• Structural Engineering and Mechanics
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• v.29 no.5
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• pp.551-565
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• 2008

Transient solution of asymmetric mechanical and thermal stresses for hollow cylinders made of functionally graded material is presented. Temperature distribution, as function of radial and circumferential directions and time, is analytically obtained, using the method of separation of variables and generalized Bessel function. A direct method is used to solve the Navier equations, using the Euler equation and complex Fourier series.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1359-1364
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• 2004

An experiment was performed to study the evaporation heat transfer and the pressure drop characteristics of liquid nitrogen in a horizontal stainless steel tube with wire coil inserts. The inner diameter of test tube is 4.3mm and the length is 1.5m. Four wire coils having different pitch and thickness were inserted into the plain test tube. The wire coil length is 1.5m and the diameter is 3.65mm with thickness of 0.5mm and 0.9mm. Experiments were conducted at saturation temperature of $-191^{\circ}C$ mass flux from 200 to 370 $kg/m^{2}s$ and heat flux of 62 $kW/m^{2}$. Direct heating method was used to apply heat to the test section. Boiling heat transfer coefficients of both the plain and the enhanced tubes were calculated. Pressure drops between inlet and outlet side of test section were also measured, and they are used to estimate EPR(Enhancement Performance Ratio).

• Journal of the Korean Society for Precision Engineering
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• v.23 no.9 s.186
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• pp.164-173
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• 2006

The cooling capacity of a micro-heat pipe is mainly governed by the magnitude of capillary pressure induced in the wick structure. For microchannel wicks, a higher capillary pressure is achievable for narrower and deeper channels. In this study, a metallic micro-heat pipe adopting high-aspect-ratio microchannel wicks is fabricated. Micromachining of high-aspect-ratio microchannels is done using the laser-induced wet etching technique in which a focused laser beam irradiates the workpiece placed in a liquid etchant along a desired channel pattern. Because of the direct writing characteristic of the laser-induced wet etching method, no mask is necessary and the fabrication procedure is relatively simple. Deep microchannels of an aspect ratio close to 10 can be readily fabricated with little heat damage of the workpiece. The laser-induced wet etching process for the fabrication of high-aspect-ratio microchannels in 0.5mm thick stainless steel foil is presented in detail. The shape and size variations of microchannels with respect to the process variables, such as laser power, scanning speed, number of scans, and etchant concentration are closely examined. Also, the fabrication of a flat micro-heat pipe based on the high-aspect-ratio microchannels is demonstrated.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.68-73
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• 2006

The present paper deals with an experimental study of boiling heat transfer characteristics of R-290, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal smooth minichannel with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method applied for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. The experiments were conducted with R-290 with purity of 99.99% at saturation temperature of 0 to $10^{\circ}C$. The range of mass flux is $50{\sim}250kg/m^2s$ and heat flux is $5{\sim}20kW/m^2$. The heat transfer coefficients of R-290 increases with increasing mass flux and saturation temperature, wherein the effect of mass flux is higher than that of the saturation temperature, whereas the heat flux has a low effect on increasing heat transfer coefficient. The significant effect of mass flux on heat transfer coefficient is shown at high quality, the effect of heat flux on heat transfer coefficient at low quality shows a domination of nucleate boiling contribution. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. Zang et al.'s correlation(2004) gave the best prediction of heat transfer coefficient.

• Proceedings of the Korean Nuclear Society Conference
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• 1999.05a
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• pp.107-107
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• 1999

Direct-contact condensation experiments of atmospheric steam and steam/air mixture on subcooled water flowing co-currently in a rectangular channel are carried out uszng an infrared thermal camera system to develop a temperature measurement method. The inframetrics Model 760 Infrared Thermal Imaging Radiometer is used for the measurement of the temperature field of the water film for various flow conditions. The local heat transfer coefficient is calculated using the bulk temperature gradient along the (low direction. It is also found that the temperature profiles can be used to understand the interfacial condensation heat transfer characteristics according to the flow conditions such as noncondensable gas effects, inclination effect, and flow rates.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.10a
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• pp.534-539
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• 1997

A CHF table method has been assessed in this study for rod bundle CHF predictions. At the conceptual design stage for a new reactor, a general critical heat flux (CHF) prediction method with a wide applicable range and reasonable accuracy is essential to the thermal-hydraulic design and safety analysis. In many aspects, a CHF table method (i.e., the use of a round tube CHF table with appropriate bundle correction factors) can be a promising way to fulfill this need. So the assessment of the CHF table method has been performed with the bundle CHF data relevant to pressurized water reactors (PWRs). For comparison purposes, W-3R and EPRI-1 were also applied to the same data base. Data analysis has been conducted with the subchannel code COBRA-IV-I. The CHF table method shows the best predictions based on the direct substitution method. Improvements of the bundle correction factors, especially for the spacer grid and cold wall effects, are desirable for better predictions. Though the present assessment is somewhat limited in both fuel geometries and operating conditions, the CHF table method clearly shows potential to be a general CHF predictor.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.1
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• pp.171-175
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• 2015

In this paper, because of global environmental problems such as energy shortage and abnormal climate, green energy development and energy saving technology development is being magnified. Heat insulation, the most basic and traditional energy saving method, is highly expected. Especially, simple and advanced heat insulation technique based on Nano material is promising future technology. The railway system also requires heat insulation. Especially, traditional heat insulator such as glass wool has been adopted frequently to rolling stock. The heat insulator is also adopted in general railway service buildings. Take account of cost-effective heat blocking performance, the heat insulator shall be adopted in diverse fields such as electric power, railroad, signal and communication. The only and direct solution for this problem is installing heat insulator with outstanding endurance, corrosion resistance and heat resistance to block outer heat Upgrading heat specification of equipment can be a solution, but since price and maintenance cost rise severely, this plan might be incongruous. In this research, energy saving effect of ultrathin heat insulator film was demonstrated by installing the film on roof of electrical room.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.2
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• pp.101-108
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• 2013

This paper is presented an experimental study of flow boiling heat transfer characteristics of ammonia, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal small tube with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method is applied for supplying heat to the refrigerant, where the test tube is uniformly heated by electric current. The local heat transfer coefficients were obtained over a heat flux range of 20 to $80kW/m^2$, a mass flux range of 50 to $500kg/m^2s$, a saturation temperature range of 0 to $10^{\circ}C$, and quality up to 1.0. The pressure drops increase with increasing mass flux and heat flux, and with decreasing saturation temperature. The heat transfer coefficients increase with increasing mass flux and saturation temperature in middle and high quality region. And the local heat transfer coefficient increase with increasing heat flux in low quality region. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. A new boiling heat transfer coefficient correlation based on the superposition model for ammonia in small tubes is developed average deviation of -0.17% and mean deviation of 10.85%.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.11
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• pp.906-914
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• 2006

The present paper dealt with an experimental study of boiling heat transfer characteristics of R-290. Pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal smooth minichannel were obtained with inner tube diameter of 3.0 mm and length of 2,000 mm. The direct electric heating method was applied for supplying a heat to the refrigerant uniformly. The experiments were conducted with R-290 purity of 99.99%, at saturation temperature of 0 to $10^{\circ}C$, a mass flux range of $50{\sim}250kg/m^2s$, and a heat flux range of $5{\sim}20kW/m^2$. The heat transfer coefficients of R-290 increased with increasing mass flux and saturation temperature, wherein the effect of mass flux was higher than that of the saturation temperature. Heat flux has a low effect on the increasing of heat transfer coefficient. The heat transfer coefficient was compared with six existing heat transfer coefficient correlations. The Zhang et al.'s correlation (2004) gave the best prediction of heat transfer coefficient. A new correlation to predict the two-phase flow heat transfer coefficient was developed based on the Chen correlation. The new correlation predicted the experimental data well with a mean deviation of 11.78% and average deviation of -0.07%.

• Korean Journal of Food Science and Technology
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• v.27 no.2
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• pp.176-180
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• 1995

Direct contact heat exchange (DCHE) method has been employed to investigate the separation of ethanol from dilute aqueous solutions. Bubbles at high temperature were dispersed into a continuous liquid phase, generating temperature gradient in air-liquid interface, which causes heat and mass transfer accordingly. The experiments were performed in the ranges of jet regime air flow. The air-water stripping coefficient increased $5{\sim}10,\;and\;1{\sim}1.5$ times as temperature and air flow rate increased, respectively. The recovery ratio based on the initial ethanol concentration reached into 80% at the air flow rate of 84.88 m/min. The initial ethanol concentration showed little effect on the stripping coefficient and the recovery ratio.