• Journal of Mechanical Science and Technology
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• v.16 no.8
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• pp.1154-1164
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• 2002

Losses on the turbine consist of the mechanical loss, tip clearance loss, secondary flow loss and blade profile loss etc.,. More than 60 % of total losses on the turbine is generated by the two latter loss mechanisms. These losses are directly related with the reduction of turbine efficiency. In order to provide a new design methodology for reducing losses and increasing turbine efficiency, a two-dimensional axial-type turbine blade shape is modified by the optimization process with two-dimensional compressible flow analysis codes, which are validated by the experimental results on the VKI turbine blade. A turbine blade profile is selected at the mean radius of turbine rotor using on a heavy duty gas turbine, and optimized at the operating condition. Shape parameters, which are employed to change the blade shape, are applied as design variables in the optimization process. Aerodynamic, mechanical and geometric constraints are imposed to ensure that the optimized profile meets all engineering restrict conditions. The objective function is the pitchwise area averaged total pressure at the 30% axial chord downstream from the trailing edge. 13 design variables are chosen for blade shape modification. A 10.8 % reduction of total pressure loss on the turbine rotor is achieved by this process, which is same as a more than 1% total-to-total efficiency increase. The computed results are compared with those using 11 design variables, and show that optimized results depend heavily on the accuracy of blade design.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.22 no.4
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• pp.114-120
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• 2008

XLPE cable is using insulator for Cross Linked Polyethylene(XLPE), because insulation Performance is superior and easy comparatively. Need systematization of accident transaction for electrical equipment accident, It is no disposal standard for defect of manufacture and second to accidents. in this paper deals with the change of XLPE cable insulation. To understand the electrical properties of XLPE insulation. Made of XLPE block sample, Penetration fracture Sample and flashover sample. Ogura needles having tip radius of $10[{\mu}m]$ were inserted into each sample. AC voltage of 1[kV/sec] increased at 60[Hz] were apple to breakdown sample and flashover sample. AC voltages of 12[kV], 17[kV] at 60[Hz] were a lied to XLPE block sample. The electrical properties of specimens were measured were measured from initiation of tree and breakdown to their characteristics were analyzed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.9
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• pp.859-869
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• 2009

The sharp indenters such as Berkovich and conical indenters have a geometrical self-similarity in theory, but different materials have the same load-depth curve in case of single indentation. In this study, we analyze the load-depth curves of conical indenter with angles of indenter via finite element method. From FE analyses of dual-conical indentation test, we investigate the relationships between indentation parameters and load-deflection curves. With numerical regressions of obtained data, we finally propose indentation formulae for material properties evaluation. The proposed approach provides stress-strain curve and the values of elastic modulus, yield strength and strain-hardening exponent with an average error of less than 2%. It is also discussed that the method is valid for any elastically deforming indenters made of tungsten carbide and diamond for instance. The proposed indentation approach provides a substantial enhancement in accuracy compared with the prior methods.

• Tribology and Lubricants
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• v.33 no.4
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• pp.127-133
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• 2017

In order to characterize the adhesive properties and failure mechanisms of diamond-like carbon (DLC) films of two different thicknesses (130 nm and $1.2{\mu}m$), deposited by plasma-enhanced chemical vapor deposition on a Si substrate, scratch testing with a micro-indenter ($12.5{\mu}m$ tip radius) was performed under a linearly increasing load. These scratch tests were conducted under the same test conditions for both films. The critical load of each film was estimated from the scratch test results, based on a sharp increase in the coefficient of friction and a clear distinction of failure modes. The critical load was the basis for evaluating the adhesion strength of the films, and the $1.2{\mu}m-thick$ DLC film had superior adhesion strength. For better understanding of the failure modes, the following analyses were conducted: friction behavior and scratch tracks analysis using scanning electron microscopy, energy-dispersive spectroscopy, and 3-D profilometry. The scratch test results showed that failure modes were related to the thickness of the films. The 130 nm-thick DLC film underwent cohesive failure modes (cracks and chipping) before reaching to a gross failure stage. On the other hand, the thicker DLC film ($1.2{\mu}m-thick$) did not exhibit micro cracks before a sudden gross failure of the film together with the evidence of cracking and chipping of the Si substrate.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.150-150
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• 2010

Carbon nanotubes have drawn attention as one of the most promising emitter materials ever known not only due to their nanometer-scale radius of curvature at tip and extremely high aspect ratios but also due to their strong mechanical strength, excellent thermal conductivity, good chemical stability, etc. Some applications of CNTs as emitters, such as X-ray tubes and microwave amplifiers, require high current emission over a small emitter area. The field emission for high current density often damages CNT emitters by Joule heating, field evaporation, or electrostatic interaction. In order to endure the high current density emission, CNT emitters should be optimally fabricated in terms of material properties and morphological aspects: highly crystalline CNT materials, low gas emission during electron emission in vacuum, optimal emitter distribution density, optimal aspect ratio of emitters, uniform emitter height, strong emitter adhesion onto a substrate, etc. We attempted a novel approach to fabricate CNT emitters to meet some of requirements described above, including highly crystalline CNT materials, low gas emission, and strong emitter adhesion. In this study, CNT emitters were fabricated by filtrating an aqueous suspension of highly crystalline thin multiwalled CNTs (Hanwha Nanotech Inc.) through a metal mesh. The metal mesh served as a support and fixture frame of CNT emitters. When 5 ml of the CNT suspension was engaged in filtration through a 400 mesh, the CNT layers were formed to be as thick as the mesh at the mesh openings. The CNT emitter sample of $1{\times}1\;cm^2$ in size was characteristic of the turn-on electrical field of 2.7 V/${\mu}m$ and the current density of 14.5 mA at 5.8 V/${\mu}m$ without noticeable deterioration of emitters. This study seems to provide a novel fabrication route to simply produce small-size CNT emitters for high current emission with reliability.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.22-29
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• 2001

The present study investigates the effects of various rib arrangements and rotating on heat/mass transfer in the cooling passage of gas turbine blades. The cooling passage has very complex flow structure, because of the rib turbulator and rotating effect. Experiments and numerical calculation are conducted to investigate the complex flow structures and heat transfer characteristics; the numerical computation is performed using a commercial code, FLUENT ver.5, to calculate the flow structures and the experiments are conducted to measure heat/mass transfer coefficients using a naphthalene sublimation technique. For the rotating duct tests, the test duct, which is the cross section of is $20mm\times40mm$ (the hydraulic diameter, $D_h$, of 26.7 mm, has two-pass with $180^{\circ}$ turning and the rectangular ribs on the wall. The rib angle of attack is $70^{\circ}$ and the maximum radius of rotation is $21.63D_h$. The partition wall has 10 mm thickness, which is 0.5 times to the channel width, and the distance between the tip of the partition wall and the outer wall of the turning region is 26.7 mm $(1D_h)$. The turning effect of duct flow makes the very complex flow structure including Dean type vortex and high turbulence, so that the heat/mass transfer increases in the turning region and at the entrance of the second pass. The Coriolis effect deflects the flow to the trailing surface, resulting in enhancement of the heat/mass transfer on the trailing surface and reduction on the leading surface in the first pass. However, the opposite phenomena are observed in the second pass. The each rib arrangement makes different secondary flow patterns. The complex heat/mass transfer characteristics are observed by the combined effects of the rib arrangements, duct rotation and flow turning.

• Transactions of the KSME C: Technology and Education
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• v.3 no.3
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• pp.233-240
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• 2015

Analysis of the aerodynamic performance of a bobsleigh has been performed for various types of bobsleigh body shape. To analyze the aerodynamic performance of the bobsleigh, three-dimensional Reynolds-averaged Navier-Stoke equations were used with the standard k-${\varepsilon}$ model as a turbulence closure. Grid structure was composed of unstructured tetrahedral grids. The radii of curvature of cowling, and height and length of front bumper at the tip on the drag coefficient were selected as geometric parameters. And, the effects of these parameters on the aerodynamic performance, i.e., the drag coefficient, were evaluated. The results shows that the aerodynamic performance is significantly affected by the height of front bumper and radius of curvature.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.51 no.5
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• pp.213-221
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• 2002

In order to improve the reliability of XLPE cables, it is necessary to find the useful diagnostic parameter according to long term aging. This paper described the change of partial discharge(PD) phase Properties of XLPE cable insulation with branch type electrical tree degradation. for long term aging. To understand the PD Properties with $\phi$ -q-n distributed shape of XLPE insulation, specimens were prepared by 22.9㎸ distribution cable and made in a type of block(16${\times}$16${\times}$3[mm]). Ogura needles having tip radius of l0${\mu}{\textrm}{m}$ were inserted into each block pieces. The measuring system was consisted of PD detector, digitizer for digital conversion, VXI system for signal processing. The PD properties of the specimens were measured from initiation of tree to breakdown and their characteristics were analyzed. We analyzed the relationship between electrical properties(PD Quantity, PD initiation angle, PD extinction angle, PD occurrence angle : (PD extinction angle - PD initiation angle)) and the normalized aging rate. We found PD parameter, PD initiation angle and occurrence angle, which are a useful diagnostic parameter in estimating the branch type electrical tree for XLPE insulation condition.

• Journal of the Korean Vacuum Society
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• v.6 no.2
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• pp.103-108
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• 1997

Silicon field emitter arrays (FEAs) have been fabricated by a novel method employing a two-step tip etch and a spin-on-glass (SOG) etch-back process using double layered thermal/tetraethylortho-silicate (TEOS) oxides as a gate dielectric. A partial etching was performed by coating a low viscous photo resist and $O_2$ plasma ashing on order to form the double layered gate dielectric. A small gate aperture with low gate leakage current was obtained by the novel process. The hight and the end radius of the fabricated emitter was about 1.1 $\mu\textrm{m}$ and less than 100$\AA$, respectively. The anode emission current from a 256 tips array was turned-on at a gate voltage of 40 V. Also, the gate current was less than 0.1% of the anode current.

• Journal of the Korean Vacuum Society
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• v.7 no.2
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• pp.88-93
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• 1998

The fabrication process and emission characteristics of gated silicon field emitter arrays(FEAs) using chemical-mechanical-polishing (CMP) method are described. Novel fabrication techniques consisting of two-step dry etching with oxidation of silicon and CMP processes were developed for the formation of sharp tips and clear-cut edged gate electrodes, respectively. The gate height and aperture could be easily controlled by varying the polishing time and pressure in the CMP process. We obtained silicon FEAs having self-aligned and clear-cut edged gate electrode opening by eliminating the dishing problem during the CMP process with an oxide mask layer. The tip height of the finally fabricated FEAs was about 1.1 $\mu$m and the end radius of the tips was smaller than 100 $\AA$. The emission current meaured from the fabricated 2809 tips array was about 31 $\mu$A at a gate voltage of 80 V.