• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.9
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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.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.14 no.4
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• pp.205-211
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• 2014

With a development of modern industry, demand for electric power is rapidly increasing and the capacity of power transfer is required to become bigger and bigger. At power station, the high-voltage power cable is used as the only method in order to transfer electric power. In this paper, we have analyzed the deterioration characteristics of 6.6kV power cable systems in operation. For the time duration of 2000 days, we have measured the cable in operation in order to extract the data for the deterioration characteristics. By analyzing the data by means of several steps of filtering processes, we could obtain the linear relations of insulation resistances as a function of time. Furthermore, we can verify that the progress characteristics in deterioration process of 6.6kV power cable systems follows the process of heat deterioration.

• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.2
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• pp.27-37
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• 2019

The shell and tube heat exchangers installed in the propulsion system test complex (PSTC) at the Naro Space Center heats cryogenic helium to 500 K with a heat transfer oil. As the experimental helium outlet temperature was lower than expected (less than 100 K), the boundary layer effect of the heat transfer oil is predicted to be the cause of the performance deterioration. A computational fluid dynamics (CFD) analysis was performed to verify where the boundary layer effect exists; however, the boundary layer effect has no significant impact on the performance of the heat exchanger. An alternative method to improve the performance of the heat exchanger by changing the heat transfer oil has been discussed in this paper. The low viscosity and high thermal conductivity at high temperature (~500 K) of heat transfer oil at the shell-side are required to improve the thermal performance of the heat exchanger. The experimental performance of the heat exchanger, used to exchange heat between the cryogenic helium and hot heat transfer oil at the PSTC are summarized in this paper.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.1
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• pp.170-178
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• 2019

Recently, as the interest in energy savings has increased, there has been increasing use of LED lighting, which is an eco-friendly device that replaces high energy consuming fluorescent lamps and incandescent lamps. In the case of a high output LED, however, the life time is shortened due to deterioration caused by heat generation. As a solution to this problem, this paper evaluated the LED life extension effect by increasing the convective heat transfer coefficient of the heat sink surface for LED packaging. A roughing process was carried out using sandpaper and sand blasting. The changes in surface roughness and surface area after each surface treatment process were evaluated quantitatively and the convective heat transfer coefficient was measured. When sandblasting and sandpaper were used to roughen the aluminum surface, a higher convection heat transfer coefficient was obtained compared to the untreated case, and a high heat dissipation efficiency of 82.76% was obtained in the sandblast treatment. Therefore, it is expected that the application of heat dissipation to the heat sink will extend the lifetime of the LED significantly and economically by increasing the heat efficiency.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.12
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• pp.82-90
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• 2019

This analysis demonstrates the effective removal of heat generated from a solar panel's output degradation factor solar cells (the solar panel's output deterioration factor), and solves the problems of oxidation and corrosion in existing metal heat sinks. The heat-dissipating test specimen was prepared using carbon materials; then, its thermal conductivity and its effectiveness in reducing temperatures were studied using heat transfer numerical analysis. As a result, the test specimen of the 30g/㎡ basis weight containing 80% of carbon fiber impregnated with carbon ink showed the highest thermal conductivity 6.96 W/(m K). This is because the surface that directly contacted the solar panel had almost no pores, and the conduction of heat to the panels appeared to be active. In addition, a large surface area was exposed to the atmosphere, which is considered advantageous in heat dissipation. Finally, numerical analysis confirmed the temperature reduction effectiveness of 2.18℃ in a solar panel and 1.08℃ in a solar cell, depending on the application of heat dissipating materials.

• Proceedings of the Korean Nuclear Society Conference
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• 2004.10a
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• pp.277-278
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• 2004

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.5
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• pp.384-392
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• 2006

Reported are the heat transfer characteristics of a two-phase loop thermosyphon (TLT) with nanofluids consisted of nano-size silver particles and distilled water as the working fluid. The nanofluids used in the present study are dispersed solutions with various amount of silver nanoparticle in distilled water. It is seen from the present study that the heat transfer performance of the test TLT with nanofluids increased as much as about 2 times higher than that of a TLT with pure water as the working fluid based on same heat flux. The study also showed that there was no deterioration of the TLT performance with time, up to a period of 8 days of continuous operation which implies that there was no coagulation of nanoparticles within the working nanofluid during the operation of the test TLT.

• Journal of Biosystems Engineering
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• v.32 no.6
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• pp.422-429
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• 2007

To prevent deterioration of agricultural products during cold transportation, optimized temperature control is essential. Because the control of temperature and thermal uniformity of transported products are mainly governed by cooling air flow pattern in the transportation equipment, the accurate understanding and removal of appearance of stagnant air zone by poor ventilation is key to design of optimized cooling environment. The objectives of this study were to develop simulation model to predict the airflow and heat transfer phenomena in the cold container and to evaluate the effect of fan blowing velocity on the temperature level and uniformity of products using the CFD approach. Comparison of CFD prediction with PIV measurement showed that RSM turbulent model reveals the more reasonable results than standard $k-{\varepsilon}$ model. The increment of fan blowing velocity improved the temperature uniformity of product and reduced almost linearly the averaged temperature of product.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.289-292
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• 2007

To prevent the damage of glass lens molds and deterioration of glass lenses using in progressive GMP process, a thermal stress and a deformation of the glass lens molds at forming temperature should be considered in the design step. In this study, as a fundamental study to develop a multi cavity mold used in an aspheric glass lens molding, a heat transfer and a thermal stress analysis were carried out for the case of one cavity glass lens mold used in progressive GMP process. Finally, using analysis results, we estimated the thermal stress in a glass lens mold and predicted a modified height of guide ring that determines the forming height of a glass lens.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.4
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• pp.243-250
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• 2011

In order to investigate the effects of PAG oil concentration on heat transfer performance and pressure drop during gas cooling process of $CO_2$, the experiments on fin-tube heat exchanger of $CO_2$ heat pump were performed. The experimental apparatus consists of a gas cooler, a heater, a chiller, a mass flow meter, a pump and measurement system. Experiments were conducted in various experimental conditions, which were inlet temperature($110^{\circ}C$), mass flow rates (50, 55, 60, 65, 70 g/s) and PAG oil concentration(0 to 2.6 wt%). Heat transfer rate decreased with the increase of the oil concentration and the decrease of inlet pressure. And pressure drop increased with the increase of the oil concentration and mass flow rate of refrigerant. The COP reduction by deterioration of gas cooler performance with oil concentration was analyzed. When inlet pressure of gas cooler is 100 bar, the COP reduction was estimated by 6% under 1 wt% of oil concentration.