• International Journal of Air-Conditioning and Refrigeration
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• 제16권4호
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• pp.130-136
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• 2008

The refrigerant R-134a flow distributions are experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations(parallel, normal, vertical) were investigated. Tests were conducted with downward flow for the mass flux from 70 to 130 $kg/m^2s$ and quality from 0.2 to 0.6. In the test section, tubes were flush-mounted with no protrusion into the header. It is shown that normal and vertical inlet yielded approximately similar flow distribution. At high mass fluxes or high qualities, however, slightly better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results.

• 대한원격탐사학회지
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• 제34권3호
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• pp.519-533
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• 2018

2014년부터 2017년까지 4년의 기간 동안 COARE 3.5 벌크 알고리즘과 위성 기반의 대기-해양 변수 자료를 이용하여 한반도 주변 해양의 현열 플럭스(Sensible Heat Flux; SHF)와 잠열 플럭스(Latent Heat Flux; LHF)를 $40W/m^2$ 산출하였다. 열 플럭스 산출에 필요한 변수 중 10-m 풍속(U)과 해수면온도($T_s$) 자료는Advanced Microwave Scanning Radiometer 2(AMSR2)와 Global Precipitation Measurement Microwave Imager(GMI) 위성 센서로부터 관측되는 값을 일 평균하여 생성하였으며, 위성으로부터 직접 관측이 되지 않는 대기 온도($T_a$)와 대기 비습($Q_a$)은 위성 기반의 W 및 $T_s$와 갖는 상관성을 이용하여 경험적 통계식을 통해 추정하였다. 추정된 $T_a$와 $Q_a$는 해양 부이에서 관측된 값과 각각 0.96 이상의 높은 상관성을 보였다. 위성 기반으로 관측 및 추정된 대기-해양 변수 자료들을 이용해 한반도 주변 해양(서해, 동해, 남해)의 SHF와 LHF를 산출하였고 월평균 시공간분포의 특성을 확인하였다. SHF는 3월부터 8월까지 한반도 전 해역에 걸쳐 $20W/m^2$의 낮은 값을 보였으며, 특히 7월에는 일부 해양에서 $0W/m^2$ 이하의 낮은 값을 보였다. SHF는 9월부터 점차 증가하여 12월에 가장 높은 값이 나타났다. LHF는 4월부터 7월까지 $40W/m^2$ 정도의 낮은 값을 보이다가 가을철부터 급격히 증가하여 SHF과 마찬가지로 12월에 남해에서 최대 $380W/m^2$ 이상의 높은 값을 보였다. 두 열 플럭스는 모두 쿠로시오 난류가 지나가는 지역에서 연중 높은 값을 나타냈다. 해양 플럭스에 영향을 미치는 대기-해양 변수의 월평균 특성을 분석한 결과 SHF와 LHF는 각각 대기-해양 온도 차이(${\Delta}T$)와 비습 차이(${\Delta}Q$)의 변화에 밀접하게 연관되며, 겨울철에는 U에 대한 민감도가 증가하여 현열 및 잠열 플럭스가 겨울철에 가장 큰 값을 보이는 것에 기여한 것으로 분석된다.

• Membrane and Water Treatment
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• 제2권3호
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• pp.159-173
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• 2011

The air gap membrane distillation (AGMD) process was applied for water desalination. The main objective of the present work was to study the heat and mass transfer mechanism of the process. The experiments were performed on a flat sheet module using aqueous NaCl solutions as a feed. The membrane employed was hydrophobic PTFE of pore size 0.22 ${\mu}m$. A mathematical model is proposed to evaluate the membrane mass transfer coefficient, thermal boundary layers' heat transfer coefficients, membrane / liquid interface temperatures and the temperature polarization coefficients. The mass transfer model was validated by the experimentally and fitted well with the combined Knudsen and molecular diffusion mechanism. The mass transfer coefficient increased with an increase in feed bulk temperature. The experimental parameters such as, feed temperature, 313 to 333 K, feed velocity, 0.8 to 1.8 m/s (turbulent flow region) were analyzed. The permeation fluxes increased with feed temperature and velocity. The effect of feed bulk temperature on the boundary layers' heat transfer coefficients was shown and fairly discussed. The temperature polarization coefficient increased with feed velocity and decreased with temperature. The values obtained were 0.56 to 0.82, indicating the effective heat transfer of the system. The fouling was observed during the 90 h experimental run in the application of natural ground water and seawater. The time dependent fouling resistance can be added in the total transport resistance.

• 한국추진공학회지
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• 제16권2호
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• pp.42-50
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• 2012

열전달 연구의 목적은 온도와 열유속 분포를 보다 정확하게 예측하는 것이다. 이를 위해 상용 CFD 코드인 FLUENT를 사용하여 2종류의 노즐에 대해 질량유속비와 압력비를 계산하였으며, 실험결과와 잘 일치하였다. 또한 1종류의 노즐에 대해 FLUENT를 사용한 노즐 벽면에서의 열전달계수 계산결과는 노즐 축소부에서 실험결과 보다 약간 크게 예측되었으나 확대부에서는 잘 일치하고 있다. Bartz식을 이용한 열전달계수 계산결과는 전체적으로 실험결과 보다 크게 예측되었다. 계산결과가 실험결과와 차이를 보이는 원인은 노즐 내 급가속 유동에 의한 층류화, 난류모델 및 격자구성 등을 고려해 볼 수 있다.

• 대한기계학회논문집B
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• 제24권2호
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• pp.316-323
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• 2000

A nonlinear low-Reynolds-number heat transfer model is developed to predict turbulent flow and heat transfer in separated and reattaching flows. The $k-{\varepsilon}-f_{\mu}$ model of Park and Sung (1997) is extended to a nonlinear formulation, based on the nonlinear model of Gatski and Speziale (1993). The limiting near-wall behavior is resolved by solving the $f_{\mu}$ elliptic relaxation equation. An improved explicit algebraic heat transfer model is proposed, which is achieved by applying a matrix inversion. The scalar heat fluxes are not aligned with the mean temperature gradients in separated and reattaching flows; a full diffusivity tensor model is required. The near-wall asymptotic behavior is incorporated into the $f_{\lambda}$ function in conjunction with the $f_{\mu}$ elliptic relaxation equation. Predictions of the present model are cross-checked with existing measurements and DNS data. The model preformance is shown to be satisfactory.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2005년도 후기학술대회논문집
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• pp.244-245
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• 2005

This paper presents the heat transfer and pressure drop characteristics during cooling process of carbon dioxide in a horizontal tube. The test section is a tube in tube type heat exchanger with refrigerant flowing in the inner tube and water flowing in the annulus. It was made of a stainless steel tube with the inner diameter of 7.75 [mm], the outer 2 diameter of 9.53 [mm] and length of 6000 [mm]. The refrigerant mass fluxes were $200{\sim}400$ [kg/$m^2s$] and the average pressure varied from 7.5 [MPa] to 10.0 [MPa]. The main results were summarized as follows The heat transfer coefficient of supercritical $CO_2$ increases in decrease of the gas cooler pressure. And the heat transfer coefficient increases with respect to the increase of the refrigerant mass flux. Among some correlations proposed in a transcritical region, Bringer-Smith's correlation has some analogy with experimental results. The pressure drop decreases in increase of the gas cooler pressure and increases with respect to increase the refrigerant mass flux.

• Journal of Mechanical Science and Technology
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• 제20권3호
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• pp.399-408
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• 2006

Nucleate pool boiling heat transfer coefficient (HTCs) were measured with one nonazeotropic mixture of propane/isobutane and two azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a. All data were taken at the liquid pool temperature of $7^{\circ}C$ on a horizontal plain tube of 19.0mm outside diameter with heat fluxes of $10\;kW/m^2\;to\;80kW/m^2$ with an interval of $10\;kW/m^2$ 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 larger gliding temperature difference. Stephan and Korner's and lung 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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• 제29권7호
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• pp.854-860
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• 2005

Pool boiling heat transfer characteristics of R-134a were investigated in titanium plain and low finned tubes. The diameter of test tube was 15.88 mm and the fin density was 33 fpi. Tests were conducted at saturation temperatures of $20^{\circ}C$ and $30^{\circ}C$. Heat fluxes varied from 5000 W/$m^2$ to 50,000 W/$m^2$ based on surface area of the plain tube. The pool boiling heat transfer coefficients of the titanium horizontal plain tube are lower than those of the copper plain tube by $8.2\%$. The boiling heat transfer coefficients of the low finned tube are averagely higher than those of the plain tubes by $34\%$. The average deviation of the Slipcevic correlation from the present data for the low finned tube is $20\%$.

• 설비공학논문집
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• 제25권9호
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• pp.484-492
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• 2013

Nucleate pool boiling heat transfer coefficients (HTCs) are measured with HFC32/HFC152a mixture at several compositions. All data are taken at the liquid pool temperature of $7^{\circ}C$, on a horizontal plain square surface of $9.53{\times}9.53$ mm, with heat fluxes of 10 $kW/m^2$ to 100 $kW/m^2$ with an interval of 10 $kW/m^2$, in the increasing order of heat flux. Test results show that the HTCs of these mixtures are up to 45% lower than those of the ideal HTCs calculated by a linear mixing rule with pure fluids' HTCs, due to the mass transfer resistance associated with non-azeotropic refrigerant mixtures. Pool boiling data show the deduction in HTCs with an increase in GTD of the mixture. The present mixture data agree well with five well known correlations, within 20% deviation.

• Nuclear Engineering and Technology
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• 제51권5호
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• pp.1251-1260
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• 2019

The water-cooled WCLL blanket is one of the possible candidates for the blanket of the fusion power reactors. The plasma-facing first wall manufactured from the reduced-activation ferritic-martensitic steel Eurofer97 will be cooled with water at a typical pressurized water reactor (PWR) conditions. According to new estimates, the first wall will be exposed to peak heat fluxes up to $7MW/m^2$ while the maximum operated temperature of Eurofer97 is set to $550^{\circ}C$. The performed analysis shows the capability of the designed flat first wall concept to remove heat flux without exceeding the maximum Eurofer97 operating temperature only up to $0.75MW/m^2$. Several heat transfer enhancement methods (turbulator promoters), structural modifications, and variations of parameters were analysed. The effects of particular modifications on the wall temperature were evaluated using thermo-hydraulic three-dimensional numerical simulation. The analysis shows the negligible effect of the turbulators. By the combination of the proposed modifications, the permitted heat flux was increased up to $1.69MW/m^2$ only. The results indicate the necessity of the re-evaluation of the existing first wall concepts.