• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.4
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• pp.429-435
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• 2015

A bellows is an important temperature control component in a Joule-Thomson micro-cryocooler. It is designed using a very thin shell, and the inside of the bellows is filled with nitrogen gas. The bellows is made of a nickel-cobalt alloy that maintains its strength and elastic properties in a wide range of temperatures from cryogenic to $300^{\circ}C$. The pressure of the gas and the volume within the bellows vary according to the temperature of the gas. As a result, the bellows contracts or expands in the axial direction like a spring. To explore this phenomenon, the deformation of the bellows and its internal volume must be calculated iteratively under a modified pressure until the state equation of the gas is satisfied at a given temperature. In this paper, the modified Benedict-Webb-Rubin state equation is adopted to describe the temperature-volume-pressure relations of the gas. Experiments were performed to validate the proposed method. The results of a numerical analysis and the experiments showed good agreement.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.1
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• pp.25-30
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• 2014

The flow characteristics of two types of overhung compressor volutes for automobile turbochargers were analyzed numerically using commercial software. For obtaining high performance from a volute, it is necessary that the volute have a high pressure recovery coefficient and a low loss coefficient. We investigated the flow characteristics of two types of overhung compressor volutes with a fixed diffuser inlet angle of $24^{\circ}$ and a mass flow rate of 0.055 kg/s. The first type is a volute with one-arc cross section (type 1) and the second type is with three-arc cross section (type 2). Our results showed that between the two types of volutes, type 2 had the higher pressure recovery coefficient and the lower loss coefficient along the entire angular position.

• Tunnel and Underground Space
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• v.21 no.2
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• pp.109-116
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• 2011

In this study, we estimated the dynamic tensile strength and strain rate from Brazilian tensile test using Split Hopkinson Pressure Bar (SHPB) system. A pulse shaping technique, which controls the shape of the impactinduce incident waves, was used for achieving the dynamic stress equilibrium and constant strain rate before fracture of rock samples. Three kinds of rock type, Inada granite, Kimachi sandstone and Tage tuff were prepared as 50mm in diameter and 26 mm in thickness. The high-speed videography system was used to observe the fracture processes of the rock samples. As the results of the tests, the ratio of dynamic tensile strength and static tensile strength was 11.9 for Inada granite, 8.5 for Kimachi sandstone and 9.2 for Tage tuff.

• Nuclear Engineering and Technology
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• v.24 no.3
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• pp.263-273
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• 1992

The study on the velocity distribution and the pressure drop characteristic of the nuclear fuel assembly is of importance for the thermal hydraulic design and safety analysis. The purpose of this experimental study is to investigate the hydraulic mixing behind the different kinds of spacer grids in the now or rod bundles. In this study, the detailed hydraulic characteristics in subchannels of 5$\times$5 PWR(Pressurized Water Reactor) rod bundles were measured using one-component He-Ne LDV(Laser Doppler Velocimeter). Measurements of the axial velocity, turbulent intensities and pressure drops were peformed Lateral velocity, turbulent intensities and Reynolds shear stress were also measured by adjust-ing LDV alignment. Friction factors in rod bundles and loss coefficients for spacer grids were evaluated from the measured pressure drops. Hydraulic mixing performance for different kinds of spacer grids could be investigated by estimating the turbulent cross-flow mixing rates between neighboring subchannels.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.9
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• pp.1760-1769
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• 1992

An experimental study of thermal transport from a uniformly heated pipe to a pulsating flow has been carried out. Surface of the pipe is imposed with constant heat flux providing by electric heating band. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. Temporal Variatiens of temperature and pressure inside the circular pipe are measured. The dependence of temperature distributions and heat transfer rate on the mean flow rate in the pipe and on the pulsating frequency is investigated in detail. The experimental results indicate that the measured temporal variations of temperature and pressure become nearly sinusoidal The amplitude of temperature variation near the pipe wall is much more substantial than that in core of the pipe. It is also found that the heat transfer rate is increased significantly as the frequency of the pulsating pressure is increased or the mean flow rate in a pipe is increased. The results obtained are also compared with those for non-pulsating flow circumstance.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.131-131
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• 2011

이종접합 태양전지 제작을 위해 기판의 buffer layer로 사용되는 기존의 a-Si 박막을 SiON 박막으로 대체하려는 연구가 진행 중이다. 기존의 a-Si 박막은 대면적에서 균일도를 담보하기 어렵고, 열적 안정성에 취약한 문제점이 있다. 이에 반해 SiON 박막은 일종의 화학 반응인 oxidation 방법으로 형성이 되기 때문에 막의 균일도를 담보 할 수 있고, $400^{\circ}C$이상의 온도에서 형성되기 때문에 열적 안정성이 우수한 장점이 있다. 이러한 장점에도 불구하고 기판위에 직접 형성이 되기 때문에 기판과 SiON 계면 사이의 pssivation이 무엇보다 중요하다. 본 연구에서는 비정질 실리콘 이종접합 태양전지에 적용키 위한 SiON 박막을 형성하고, 기판과 SiON 계면에서의 passivation 향상을 위한 계면 결함 감소에 대한 연구를 진행하였다. 실험을 위한 SiON 박막은 공정온도 $450^{\circ}C$, 공정압력 100 mTorr, 증착파워 120 mW/cm2에서 5분간 증착하였으며, 이때 50 sccm의 N2O 가스를 주입하였다. 증착된 박막은 2~4 nm의 두께로 증착이 되었으며, 1.46의 광학적 굴절률을 가지는 것으로 분석되었다. 계면의 결함을 줄이기 위해 PECVD를 이용한 NH3 plasma treatment를 실시하였다. 공정온도 $400^{\circ}C$, 공정압력 150mTorr~450 mTorr, 플라즈마 파워 60mW/cm2에서 30분간 진행하였으며, 50 sccm의 N2O 가스를 주입하였다. 계면의 결함이 줄었는지 확인하기 위해 C-V 측정을 위한 시료를 제작하여 분석을 하였다. 실험 결과 VFB가 NH3 plasma treatment 이후 positive 방향으로 shift 됨을 알 수 있었다. Dit 분석을 통해 공정 압력 450 mTorr에서 $4.66{\times}108$[cm2/eV]로 가장 낮은 계면 결함 밀도를 확인 할 수 있었다. 결과적으로 NH3 plasma 처리를 통해 positive charge를 갖는 N-content가 형성되었음을 예측해 볼 수 있으며, N-content가 증가하면, 조밀한 Si-N 결합을 형성하면서, boron 및 phosphorus diffusion을 막는데 효과적이다. 또한, plasma treatment 과정에서 H-content에 의한 passivation 효과를 기대할 수 있다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.9
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• pp.745-751
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• 2014

For the multi-nozzle propulsion, the rupture pressure of nozzle closure has an effect on the initial strain rate of ignition. Moreover, the deviation of rupture pressure for each nozzle closure leads to side forces which can disturb the attitude control of rocket. When designed, it should be considered whether nozzle closures are ruptured equally and exactly in the intented pressure. In this paper, the rupture behavior is analyzed by analytical and experimental methods for plate and "+" notched nozzle closures. The rupture pressure and deviation for operating temperature, whether notched or not and notched directions are analyzed. This paper provides a comparison between rupture pressure prediction of finite elements method which tool is Abaqus/Explicit and results of the rupture test. Jonson-Cook shear failure model which corresponds to the damage initiation criterion were used in this simulation.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.8
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• pp.735-743
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• 2011

In this study, the distribution of surface pressure was measured in a steady state on a turbine blade which was moved the injected region and receded the stagnation region using a linear cascade apparatus. Axial-type blades were used and the blade chord was 200mm. The rectangular nozzle was applied and its size was $200mm{\times}200mm$. The experiment was done at $3{\times}10^5$ of Reynolds number based on the chord. The surface pressures on the blade were measured at three different nozzle angles of $58^{\circ}$, $65^{\circ}$ and $72^{\circ}$ for off-design performance test. In addition, three different solidities of 1.25, 1.38 and 1.67 were applied. From the results, the low solidity caused the low pressure on the blade suction surface at entering region and the reverse rotating force was generated at the low nozzle angle. The positive incidence also made the pressure lower on the suction surface at entering region.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.4
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• pp.1-8
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• 2014

Overexpanded flow of an axisymmetric thruster nozzle is numerically simulated to investigate effects of nozzle pressure ratio (NPR) on the shock structure and thrust performance. The Reynolds-averaged Navier-Stokes equations with k-${\omega}$ SST turbulence model are solved utilizing FLUENT solver. As the NPR is raised, thrust performance monotonically increases with the shock structure and flow-separation point being pushed toward the nozzle exit. It is also discussed that the flow structure at nozzle-exit plane which is immediately affected by a position of nozzle-interior shocks and expansion waves, has strong influence upon the thrust performance of thruster nozzle.

• Journal of Advanced Marine Engineering and Technology
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• v.37 no.2
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• pp.156-162
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• 2013

The characteristics of the unsteady flowfield of a square prism having a detached splitter plate at the wake side were investigated by advanced vortex method. The instantaneous and average velocity field and pressure field around a square prism without and having splitter plate were calculated by forcing the gap ratio having the maximum drag reduction rate, at Reynolds number $Re=1.0{\times}10^4$ and the width ratio H/B=1.0 of splitter to the prism width. The drag and lift coefficients on the square prism were also obtained. The calculated results agree with the measured drag coefficients and pressure distributions on the square prism. The vortices of the opposite direction at upside and down side of the splitter plate were generated by installing of the plate. And the drag on the square prism was decreased by increasing of the pressure of back face of the prism with the vortices.