• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.425-430
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• 1998

SMART 관류형증기발생기의 유동 불안정성을 분석할 수 있는 시간영역-비선형 해석모델을 개발하였다. 해석모델은 일차계통 모델을 포함하고 있으며 이차측 튜브 양단에 일정압력 경계조건을 이용하고 내부에서는 평형 균질 이상유동 모델을 도입하였다. 기존의 정상 상태 및 임계조건에 대한 실험 결과와 개발된 해석모델 모델을 이용한 계산 결과를 비교한 뒤 임계점 이후 나타나는 진동의 특성을 분석하였다. 개발된 해석모델은 SMART 관류형증기발생기에서 발생할 수 있는 유동 진동의 특성과 영향을 파악하고 유동 불안정성을 막기 위한 입구 오리피스 설계의 목적에 활용할 수 있을 것이다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.275-278
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which ran be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• Proceeding of EDISON Challenge
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• 2012.04a
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• pp.33-36
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• 2012

풍력발전기 성능은 유동의 안정성과 풍속에 의해 결정되는데, 이때 유동 불안정성은 풍력발전기의 성능뿐만 아니라 구조적 문제를 함께 유발시킨다. 본 연구에서는 풍력발전기 타워 후류에서의 불안정성을 최소화시키기 위하여 타워 단면의 기초 형상설계 연구를 수행하였다. 기존의 풍력발전기 타워 형상에 부가 구조물을 설치함으로써 Karman vortex의 생성을 지연시키고 와류 간섭현상을 줄여 풍력발전기의 안정성을 증대시키고자 하였다. 이를 위해 다양한 타워 단면 형상에 대하여 양력계수 및 항력계수를 비교 분석하였다. 그 결과 반지름의 1/2 길이의 자유류 방향 tip과 splitter plate를 후방에 설치하는 것이 후류 불안정성을 억제하는데 가장 효율적인 것으로 나타났다.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05a
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• pp.463-468
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• 1998

수직 관다발형 비등관에서의 밀도파 진동 및 유동 폭주형 이상유동 불안정성을 해석하기 위하여 선형화 기법 및 D-partition 방법론에 근거한 해석 코드(ALFS)를 개발하고 기존 실험자료 분석을 통해 코드의 예측 성능을 평가하였다. 그 결과 이상유동이 평형상태에 있는 것으로 가정하는 가장 단순한 모델인 HEM은 전반적으로 유동 불안정성 발생 시점의 열출력을 실험치보다 약 20% 정도 낮게 예측하였으며, 이상 유동의 속도 및 온도의 비평형 상태를 고려하는DEM과 DNEM에 의한 예측 결과는 7∼15%의 평균 오차 범위에서 실험 자료를 예측하는 것으로 나타났다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.12
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• pp.1193-1199
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• 2012

A cup burner experiment was performed to investigate the effect of the oxidizer velocity and concentration on flame instability near extinction. Heptane was used as a fuel and air diluted by nitrogen and carbon dioxide was used in the oxidizer stream. Two types of flame instabilities at the flame base and at axial downstream were observed near extinction. The instability at the flame base could be characterized by cell, swing, and rotation modes, and the cell mode changed to the rotation mode through the swing mode as the oxidizer velocity increased. To assess the parameters for the flame instability, the initial mixture strengths, Lewis number, and adiabatic flame temperature were investigated under each condition. The Lewis number might be the most important among them, but it is impossible to generalize because of the insufficient number of cases. Furthermore, the axial periodic flickering motion disappeared at low and high oxidizer velocities near extinction. This resulted from the fact that low oxidizer velocity induced evaporated fuel velocity below the critical velocity and high velocity made the reacting fuel velocity comparable.

• Journal of Aerospace System Engineering
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• v.14 no.5
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• pp.100-106
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• 2020

Turbopumps for liquid rocket engines are exposed to various cavitation instabilities under their operating conditions. The instabilities affect the stability of the turbopumps. To make sure of the stability of the turbopump of KSLV-II, the present work examined the characteristics of the cavitation instabilities during the turbopump assembly test. In the test, the LOx pump was operated under super-synchronous rotating cavitation and attached to uneven cavitation. In the vibration analysis of the fuel pump, the characteristic frequency by the super-synchronous cavitation of the LOx pump was clearly shown.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2013.10a
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• pp.182-183
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• 2013

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.8.1-8.1
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• 2005

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.11b
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• pp.1400-1405
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• 2001

In this study, an analysis program to assess the susceptibility of steam generator tubes due to the flow-induced vibration was developed. Analysis of fluid-elastic instability and random turbulence excitation for the U-tube bundle in CE-type steam generator was accomplished. The effective mass distribution along the U-tube was obtained to calculate the natural frequency and dynamic mode shape. Finally, stability ratios and rms vibration amplitude for selected tubes are obtained.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.1948-1966
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• 1991

Two-phase cross-flow exists in many shell-tube heat exchangers such as condensers, reboilers and nuclear steam generators. To avoid problems due to excessive vibration, information on vibration excitation in two-phase cross-flow is required. Fluid-elastic instability is discussed in this paper. Four tube bundle configurations were subjected to increasing flow up to the onset of fluid-elastic instability. The tests were done on bundles with one flexible tube surrounded by rigid tubes. The fluid-elastic instability behavior is different for intermittent flows than for bubbly flows. For bubbly flows, the observed instabilities satisfy the relationship V/fd=K(2.pi..zeta. m/rho. $d^{21}$)$^{0.51}$ in which the minimum instability factor K was found to be 2.3 for bundles of p/d=1.22. The lowest critical velocities for fluid-elastic instability were experienced with parallel-triangular tube bundles. For intermittent flow, the observed instabilities did not follow the forgoing relation-ship. Significantly lower flow velocities were required for instability..