• 한국소음진동공학회논문집
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• 제14권11호
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• pp.1066-1074
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• 2004

A vibration isolation system for a large reverberation chamber (1,228 $m^3$ and 1,000 ton) has been installed and verified. The reverberation chamber generates loud noise and induces high level of vibration while performing spacecraft acoustic reliability tests. The isolation system prevents vibration transfer from the chamber to the enclosure buildings. This paper describes design process and commissioning experiments of the system. Design criteria have been derived from rigid body model of the chamber. The stiffness of neoprene pads has been selected by employing finite element analysis of the reverberant chamber and isolation system. A total of 21 neoprene pads have been installed between the chamber and supporting Pedestals. A sand bag of 800kg was dropped on the chamber floor to measure the natural frequency of the isolation system. While 136.9 dB noise is generated in the chamber, absolute transmissibility of the isolation system has been measured. The measured natural frequency of the chamber is 10.2Hz, which is 80% of the predicted value. Overall transmissibility at working frequency range (25∼10.000 Hz) is less than -12.4 dB.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2003년도 제21회 추계학술대회 논문집
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• pp.153-156
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• 2003

스월 인젝터의 분무특성을 변화시키는 기하학적인 변수인 백홀은 스월 인젝터의 내부유동이나 음향학적인 특성에도 큰 영향을 주게 된다. 백홀이 연소불안정 제거를 위하여 연소실에 부가적으로 부착하는 음향공(Acoustic cavity)과 유사한 역할을 할 수 있는지 알아보기 위하여 백홀을 Helmholtz resonator나 Quarter-wave resonator로 해석하였다. 그 결과 백홀을 가지는 스월 인젝터는 연소실내의 파동과 상쇄될 수 있는 고유주파수를 가지게 될 수 있었다. 또한 이를 위한 설계식을 제시하였다.

• 한국음향학회:학술대회논문집
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• 한국음향학회 1994년도 FIFTH WESTERN PACIFIC REGIONAL ACOUSTICS CONFERENCE SEOUL KOREA
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• pp.770-774
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• 1994

The volume of an acoustic source is important in determining various acoustic parameters. One of the suggested techniques is the internal pressure method incorporating a loudspeaker attached to a chamber wall and a microphone inserted into the cavity. Although the method is easy to handle with a very simple measurement setup, the coupling effects between the dynamic system of the loudspeaker and acoustic field, and the effects of higher order modes introduced by the discontinuities in the acoustic field, and the effects of higher order modes introduced by the discontinuities in the acoustic field should be considered for precise result. In this study, higher order modes due to the discontinuities of loudspeaker and microphone boundaries are included and the electro-acoustic coupling effects are compensated for by using the results of two cylinders with different lengths. The volume velocity of a loudspeaker thus obtained agrees very with that measured by laser sensor.

• 한국추진공학회지
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• 제16권5호
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• pp.29-36
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• 2012

연소시험을 통한 성공/실패 사례가 있는 FASTRAC 연소기에 대해, 효용성이 검증된 3차원 선형 음향해석을 수행하여 음향공의 물성치 변화에 따른 감쇠 특성을 파악하였다. 음향공의 동조주파수와 감쇠능력을 정량화하기 위해 음향 임피던스 개념을 도입하였다. 기하학적 형상이 주어진 상태에서, 음향공 내부의 음속 변화에 따라 최적의 동조가 가능한 음속을 파악하였다. 최적의 동조가 이루어지기 위한 음향공 내부의 음속 예측 방법을 제시하였다. 이를 통하여 여러 기하학적 형상을 가지는 음향공에 대하여 적용하여 감쇠능력을 정량화함으로써 연소안정성의 확보 여부에 대한 검토가 가능할 것이며, 안정성이 확보되지 못한 경우에 대해서는 그 이유를 명확하게 규명할 수 있을 것이다.

• 대한기계학회논문집B
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• 제33권7호
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• pp.552-558
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• 2009

An experimental study using the combustor with branch tube was conducted in order to model the industry combustor with FGR (flue gas recirculation) system and to study a thermo-acoustic instability generated by a branch tube. The branch tube is a structure used to modify a system geometry and then to change its pressure field, and the thermo-acoustic instability, usually occurs in a confined geometry, can result in serious problems on industrial combustors. Thus understanding of the instability created by modifying geometry of combustor is necessary to design and operate combustor with FGR system. Pressure fluctuation in the combustion chamber was observed according to diameter and length of branch and it was compared with the solution of 1-D wave equation. It was found that branch tube affects the pressure field in the combustion chamber, and the pressure fluctuation in the combustion chamber was reduced to almost zero when phase difference between an incipient wave in the combustion chamber and a reflected wave in the branch tube is $\pi$ at the branch point. Also, the reduction of pressure fluctuation is irrespective of the installed height of branch tube if it is below $h^*=0.9$ in the close-open tube and open-open tube.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2007년도 제29회 추계학술대회논문집
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• pp.198-203
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• 2007

본 연구의 목표는 air core의 길이 조절이 가능한 스월인젝터의 음향학적인 감쇠기능을 통해 고주파 연소불안정을 억제하는 것이다. air core(길이, 형상, 부피)와 인젝터의 위치에 대한 음향학적인 감쇠 효과는 선행 연구를 통해 실험되었다. 이러한 결과들을 바탕으로 다중 인젝터들의 효과에 대한 연구를 진행하였다. 실험 결과로부터 각 모드의 배(anti-node)에 장착된 인젝터들의 수의 증가로 감쇠효과가 증가하는 것을 확인하였다. 또한 각 모드들의 배(anti-node)에 장착되어 동조된 인젝터들이 동시에 개별적인 모드 감쇠 성능을 보일 수 있음을 확인했다.

• 한국항공우주학회지
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• 제30권5호
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• pp.79-87
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• 2002

로켓엔진에서 발생하는 음향 불안정성의 정성적 경향성을 파악하기 위해 축대칭 연소실에서의 음향파 응답 특성을 수치해석적으로 조사하였다. 주로 작동조건의 변화와 외부교란의 가진 주파수 변화에 따른 응답 특성을 계산하였다. 외부교란으로 연소실 입구부분의 전압이 정현파 형태로 섭동하도록 인위적으로 부여하였다. 음향 응답의 정성적 경향성이 유지되는 범위내에서, 분무 연소과정에 수반되는 여러 가지 물리 화학적인 과정들을 단순화하였다. 먼저, 주어진 작동조건에서 정상상태의 연소장을 구하고, 다음 단계로 압력섭동에 대한 연소장의 거동을 시간의 경과에 따라 구하였다. 작동조건의 변화에 따른 음향파 응답의 계산 결과, 반응 지역에서 약한 강도의 화염이 형성되는 경우 민감한 응답이 나타나 불안정안 압력분포를 나타냈다. 또한, 가진되는 압력 섭동의 주파수에 따라 응답 특성이 변하였고, 특히 높은 가진 주파수의 압력 섭동에 대해서 불안정한 응답이 나타났다. 해석 결과로 나타난 거시적 현상의 이해를 돕기위해, 기존의 화염소 모델을 채택한 음향파 응답 연구 결과와의 상관성을 찾아 본 해석 결과를 분석하였다.

• 한국연소학회:학술대회논문집
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• 한국연소학회 제26회 KOSCO SYMPOSIUM 논문집
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• pp.167-171
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• 2003

Acoustic characteristics of baffled combustion chamber to elucidate suppressing effect of baffle on combustion instability are numerically investigated by linear acoustic analysis. A hub-blade baffle of 5 blades is selected as a candidate one and five variants of baffles with various configuration are designed. Resonant-frequency shift and damping factor are analyzed quantitatively as damping parameters. When the hub is located radially at the pressure node, the decrease of resonant frequency and increase of damping factor in 1R mode are dominant. But sub-1T mode is formed within hub, therefore, there would be a possibility of initiating 1T mode in unbaffled region, which would occur another problem. For smaller hub size, four kinds of axial baffle length is selected. As the axial baffle length increases, resonant frequency shift and increase of damping factor of transverse acoustic modes is obtained. Especially, two close acoustic modes such as 1L and 1T could be overlapped for a certain axial length, resulting in extreme increase of damping factor. The present study based on linear acoustic analysis is expected to be a useful confirming tool to predict acoustic field and design a passive control devices such as baffle and acoustic cavity.

• 한국연소학회지
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• 제23권1호
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• pp.13-20
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• 2018

Transverse acoustic mode in annular combustion chambers affects air-fuel mixing characteristics in the nozzle and can result in heat release fluctuations in the combustor. In addition, the acoustic mode coupling between the nozzle and the combustion chamber is one of the key parameters determining combustion instability phenomenon in the annular combustor. In this study, acoustic coupling between the nozzle and annular combustor was numerically analyzed using 3D-based in house FEM code. As a result, it was found that the acoustic mode inside the combustion chamber at anti-node locations of the transverse mode was strongly influenced by the nozzle inlet boundary conditions.

• International Journal of Precision Engineering and Manufacturing
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• 제9권2호
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• pp.28-33
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• 2008

In a scanning electron microscope (SEM), outside acoustic noise causes image noise that distorts observations of the specimen being examined. A SEM that is less sensitive to acoustic noise is highly desirable. This paper investigates the image noise problem by addressing the mode shapes of the base plate and the transmission path of the acoustic noise and vibration. By arranging the position of the rib, a new SEM base plate was developed that had twisting as the 1st and 2nd modes. In those two twisting modes, vibration nodes existed near the center of the base plate where the specimen chamber is placed. Less vibration was transmitted to the chamber and to the specimen by the twisting modes compared to bending ones, which are the 2nd and 3rd modes for a rectangular plain base plate. An SEM with the developed base plate installed exhibited a significant reduction of image noise when exposed to acoustic noises below 250 Hz.