• Journal of Radiation Protection and Research
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• v.3 no.1
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• pp.6-22
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• 1978

High energy electron beams took effect for tumor radio-therapy, however, had a lot of problems in clinical application because of various conversion factors and complication of physical reactions. Therefor, we had experimentally studied the important properties of high energy electron beams from the linear accelerator, LMR-13, installed in Yonsei Cancer Center. The results of experimental studies on the problems in the 8, 10, 12 Mev electron beam therapy were reported as following. 1. On the measurements of the outputs and absorbed doses, the ionization type dosimeters that had calibrated by $^{90}Sr$ standard source were suitable as under 3% errors for high energy electrons to measure, but measuring doses in small field sizes and the regions of rapid fall off dose with ionization chambers were difficult. 2. The electron energy were measured precisely with energy spectrometer consisted of magnet analyzer and tele-control detector and the practical electron energy was calculated under 5% errors by maximum range of high energy electron beam in the water. 3. The correcting factors of perturbated dose distributions owing to radiation field, energy and material of the treatment cone were checked and described systematically and variation of dose distributions due to inhomogeneous tissues and sloping skin surfaces were completely compensated. 4. The electron beams, using the scatterers; ie., gold, tin, copper, lead, aluminium foils, were adequately diffused and minimizing the bremsstrahlung X-ray induced by the electron energy, irradiation field size and material of scatterers, respectively. 5. Inproving of the dose distribution from the methods of pendulum, slit, grid and focusing irradiations, the therapeutic capacity with limited electron energy could be extended.

• Proceedings of the Korea Contents Association Conference
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• 2010.05a
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• pp.277-280
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• 2010

The prevention of disasters is important to prepare in advance through analysis and an estimate. But for all the efforts of the government to stave off disasters, the damage out of a guerilla localized heavy rain caused the global warming, a landslide and inundation is growing. To prevent these damages, the basic data and system through systematic research and analysis should be set up. But it is true that collecting of the basic data and the system for preventing disasters are either constructing or insufficient so far. In this research, by using topography spatial data including LiDAR data including the aerial photo and digital maps, and etc. the factor of a disaster, the disaster risk element was extracted. Moreover, the disaster region about the disaster generation available region was evaluated in advance using the easy disaster analysis of current situation photo map which made with the grid analysis method and weighted value estimate technique.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.5
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• pp.411-420
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• 2009

This study presents an extended finite difference method based on moving least squares(MLS) method for solving potential problems with interfacial boundary. The approximation constructed from the MLS Taylor polynomial is modified by inserting of wedge functions for the interface modeling. Governing equations are node-wisely discretized without involving element or grid; immersion of interfacial condition into the approximation circumvents numerical difficulties owing to geometrical modeling of interface. Interface modeling introduces no additional unknowns in the system of equations but makes the system overdetermined. So, the numbers of unknowns and equations are equalized by the symmetrization of the stiffness matrix. Increase in computational effort is the trade-off for ease of interface modeling. Numerical results clearly show that the developed numerical scheme sharply describes the wedge behavior as well as jumps and efficiently and accurately solves potential problems with interface.

• 한국지구물리탐사학회:학술대회논문집
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• 2003.11a
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• pp.390-397
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• 2003

A three-dimensional (3D) inversion technique has been developed for interpretation of magnetotelluric (MT) data. The inversion method is based on the linearized least-squares (Gauss-Newton) method with smoothness regularization. In addition to the underground 3D resistivity distribution, static shifts are also treated as unknown parameters in the inversion. The forward modeling is by the staggered-grid finite difference method. A Bayesian criterion ABle is applied to search the optimum trade-off among the minimization of the data misfit, model roughness and static shifts. The method has been applied to several MT datasets obtained at geothermal fields in Japan and other Asian countries. In this paper, two examples will be discussed: one is the data at the Ogiri geothermal area, southwestern Japan, and the other is at the Pohang low-enthalpy geothermal field, southeastern Korea. The inversion of the Ogiri data has been performed stably, resulting in a good fitting between the observed and computed apparent resistivities and phases. The recovered 3D resistivity structure is generally similar to the two-dimensional (2D) inversion models, although the deeper portion of the 3D model seems to be more realistic than that of the 2D model. The 3D model is also in a good agreement with the geological model of the geothermal reservoirs. 3D interpretation of the Pohang MT data is still preliminary. Although the fitting to the observed data is very good, the preliminary 3D model is not reliable enough because the station coverage is not sufficient for a 3D inversion.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.04b
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• pp.7-7
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• 2009

Lead telluride (PbTe) is a very promising thermoelectric material due to its narrow band gap (0.31 eV at 300 K), face-centered cubic structure and large average excitonic Bohr radius (46 nm) allowing for strong quantum confinement within a large range of size. In this work, we present the thermoelectric properties of individual single-crystalline PbTe nanowires grown by a vapor transport method. A combination of electron beam lithography and a lift-off process was utilized to fabricate inner micron-scaled Cr (5 nm)/Au (130 nm) electrodes of Rn (resistance of a near electrode), Rf (resistance of a far electrode) and a microheater connecting a PbTe nanowire on the grid of points. A plasma etching system was used to remove an oxide layer from the outer surface of the nanowires before the deposition of inner electrodes. The carrier concentration of the nanowire was estimated to be as high as $3.5{\times}10^{19}\;cm^{-3}$. The Seebeck coefficient of an individual PbTe nanowire with a radius of 68 nm was measured to be $S=-72{\mu}V/K$ at room temperature, which is about three times that of bulk PbTe at the same carrier concentration. Our results suggest that PbTe nanowires can be used for high-efficiency thermoelectric devices.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1884-1893
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• 2016

The ride-through control of a doubly-fed induction generator (DFIG) for the voltage sags on wind farms utilizing crowbar circuits by which the rotor side converter (RSC) is disabled has being reported in many literatures. An analysis and calculation of the transient current when the RSC is switched off are of significance for carrying out the low voltage ride through (LVRT) of a DFIG. The mathematical derivation is highlighted in this paper. The zero-state and zero-input responses of the transient current in the frequency domain through a Laplace transformation are investigated, and the transient components in the time domain are achieved. With the characteristics worked out from the linear resolving without modeling simplification, the selection of the resistance in the linear crowbar circuit and the value conversion from a linear circuit to a nonlinear one is proposed to setup the attenuation rate. In terms of grid code requirements, the theoretical analysis for the time constant of the transient components attenuation insures the controllability when the excitation of the RSC is resumed and it guarantees the reserved time for the response of the reactive power compensation. Simulations are executed in MATLAB/SIMPOWER and experiments are carried out to validate the theoretical analysis. They indicate that the calculation method is effective for selection of the resistance in a crowbar circuit for LVRT operations.

• KIEE International Transaction on Electrical Machinery and Energy Conversion Systems
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• v.5B no.4
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• pp.366-373
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• 2005

This paper presents two new circuit topologies of the dc busline side active resonant snubber assisted voltage source high frequency link soft switching PWM full-bridge dc-dc power converters acceptable for either utility ac 200V-rms or ac 400V-rms input grid. These high frequency switching dc-dc converters proposed in this paper are composed of a typical voltage source-fed full-bridge PWM inverter, high frequency transformer with center tap, high frequency diode rectifier with inductor input filter and dc busline side series switches with the aid of a dc busline parallel capacitive lossless snubber. All the active switches in the full-bridge arms as well as dc busline snubber can achieve ZCS turn-on and ZVS turn-off transition commutation with the aid of a transformer leakage inductive component and consequently the total switching power losses can be effectively reduced. So that, a high switching frequency operation of IGBTs in the voltage source full bridge inverter can be actually designed more than about 20 kHz. It is confirmed that the more the switching frequency of full-bridge soft switching inverter increases, the more soft switching PWM dc-dc converter with a high frequency transformer link has remarkable advantages for its power conversion efficiency and power density implementations as compared with the conventional hard switching PWM inverter type dc-dc power converter. The effectiveness of these new dc-dc power converter topologies can be proved to be more suitable for low voltage and large current dc-dc power supply as arc welding equipment from a practical point of view.

• The KSFM Journal of Fluid Machinery
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• v.18 no.3
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• pp.46-52
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• 2015

Pump-turbine system is widely used by the hydropower industry for stabilizing the electrical grid in the vast growing economy of most developed countries. This study only investigates the Fluid-structure Interaction (FSI) analysis of the pump-turbine system at various operating conditions. The FSI analysis can show how reliable each component of the system is by providing the engineer with a better understanding of high stress and deformation points, which could reduce the lifespan of the pump-turbine. Pump-turbine components are categorized in two parts, pressurized static parts and movable stressed parts. The fixed parts include the spiral casing, top and bottom cover, stay vane and draft tube. The movable parts include guide vanes and impeller blades. Fine hexahedral numerical grids were used for CFD calculation and fine tetrahedral grids were used for structural analysis with imported load solution mapping greater than 90 %. The maximum equivalent stress are much smaller than the material yield stress, and the maximum equivalent stress showed an increasing tendency with the varying of operating conditions from partial to excessive at both modes. In addition, the total deformation of all the operating conditions showed a small magnitude, which have quite small influence on the structural stability. It can be conjectured that this system can be safely implemented.

• Journal of the Korean Society of Safety
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• v.31 no.3
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• pp.22-27
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• 2016

The use of electromagnetic wave has been interested in various energy industry because it enhances a flame stability and provides higher safety environments. However it might increase the pollutant emissions such as NOx and soot, and have harmful influence on human and environments. Therefore, it is very important to understand interaction mechanism between flame and electromagnetic wave from environmental point of view. In this study, an experiment was performed with jet diffusion flames induced by electromagnetic wave. Microwave was used as representative electromagnetic wave and a flickering flame was introduced to simulate the more similar combustion condition to industry. The results show that the induced microwave enhances the flame stability and blowout limit. The unstable lifted flickering flames under low fuel/oxidizer velocity is changed to stable attached flames or lift-off flames when microwave applied to the flames, which results from the abundance of radical pool. However, NOx emission was increased monotonically with increasing the microwave power as microwave power increased up to 1.0 kW. The effects might be attributed to the heating of combustion field and thermal NOx mechanism will be prevailed. Soot particle was examined at the post flame region by TEM grid. The morphology of soot particle sampled in the microwave induced flames was similar to the incipient soot that is not agglomerated and contain a lots of liquid phase hydrocarbon such as PAH, which soot particle formed near reaction zone is oxidized on the extended yellow flame region and hence only unburned young particles are emitted on the post flame region.

• Journal of Power Electronics
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• v.15 no.2
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• pp.455-468
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• 2015

A microgrid (MG) with integrated renewable energy resources can benefit both utility companies and customers. As a result, they are attracting a great deal of attention. The control of a MG is very important for the stable operation of a MG. The droop-control method is popular since it avoids circulating currents among the converters without using any critical communication between them. Traditional droop control methods have the drawback of an inherent trade-off between power sharing and voltage and frequency regulation. An adaptive droop control method is proposed, which can operate in both the island mode and the grid-connected mode. It can also ensure smooth switching between these two modes. Furthermore, the voltage and frequency of a MG can be restored by using the proposed droop controller. Meanwhile, the active power can be dispatched appropriately in both operating modes based on the capacity or running cost of the Distributed Generators (DGs). The global information (such as the average voltage and output active power of the MG and so on) required by the proposed droop control method to restore the voltage and frequency deviations can be acquired distributedly based on the Multi Agent System (MAS). Simulation studies in PSCAD demonstrate the effectiveness of the proposed control method.