용한 비등 조건 개선에 대한 연구는 한가지 물질의 표면 구조를 개질하는데 국한되었다. 본 논문에서는 최적화된 비등 조건을 이룰 수 있는 표면 젖음성을 찾기 위한 연구의 일환으로 소수성 물질과 친수성 물질의 혼합을 시도하였다. 가열 표면은 표면 접촉각이 $60^{\circ}$인 친수성 표면위에 표면 접촉각 $120^{\circ}$의 소수성 물질 점이 생기도록 개질되었다. 개질된 소수성 점은 마이크로 단위와 밀리단위로 그 크기를 변화시켜 가며 풀 비등 성능을 평가하였다.

• 설비공학논문집
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• 제12권7호
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• pp.642-648
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• 2000

Falling film heat transfer analyses with aqueous lithium bromide solution were peformed to investigate the transfer characteristics of the copper tubes. Finned(knurled) tube and a smooth tube were selected as test specimens. Averaged generation fluxes of water and the heat transfer performances(heat flux, heat transfer coefficient) were obtained. The results of this work were compared with the data reported previously. As the film flow rate of the solution increased, the generation fluxes of water decreased for both tubes. The reason is estimated by the fact that the heat transfer resistance with the film thickness increased as the film flow rate increased. The effect of the enlarged surface area at the knurled tube was supposed to be dominant at a small flow rate. The generation fluxes of water increased with the increasing degree of tube wall superheat. Nucleate boiling is supposed to occur at a wall superheat of 20 K for a smooth tube, and at 10 K for a knurled tube. The heat transfer performance of the falling film was superior to pool boiling at a low wall superheat below 10 K for both tubes tested. The knurled tube geometry showed good performance than the smooth tube, and the increased performance was mainly came from the effect of the increased heating surface area.

• Nuclear Engineering and Technology
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• 제41권5호
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• pp.561-574
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• 2009

First ex-vessel core catcher has been applied to the practical design of NPPs with VVER-1000 reactors built in China (Tyanvan) and India (Kudankulam) for severe accident management (SAM) and mitigation of SA consequences. The paper presents the concept and basic design of this crucible-type core catcher as well as an evaluation of its efficiency. The important role of oxidic sacrificial material is discussed. Insight into the behaviour of the molten pool, which forms in the catcher after core relocation from the reactor vessel, is provided. It is shown that heat loads on the water-cooled vessel walls are kept within acceptable limits and that the necessary margins for departure from nucleate boiling (DNB) and of vessel failure caused by thermo-mechanical stress are satisfactorily provided for.

• 대한기계학회논문집B
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• 제22권7호
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• pp.980-991
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• 1998

In this study, nucleate pool boiling heat transfer coefficients of alternative refrigerants on a plain, low fin, and two enhanced tubes were measured and compared against each other. To obtain data at conditions similar to the actual evaporator, a fluid heating method was employed instead of an electric heating method in the experiments. R123, R134a, R22 and R410a were used as working fluids and data were taken at 7 deg.C ar heat fluxes of 20 ~ 100 kW/m$\^$2/. Comparison of the plain tube data against some correlations showed that the simplest correlation of Cooper based on reduced pressure predicted the data for all fluids tested with a 10% deviation. For all refrigerants, enhanced tubes composed of subsurface and subtunnels, especially Thermoexcel-E tube, showed the highest heat transfer coefficients among the tubes tested with one exception that the low fin tube's performance was better than those of enhanced tubes for high vapor pressure fluid such as R410a at high heat flux. Finally, a low fin and enhanced tubes showed higher heat transfer enhancement for low vapor pressure of R123 than for high vapor pressure fluisd. For R123, the enhancement factors for Turbo-B and Thermoexcel-E tubes were 2.8 ~ 4.8 and 4.6 ~ 8.1 respectively.

• 에너지공학
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• 제2권2호
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• pp.179-186
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• 1993

평판형 태양열 집열기에 적용되는 열파이프는 증발부에 상대적인 낮은 열유속으로 길이가 길고 가느다란 형상을 갖는다. 이러한 열파이프는 증발부의 하단부에서 액체 Pool이 형성하는 경향이 있다. 그리고 이 Pool에서 과열, 급속한 기포의 생성, 기포의 폭발적인 성장과정과 Flooding 등의 복잡한 증발과 유체 역학적 현상이 발생한다. 본 논문에서는 4개의 열파이프와 3개의 열사이폰을 이용하여 주 설계변수인 작동유체의 충전량과 위크의 설치 영역을 조절함으로써 이러한 문제를 해결하도록 하였다. 이에 대한 결과들은 다음과 같이 요약할 수 있다$^{1)}$ . 열파이프의 유효 열전도도는 단열부와 응축부에 위크를 제거함으로써 상당히 개선할 수 있었다$^{2)}$ .액체의 충전량은 위크를 적실수 있는 양보다 약 40%정도 증가시켜야 한다$_{3)}$ . 증발부에서 위크는 핵비 등의 단속적 발생에 의한 불안정한 작동과 포기 시동과정에서 응답시간을 줄이는데 유용한 효과를 갖는다.

• 에너지공학
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• 제24권1호
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• pp.149-163
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• 2015

본 연구에서는 전산유체역학(Computational Fluid Dynamics, CFD) 해석코드를 사용하여 마스트집합체의 열수력적 안전성에 대한 연구를 수행하였다. 이를 위해 자연대류 벤치마크 문제를 선정하여 CFD 코드의 물리모델을 선정 및 해석 능력을 검증하고 이를 이용하여 마스트집합체에 대한 자연대류 열전달 해석을 수행하였다. 본 연구에서는 Betts et al.의 사각 수직공동에서 난류 자연대류 실험결과를 대상으로 CFD 해석을 수행하여 자연대류 조건에 적용하기 위한 난류 모델로 표준 $k-{\omega}$ 모델을 선정하였다. 이렇게 도출된 난류모델을 CFD코드에 적용하여 Bates et al.에 의해 수행된 PNL(Pacific Northwest Laboratory)의 $2{\times}6$ 번들 실험과 이에 대한 Kwon et al.의 MATRA, Fluent 코드의 해석과 비교 계산을 수행하여 CFD코드의 부수로조건 자연대류 열전달 해석 능력을 검증하였다. 최종적으로 도출된 $k-{\omega}$ 난류 모델을 사용하여 마스트집합체 및 핵연료 집합체에 대한 자연대류 해석을 수행하였다. 해석 결과 수조 내부 및 부수로 내에서 안정적인 자연대류 유동이 발생함을 확인하였으며, 본 유동 조건에서 핵비등이탈비를 계산함으로써 열수력적 안전성을 정량적으로 평가하였다.