• 한국전자통신학회논문지
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• 제13권1호
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• pp.265-276
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• 2018

지상에 고정된 기기로 감시하기 어려운 대기 하층의 관측 사각 지역에서 유용한 관측 및 자료 분석 기술을 개발하였다. 상층 기상 관측에 사용되는 UHF 라디오존데를 UAV에 탑재하여 기온과 상대습도의 연직 자료를 원격 수집하여 10 m 기상 타워에서 측정한 자료와 비교하였다. 검증된 연직 분포로부터 외삽된 지표 기온과 총체 전달 방법을 이용하여 대기 안정도 변화에 따른 현열 플럭스를 추정하였다. 지상에서 3차원 초음파 풍속계로 측정한 현열 플럭스와 비교한 결과는 원격탐사로 산출되는 현열 플럭스에 허용되는 오차 범위 30%보다 작은 23% 이내의 오차를 보였다. UAV 관측 현열 플럭스로부터 추정한 대기경계층 고도는 대기 오염 확산 모델에 유용한 자료를 실시간, 경제적으로 제공할 수 있다.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2005년도 동계학술발표대회 논문집
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• pp.353-358
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• 2005

The heat transfer and pressure drop of supercritical $CO_2$ cooled in a helically coiled tube was investigated experimentally. The experiments were conducted without oil in the refrigerant loop. The experimental apparatus of the refrigerant loop consist of receiver, a variable speed pump, a mass flowmeter, a pre-heater, a gas cooler(test section) and an isothermal tank. The test section is a helically coiled tube in tube counter flow heat exchanger with $CO_2$ flowed inside the inner tube and coolant( water) flowed along the outside annular passage, It was made of it copper tube with the inner diameter of 4.55[mm]. the outer diameter of 6.35 [mm] and length of 10000 [mm]. The refrigerant mass fluxes were $200^{\sim}600$ [kg/m2s] and the inlet pressure of gas cooler varied from 7.5 [MPa] to 10.0 [MPa]. The main results are summarized as follows : The heat transfer coefficient of supercritical $CO_2$ increases, as the cooling pressure of gas cooler decreases. And the heat transfer coefficient increases with the increase of the refrigerant mass flux. The pressure drop decreases in increase of the gas cooler pressure and increases with increase the refrigerant mass flux.

• Nuclear Engineering and Technology
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• 제44권4호
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• pp.429-436
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• 2012

Critical heat flux (CHF) is the thermal limit of a phenomenon in which a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the heat transfer efficiency, thus causing localized overheating of the heating surface. The enhancement of CHF can increase the safety margins and allow operation at higher heat fluxes; thus, it can increase the economy. A very interesting characteristic of nanofluids is their ability to significantly enhance the CHF. Nanofluids are nanotechnology-based colloidal dispersions engineered through the stable suspension of nanoparticles. All experiments were performed in round tubes with an inner diameter of 0.01041 m and a length of 0.5 m under low pressure and low flow (LPLF) conditions at a fixed inlet temperature using water, 0.01 vol.% $Al_2O_3$/water nanofluid, and SiC/water nanofluid. It was found that the CHF of the nanofluids was enhanced and the CHF of the SiC/water nanofluid was more enhanced than that of the $Al_2O_3$/water nanofluid.

• 설비공학논문집
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• 제18권12호
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• pp.955-963
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• 2006

Nucleate pool boiling heat transfer coefficients (HTCs) were measured with one nonazeotropic mixture of Propane/Isobutane and two azeotropic mixtures of HFC134a/Isobutane and Propane/HFC134a. All data were taken at the liquid pool temperature of $7^{\circ}C$ on a horizontal plain tube with heat fluxes of $10kW/m^2\;to\;80kW/m^2$ with an interval of $10kW/m^2$ in the decreasing order of heat flux. The measurements were made through electrical heating by a cartridge heater. The nonazeotropic mixture of Propane/Isobutane showed a reduction of HTCs as much as 41% from the ideal values. The azeotropic mixtures of HFC134a/Isobutane and Propane/HFC134a showed a reduction of HTCs as much as 44% from the ideal values at compositions other than azeotropic compositions. At azeotropic compositions, however, the HTCs were even higher than the ideal values due to the increase in the vapor pressure. For all mixtures, the reduction in heat transfer was greater with a larger gliding temperature difference. Stephan and $K{\ddot{o}}rner's$ and Jung et al's correlations predicted the HTCs of mixtures with a mean deviation of 11%. The largest mean deviation occurred at the azeotropic compositions of HFC134a/Isobutane and Propane/HFC134a.

• 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.

• 한국수산과학회지
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• 제29권1호
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• pp.84-91
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• 1996

각종 해${\cdot}$기상 자료와 위성관측에 의한 운량 자료를 이용하여 벌크법에 의해 동지나해에 있어서 대기와 해양간의 열속을 추정하였으며, 그 결과를 요약하면 다음과 같다. 1) 태양복사량은 5월에 $255Wm^{-2}$로 최대, 12월에 $111Wm^{-2}$로 최소이며, 그 분포는 동절기에는 남쪽으로 갈수록 증가하고, 하절기에는 장마전선의 영향으로 북쪽으로 갈수록 증가하는 경향이 있었다. 장파복사량의 경우, 공간적 분포의 차이는 작으나 계절에 따른 차이는 커서 최대인 2월의 값은 (약 $70Wm^{-2}$) 최소인 7월의 2배에 이른다. 2) 현열과 잠열의 공간적 분포양상은 조사해역내의 해류 분포와 비슷하였다. 겨울철에 이 두 요소에 의한 해양의 열손실량은 대단히 커서 $830Wm^{-2}$ 이상이고, 이것은 같은 기간중 순폭사량의 5배에 이른다. 3) 연평균 순열플럭스의 분포는 전역에서 부 값을 보였으며, 최대 열손실 해역은 큐슈 남단으로 1월에 $400Wm^{-2}$ 이상이었다. 4) 조사해역의 태양복사량, 장파복사량, 현열 및 잠열의 연평균치는 각각 187, -52, -30, $-137Wm^{-2}$이고, 결과적으로 연간 약 $32Wm^{-2}(2.48\times10^{13}\;W)$의 에너지를 손실하고 있는 것으로 추정되었다. 5) Fig. 1에 표시되어 있는 A 해역 (황해)은 대기와의 열교환을 통하여 오히려 연간 $10Wm^{-2}(0.4\times10^{13}\;W)$의 에너지를 얻고, B 해역 (동지나해)은 $48Wm^{-2}(2.1\times10^{13}\;W)$,그리고 C 해역 (쿠로시오역)은 $39Wm^{-2}(1.7\times10^{13}\;W)$의 열을 손실하고 있는 것으로 추정되었다.

• Asian Journal of Atmospheric Environment
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• 제10권4호
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• pp.179-189
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• 2016

Here we evaluated the effect of using water retentive pavement or WRP made from fly ash as material for main street in a real city block. We coupled computational fluid dynamics and pavement transport (CFD-PT) model to examine energy balance in the building canopies and ground surface. Two cases of 24 h unsteady analysis were simulated: case 1 where asphalt was used as the pavement material of all ground surfaces and case 2 where WRP was used as main street material. We aim to (1) predict diurnal variation in air temperature, wind speed, ground surface temperature and water content; and (2) compare ground surface energy fluxes. Using the coupled CFD-PT model it was proven that WRP as pavement material for main street can cause a decrease in ground surface temperature. The most significant decrease occurred at 1200 JST when solar radiation was most intense, surface temperature decreased by $13.8^{\circ}C$. This surface temperature decrease also led to cooling of air temperature at 1.5 m above street surface. During this time, air temperature in case 2 decreased by $0.28^{\circ}C$. As the radiation weakens from 1600 JST to 2000 JST, evaporative cooling had also been minimal. Shadow effect, higher albedo and lower thermal conductivity of WRP also contributed to surface temperature decrease. The cooling of ground surface eventually led to air temperature decrease. The degree of air temperature decrease was proportional to the surface temperature decrease. In terms of energy balance, WRP caused a maximum increase in latent heat flux by up to $255W/m^2$ and a decrease in sensible heat flux by up to $465W/m^2$.

• 대한기계학회논문집B
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• 제28권3호
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• pp.289-295
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• 2004

The heat transfer coefficient and pressure drop during gas cooling process of carbon dioxide in a horizontal tube were investigated. The experiments were conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flowmeter, an evaporator, and a gas cooler(test section). The main components of the water loop consist of a variable-speed pump, an isothermal tank, and a flowmeter. The gas cooler is a counterflow heat exchanger with refrigerant flowing in the inner tube and water flowing in the annulus. The test section consists of smooth, horizontal stainless steel tube of the outer diameter of 9.53mm and of the inner diameter of 7.75mm. The length of the test section is 6m. The refrigerant mass fluxes were 200∼300kg/(m2$.$s) and the inlet pressure of the gas cooler varied from 7.5㎫ to 8.5㎫. The main results were summarized as follows : Pressure drop of CO2 increases with increasing gas cooler pressure. The friction factors of CO2 in a horizontal tube show a relatively good agreement with the correlation by Blasius. The heat transfer coefficient of CO2 in transcritical region increases with decreasing gas cooler pressure and decreasing mass flux of CO2. Most of correlations proposed in a transcritical region showed significant deviations with experimental data except for those predicted by Gnielinski.

• 한국화재소방학회논문지
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• 제33권3호
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• pp.29-36
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• 2019

다층·다성분 난연성 케이블의 화재시뮬레이션에서 요구되는 연소물성이 콘 칼로리미터를 통해 측정되었다. 난연성 케이블의 주요 재질에 따른 CO 및 Soot yields 그리고 연소열이 검토되었다. 케이블의 난연성능이 우수한 TFR-8(고난연성 PCV 및 XLPE 첨가), TFR-CVV-SB(고난연성 PCV 및 일반 PVC로 구성) 및 VCTF가 각각 대상으로 고려되었다. 주요 결과로서, 난연성케이블인 TFR-8과 TFR-CVV-SB는 입사 복사열유속이 25 kW/㎡에서 50 kW/㎡으로 증가됨에 따라 CO yield(y_CO) 는 각각 23% 와 16% 증가한다. 반면에 VCTF의 CO yield는 복사 열유속의 변화에 큰 영향을 받지 않는다. 마지막으로 Soot yield 및 연소열은 시스의 재질(난연성능)이 강화될수록 복사 열유속에 의한 차이가 증가됨이 확인되었다. 따라서 다양한 열유속이 공존하는 화재환경에서 난연성 케이블의 연소물성의 적용에는 상당한 주의가 요구된다.

• 한국가스학회지
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• 제12권1호
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• pp.48-53
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• 2008

본 연구는 세관내 R-22와 R-410A의 응축 열전달 계수를 실험적으로 조사하였다. 냉매 순환루프의 주요 구성품은 수액기, 변속 액펌프, 질량유량계, 증발기(예열기), 응축기(시험부)로 구성된다. 시험부는 평활, 수평 동관으로 내관의 내경과 외경이 각각 1.77 mm와 3.38 mm이다. 냉매 질량유속은 $450{\sim}1050\;kg/(m^2s)$이고, 입출구 평균건도는 $0.05{\sim}0.095$이다. 주요 실험결과를 요약하면, 응축 열전달 계수는 질량유속과 건도이 증가할수록 증가하였고, R-410A의 응축 열전달 계수가 R-22에 비해 약간 높았다. 종래의 상관식과 비교한 결과, 저건도와 저질량유속을 제외하고는 실험값과 종래의 상관식으로 예측한 값의 차이가 큰 것으로 나타났다.