• Tribology and Lubricants
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• v.10 no.2
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• pp.43-50
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• 1994

The hydrodynamic bearing is accepted in many rotating systems because it has a large load carrying capacity. But the anisotropic pressure distribution of the bearing can arise the unstable vibration phenomenon over a certain speed. The magnetic bearing is an active element so that the unstable phenomenon of the hydrodynamic bearing, which is induced by the anisotropic support pressure of the oil film, can be controlled if the control algorithm and the controller gains are chosen appropriately. In this study, we investigate the stabilization method of the hydrodynamic bearing system composing the hybrid bearing which is the single unit of hydrodynamic bearing and magnetic bearing. The load carrying conditions of the hybrid bearing is modelled by the sum of the stiffness and damping coefficients of the hydrodynamic and the magnetic bearings in each direction. The dynamics of the rotor is analyzed by the Finite Element Method and the stability limit is determined by the eigenvalues of the hybrid bearings and shaft system. The eigenvalue study of the system shows that the stability limit of the hybrid bearing is increased compared to that of the hydrodynamic bearing. A Small increment of the stiffness and damping coefficient of the hybrid bearings by the magnetic actuators can increase the stability limit of the system. In this paper we tried to show the design references of the hybrid bearings by using the nondimensional bearing parameters. The analysis results show the possibilities of the stability limit increment of the hydrodynamic bearing system by combining the magnetic bearing.

• 한국신재생에너지학회:학술대회논문집
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• 2005.11a
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• pp.140-148
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• 2005

The molten carbonate fuel cell (MCFC) is endowed with the high potential especially in future electric power generation industry by its own outstanding characteristics. KEPCO (KEPRI) started a 100 kW MCFC system development program in 1993 and has been executed 100kW system develpilot plant successfully completed first phaseopment by 2005 on the basis of successful results of 25kW system development. In this program, the components and mechanical structure for 100 kW stack and system construction were completed on last year and now system pre - commissioning was being executed. A 100 kW MCFC power plant was constructed at the site of Boryeong Thermal Power Plant. A 100 kW MCFC system has characterized as a high pressure operation mode, CO2 recycle, and externally reforming power generation system. The 100 kW MCFC system consisted with stacks which was made by two 50 kW sub-stacks, 90 cells with 6,000 cm2 active area and BOP including a reformer, a recycle blower, a catalytic burner, an inverter, and etc. The system has been operated from 13th of September on this year and produced 50 kW AC under atmospheric pressure condition and expected to operate by the end of this year.

• Journal of Korea Foundry Society
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• v.31 no.2
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• pp.71-78
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• 2011

This study aims to investigate the formability of bipolar plates for fuel cell fabricated by vacuum die casting of ALDC 6. Cavity shape of mold is thin walled plate (size: $200mm{\times}200mm{\times}0.8mm$) with a serpentine channel (active area: $50mm{\times}50mm$). Before bipolar plate was made by HPDC, computational filling behavior and solidification was performed by MAGMA soft. The final mold design for location and direction of channel was determined by computational simulation. Also, according to injection speed conditions, simulation result was compared to actual die casting experimental result. When vacuum pressure, injection speed of low and high region is 350 mbar, 0.3 m/s and 2.5 m/s respectively, products had few casting defects. On the other hand, at the same as injection speed, without vacuum pressure, products had many casting defects between end of the channel and overflow.

• Proceedings of the KIEE Conference
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• 1997.07d
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• pp.1433-1435
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• 1997

ZnO shows the properties of wide conductivity variation, high optical transmittance, and excellent piezoelectricity. Using these properties of ZnO, the material applications were extended to sensors, SAW filters, solar cells, and display devices. This paper investigated transmittance influencing factors for thin film ZnO grown by RF magnetron sputtering. The growth rate and structural investigation were carried out in conjunction with optical transmittance characteristics of thin film ZnO. The glass substrate temperature of $175^{\circ}C$ exhibited a preferential crystallization along (002) orientation. Transmittance of ZnO film deposited at the substrate temperature of $175^{\circ}C$ showed higher than 92%. An active sputter gas was investigated with a variation of $O_2$ partial pressure from 0 to 10% in an Ar atmosphere. ZnO film grown in 100% Ar gas shows that a reduced transmittance of 82% at the short wavelengths and decreased resistivity value. As the partial pressure of $O_2$ gas increased, the optical transmittance was increased above 90% at the short wavelengths, however, resistivity was drastically increased to higher than $10^4{\Omega}$-cm.

• Journal of Korean Neurosurgical Society
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• v.58 no.1
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• pp.76-78
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• 2015

This report details a case of unexpected, severe post-operative cerebral edema following cranioplasty. We discuss the possible pathological mechanisms of this complication. A 50-year-old female was admitted to our department with sudden onset of stuporous consciousness. A brain computed tomography (CT) revealed a subarachnoid hemorrhage with intracranial hemorrhage and subdural hematoma. Emergency decompressive craniectomy and aneurysmal neck clipping were performed. Following recovery, the decision was made to proceed with an autologous cranioplasty. The cranioplasty procedure was free of complications. An epidural drain was placed and connected to a suction system during skin closure to avoid epidural blood accumulation. However, following the procedure, the patient had a seizure in the recovery room. An emergency brain CT scan revealed widespread cerebral edema, and the catheter drain was clamped. The increased intracranial pressure and cerebral edema were controlled with osmotic diuretics, corticosteroids, and antiepileptic drugs. The edema slowly subsided, but new low-density areas were noted in the brain on follow-up CT 1 week later. We speculated that placing the epidural drain on active suction may have caused an acute decrease in intracranial pressure and subsequent rapid expansion of the brain, which impaired autoregulation and led to reperfusion injury.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1253-1263
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• 1994

The experimental study is carried out to identify the combustion generated noise mechanism in free turbulent jet diffusion flames. Axial mean fluctuating velocities in cold and reacting flow fields were measured using hot-wire anemometer and LDv.The overall sound pressure level and their spectral distribution in far field with and without combustion were also measured in an anechoic chamber. The axial mean velocity is 10-25% faster and turbulent intensities are about 10 to 15% smaller near active reacting zone than those in nonreacting flow fields. And sound pressure level is about 10-20% higher in reacting flow fields. It is also shown that the spectra of the combustion noise has lower frequency characteristics over a broadband spectrum. These results indicate that the combustion noise characteristics in jet diffusion flames are dominated by energy containing large scale eddies and the combusting flow field itself. Scaling laws correlating the gas velocity and heat of combustion show that the acoustic power of the combustion noise is linearly proportional to the 3.8th power of the mean axial velocity rather than 8th power in nonreacting flow fields, and the SPL increases linearly with logarithmic 1/2th power of the heat of combustion.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.4
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• pp.288-297
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• 2009

A serpentine channel geometry often used in a fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from the intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with high aspect ratio of active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compressive forces. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds that of dropwise condensation in cathode channels.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.11
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• pp.1630-1636
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• 2003

Automobile companies and research institutions in leading automobile-manufacturing nations have recently been very active with research regarding the HCCI engine for use in future vehicles. Because HCCI engines take advantage of high compression ratio and heat release rate, they exhibit high efficiency found in compression ignition engines. HCCI engines also utilize a lean air/fuel ratio resulting in low emissions of NO$_{x}$ and PM (particulate matter). The objective of this research is to determine the effects of EGR rate on the combustion processes of HCCI. for this purpose, a 4-cylinder, compression ignition engine was converted into a HCCI engine, and a heating device was installed to raise the temperature of the intake air and also to make it more consistent. In addition, a pressure sensor was inserted into each of the cylinders to investigate the differences in characteristics among the cylinders. The experimental study of the effects of EGR rate on various gas emissions, engine performance, etc. should prove to be a valuable source of information for the development of the HCCI engine.e.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.132-132
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• 2013

Colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has brought new opportunities to unravel the surface structure of working catalysts. Recent studies suggest that surface oxides on transition metal nanoparticles play an important role in determining the catalytic activity of CO oxidation. In this talk, I will outline the recent studies on the influence of surface oxides on Rh, Pt, Ru and Co nanoparticles on the catalytic activity of CO oxidation [1-3]. Transition metal nanoparticle model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir-Blodgett deposition technique. APXPS studies exhibited the reversible formation of surface oxides during oxidizing, reducing, and CO oxidation reaction [4]. General trend is that the smaller nanoparticles exhibit the thicker surface oxides, while the bigger ones have the thin oxide layers. Combined with the nature of surface oxides, this trend leads to the different size dependences of catalytic activity. Such in situ observations of metal nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications. I will also show that the surface oxide can be engineered by using the simple surface treatment such as UV-ozone techniques, which results in changing the catalytic activity [5]. The results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.9 no.1
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• pp.40-47
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• 2013

The capacity factor of nuclear power plant in Korea is the highest level in the world. However, the integrity assessment of nuclear power plant is depended on foreign country. Especially, most eddy current testing systems for inspecting steam generators in nuclear power plant are currently imported from USA, Canada, and so on. Therefore, the eddy current testing system can react more active and adaptive from economic and managerial standpoint for actual nuclear power plants in Korea is required. In this paper, an eddy current testing system for inspecting steam generators in nuclear power plants is introduced. Frequency generator, analog circuit, analog digital converter circuit, and digital control circuit are composed in eddy current testing system. A benchmarking of acquisition system and acquisition software, eddynet 11i made by Zetec, and modifications are carried out based on the test environment of Korea nuclear power plants. Finally, all eddy current apparatus are integrated to inspect steam generator tubes in nuclear power plants.