• Proceedings of the SAREK Conference
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• 2005.11a
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• pp.268-273
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• 2005

Because of environmental issues caused by CFC, HCFC or HFC refrigerants, new alternative refrigerants has gained a significant attention. This paper presents experimental information on heat transfer coefficient and pressure drop behavior during evaporation process of R32/290 mixtures in a horizontal smooth tube. A smooth tube with outer diameter of 5 mm and length of 5 m was selected as a test tube. Heat transfer coefficients and pressure drop characteristics were measured for a range of mass fluxes from 497 to 994 $kg/m^2s$, heat fluxes from 12 to 20 $kW/m^2$ and for several mixture compositions(100/0, 75/25, 58.4/41.6, 2s/75, 100/0 by wt% of R32/290). The differences of measured heat transfer characteristics among various R32/290 refrigerant mixtures were analyzed for various compositions.

• Proceedings of the SAREK Conference
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• 2008.06a
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• pp.154-159
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• 2008

Two-phase flow boiling heat transfer of R-410A in horizontal small tubes was reported in the present experimental study. The local heat transfer coefficients were obtained over a heat flux range of 5 to 40 kW/$m^2$, a mass flux range of 170 to 600 kg/$m^2s$, a saturation temperature range of 3 to $10^{\circ}C$, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5 and 3.0 mm, and lengths of 330 and 3000 mm, respectively. The section was heated uniformly by applying a direct electric current to the tubes. The effects on heat transfer of mass flux, heat flux, inner tube diameter, and saturation temperature were presented. The experimental heat transfer coefficient is compared with six existing heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model for R-410A in small tubes was developed with mean deviation of 10.13%.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.1
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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.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.10
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• pp.932-940
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• 2000

An experimental study on evaporative heat transfer characteristics in micro-fin tubes before and after expansion process has been performed with R-22. Single-grooved micro-fin tubes with outer diameter of 9.52 mm were used as test sections, and it was uniformly heated by applying direct current to the test tubes. Experiments were conducted at mass flow rates of 20 and 30 kg/hr. For each mass flow rate condition, evaporation temperature was set at 5 and $15^{\circ}C$and heat flux was changed from 6 to 11 kW/$m^2$ The evaporative heat transfer coefficient of micro-fin tubes after expansion is decreased because of the crush of fins and enlargement of inner diameter compared to that before expansion. Convective boiling effect decreased remarkably at higher quality range in the micro-fin tube after expansion, and the difference of the heat transfer coefficient in micro-fin tubes before and after expansion was greater for higher quality region. The evaporative heat transfer coefficient of the micro-fin tube after expansion was 19.9% smaller on the average than that before expansion.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.2
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• pp.161-167
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• 2002

In this study, external condensation heat transfer coefficients (HTCs) of two non-azeotropic refrigerant mixtures of HFC32/HFC134a and HF0134a/HCF0123 at various compositions were measured on both low fin and Turbo-C enhanced tubes of 19.0 mm outside diameter All data were taken at the vapor temperature of 39$^{\circ}C$ with a wall subcooling of 3- 8 K. Test results showed that HTCs of the tested mixtures on the enhanced tubes were much lower than the ideal values calculated by the mass fraction weighting of the pure compo- nents'HTCs. Also the reduction of HTCs due to the diffusion vapor film was much larger than that of a plain tube. Unlike HTCs of pure fluids, HTCs of the mixtures measured on enhanced tubes increased as the wall subcooling increased, which was due to the sudden break up of the vapor diffusion film with an increase in wall subcooling. Finally, heat transfer enhancement ratios for mixtures were found to be much lower than those of pure fluids.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.46-46
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• 2008

스텔스 기능을 가진 군사용 항공기는 레이다 망의 추적을 피하기 위해 일반 냉각수 대신에 절연유를 냉각매체로 사용한다. 그러나 절연유는 물에 비하여 열전달특성이 매우 낮기 때문에 항공기 전기전자 기기/부품 발열부를 효과적으로 냉각시키지 못하는 한계성을 가지고 있다. 따라서 본 연구에서는 나노유체(Nanofluid) 개념을 이용하여 절연유에 알루미나 및 질화알루미늄 나노분말을 미랑 분산시킨 나노절연유를 제조하고 이것의 열전달특성을 순수 절연유의 그것과 비교 평가함으로써, 냉각특성이 크게 개선된 새로운 냉매로서의 적용 가능성을 확인하고자 하였다. 다만 나노절연유를 제조함에 있어서 가장 큰 장애물은 오일에 대한 분산성 확보에 있기 때문에, 비드밀 및 초음파를 이용한 나노분말 응집체의 습식분산 및 분산제를 이용한 친유성 표면개질을 동시에 수행함으로써 장시간 안정된 분산성을 확보하도록 노력하였다. 나노유체의 열전도도 및 대류열전달계수는 비정상열선법을 이용하여 측정하였으며, 유체 속의 분말 분산 상태는 원심력을 이용한 분산안정성 평가장치 및 cryo FE-SEM을 이용하여 확인하였다. 또한 분말 형상이 대류열전달에 미치는 영향을 조사하기 위하여 알루미나 나노분말은 구상과 침상의 분말을 모두 사용하였고, 질화알루미늄 외에 다이아몬드 나노절연유도 함께 제조, 평가함으로써 냉각 및 절연특성, 그리고 물리화학적 안정성이 우수하고 실적용 가능성이 가장 높은 재료를 도출하고자 하였다.

• Korean Chemical Engineering Research
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• v.45 no.1
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• pp.17-24
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• 2007

The purpose of the present study is to identify the drying cylinder model in paper plants and to analyze characteristics of process responses for changes in input variables. The model developed in this work is based on actual plant operation data where the steam pressure applied to the cylinder behaves as the major variable. It is found that heat transfer coefficients from the condensate to the canvas could be represented as empirical relations based on heat conductivities and operation data. The effectiveness of the cylinder model is demonstrated by the measured moisture contents and web temperature. Stability of the drying process is analyzed based on the transfer functions derived from the cylinder model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.9
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• pp.695-703
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• 2008

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in a compact heat exchanger with flat tubes and continuous plate fins according to the aspect ratio. RNG k-$\varepsilon$ model is applied for turbulence analysis. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous correlations for circular tubes. The numerical conditions are considered for the aspect ratios ranging from 3.06 to 5.44 and Reynolds number ranging from 1000 to 10,000. The results showed that heat transfer coefficients decreased with the increase of aspect ratio. From the calculated results a correlation of Colburn j factor for the considered aspect ratio in the compact heat exchanger system is suggested. The predicted results in this study can be applied to the optimal design of air conditioning system.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.12
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• pp.994-1001
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• 2007

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in a compact heat exchanger with circular tubes and continuous plate fins. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous experimental correlations. Three models of standard and RNG $k-{\varepsilon}$, and Reynolds stress are applied for turbulence model applicability. Predicted heat transfer coefficient from the models of standard and RNG $k-{\varepsilon}$ are very close to those of the heat transfer correlations while there are relatively large difference, more than 17 percentage in the result from the Reynolds stress model. From the calculated results a correlation for Colburn j factor in the compact heat exchanger system is suggested.

• Proceedings of the SAREK Conference
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• 2007.11a
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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.