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격자볼츠만법을 이용한 능동형 미소 혼합기의 혼합 특성 및 성능에 관한 연구

Title
격자볼츠만법을 이용한 능동형 미소 혼합기의 혼합 특성 및 성능에 관한 연구
Other Titles
A study for mixing characteristics and performance of an active micro-mixer by the lattice Boltzmann method
Author
김용대
Alternative Author(s)
Kim, Yong-Dae
Advisor(s)
맹주성
Issue Date
2007-02
Publisher
한양대학교
Degree
Doctor
Abstract
최근 극도로 작은 장치를 제작할 수 있는 제조공정이 개발됨에 따라 집적도, 정밀도, 이동성 등이 뛰어난 Micro-Electro Mechanical System(MEMS) 장치들의 연구와 개발이 활발하게 이루어지고 있다. MEMS 장치는 매우 작은 크기로 제작된 완전한 시스템으로 마이크로 채널, 펌프, 믹서, 반응기 등의 다양한 유동 장치들로 구성되어있다. 각 장치들의 성능은 MEMS 시스템의 성능을 결정하는 매우 중요한 인자로 작용한다. 그 중에서도 마이크로 믹서의 성능은 결과의 신뢰성 및 효율에 중대한 영향을 미치는 장치이다. 마이크로 믹서는 작은 크기로 인하여 필연적으로 층류 유동영역에서 작동하기 때문에 분자확산에 의해 혼합이 이루어지며 혼합 속도가 매우 느리다. 그러므로 혼합속도를 가속하는 것이 매우 중요하다. 빠른 혼합을 성취하는 가장 효과적인 방법은 혼돈류를 이용하는 것이다. 본 연구에서는 진동하는 교반기를 이용하여 유체에 주기적인 섭동을 가하여 혼합을 가속하는 새로운 능동형 혼합기를 제안하고 격자볼츠만법(LBM)의 D2Q9 모델을 사용하여 교반기 유동 및 혼합을 모사하여 진동 교반기의 성능 및 혼합 특성을 분석하였다. 격자볼츠만법은 연속체 가정에 의하여 도출된 기존의 해석방법과 달리 분자동역학에 기초한 볼츠만 방정식에 의한 해석기법으로 마이크로 유동의 해석에서 물리적, 수학적으로 더욱 타당한 해법이며, 또한 교반기 진동과 같은 이동하는 경계를 포함하는 복잡한 유동의 모사에 있어서 기존 CFD에 비하여 접근이 용이한 장점을 가지고 있다. 먼저, 기존 회전 교반기와 제안된 진동 교반기의 혼합을 해석하여 교반속도 및 레이놀즈수에 따른 혼합 특성 및 성능을 비교하였다. 10부터 80까지의 레이놀즈수에 대해 교반속도를 변화하여 혼합을 분석하였다. 레이놀즈수 20이하에서는 임계 교반속도에서 진동교반기가 우수한 성능을 나타내었고, 레이놀즈수 30이상에서는 교반속도가 커질수록 진동교반기의 혼합성능이 더 뛰어남을 확인하였다. 다음으로 제안된 진동 교반기의 상류에 원형 실린더를 설치하여 실린더에서 발생하는 Karman 와의 영향을 고찰하였다. 혼합 성능은 원형 실린더 및 진동 교반기의 단일 혼합효과와 비교하였으며, 그 결과 상류의 원형 실린더는 교반기의 혼합 성능을 약 27% 향상시키는 효과적인 예혼합방법임을 확인하였다. 마지막으로 혼돈류를 정량적으로 분석할 수 있는 Poincare mapping과 Lyapunov exponent를 이용한 -map을 통하여 단일 진동교반기의 혼합 특성을 분석하였다. 일반 MEMS 장치의 작동 영역인 1-100의 레이놀즈수에 대하여 교반기의 교반속도를 환원진동수(k) 0-6의 다양한 진동조건으로 구동하여 교반기 섭동에 의해 발생하는 혼돈류 특성을 분석하고 혼돈류와 혼합율과의 관계를 고찰하였다. 레이놀즈수가 증가함에 따라 혼합양상은 초승달 형태에서 버섯모양의 말림현상으로 발달한다. 또한 교반기 환원진동수는 혼합 성능을 결정하는 매우 중요한 파라미터로 혼합효율이 최대가 되는 최적의 환원진동수가 존재한다. 레이놀즈수와 최적의 환원진동수는 일정한 로그 형태의 함수적 관계가 있음을 확인하였다. 최적의 환원진동수를 적용하여 교반기를 구동할 경우, 단일 교반기 만으로도 레이놀즈수 100에서 단순 직선채널에 비해 약 16배의 혼합성능 향상 효과가 있고, 40의 레이놀즈수에서도 약 10배의 성능 향상효과가 있어서 매우 다양한 레이놀즈수 범위에서 효율적인 혼합기를 구성할 수 있는 효율적인 혼합 방법이다.; Recently, as the manufacturing processes that can create extremely small devices have been developed, a research and development of microelectromechanical system(MEMS) are explosively increased. The MEMS devices are small size complete system constructed with various microfluidic components such as micro-channel, micro-pump, micro-mixer, micro reactor and etc. The performance of each device is very important factor which affect the capacity of whole system. The mixing performance of micro-mixer decisively affect the reliability of analysis and the system efficiency. In the micro-mixer, the flow is necessarily limited laminar flow regime due to extremely small size and flow speed, therefore mixing is governed by molecular diffusion and the speed of mixing is severely limited. Consequently, time inefficiency is the major problem in micro-mixer. The most efficient method for rapid mixing in laminar flow regime is chaotic stirring. In this study, we propose the novel active micro-mixer using an oscillating microstirrer and the mixing and flow field are simulated using D2Q9 lattice Boltzmann method(LBM), which is mathematically and physically proper simulation method in MEMS. Firstly, the mixing characteristics and performance of proposed oscillating stirrer compare with that of rotating stirrer. Within the range of Reynolds numbers from 10 to 80. The mixing behaviors of the rotating and the oscillating stirrer are analyzed for different stirring speeds. From the variation of the calculated We found that the critical values for the stirring speed, which have a maximum mixing efficiency exist. In addition, the oscillating stirrer was found to have a better mixing efficiency than the rotating stirrer at the critical stirring speed for each Reynolds number. Secondly, an active micro-mixer equipped successively with a circular cylinder and proposed oscillating stirrer is designed by analyzing the mixing behaviors, i.e. the effect of the Karman vortex, the flow structure after the stirrer, etc. Consequently, the Karman vortex generated by circular cylinder improves 27% in mixing efficiency over active mixer with only an oscillating stirrer, therefore it is suitable pre-mixing method. Finally, the mixing and chaotic characteristics are analyzed through Poincare section and Lyapunov exponent which can quantify the rate of chaotic advection. In Re=1-100 as flow regime of MEMS devices and reduced frequency of stirrer, k=0-6, the chaotic characteristics of flow are analyzed and the relations between chaotic advection and mixing rate are considered. As the Reynolds number is increased, the mixing behaviors are developed from crescent shapes to scroll behavior of mushroom shape. The optimum reduced frequency of stirrer for the maximum mixing efficiency exist at each Reynolds number and the optimum frequency has logarithmic relation with Reynolds number. Adapting the optimum frequency improve the mixing efficiency about 16 and 10 times for that of a simple straight channel in Re=100 and 40, respectively. Consequently, the oscillating stirrer driving with the optimum frequency is very efficient mixing mechanism for various flow condition in MEMS.Recently, as the manufacturing processes that can create extremely small devices have been developed, a research and development of microelectromechanical system(MEMS) are explosively increased. The MEMS devices are small size complete system constructed with various microfluidic components such as micro-channel, micro-pump, micro-mixer, micro reactor and etc. The performance of each device is very important factor which affect the capacity of whole system. The mixing performance of micro-mixer decisively affect the reliability of analysis and the system efficiency. In the micro-mixer, the flow is necessarily limited laminar flow regime due to extremely small size and flow speed, therefore mixing is governed by molecular diffusion and the speed of mixing is severely limited. Consequently, time inefficiency is the major problem in micro-mixer. The most efficient method for rapid mixing in laminar flow regime is chaotic stirring. In this study, we propose the novel active micro-mixer using an oscillating microstirrer and the mixing and flow field are simulated using D2Q9 lattice Boltzmann method(LBM), which is mathematically and physically proper simulation method in MEMS. Firstly, the mixing characteristics and performance of proposed oscillating stirrer compare with that of rotating stirrer. Within the range of Reynolds numbers from 10 to 80. The mixing behaviors of the rotating and the oscillating stirrer are analyzed for different stirring speeds. From the variation of the calculated We found that the critical values for the stirring speed, which have a maximum mixing efficiency exist. In addition, the oscillating stirrer was found to have a better mixing efficiency than the rotating stirrer at the critical stirring speed for each Reynolds number. Secondly, an active micro-mixer equipped successively with a circular cylinder and proposed oscillating stirrer is designed by analyzing the mixing behaviors, i.e. the effect of the Karman vortex, the flow structure after the stirrer, etc. Consequently, the Karman vortex generated by circular cylinder improves 27% in mixing efficiency over active mixer with only an oscillating stirrer, therefore it is suitable pre-mixing method. Finally, the mixing and chaotic characteristics are analyzed through Poincare section and Lyapunov exponent which can quantify the rate of chaotic advection. In Re=1-100 as flow regime of MEMS devices and reduced frequency of stirrer, k=0-6, the chaotic characteristics of flow are analyzed and the relations between chaotic advection and mixing rate are considered. As the Reynolds number is increased, the mixing behaviors are developed from crescent shapes to scroll behavior of mushroom shape. The optimum reduced frequency of stirrer for the maximum mixing efficiency exist at each Reynolds number and the optimum frequency has logarithmic relation with Reynolds number. Adapting the optimum frequency improve the mixing efficiency about 16 and 10 times for that of a simple straight channel in Re=100 and 40, respectively. Consequently, the oscillating stirrer driving with the optimum frequency is very efficient mixing mechanism for various flow condition in MEMS.
URI
https://repository.hanyang.ac.kr/handle/20.500.11754/150536http://hanyang.dcollection.net/common/orgView/200000405653
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GRADUATE SCHOOL[S](대학원) > MECHANICAL ENGINEERING(기계공학과) > Theses (Ph.D.)
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