• 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.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2007년도 동계학술발표대회 논문집
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• pp.247-251
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• 2007

Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristics of carbon dioxide flowing upward in a vertical micro-fin tube have been investigated by experiment. Before a test section, a pre-heater is installed to adjust the inlet quality of the refrigerant to a desired value. The micro-fin tube with outer diameter of 5 mm and length of 1.44 m was selected as the test section. The test was conducted at mass fluxes of 318 to $530\;kg/m^2s$, saturation temperature of -5 to $5^{\circ}C$, and heat fluxes of 15 to $30\;kW/m^2$. As the vapor quality increases, the heat transfer coefficients of carbon dioxide are increased, and the heat transfer coefficients increase when the heat fluxes and saturation temperatures increase, and there was not much of influence of mass flux on the heat transfer coefficients.

• 설비공학논문집
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• 제14권1호
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• pp.38-45
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• 2002

Evaporative heat transfer characteristics of carbon dioxide have been investi- gated by experiment. The experiments have been carried out for a seamless stainless steel tube of the outer diameter of 9.55 mm, the inner diameter of 7.75 mm and the length of 5.0 m. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Experiments were conducted with$CO_2$of purity 99.99% at saturation temperatures of 0.0 to 10.5$^{\circ}C$, heat fluxes of 12 to 27kW/$m^2$s and mass fluxes of 212 to 530 kg/$m^2$s. The heat transfer coefficients of $CO_2$are decreased as the vapor quality increases and these phenomena are explained by dimensionless Weber and Bond numbers. The heat transfer coefficients of$CO_2$increase when the heat and mass fluxes increase, and the saturation temperature effects are minor in the test range of this study. The present experimental data are compared with six renowned correlations with root-mean-squared deviations ranging from 23.0 to 94.9% respectively.

• 수산해양교육연구
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• 제27권5호
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• pp.1310-1317
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• 2015

Two-phase flow boiling heat transfer in micro-channels was experimently investigated. The test section consisted of 15 rectangular micro-channels with a depth of 0.45mm, width of 0.20mm. The experiments were performed for heat fluxes ranging from 5.6 to 46.1kW/m2 and mass fluxes from 150 to 450kg/m2s using FC-72 as the working fluid. According to the results, at the low heat flux region, heat transfer coefficient strongly depends on the heat flux, while heat transfer coefficient at the high heat flux region was independent on the heat flux. Four correlations were used to predict the heat transfer coefficient. The measured heat transfer coefficient was compared with four correlations. It was found that Kaew-On and Wongwises's correlation well predicted the measured data, within the MAE of 40.3%.

• 동력기계공학회지
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• 제14권4호
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• pp.11-16
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• 2010

The evaporation heat transfer characteristics of propane(R-290) in horizontal small diameter tubes were investigated experimentally. The test tubes have inner diameters of 1 mm and 4 mm. Local heat transfer coefficients were measured at heat fluxes of 12, $24\;kW/m^2$, mass fluxes of 150, $300\;kg/m^2s$, and evaporation temperature of $15^{\circ}C$. The experimental results showed that the evaporation heat transfer coefficient of R-290 has an effect on heat flux, mass flux, tube diameter, and vapor quality. The evaporation heat transfer of R-290 has an influenced on nucleate boiling at low quality and convective boiling at high quality. The evaporation heat transfer coefficient of R-290 increases with decreasing inner tube diameter. And the evaporation heat transfer coefficient of R-290 is about 1~3 times higher than that of R-134a.

• 대한기계학회논문집B
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• 제28권5호
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• pp.526-534
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• 2004

A new second-moment closure model for turbulent heat fluxes is proposed on the basis of the elliptic equation. The new model satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. The predictions of turbulent heat transfer in a channel flow have been carried out with constant wall heat flux and constant wall temperature difference boundary conditions respectively. The velocity field variables are supplied from the DNS data and the differential equations only fur the mean temperature and the scalar flux are solved by the present calculations. The present model is tested by direct comparisons with the DNS to validate the performance of the model predictions. The prediction results show that the behavior of the turbulent heat fluxes in the whole region is well captured by the present model.

• International Journal of Air-Conditioning and Refrigeration
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• 제9권3호
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• pp.57-62
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• 2001

Falling film heat transfer analyses with aqueous lithium bromide solution were performed 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 flux were obtained. As the film flow rate of the solution increased, the generation fluxes of water decreased for both tubes due to the fact that the heat transfer resistance increased with the film thickness. 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 20K for a smooth tube, and at 10K for a knurled tube. The increased performance of the knurled tube was supposed to mainly come from the effect of the increased heating surface area.

• 한국해양학회지:바다
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• 제19권3호
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• pp.169-179
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• 2014

황해 남동부 해역에 설치한 해양부이(YSROB)에서 약 27개월간 관측된 장파, 단파 복사량을 포함한 대기, 해양 변수와 COARE 3.0 알고리즘을 이용하여 월평균 해양-대기간 열속을 산출하고 기존 연구결과와 비교하였다. YSROB 위치에서 열속은 순 단파복사(Qi)에 의해 해양은 대기로부터 열을 얻고 순 장파복사($Q_b$), 현열($Q_h$), 잠열($Q_e$)에 의해서 열손실이 일어난다. 전체 열손실 중 $Q_e$에 의한 손실이 51%로 가장 크게 나타났으며 $Q_b$와 $Q_h$에 의한 손실은 각각 34%, 15% 이다. 순열속($Q_n$)은 $Q_i$가 최대인 5월에 최대($191.4W/m^2$)이며 모든 열속 성분이 최소인 12월에 최소($-264.9W/m^2$)이다. 연평균 $Q_n$은 $1.9W/m^2$ 이지만 관측기기의 정확도에 의한 오차산정 결과(최대 ${\pm}19.7W/m^2$)를 고려하면 무시할 정도로 작다. YSROB과 동일한 위치에서의 기존 월별 열속 산출 결과는 YSROB에서 실측값에 기반한 열속에 비해 여름철 $Q_i$가 약 $10{\sim}40W/m^2$ 과소 평가된 반면에 겨울철에는 $Q_e$와 $Q_h$에 의한 열 손실이 각각 약 $50W/m^2$, $30{\sim}70W/m^2$ 과다하게 산출되었다. 이로 인하여 해양이 열을 얻는 4월~8월에는 기존 연구에서의 열 획득량이 본 연구 결과보다 적게 나타나며, 해양이 열을 잃는 겨울철에는 기존 연구에서의 해양으로부터의 열 손실이 본 연구 결과에 비해 크게 나타난다. 특히, 12월과 1월의 $Q_n$ 차이는 약 $70{\sim}130W/m^2$에 달한다. 장기적인 재분석장(MERRA) 분석 결과에 의하면 이와 같은 월평균 열속의 차이는 연변동 등 시간 변동에 의한 것이 아니라 열속 산출 시 사용된 자료의 부정확성에 기인하는 것으로 판단된다. 본 연구 결과로부터 기존의 기후적인 열속을 연구에 활용하거나 수치모델에 사용함에 있어 주의가 요망된다.

• 한국습지학회지
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• 제18권2호
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• pp.166-172
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• 2016

전 지구 지표 순환 분석을 위하여 지표와 대기 사이의 에너지 교환에 대한 분석이 필수적이다. 이러한 에너지 교환의 정량화를 위하여 다양한 지면 모형에 대한 연구가 진행되고 있다. 다양한 모형들 중 Common Land Model(CLM)과 Variable Infiltration Capacity(VIC) 모형을 활용한 연구가 활발히 수행되고 있다. CLM은 발전된 지면 모형의 형태로 적은 사용자 변수로 현실적인 결과를 산출한다는 장점이 있다. VIC 모형 또한 대표적인 지면 모형 중 하나로 에너지 인자 및 유출량 모의를 위하여 전 세계적으로 다양한 분야에서 활용되고 있다. 본 연구에서는 미국 캘리포니아 주 SS-CZO 사이트를 대상으로 CLM과 VIC 모형을 활용하여 주요 에너지 인자 인 순복사량, 현열, 잠열을 모의하였다. 순복사량과 현열 모두 두 모형에서 양호한 결과를 보이나, 강우 발생 시 CLM은 잠열과 현열을 과소모의하는 경향을 나타내었다. 잠열은 CLM의 모의 결과가 잠열을 과소모의 한 VIC 모형에 비하여 관측된 잠열의 경향을 더 잘 모의하는 것으로 나타났다. 이러한 에너지 인자 모의 및 모형의 장단점에 대한 분석을 통하여 CLM과 VIC 모형의 활용가능성 및 다양한 모형 활용의 필요성을 확인하였다.

• 대기
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• 제26권3호
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• pp.473-482
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• 2016

An accurate determination of turbulent fluxes over an urban area is a challenging task due to its morphological diversity and associated flow complexity. In this study, an eddy covariance (EC) method is applied over a highly heterogeneous urban area in a small city (Gongju), South Korea to investigate the quantitative influence of 'coordinate tilt' in determining the turbulent fluxes of sensible heat, latent heat, momentum, and carbon dioxide mass. Two widely-used coordinate transform methods are adopted and applied to eight directional sections centered on the site to analyze a 1-year period EC measurement obtained from the urban site: double rotation (DR) and planar fit (PF) transform. The results show that mean streamline planes determined by the PF method are distinguished from the sections, representing morphological heterogeneity of the site. The sectional pitch angles determined by the DR method also compare well with those in the PF method. Both the PF and DR methods show large variabilities in the determined streamline planes at each directional section, implying that flow patterns may form in a complicate way due to the surface heterogeneity. Resulting relative differences of the turbulent fluxes, defined by $(F_{DR}-F_{PF})/F_{DR}$, are found on average +13% in sensible heat flux, +21% in latent heat flux, +37% in momentum flux, and +26% in carbon dioxide mass flux, which are larger values than those reported previously for fairly homogeneous natural sites. The fractional differences depend significantly on wind direction, showing larger differences in northerly winds at the measurement site. It is also found that the relative fractional differences are negatively correlated with the mean wind speed at both stable/unstable atmospheric conditions. These results imply that EC turbulent fluxes determined over heterogeneous urban areas should be carefully interpreted with considering the uncertainty due to 'coordinate tilt' effect in their applications.