• International Journal of Aeronautical and Space Sciences
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• v.11 no.1
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• pp.19-25
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• 2010

A computational study for the optimal design of heat exchangers (HX) used in a high temperature and high pressure system is presented. Two types of air to air HX are considered in this study. One is a single-pass cross-flow type with straight plain tubes and the other is a two-pass cross-counter flow type with plain U-tubes. These two types of HX have the staggered arrangement of tubes. The design models are formulated using the number of transfer units ($\varepsilon$-NTU method) and optimized using a genetic algorithm. In order to design compact light weight HX with the minimum pressure loss and the maximum heat exchange rate, the weight of HX core is chosen as the object function. Dimensions and tube pitch ratio of a HX are used as design variables. Demanded performance such as the pressure loss (${\Delta}P$) and the temperature drop (${\Delta}T$) are used as constraints. The performance of HX is discussed and their optimal designs are presented with an investigation of the effect of design variables and constraints.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.3
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• pp.223-231
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• 2005

When test tubes for PCM with melting point lower than a room temperature are installed vertically as the T-history method proposes, there exists a temperature distribution in the longitudinal direction by natural convection, which lowers the precision of measured heat-of-fusion. The purpose of the present work is to improve the precision by arranging the test tubes horizontally, while maintaining the simplicity and convenience. Assuming that the amount of heat transfer is very small through the vapor space formed in the upper part of the tubes by volumetric change, the obtained value by the T-history method using the latent heat period bounded by two inflection points is in good agreement with that of the literature. Also, the scattering of measured values by the proposed method decreases markedly compared to that of vertical positioning.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.3
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• pp.220-227
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• 2007

Heat transfer and momentum transfer under conditions of both oscillating flow and oscillating pressure within pulse tubes show very different behavior from those for steady state conditions. The analytic solutions of axial velocity and temperature of the gas within pulse tubes were obtained by assuming that the variations in pressure and temperature were purely sinusoidal and small. The shear stress and the heat flux at the tube wall obtained from the solutions are expressed in terms of the cross-sectional averaged velocity, the difference between mean temperature and instantaneous cross-sectional averaged temperature and the difference between mean pressure and instantaneous pressure. It is shown that the complex shear factor, which has been applied to momentum transfer of incompressible oscillating flow, and the complex Nusselt number, which has been applied to either heat transfer with oscillating pressure only or heat transfer of incompressible oscillating flow, could also be used for momentum transfer and heat transfer subjected to both oscillating flow and oscillating pressure, respectively.

• Proceedings of the SAREK Conference
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• 2008.11a
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• pp.531-536
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• 2008

This paper presents experimental result of characteristics about Closed circuit Cooling Tower having a rated 2RT. The experimental apparatus has been set-up with a conventional type system. The test section is heat exchangers of cooling tower that consist of different vertical tubes, 15.88mm with 10 rows and columns and 19.05 mm with 8 rows and 12 columns. The main results were summarized as follows : The values of heat and mass transfer coefficients of cooling tower operating with two paths are higher than these with one path. Cooling capacity per unit volume using 15.88mm tubes are higher than 19.05mm tubes.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.11
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• pp.1028-1034
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• 2004

Recently, the ice storage system using ice slurry has been used increasingly since it has been introduced where the rapid cooling load change is required. Because it overcomes a decrease of the melting performance and an increase of the thermal resistance on the ice layer in static ice thermal storage system. This study is performed to understand the effects of transporting ice slurry through horizontal, vertical and inclined tubes ($30^{\circ},\;45^{\circ}$). It used propylene glycol-water solution and ice particles (diameter of about 2 mm) in this experiment. The experiments were carried out under various conditions, with concentration of water solution ranging from 0 to $20wt\%$, and velocity of water solution at the entry ranging from 1.5 to 2.5 m/s. The results were as follows: Regarding the angle of inclined tube, the highest pressure loss was measured for vertical tube and the pressure loss for $45^{\circ},\;30^{\circ}$, horizontal straight tubes were lower successively. The lowest pressure loss in these tubes was measured at velocity of $2.0{\sim}2.5m/s$ and concentration of $10wt\%$. The outlet IPF was likewise stable in these ranges.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.9
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• pp.756-765
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• 2002

Experimental work was performed on the flow characteristics of R-22 and R-l34a in small diameter tubes. The experimental apparatus was made up of liquid pump, pre-heater, sight-glass, condenser and measurement instruments. The sight-glass for flow pattern observations was located at the outlet of the pre-heater. The experiment was carried out to show the flow characteristics of R-22 and R-l34a. Data were taken with test conditions in the following ranges; the mass flux was ranged from 100 to 1,000 kg/$m^2s$, the saturation temperature was $30^{\circ}C$ and the vapor quality was ranged from 0.1 to 0.9. The main results were summarized as follows; In the flow patterns during evaporation, the annular flow in a 2 mm inner diameter tube occurred at a relatively lower quality and mass velocity, compared to the flow in a 8mm inner diameter tube. The evaporation flow in small diameter tubes has been shown major deviations with the Mandhane, Taitel-Dukler's and Wambs-ganss' flow pattern maps but it was similar to the Dobson's flow pattern map.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.1
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• pp.48-56
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• 2021

In this study, the heat transfer process around the finned channel tubes is numerically examined. Serially arranged tubes of an evaporator were used for heat exchange. The numerical analysis results confirmed that the vortex generated at the rear of the channel pipe was caused by the fin. Furthermore, it was also confirmed that the temperature difference was large between the inlet and outlet ends of the fin. The temperature of the location where the fin was attached to the channel pipe was found to be close to the surface temperature of the channel wall. However, the temperature rose rapidly closer to the ambient air temperature of 350 K towards the fin end, located at a distance of 0.035 m; it was found to have a significant influence on the heat transfer around the fin-attached channel tube. The wider the vertical flow path, the lower the total heat transfer coefficient. However, the overall heat transfer coefficient increased as the horizontal flow path narrowed. The increment is attributed to an increase in the heat transfer amount due to increased heat transfer surface.

• Journal of the Korean association of regional geographers
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• v.13 no.6
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• pp.609-622
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• 2007

This paper examines the lava tubes designated as a natural monument in Jeju Island, and then makes the several suggestions for conserving and managing them. The cave entrances should be scientifically and strictly managed to prevent the deterioration, and to preserve the interior ecosystem, of the lava tubes. The land use and vegetation of the ground surface over the lava tubes should be also systematically managed; the destroyed sand dunes must be recovered for the development of speleothem in the lava tubes, and the plants with deep roots must be removed for the prevention of ceiling collapse. The provincial authorities should manage the ground surface over the lava tubes as a public land to remove the factors threatening the stability of the lava tubes. Since the roads crossing the ground surface are likely to cause the collapse of the lava tubes, new roads bypassing the caves need to be constructed, and warning signs should be also installed at the traverse points. A monitoring system on the cultural properties protection area over the lava tubes could be effectively established by encouraging residents to participate the conservation program of lava tubes. Since the falling blocks from ceilings have been usually observed in the lava tubes, it is urgent to examine their stability. In particular, the dangerous sections must be continuously monitored for taking a quick measure against the collapse. The air environments of lava tubes need to be scientifically analyzed based upon the long-term observations. The lighting equipments must be improved to minimize the green pollution in the lava tubes open to the public. The numbers of visitors should be also controlled to prevent the destruction and contamination of lava tubes caused by a large number of visitors.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.1199-1204
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• 2006

The pressure drop and heat transfer coefficient were measured at room temperature in CFB heat exchanger with multiple vertical tubes. Also the circulation rate of solid particles was measured. The theoretical model for predicting heat transfer coefficient using the solid flowrate was developed in this study. The model predictions were compared with the measured heat transfer coefficient to show relatively good agreement.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.12 no.1
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• pp.78-84
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• 2016

The FHX (Forced-draft sodium-to-air Heat Exchanger) employed in the ADHRS (active decay heat removal system) is a shell-and-tube type counter-current flow heat exchanger with M-shape finned-tube arrangement. Liquid sodium flows inside the heat transfer tubes and atmospheric air flows over the finned tubes. The unit is placed in the upper region of the reactor building and has function of dumping the system heat load into the final heat sink, i.e., the atmosphere. Heat is transmitted from the primary cold sodium pool into the ADHRS sodium loop via DHX (decay heat exchanger), and a direct heat exchange occurs between the tube-side sodium and the shell-side air through the FHX tube wall. This paper describes the DHRS and the structural design of the FHX.