• Proceedings of the KWS Conference
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• 2009.11a
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• pp.44-44
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• 2009

While the pinch instability theory (PIT) has been widely employed to analyze the spray transfer mode in the gas metal arc welding (GMAW), it cannot predict the detaching drop size accurately. The PIT is modified in this work to increase the accuracy of prediction and to simulate the molten tip geometry to be more physically acceptable. Since the molten tip becomes a cone shape in the spray mode, the effective wire diameter is formulated that the effective diameter is inversely proportional to current square. Modifications are also made to consider the finite length of the liquid column and current leakage through the arc. While the effective diameter influences drop transfer significantly, the current leakage has negligible effects. The effects of modifications on drop transfer are analyzed, and the predicted drop diameters show good agreements with the experimental data of the steel wire.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.26 no.11
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• pp.522-528
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• 2014

This experimental study measured and analyzed the heat transfer and pressure drop performance of brazed plate heat exchangers. Heat exchangers with different plate numbers of 10, 20, 30, and 40 were used for measurement, and their plate pattern was identical. The experiment was performed with distilled water, by changing the hot-side flow rate, with the cold-side flow rate fixed. The experimental results were compared with the calculated values by correlations based on gasketed plate heat exchangers, and showed that the heat transfer performance and pressure drop of brazed plated heat exchangers were higher than those of gasketed plate heat exchangers. From the variation of the friction factor, it could be inferred that in the flow channels of brazed plate heat exchangers, turbulent flow occurred at Reynolds numbers higher than 500. A new correlation to predict the Nusselt number was developed, and its absolute average deviation was 2.62%, compared with the values from the experimental data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.9
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• pp.676-682
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• 2008

Numerical analysis was carried out to examine the heat transfer and pressure drop characteristics of plate heat exchangers for absorption application using Computational Fluid Dynamics(CFD) technique. A commercial CFD software package, FLUENT was used to predict the characteristics of heat transfer, pressure drop and flow distribution within plate heat exchangers. In this paper, a welded plate heat exchanger with the plate of chevron embossing type was numerically analyzed by controlling mass flow rate, solution concentration, and inlet temperatures. The working fluid is $H_2O$/LiBr solution with the LiBr concentration of 50-60% in mass. The numerical simulation shows reasonably good agreement with the experimental results. Also, the numerical results show that plate of the chevron shape gives better results than plate of the elliptical shape from the view points of heat transfer and pressure drop. These results provide a guideline to apply the welded PHE for the solution heat exchanger of absorption systems.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.5
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• pp.453-460
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• 2013

The surface roughness and heat transfer enhancement devices are known to increase the performance of a flat plate soar collector. This study includes the experiments on the effect of the several heat transfer enhancement devices inserted in duct to simulate the flat-plate solar collector. Experiment was basically at a constant heat flux on the upper duct wall. Inserted heat transfer enhancement devices are Chamfered rib $10^{\circ}$, Chamfered rib $20^{\circ}$, Rib & Groove and Rib & Dimple. Reynolds number is in the range of 2,300 to 22,000 which corresponds to turbulent regime. With the heat transfer enhancement devices, heat transfer would increase by the secondary flow and the increase of the heat transfer area. Pressure drop also increases with the insertion of the enhancement devices. Rib & Dimple model is the best in heat transfer enhancement, however, Chamfered rib $10^{\circ}$ model is the lowest in the pressure drop. Considering the heat transfer enhancement simultaneously with low pressure drop increase, performance factor was the best for the Chamfered rib $10^{\circ}$.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.05a
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• pp.70-73
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• 2006

Recently, several researches have been performed on the prediction of vertical vibration on structures by using an analytical method. However, these studies have been focused on mainly the vibration analysis through analytical modeling of structures. This study aims to investigate the characteristics of vertical vibration transfer in terms of the directions of transfer(upward transfer and downward transfer) on the shear wall building structures due to heel-drop impact forces. In order to examine the characteristics of vertical vibration transfer, the mode analysis and the impact experiment were conducted several times on two shear wall building structures. The results of this study suggest that the characteristics of vertical vibration transfer are similar in terms of the directions of transfer.

• International Journal of Highway Engineering
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• v.17 no.4
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• pp.33-40
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• 2015

PURPOSES: Evaluation of the wind speed effect on the temperature drop of an asphalt mixture during construction, by using the transient heat transfer theory and dominant convective heat transfer coefficient model. METHODS: Finite difference method (FDM) is used to solve the transient heat transfer difference equation numerically for various wind speeds and initial temperature conditions. The Blasius convective heat transfer coefficient model is adapted to account for the effect of wind speed in the temperature predictions of the asphalt mixture, and the Beaufort number is used to select a reasonable wind speed for the analysis. As a function of time and depth, the temperature of the pavement structure is predicted and analyzed for the given initial conditions. RESULTS : The effect of wind speed on the temperature drop of asphalt mixture is found to be significant. It seems that wind speed is another parameter to be accounted for in the construction specifications for obtaining a better quality of the asphalt mixture. CONCLUSIONS: It is concluded that wind speed has a significant effect on the temperature drop of the asphalt layer. Although additional field observations have to be made to reflect the effect of wind speed on the construction specifications, it appears that wind speed is a dominant variable to be considered, in addition to the atmospheric temperature.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.425-432
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• 2001

Single-phase heat transfer coefficients and pressure drops of R-22 were measured in smooth, horizontal copper tubes with inner diameters of 3.36 mm, 5.35 mm, 6.54 mm and 8.12 mm, respectively. The experiments were conducted in the closed loop, which was driven by a magnetic gear pump. Data are presented for the following range of variables: Reynolds from 1000 to 20000. Single-phase heat transfer coefficients increased by $10{\sim}30%$ as the inner diameter of tube was reduced and it was found that a well-known previous correlation, Gnielinski's correlation, was not suitable for the small diameter tubes. But the pressure drop in the small diameter tubes have been shown slightly deviations with Blauius' correlation. Based on an analogy between heat and mass transfer, the new heat transfer correlation is proposed to predict the experimental data successfully.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.7
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• pp.681-687
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• 2005

Experimental results for heat transfer characteristic and pressure gradients of HCs refrigerants R-290, R-600a, R-1270 and HCFC refrigerant R-22 during evaporating inside horizontal double pipe heat exchangers are presented. The test sections which has one tube diameter of 12.70 m with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness are used for this investigation. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were higher than that of R-22. The average evaporating heat transfer coefficient increased with the increase of the mass flux. It showed the higher values in hydrocarbon refrigerants than R-22. Hydrocarbon refrigerants have higher pressure drop than R-22 in 12.7 mm and 9.52 mm. This results form the investigation can be used in the design of heat transfer exchangers using hydrocarbons as the refrigerant for the air- conditioning systems.

• Journal of Mechanical Science and Technology
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• v.15 no.8
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• pp.1156-1164
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• 2001

Evaporation heat transfer coefficients and pressure drops were measured for smooth and micro-fin tubes with R-22 and R-410A. Heat transfer measurements were performed for 3.0m long horizontal tubes with nominal outside diameters of 9.52 and 7.0mm over an evaporating temperature range of -15 to 5$\^{C}$, a mass flux range of 68 to 211kg/㎡s, and a heat flux range of 5 to 15kW/㎡. It was observed that the heat transfer coefficient increased with mass flux. Evaporation heat transfer coefficients of R-22 and R-410A increased as the evaporating temperature dropped at a lower heat flux. Generally, R-420A showed the higher heat transfer coefficients than R-22 in the range of low mass flux, high heat flux and high evaporating temperature. Pressure drop increased with a decrease of evaporating temperature and a rise of mass flux. Pressure drop of R-22 was higher than that of R-410A at the same mass flux.

• Proceedings of the SAREK Conference
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• 2009.06a
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• pp.1141-1146
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• 2009

Experimental investigation on two-phase flow boiling heat transfer of R-410A and R-134a in horizontal small tubes is reported. The pressure drop and local heat transfer coefficients were obtained over heat flux range of 5 to $40\;kW/m^2$, mass flux range of 70 to $600\;kg/m^2s$, saturation temperature range of 2 to $12^{\circ}C$, and quality up to 1.0 in test section with inner tube diameters of 3.0 and 0.5 mm, and lengths of 2000 and 330 mm, respectively. The section was heated uniformly by applying a direct electric current to the tubes. The effects of mass flux, heat flux, and inner tube diameter, on pressure drop and heat transfer coefficient are presented. The experimental results are compared against several existing correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in small tubes is developed.