• Proceedings of the KIEE Conference
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• 2000.11c
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• pp.556-558
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• 2000

유한요소법을 이용하여 대전력용 트리가트론의 전극 전압의 극성 변화와 전극 구조의 변화에 따른 전계 분포를 해석하였다. 시동 특성을 좋게 하기 위해서는 트리거 전극과 접지 전극 사이의 적절한 거리를 찾아야 한다. 유한요소법을 이용한 전계 분포를 가지고 이 거리를 정할 수 있다. 본 논문은 트리거 갭과 주 갭(트리거 전극과 반대 주전극 사이)에서 동시에 절연 파괴가 일어날 때 트리가트론이 최적으로 동작한다는 연구 결과를 바탕으로 한다. 본 논문은 실험을 위한 예비 단계로서, 시뮬레이션을 통하여 최적의 전극 구조를 찾는데 드는 비용과 시간을 줄일 수 있다.

• Proceedings of the KIEE Conference
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• 2006.07b
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• pp.1219-1220
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• 2006

본 연구의 목적은 Pspice를 이용하여 SI Thyristor의 구조적인 특징과 스위칭 동작을 설명하면서도 비교적 간략화된 등가 모델을 개발하는 것에 있다. 이러한 목표로 등가모델은 SI Thyristor의 구조적 형태에 기반을 두어 BJT 소자를 이용한다. 또 게이트 구조와 스위칭 매커니즘을 고려한 MOSFET, Steady state Turn on 상태에서 dominant 모델인 PIN Diode로 구성되어 있다. 개발된 등가모델을 스너버회로와 함께 스위칭 과도응답을 시뮬레이션하였으며 그 결과는 실제 실험결과와 비교하여 검증하였다. 비교적 간단하게 고안된 회로를 통해 Turn On/Off 동작에서 스위칭 특성을 예측할 수 있으므로 펄스파워용 스위치로서 SI Thyristor의 시동특성을 해석하는 데 본 등가모델을 활용할 수 있을 것으로 전망한다.

• Proceedings of the KIEE Conference
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• 2006.07c
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• pp.1679-1680
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• 2006

본 연구의 목적은 Pspice를 이용하여 SI Thyristor의 구조적인 특징과 스위칭 동작을 설명하면서도 비교적 간략화된 등가 모델을 개발하는 것에 있다. 이러한 목표로 등가모델은 SI Thyristor의 구조적 형태에 기반을 두어 BJT 소자를 이용한다. 또 게이트 구조와 스위칭 매커니즘을 고려한 MOSFET, Steady state Turn on 상태에서 dominant 모델인 PIN Diode로 구성되어 있다. 개발된 등가모델을 스너버회로와 함께 스위칭 과도응답을 시뮬레이션하였으며 그 결과는 실제 실험결과와 비교하여 검증하였다. 비교적 간단하게 고안된 회로를 통해 Turn On/off 동작에서 스위칭 특성을 예측할 수 있으므로 펄스파워용 스위치로서 SI Thyristor의 시동특성을 해석하는 데 본 등가모델을 활용할 수 있을 것으로 전망한다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.363-366
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• 2010

Among the many different types of wind tunnels, Ludwieg Tube(LT) is the most suitable facility for high Reynolds number testing. Depending on the location of diaphragm, there are two types of LTs. In the present study, a computational work has been carried out to compare the operation characteristics of upstream- and downstream-diaphragm LTs. Two-dimensional, axisymmetric, unsteady, compressible Navier-Stokes equations were solved using a fully implicit finite volume scheme. Based on the present results, the flow mechanism of the starting process was discussed in detail using wave diagrams and characteristics of starting time and working time were investigated.

• The Proceedings of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.10 no.5
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• pp.52-56
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• 1996

Fluorescent lamps are widely accepted to energy efficient commercial lighting applications. In designing a fluorescent lamp system, a ballast design heavily relies on the characteristic of a fluorescent lamp under consideration. Especially, at startup time, the transient characteristic of a fluorescent lamp puts much tighter specification of a design. In this paper, based on the transient characteristic at the startup time, a transient behavioral model of a fluorescent lamp is presented with an equivalent circuit. The model is applicable to the wide range of fluorescent lamps provided by different manufacturers. The experimental results are compared with the results provided by PSPICE simulation. The result shows the model is effective In practice. As a result, we could identify more accurate startup constraints to decide the design of either an electro mechanical or an electronic ballast.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.213-217
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• 2017

Since more than 30% of the liquid rocket engine failures occur during the start-up process, and the Space Shuttle Main Engine (SSME) is especially sensitive to small changes in propellant conditions, a 2% error in the valve position or a 0.1sec timing error could lead to significant damage of the engine, simulation modeling of start-up process is important. However, there are many difficulties associated with engine start-up process caused by nonlinear mass flow and heat transfer characteristics associated with filling an unconditioned engine system with cryogenic propellants. In this paper, we modelled a SSME simulation model using partially Computational Fluid Dynamics (CFD) method to solve these problems and checked the performance by comparing with the performance of the simulation model using the lumped method under the state of normal condition.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.609-611
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• 2017

The hysteresis phenomena are frequently encountered in the wide variety of fluid flow systems of industrial and engineering applications. Hysteresis mainly appears during the transient change of pressure ratios, and this, in turn, influences the performance the supersonic wind tunnel. However, investigations on the hysteresis phenomenon particularly inside the supersonic wind tunnel are rarely studied. In the present study, numerical simulations are carried out to investigate hysteresis phenomenon of the shock waves inside the Supersonic Wind Tunnel. The unsteady, compressible flow through the supersonic wind tunnel is computationaly analyzed with an symmetric model. The Navier-Stokes equations are solved with Spalart-Allmaras turbulence model using a fully implicit finite volume scheme. The variaton in the flow field between the starting pressure ratio and operating pressure ratio of a supersonic wind tunnel is investigated in terms of hysteresis phenomenon.

• Tribology and Lubricants
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• v.31 no.3
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• pp.125-140
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• 2015

In this paper, we present the results of the wear analysis of journal bearings on a stripped-down single-cylinder engine during start-up and coast-down by motoring. We calculate journal bearing wear by using a modified specific wear rate considering the fractional film defect coefficient and load-sharing ratio for the asperity portion of a mixed elastohydrodynamic lubrication (EHL) regime coupled with previously presented graphical data of experimental lifetime linear wear in radial journal bearings. Based on the calculated wear depth, we obtain a new oil film thickness for every crank angle. By examination of the oil film thickness, we determine whether the oil film thickness at the wear scar region is in a mixed lubrication regime by comparing dimensionless oil film thickness, h/σ, to 3.0 at every crank angle. We present the lift-off speed and the crank angles involved with the wear calculation for bearings #1 and #2. The dimensionless oil film thickness, h/σ, illustrates whether the lubrication region between the two surfaces is still within the bounds of the mixed lubrication regime after scarring of the surface by wear. In addition, we present in tables the asperity contact pressure, the real minimum film thickness at the wear scar region, the modified specific wear rate, and the wear angle, α, for bearings #1 & #2. To show the real shape of the oil film at wear scar region, we depict the actual oil film thickness in graphs. We also tabulated the ranges of bearing angles related with wear scar. We present the wear volume for bearings #1 and #2 after one turn-on and turn-off of the engine ignition switch for five kinds of equivalent surface roughness. We show that the accumulated wear volume after a single turn-on and turn-off of an ignition switch normally increases with increasing surface roughness, with a few exceptions.

• Tribology and Lubricants
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• v.31 no.3
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• pp.109-124
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• 2015

This paper presents a wear analysis procedure for the journal bearings on a stripped-down single-cylinder engine during start-up and coast-down by motoring. A journal bearing is in the mixed elastohydrodynamic (EHL) lubrication region when the shaft speed is less than the corresponding lift-off speed. Below the lift-off speed, a wear scar can form on bearing surfaces. In part 1 of this paper, we develop the appropriate formulations and the calculation procedure for the analysis. Specifically, we formulate an equation for modified film thickness in a journal bearing considering the additional wear volume. In order to obtain the modified specific wear rate induced by the modified Archard’s wear coefficient, we utilized the extended non-dimensional diagram for the specific wear rate, k, the fractional film defect coefficient, Ψ and the asperity load sharing factor, γ₂. This asperity load sharing factor is newly calculated by setting the Zhao-Maietta-Chang (ZMC) asperity contact pressure equation coupled with the central film thickness equation derived by using the ZMC asperity contact model equal to the modified central contact pressure derived by using the central (or maximum) contact pressure at the dry rough line-contact configuration. We can use the procedure introduced in this paper to determine the lifetime (or longterm) linear wear in radial journal bearings that is a result of repeated stop-start cycles.

• Tribology and Lubricants
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• v.33 no.4
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• pp.168-186
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• 2017

This paper presents a wear analysis procedure for calculating the wear of journal bearings during the start-up and coast-down cycles of a motoring stripped-down single cylinder engine operating with a tilted shaft. In order to decide whether the lubrication state of a journal bearing is in the mixed-elasto-hydrodynamic lubrication regime, we utilize lift-off speed and MOFT (most oil film thickness) under mixed-elasto-hydrodynamic lubrication regime at the corresponding aligned shaft. We formulate an equation for the modified film thickness in a misaligned journal bearing considering the additional wear volume described in Part I of this study. For this, we use the calculation results of the degree of misalignment and tilting angle obtained after finding the eccentricities of the two bearings supporting the crankshaft of a single cylinder engine. In this Part II, we calculate the wear of journal bearings using the fractional film defect coefficient, the asperity load sharing factor, and the modified specific wear rate for the application of mixed-elasto-hydrodynamic lubrication regime. We show that the accumulated wear volume after turning the ignition switch on and off once, increases to ${\sigma}=39{\mu}m$ and then decreases from ${\sigma}=39{\mu}m$ with increasing in surface roughness.