• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2002년도 추계학술대회논문초록집
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• pp.364.2-364
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• 2002

Thermoacoustic oscillation, which stems from phase correlation between unsteady heat release and acoustic fluctuation, can cause severe vibration and incite the excessive local heat transfer inside the rocket engine. It is very important to understand and prevent this phenomenon in the way of rocket engine development. In this study, the propulsion test facility of KSR-III, which is the first liquid propellant rocket developed by KARI, will be introduced. and the characteristics of thermoacoustic ocillation occurred at the facility will be examined.

• 한국연소학회지
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• 제21권2호
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• pp.29-37
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• 2016

This study described the experimental investigations of combustion instability in a model gas turbine combustor. Strong coupling between pressure oscillations and unsteady heat release excites a self-sustained acoustic wave, which results in a loud and annoyed sound, and may also lead to a structural damage to the combustion system. In this study, in order to examine the combustion instability phenomenon of a dual swirling combustor configuration, the information of heat release and pressure fluctuation period with respect to the variation in both thermal power and combustor length was collected experimentally. As a result, the fundamental acoustic frequency turned out to increase with the increasing thermal power without respect to the combustor length. The frequency response to the combustor length was found to have two distinct regimes. In a higher power regime the frequency significantly decreases with the combustor length, as it is expected from the resonance of gas column. However, in a lower power regime it is almost insensitive to the combustor length. This insensitive response might be a result of the beating phenomenon between the interacting pilot and main flames with different periods.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2012년도 제38회 춘계학술대회논문집
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• pp.555-561
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• 2012

Empirical Mode Decomposition(EMD)을 통한 Hilbert Huang Transform(HHT)은 시간-주파수 영역분석 방법 중 하나로 기존의 다른 분석 방법에 비해 비선형, 비정상 신호를 해석 가능하다는 등 여러가지 이점이 있다. 그러나 인접한 주파수를 분별하기 힘들고 잡음에 취약하다는 결점이 있다고 알려져 있다. 본 논문에서는 HHT와 정상신호 분석에 효과적인 Short-Time Fourier Transform(STFT)을 비교하여 각 방법의 장 단점을 분석하고 Rijke 튜브 실험에서 얻은 열음향학적 불안정 데이터에 적용하여 잡음에 취약한 점을 보완한 Improved HHT와 비교한다. 그 결과, EMD를 이용한 Original HHT보다 EEMD를 이용한 Improved HHT가 잡음의 영향을 적게 받아 보다 정확한 신호분석이 가능하다는 것을 알 수 있었다.

• Structural Engineering and Mechanics
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• 제89권3호
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• pp.265-281
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• 2024

The sound radiation responses of multi-layer composite plates subjected to harmonic mechanical excitation in hygrothermal environment is numerically investigated. A homogenized micromechanical finite element (FE) based on the higher-order mid-plane kinematics replicating quadratic function as well as the through the thickness stretching effect together with the indirect boundary element (IBE) scheme has been first time employed. The isoparametric Lagrangian element (ten degrees of freedom per node) is used for discretization to attain the hygro-thermo-elastic natural frequencies and the modes of the plate via Hamilton's principle. The effective material properties under combined hygrothermal loading are considered via a micromechanical model. An IBE method is then implemented to attain structure-surrounding coupling and the Helmholtz wave equation is solved to compute the sound radiation responses. The effectiveness of the model is tested by converging it with the similar analytical/numerical results as well as the experimentally acquired data. The present scheme is further hold out for solving diverse numerical illustrations. The results revealed the relevance of the current higher-order FE-IBE micromechanical model in realistic estimation of hygro-thermo-acoustic responses. The geometrical parameters, volume fraction of fiber, layup, and support conditions alongside the hygrothermal load is found to have significant influence on the vibroacoustic characteristics.

• 마이크로전자및패키징학회지
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• 제15권1호
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• pp.7-18
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• 2008

This paper reviews the current development of an ultra-slim SiP for Radio Frequency (RF) application, in which three flip chips, additional passive components and Surface Acoustic Wave (SAW) filters are integrated side-by-side. A systematic investigation is carried out for the design optimization, process and reliability improvement of the package, which comprises several aspects: a design study based on the 3D thermo-mechanical finite element analysis of the packaging, the determination of stress, warpage distribution, critical failure zones, and the figuration of the effects of material properties, process conditions on the reliability of package. The optimized material sets for manufacturing process were determined which can reduce the number of testing samples from 75 to 2. In addition the molded underfilling (MUF) process is proposed which not only saves one manufacturing process, but also improves the thermo-mechanical performance of the package compared with conventional epoxy underfilling process. In the end, JEDEC's moisture sensitivity test, thermal cycle test and pressure cooker tests have also been carried out for reliability evaluation. The test results show that the optimized ultra-slim SiP has a good reliability performance.

• 한국태양에너지학회 논문집
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• 제34권1호
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• pp.48-57
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• 2014

The transient and synchronization behaviors of a TA (thermo acoustic) laser pair were investigated experimentally for various crossing angles and different separation distances between the laser openings. Sound waves generated by the lasers were measured and analyzed at or near the focusing point by means of microphones, SPL meters, and a commercial software called Signal-Express. The two TA lasers were acoustically coupled through the air mass between their openings, and the only mode-locking operation that could be achieved was the one that was nearly $180^{\circ}C$ out of phase. The time to achieve synchronization was found to be dependent upon the initial mistuning of the frequencies and the crossing angle between the laser axes. The synchronization process could also be accelerated by turning on the laser with the lower power input first.

• 한국분무공학회지
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• 제25권3호
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• pp.119-125
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• 2020

In this paper, we used Helmholtz solver based on 3D finite element method to quantitatively analyze the effects of change of gain, time delay and time delay spread, which are the main variables of flame transfer function, on combustion instability in gas turbine combustor. The effects of the variable of flame transfer function on the frequency and growth rate, which are the main results of combustion instability, were analyzed by applying the conventional heat release fluctuation model and modified one considering the time spread. The analysis results showed that the change of gain and time delay in the same resonance mode affected the frequency of the given resonance modes as well as growth rate of the feedback instability, however, the effect of time delay spread was not relatively remarkable, compared with the dominant effect of time delay.

• International Journal of Aeronautical and Space Sciences
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• 제16권2호
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• pp.165-176
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• 2015

A numerical analysis was conducted to investigate the inlet buzz and combustion oscillation in an axisymmetric ramjet engine with wedge-type flame holders. The physical model of concern includes the entire engine flow path, extending from the leading edge of the inlet center-body through the exhaust nozzle. The theoretical formulation is based on the Farve-averaged conservation equations of mass, momentum, energy, and species concentration, and accommodates finite-rate chemical kinetics and variable thermo-physical properties. Turbulence closure is achieved using a combined scheme comprising of a low-Reynolds number k-${\varepsilon}$ two-equation model and Sarkar's compressible turbulence model. Detailed flow phenomena such as inlet flow aerodynamics, flame evolution, and acoustic excitation as well as their interactions, are investigated. Mechanisms responsible for driving the inlet buzz are identified and quantified for the engine operating at subcritical conditions.

• Structural Engineering and Mechanics
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• 제76권5호
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• pp.613-618
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• 2020

Exhausting oil resources and increasing pollution around the world are forcing researchers to look for new, renewable, biodegradable materials to lead sustainable development. The use of fiber reinforced composites based on natural fibres has increasingly begun as prospective materials for various engineering applications in the automotive, rail, construction and aerospace industries. The natural fiber chosen to make the composite material is plant-based fibre, e.g. jute fibre, and hemp fibre. Thermosetting polymer based Epoxy (LY556) was utilized as matrix material and The composites were produced using hand lay-up technique. The fabricated composites were tested for acoustic testing, thermo-gravimetric analysis (TGA) and flammability testing to asses sound absorption, thermal decomposition and fire resistivity of the structures. Hemp fibre composites have shown improved thermal stability over Jute fibre composites. However, the fire resistance characteristics of jute fibre composites are better as compared to hemp fibre composites. The sound absorption coefficient of composites was found to enhance with the increase of frequency.

• 한국분무공학회지
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• 제27권1호
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• pp.36-41
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• 2022

In this study, the combustion instability characteristics according to the change in the hydrogen ratio in the fuel in the single nozzle system of the hydrogen-natural gas mixed gas turbine for power generation was analyzed using a three-dimensional finite element analysis-based Helmholtz solver. This combustor shows the instability characteristics in which mode transition occurs from a mode having a low amplitude near 70 Hz to a mode having a high amplitude of 250 Hz or higher as the hydrogen fraction in the fuel increases. The current modeling results are found to reasonably predict the main characteristics of the change in measured instability frequency and growth rate with the change in fuel composition.