• 제목/요약/키워드: Local Heat Transfer Coefficient

검색결과 201건 처리시간 0.023초

자유유동 난류강도가 터빈 동익 표면에서의 열(물질)전달 특성에 미치는 영향 (Free-Stream Turbulence Effect on the Heat (Mass) Transfer Characteristics on a Turbine Rotor Surface)

  • 이상우;박진재;권현구;박병규
    • 대한기계학회:학술대회논문집
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    • 대한기계학회 2004년도 춘계학술대회
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    • pp.1442-1446
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    • 2004
  • The heat (mass) transfer characteristics on the blade surface of a first-stage turbine rotor cascade has been investigated by employing the naphthalene sublimation technique. A four-axis profile measurement system is employed for the measurements of the local heat (mass) transfer coefficient on the curved blade surface. The experiments are carried out for two free-stream turbulence intensities of 1.2% and 14.7%. The high free-stream turbulence results in more uniform distributions of heat load on the both pressure and suction surfaces and in an early boundary-layer separation on the suction surface. The heat (mass) transfer enhancement on the suction surface due to the endwall vortices is found to be relatively small under the high free-stream turbulence.

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경사진 헬리컬 코일 열교환기의 열전달 특성에 관한 연구 (Heat Transfer Characteristics of Inclined Helical Coil Type Heat Exchanger)

  • 손창효;전민주;장승일;오후규
    • Journal of Advanced Marine Engineering and Technology
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    • 제31권6호
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    • pp.707-714
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    • 2007
  • The heat transfer coefficient and Pressure drop during gas cooling process of $CO_2$ (R-744) in inclined helical coil copper tubes were investigated experimentally. The main components of the refrigerant loop are a receiver. a variable-speed pump. a mass flow meter, a pre-heater and a inclined helical coil type gas cooler (test section). The test section consists of a smooth copper tube of 2.45mm inner diameter. The refrigerant mass fluxes were varied from 200 to $600[kg/m^2s]$ and the inlet Pressures of gas cooler were 7.5 to 10.0 [MPa]. The heat transfer coefficients of $CO_2$ in the inclined helical coil tubes increases with the increase of mass flux and gas cooling pressure of $CO_2$. The pressure drop of $CO_2$ in the gas cooler shows a relatively good agreement with those Predicted by Ito's correlation developed for single-phase in a helical coil tube. The local heat transfer coefficient of $CO_2$ agrees well with the correlation by Pitla et al. However, at the region near pseudo-critical temperature. the experiments indicate higher values than the Pitla et al. correlation. Therefore. various experiments in the inclined helical coil tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of $CO_2$ in the inclined helical coil tubes.

냉동유가 수평 마이크로 핀관내 R-134a의 증발열전달에 미치는 영향 (Influence of Refrigeration Oil on Evaporation Heat Transfer of R-134a in a Horizontal Micro-Fin Tube)

  • 배상철;강태욱;김정훈;정찬영;김종수
    • 설비공학논문집
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    • 제8권1호
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    • pp.140-150
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    • 1996
  • CFC-12, which has been used most widely in automobile air conditioners and household refrigerators is scheduled to be phased out soon because of its high ozone depletion potential. Now HFC-134a is suggested as an alternative refrigerant for CFC-12. In this Study, we intended to investigate how PAG oil influence evaporation heat transfer and flow pattern, using R-134a and PAG oil influences evaporation heat transfer and flow pattern, using R-134a and PAG oil in the horizontal miro-fin evaporation tube. Experiments were conducted under the flowing est conditions : mass velocity 86-250kg/$m^2$s, heat flux 5-30 ㎾/$m^2$, oil concentration 0-21 wt.% and saturation temperature 5$^{\circ}C$. Local evaporation heat transfer coefficients were found to be higher at the top, side and bottom of the tube in this order. Average heat transfer coefficients turned out to increase with oil concentration increment up to 3 wt.% oil concentration, whereas heat transfer coefficients gradually decreased over 3 wt.% oil concentration, because of oil-rich liquid film was formed on the heat transfer surface. Flow patterns were rapidly transitioned to annular regimes up to 3 wt.% oil concentration. In case of pure refrigerant, measured heat transfer coefficients in the experiments were similar to those of Kandlikar's correlation.

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충돌제트의 유동 및 열전달 특성에 미치는 맥동의 영향 (Effect of Pulsations on Flow and Heat Transfer Characteristics of an Impinging Jet)

  • 이은현;이성혁;이준식
    • 대한기계학회논문집B
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    • 제25권12호
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    • pp.1869-1878
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    • 2001
  • Experiments are carried out to investigate the effect of pulsations on the flow and heat transfer characteristics of an axisymmetric impinging jet on a flat plate heated by using a gold coated aim. Vertex motion in the impinging jet is visualized using a fog generator, and a thermochromatic liquid crystal (TLC) technique is used to measure the time averaged local temperature distributions on the impingement plate. In addition, the quantitative data for mean velocity and turbulence intensity are obtained employing hot-wire anemometer. Parameters such as pulsating frequency (f = 0, 10 and 20 Hz) and the nozzle-to-palate spacing (H/D = 2, 10) are considered at the jet Reynolds number of 20,000. Consequently, the significant changes of flow structure and local Nusselt number distribution due to pulsations are observed. In the case of H/D = 2, the enhanced heat transfer coefficient exceeding 30 % is observed at the stagnation point. At the high H/D, heat transfer rate increases with pulsation frequency.

팁간극이 고선회각 터빈 동익 평면팁 표면에서의 열전달에 미치는 영향 (Effect of Tip Clearance Height on Heat Transfer Characteristics on the Plane Tip Surface of a High-Turning Turbine Rotor Blade)

  • 문현석;이상우
    • 유체기계공업학회:학술대회논문집
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    • 유체기계공업학회 2005년도 연구개발 발표회 논문집
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    • pp.173-177
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    • 2005
  • The heat/mass transfer characteristics on the plane tip surface of a high-turning first-stage turbine rotor blade has been investigated by employing the naphthalene sublimation technique. The heat/mass transfer coefficient is measured for four tip clearance height-to-chord ratios of h/c = 1.0%, 2.0%, 3.0%, and 4% at the Reynolds number of $2.09{\times}105$. The result shows that at lower h/c, there exists a strong flow separation/re-attachment process, which results in severe thermal load along the pressure-side comer. As h/c increases, the re-attachment is occurred further downstream of the pressure-side comer with lower thermal load. At higher h/c, a pair of vortices on the tip surface near the leading edge are found along the pressure-side and suction-side comers, and the pressure-side tip vortex have significant influence even on the mid-chord local heat transfer.

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전폐형 유도전동기 엔드와인딩 표면의 열전달에 미치는 회전자팬의 영향 (Rotor fan effects on end winding heat transfer in totally enclosed fan cooled induction motor)

  • 윤명근;고상근
    • 대한기계학회논문집B
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    • 제21권7호
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    • pp.928-938
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    • 1997
  • The end winding is an important part in induction motor for thermal analysis. But there is little information on the heat transfer coefficient of that surfaces because of geometrical complexity. So our experimental object is to know the heat transfer coefficient of end winding and find the optimum design parameter of rotor fan. Carbon coated papers were used for a uniform heat generating surfaces which were easy to fabricate. The experiments of some parameters were performed as varying rotation speed of rotor fan. We obtained the local and average Nusselt number of the end winding surfaces by correcting radiation and conduction losses errors. The results showed that the average Nusselt number increased with rotor fan blade number and width but decreased with end winding length. However, the increasing limits existed in the case of rotor fan width and blade number. So optimum design value were obtained for rotor fan width and blade numbers.

공기를 함유한 프레온 증기의 수평관 외부에서의 막응축 (Filmwise Condensation of Freon Vapor Including Air on a Horizontal Tube)

  • 김경훈;고형종
    • 설비공학논문집
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    • 제7권1호
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    • pp.20-29
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    • 1995
  • A theoretical model for film condensation of a vapor including a relatively lighter noncondensable gas on a horizontal tube has been formulated on the basis of the conservation laws and other fundamental physical principles. The model is applied to the prediction of the condensation heat transfer characteristics for the Freon vapor in the presence of air on a horizontal tube. Calculated results for the mean heat transfer coefficient, which is shown to depend strongly on the bulk concentration of air, are in good agreement with the available experimental results for a range of operating conditions. The distributions of physical quantities along the surface of tube are also calculated, such as the boundary layer thickness and local heat transfer coefficient. The present model is readily reduced to the Nusselt model as the bulk concentration of air decreases to zero. Therefore, the transition from the condensation of pure vapor to that of vapor-air mixture occurs continuously not abruptly.

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수직 벽에서 과열증기의 막응축에 대한 열물성의 영향 (The Influence of Variable Thermophysical Properties for Filmwise Condensation of Superheated Vapor on a Vertical Wall)

  • 김경훈;성현찬
    • 설비공학논문집
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    • 제12권3호
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    • pp.235-243
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    • 2000
  • A theoretical model for laminar filmwise condensation along an isothermal vertical wall at constant pressure has been formulated on the basis of conservation laws and other fundamental physical principles. The model was applied to the prediction of the influences of variable thermophysical properties of liquid and vapor layers in the filmwise condensation of superheated vapor of Rl2, R134a, R142b and R152a. The dimensionless velocity component method was employed in the transformation of the governing equations and their boundary conditions, and the polynomial method was used for treating variable thermophysical properties of liquid and vapor. Physical quantities, such as the dimensionless thickness of the liquid layer, local heat transfer rate and mean heat transfer coefficient, were investigated for different values of the superheated temperature of the stagnant vapor far from the wall. It was found that the value of mean heat transfer coefficient of R134a was higher than other refrigerants for the change of the superheated temperature.

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An Experimental Study on Heat Transfer and Pressure Drop Characteristics of Carbon Dioxide During Gas Cooling Process in a Hellically Coiled Tube

  • Oh, Hoo-Kyu;Son, Chang-Hyo;Yu, Tae-Geun;Kim, Dae-Hui
    • Journal of Advanced Marine Engineering and Technology
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    • 제31권3호
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    • pp.263-271
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    • 2007
  • The heat transfer coefficient and pressure drop during 9as cooling process of $CO_2$ (R744) in a helically coiled copper tube with the inner diameter of 4.55 mm and outer diameter of 6.35 mm were investigated experimentally. The main components of the refrigerant loop are a receiver, a variable-speed pump. a mass flow meter a pre-heater and a helically coiled type gas cooler (test section). The refrigerant mass fluxes are varied from 200 to $800kg/m^2s$ and the inlet pressures of gas cooler are 7.5 to 10.0 MPa. The heat transfer coefficients of $CO_2$ in a helically coiled tube are higher than those in a horizontal tube. The Pressure drop of $CO_2$ in the gas cooler shows a relatively good agreement with those predicted by Ito's correlation developed for single-phase in a helically coiled tube. The local heat transfer coefficient of $CO_2$ agrees well with the correlation by Pitla et al. However. at the region near pseudo-critical temperature. the experiments indicate higher values than the Pitla et al correlation. Therefore, various experiments in helically coiled tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of $CO_2$ in a helically coiled tube.

상변화 물질을 이용한 에너지의 저장에 관한 연구 - 핀이 부착된 열싸이폰의 이용에 관하여 - (Thermal Energy Storage in Phase Change Material - by Means of Finned Thermosyphon -)

  • 김권진;유재석;김기현
    • 태양에너지
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    • 제11권1호
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    • pp.69-77
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    • 1991
  • 파라핀 왁스에 열에너지를 저장하기 위해 열전달 기구로 원판형 핀이 달린 열싸이폰을 사용하였다. 실험은 4, 6, 8개의 핀에 대해 수행되었으며 각각의 경우 왁스의 초기온도와 주입열량을 변화시키면서 실험을 반복하였다. 실험을 통하여 특히 잠열 축열계의 전열경로의 주요지점에서의 열전달 특성을 규명하고자 하였으며 주요 결론은 다음과 같다. (1) 열싸이폰의 열관류율과 총합열관류율은 핀의 수가 증가함에 따라 커지나 핀과 왁스사이의 열전달계수는 감소하였다. (2) 핀에 의해 열전달이 촉진됨으로서 핀이 없는 열싸이폰에서 일어나는 dry-out 현상이 제거되었다. (3) 수평형의 핀은 왁스의 큰 규모의 대류를 억제하며 핀의 수가 증가함에 따라 핀 사이에서의 국소대류도 더욱 억제되었다.

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