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박태정(Tae-jung Park),이학성(Hak-sung Lee),배주한(Joo-han Bae),김석(Seok Kim),이상용(Sang Yong Lee) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
Phenomena of flow instabilities in boiling two-phase natural circulation systems have been of interest to the power-plant engineers for several decades. The steam generator is a typical component of the pressurized water reactor (PWR), where the shell-side fluid is evaporated by the hot RCS coolant and circulates naturally within it. In the present paper, considering the circulation characteristics of flow in the steam generator, an attempt has been made to identify the dominant flow instability. Sensitivity of the operating parameters and geometry of the system to the flow instabilities has been tested by using Nyquist stability criterion. In addition, appropriateness the two-phase flow model has been studied, especially for homogeneous and drift-flux flow models. The drift-flux model by Takeuchi turned out to be pertinent to prediction of the two-phase flow through the tube bundle in the steam generator.
확장 면적을 이용한 안정된 증발 마이크로채널 시스템의 설계
이희준(Hee Joon Lee),Shi-chune Yao 대한기계학회 2011 大韓機械學會論文集B Vol.35 No.8
마이크로채널 표면의 핵에서 생성된 기포는 물을 작동유체로 이용하는 경우 채널의 좁은 단면적에 의해 압착되어 유동불안정성을 유발한다. 직관 마이크로채널에서 압착된 기포는 관성 유동의 역방향인 채널 상류로 진행한다. 마이크로채널에서 역방향 유동을 없애기 위해 채널 하류의 단면적을 확장시키는 것이 효과적이다. 그 이유는 압착된 기포의 전후단 계면의 표면장력에 의한 압력차이가 계면의 반지름에 역비례해서 자연적으로 기포가 채널 하류로 이동하려는 힘이 발생하기 때문이다. 확장 증발 마이크로채널에서 정적 유동불안정성 모델이 제시되었으며, 실험으로 모델을 검증하였다. 또한, 안정된 확장 시스템을 설계하기 위해 국부 설계 개념을 도입하였다. 검증된 모델과 개념을 바탕으로 안정된 확장 증발 마이크로채널 설계를 성공적으로 수행하였다. A growing bubble can be squeezed for water, and it will then encounter flow instability, which reverses toward upstream in straight micro-channels. To reduce the flow instability, a micro-channel that expands at the downstream end has been found to be effective. In the expanding area, a growing bubble will tend to move downstream because the net surface tension force of a vapor-liquid interface is inversely proportional to the local radius of curvature. We propose a static flow instability model and validate it experimentally. Moreover, we apply the local-instability parameter concept to the real design of a stable evaporative micro-channel with an expanding area. Based on the localinstability model, we establish a static design for stable expanding evaporative micro-channels.
원형 실린더가 주기적으로 배열된 채널 유동의 주 유동 불안정성
윤동혁(D.-H. Yoon),양경수(K.-S. Yang),강창우(C. Kang) 한국전산유체공학회 2010 한국전산유체공학회지 Vol.15 No.3
A parametric study has been carried out to elucidate the characteristics of channel flow with a stream wise-periodic array of cylinders. This flow configuration is relevant to heat exchanger applications. The presence of cylinders in channel flow causes the attached wall boundary layer to separate, leading to significant change in flow instabilities. There exist two kinds of instabilities; flow undergoes a primary instability (Hopf bifurcation) at a lower Reynolds number, and the unsteady two-dimensional flow becomes unstable to three-dimensional disturbances at a higher Reynolds number. We report here the dependencies of the prim my instability as well as the flow characteristics of the subsequent unsteady flow, including flow-induced forces and Strouhal number of vortex shedding, on the distance between the cylinder and the channel wall.
윤동혁(D.-H. Yoon),양경수(K.-S. Yang),강창우(C. Kang) 한국전산유체공학회 2010 한국전산유체공학회 학술대회논문집 Vol.2010 No.5
A parametric study has been carried out to elucidate the characteristics of channel flow with a streamwise-periodic array of cylinders. This flow configuration is relevant to heat exchanger applications. The presence of cylinders in channel flow causes the attached wall boundary layer to separate, leading to significant change in flow instabilities. There exist two kinds of instabilities; flow undergoes a primary instability (Hopf bifurcaiton) at a lower Reynolds number, and the unsteady two-dimensional flow becomes unstable to three-dimensional disturbances at a higher Reynolds number. We report here the dependencies of the primary instability as well as the flow characteristics of the subsequent unsteady flow, including flow-induced forces and Strouhal number of vortex shedding, on the distance between the cylinder and the channel wall.
이중원통형 유체층에서 용기의 회전이 열모세관 유동에 미치는 영향
곽호상(Ho Sang Kwak),우마로프알리세르(Alisher Umarov),김경진(Kyoungjin Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
A numerical study is conducted on thermocapillary flow of thin fluid layer in an annulus with an imposed horizontal temperature difference. In this flow configuration simulating a semiconductor materials processing, the flow is affected by the interaction between buoyancy and surface tension. In a very thin fluid layer, hydrothermal waves occur due to the dominant thermocapillary force. In a relatively thick fluid layer, the flow exhibits long convective rolls formed due to the buoyant flow instability. As a means to suppress these three-dimensional instabilities, rotation of the system is introduced. Numerical results show that the careful use of rotation may be an efficient way to reduce or eliminate hydrothermal waves. For the buoyancy-domanated flow regime, the rotation suppresses the growth of convective instability, but it may invoke other type of three-dimensional waves driven by baroclinic instability.
강창우(C. Kang),양경수(K.S. Yang) 한국전산유체공학회 2011 한국전산유체공학회지 Vol.16 No.1
Flow instability is investigated in a two-dimensional channel with thin baffles placed symmetrically in the vertical direction and periodically in the streamwise dircetion. At low Reynolds numbers, the flow is steady and symmetric. Above a critical Reynolds number, the steady flow undergoes a Hopf bifurcation leading to unsteady periodic flow. As Reynolds number further increases, we observe the onset of secondary instability. At high Reynolds numbers, the two-dimensional periodic flow becomes three dimmensional. To identify the onset of secondary instability, we carry out Floquet stability analysis. We obseved the transition to 3D flow at a Reynolds number of about 125. Also, we computed dominant spanwise wavenumbers near the critical Reynolds number, and visualized vortical structures associated with the most unstable spanwise wave.