• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.200-214
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• 1993

This paper presents the results of fluidelastic instability analysis performed for the U-tube bundle of a Westinghouse model 51 steam generator, one of the recirculating types designed at an early stage, in which the principal region of external cross-flow is associated with the U-bend portion of tube. The prerequisites for this analysis are detailed informations of the secondary side flow conditions in the steam generator and the free vibration behaviours of the U-tubes. In this study, the three-dimensional two-phase flow field in the steam generator has been calculated employing the ATHOS3 steam generator two-phase flow code and the ANSYS engineering analysis code has been used to calculate the free vibration responses of specific U tubes under consideration. The assessment of the potential instability for the suspect U-tubes, which is the final analysis process of the present work, has been accomplished by combining the secondary side velocity and density distributions obtained from the ATHOS3 prediction with the relative modal displacement and natural frequency data calculated using the ANSYS code. The damping of tubes in two-phase flow has been deduced from the existing experimental data by taking into account the secondary side void fraction effect. In operation of the steam generator, the tube support conditions at the tube-to-tube support plate intersections due to either tube denting degradation or deposition of tube support plate corrosion products or ingression of dregs. Thus, various hypothetical cases regarding the tube support conditions at the tube-to-tube support plate intersections have been considered to investigate the clamped support effects on the forced vibration response of the tube. Also, the effect of anti-vibration bars support in the curved portion of tube has been examined.

• The Transactions of the Korean Institute of Power Electronics
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• v.10 no.3
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• pp.257-263
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• 2005

Short-circuit rotor windings on a generator causes unstable oscillation of unbalance of flux, asymmetrical heat. In order to prevent serious accidents of short-circuit rotor windings, it is important to study the shorted-turn diagnosis method for rotor windings of the generator. To improve the defects of the diagnosis with sensors, the new sensorless method for rotor shorted-turn diagnosis is proposed, which is to measure the electrical values of the voltage and current at the generator and then to detect if the shorted-turned phenomena would occurred. For the feasibility of the suggested method the theoretical results are shown in the aspects of the air-gap flux density, the flux leakage, the generated output voltage and the shorted field current through the digital simulation. Also the possibility of decision for the suggested sensorless method could be shown in this paper.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.219-219
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• 2008

Energy harvesting from the environment has been of great interest as a standalone power source of wireless sensor nodes for ubiquitous sensor networks (USN). There are several power generating methods such as thermal gradients, solar cell, energy produced by human action, mechanical vibration energy, and so on. Most of all, mechanical vibration is easily accessible and has no limitation of weather and environment of outdoor or indoor. In particular, the piezoelectric energy harvesting from ambient vibration sources has attracted attention because it has a relative high power density comparing with other energy scavenging methods. Through recent advances in low power consumption RF transmitters and sensors, it is possible to adopt a micro-power energy harvesting system realized by MEMS technology for the system-on-chip. However, the MEMS energy harvesting system hassome drawbacks such as a high natural frequency over 300 Hz and a small power generation due to a small dimension. To overcome these limitations, we devised a novel power generator with a spiral spring structure. In this case, the energy harvester has a lower natural frequency under 200 Hz than a normal cantilever structure. Moreover, it has higher an energy conversion efficient because shear mode ($d_{15}$) is much larger than 33 mode ($d_{33}$) and the energy conversion efficiency is proportional to the piezoelectric constant (d). We expect the spiral type MEMS power generator would be a good candidate as a standalone power generator for USN.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.03a
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• pp.7-7
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• 2010

Energy harvesting from the environment has been of great interest as a standalone power source of wireless sensor nodes for Ubiquitous Sensor Networks(USN). In particular, the piezoelectric energy harvesting from ambient vibration sources has intensively researched because it has a relatively high power density comparing with other energy scavenging methods. Through recent advances in low power consumption RF transmitters and sensors, it is possible to adopt a micro-power energy harvesting system realized by MEMS technology for the system-on-chip. However, the MEMS energy harvesting system has some drawbacks such as a high natural frequency over 300 Hz and a small power generation due to a small dimension. To overcome these limitations, we devised a novel power generator with a spiral spring structure as shown in the figure. The natural frequency of a cantilever could be decreased to the usable frequency region (under 300 Hz) because the natural frequency depends on the length of a cantilever. In this study, the natural frequency of the energy harvester was a lower than a normal cantilever structure and sufficiently controllable in 50 - 200 Hz frequency region as adjusting weight of a proof mass. Moreover, the MEMS energy harvester had a high energy conversion efficiency using a shear mode ($d_{15}$) is much larger than a 33 mode ($d_{33}$) and the energy conversion efficiency is proportional to the piezoelectric constant (d). We expect the spiral type MEMS power generator would be a good candidate for a standalone power generator for USN.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.176.2-176.2
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• 2013

Property of optical diagnostics for pulse-discharged plasma in liquid and its biological applications to proteins are investigated by making use of high voltage Marx generator. The Marx generator has been consisted of 5 stages, where each charging capacitor is 0.5 ${\mu}F$, to generate a high voltage pulse with rising time of $1{\mu}s$. We have applied an input voltage of 6 kV to the each capacitor of 0.5 ${\mu}F$. High voltage pulsed plasma has been generated inside a polycarbonate tube by a single-shot operation, where the breakdown voltage is measured to be 7 kV, current of 1.2 kA, and pulse width of ~ 1 ${\mu}s$ between the two electrodes of anode-cathode whose material is made of tungsten pin, which are immersed into the liquids. We have investigated the emitted hydrogen lines for optical diagnostics of high voltage pulsed plasma. The emission line of 656.3 nm from $H-{\alpha}$ and 486.1 nm from $H-{\beta}$ have been measured by a monochromator. If we assumed that the focused plasma regions satisfy the local thermodynamic equilibrium conditions, the electron temperature and density of the high voltage pulsed plasma in liquid could be obtained by the Stark broadening of optical emission spectroscopy. For the investigation of the influence of pulsed plasma on biological proteins, we have exposed it onto the proteins such as hemoglobin and myoglobin. The structural changes in these proteins and their analysis have also been obtained by circular dichroism (CD) and ultraviolet (UV) visible spectroscopy.

• Journal of the Korean Ceramic Society
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• v.39 no.2
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• pp.120-125
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• 2002

AC powder EL devices were fabricated by screen printing method with the dielectric materials in insulating layer and the electrical resistivity of rear electrode. Brightness and current density were measured at voltage range of 50∼300 $V_{rms}$ to estimate optoelectrical properties of AC powder EL devices, respectively. Frequency generator was used as system producing frequency and voltage of a sine wave. Brightness and current density were measured by luminometer and multimeter. Also, dielectric constant for dielectric layer was measured by impedance analyser after preparing thick film. Dielectric constant was improved with amount of $TiO_2$ to $BaTiO_3$ powder. By applying such a process to dielectric layer of low cost AC powder EL device, brightness was improved to 50 cd/$m^2$ at similar current density. Dielectric constant $BaTiO_3$ powder by solution combustion process is better than commercial $BaTiO_3$ powder. By applying to that of low power consumption AC powder EL device, brightness was improved to 85 cd/$m^2$. Brightness of AC powder EL device was relatively decreased by control of electrical resistivity of rear electrode, current density was also decreased.

• Journal of the Korean Ceramic Society
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• v.38 no.12
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• pp.1085-1092
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• 2001

The high efficient AC powder EL devices classified by low cost and low power consumption type fabricated using screen printing method with film thickness and organic binder. Brightness and current density were measured at frequency range of 400Hz∼1kHz and voltage range of 50∼300V$\_$rms/ to estimate optoelectrical properties of AC powder EL devices, respectively. Frequency generator was used as system producing frequency and voltage of a sine wave. Also brightness and current density were measured by luminometer and multimeter. In the case of low cost type AC powder EL device, brightness and current density were about 43 cd/m$^2$and 20$\mu$A/cm$^2$when the thickness of phosphor and dielectric layer was 45∼50$\mu$m under no addition of plasticizer respectively. In the case of low power consumption type AC powder EL device, brightness and current density were about 74 cd/m$^2$and 30∼40$\mu$A/cm$^2$when the thickness of phosphor and dielectric layer was 45∼50$\mu$m and 15∼20$\mu$m under addition of 15wt% plasticizer respectively. Also, AC powder EL device fabricated in this study showed absolutely excellent characteristics as the lifetime was longer than products of other company.

• Bulletin of the Korean Mathematical Society
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• v.38 no.4
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• pp.635-641
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• 2001

A limit point p of a Mobius group acting on$ B^m$ is called a concentration point if for every sufficiently small connected open neighborhood of p, the set of translates contains a local basis for the topology of p. For the case of two generator Schottky groups acting on $B^2$, we give characterizations for several different kinds of limit points.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• v.12 no.2
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• pp.109-118
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• 2008

We present a new version of the first return time test for pseudorandomness. Let $R_n$ be the first return time of initial n-block with overlapping. An algorithm to calculate the probability distribution of the first return time $R_n$ for each starting block is presented and used to test pseudorandom number generators. The standard Z-test for log $R_n$ is applied to test the pseudorandom number generators.

• Proceedings of the KIEE Conference
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• summer
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• pp.1229-1231
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• 2004

A resonant type voltage source and power device and a control method using Pulse Density Modulation(PDM) power control and Pulse Width Modulation(PWM) voltage control for plasma sterilization are described. For the stability of discharge in the generating tube