微纳系统中三类典型流动与传热机理及分析

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	微纳系统中三类典型流动与传热机理及分析
其他题名	Study of Three Typical Flow and Heat Transfer in Thermal Micro System
	宋延熙
导师	徐进良
	2009-06-02
学位授予单位	中国科学院广州能源研究所
学位授予地点	广州能源研究所
学位名称	博士
学位专业	热能工程
关键词	Mems 微流控连续流混沌数字化流
摘要	Flow in micro scale is the fundamental to study micro-fluidic devices and micro actuators such as micro controller, micro reactor and micro fuel cells. It is important to probe into micro-flow to improve performance and design of micro-fluidic devices or micro actuators. Traditionally flow is categorized into two categories: single-phase flow and multiphase flow, or laminar flow and turbulent. According to different application of the micro flow, we divide micro-flow into three typical flows: continuous flow, chaotic flow and digital flow. We thoroughly studied the mechanism for the three flows by experimentation, numerical simulation and theoretical modeling. Electronic cooling is major application for continuous flow in micro scale. Base on theory of thermal boundary layer redevelopment, we study the possibility and mechanism for heat transfer enhancement and resistance reduction in micro scale. Two silicon micro heat sinks made of MEMS technology are used in experiments. They are of the same size of 30 x 7 x 0.525. The one is etched with straight microchannels, and the other is etched with periodically interruption structure in the microchannels. The hydraulic diameters of the microchannels are 155μm. Our numerical simulation agrees perfectly with the experiments. Both experiments and simulation show that heat transfer performance of interrupted microchannels exceeds by a large margin. Meanwhile, flow resistance in interrupted microchannels doesn’t increase. In some cases, the flow resistance even decreases. The flow and temperature fields in microchanels are obtained and analyzed. The numerical results show that the interruption structure of the micro channels makes the hydraulic and thermal boundary layer redevelop simultaneously. Thermal boundary layer develops slower than the hydraulic one, when Prandtl number of the working fluid is larger that unity. It is a prerequisite to benefit from interrupted microchannels as long as Prandtl number is larger than unity. Electronic cooling is major application for continous flow in micro scale. We study the possibility and mechanism for heat transfer enhancement and resistance reduction in micro scale. The exprimental aperfor a silicon heat sink with parallel microchannels, based on the concept of hydraulic and thermal boundary layer redevelopment. The experimental apparatus are two heat sinks made by MEMS fabrication technology. The two heat sinks are of the same size of 30 x 7 x 0.525. The one is etched with straight microchannels, and the other is etched with periodically interruption structure in the microchannels. The hydraulic diameters of the microchannels are 155μm. Our numerical simulation agrees perfectly with the experiments. Both experimental data and numerical results show that heat transfer performance in interrupted microchannels far exceeds that in straight microchannels while flow resistance doesn’t increase or even decreases. The numerical results also show that the interruption structure of the micro channels makes the hydraulic and thermal boundary layer redevelop simultaneously. Heat transfer can be enhanced while friction be reduced through optimizing the interruption structure of the microchannels, as long as Prandtl number is larger than unity, i.e., thermal boundary layer develops slower that hydraulic boundary layer. We design several pulsating heat pipes with different size to study chaotic behavior for flow and heat transfer in micro scale. We obtain time series of the pulsating heat pipes, by which we calculate chaotic characteristics like Kolmogorov entropy, correlation dimension of the strange attractors and Hurst exponent. We prove that the flow inside pulsating heat pipes is deterministic chaotic behavior. By comparing heat transfer performance (with different working parameters, such as heat power, inclination angle, charge ratio, and working fluids) with these chaotic characteristics, we find that heat transfer performance increases as the system becomes more chaotic. Moreover, we can predict different dynamics and flow patterns inside the pulsating heat pipes by the aid of phase portrait and correlation dimension. Finally we exemplify a micro-fluidic device based on a cascade of co-flows in co-axial capillary tubes to study the digital flow. By this device, we obtain highly mono-disperse and controllable compound drops, of which the size and generation frequency can be regulated by adjusting flow rate. We make thorough numerical investigation on such process for generation of compound drops. The effect of wall thickness, Reynolds number, Weber number, viscosity and density ratio, are investigated and discussed. We also analyze the stability issue for both axisymmetric and asymmetric devices. In addition, Based on low Reynolds number theory, we establish the theoretical model for a single compound drop translating inside a long circular tube. The exact solution in the form of stream function is obtained through combination and transformation between cylindrical and bipolar coordinate systems. The exact solution may be employed to further study the phenomena of the digital flow of double emulsions. The study may provide useful information for design and operation of micro electronic cooling and micro-fluidic devices.
其他摘要	微尺度流动是研究微控制器、微反应器、微换热器、微燃料电池等微流控与微执行器件的基础。深入了解微尺度下的流动对微流控与微执行器的设计与运行有重要指导意义。通常情况下，流动可划分为单相和多相流，层流和湍流。基于微尺度流动的应用，本文把微尺度下的流动划分为连续流，混沌流动和数字化流。通过实验、数值模拟和理论建模，对这三种典型流动的现象和机理作了深入细致的研究。微尺度下连续流的主要应用是电子冷却。本文研究了微尺度下，利用流动和热边界的再发展概念强化换热和降低流动阻力的可能性和相关机理。实验件为标准MEMS微加工工艺制作的两个尺寸为30 x 7 x 0.525的硅基热沉。在热沉上分别刻蚀着普通微通道和具有周期性中断结构的微通道（水力直径155μm）。本文的数值模拟和实验数据吻合。数值模拟和实验结果表明：采用边界层中断概念制作的微通道传热性能大大提高，相应的流动阻力却没有增加，甚至在某些情况下降低。通过数值模拟，得到了流体内部流场和温度场。研究发现在中断微通道内，流动和温度边界层出现周期性发展。当工质的Prandtl数大于1时，热边界层发展得比流动边界层慢。因此，Pr>1是中断微通道有实用价值的必要条件。为研究微尺度下流动和传热中的混沌现象，本文设计了几个不同尺寸的脉冲热管，获得了它们在不同工况下的温度时间序列。根据温度时间序列，计算了混沌特征量如Kolmogorov熵，奇怪吸引子的关联维和Hurst指数，从而证实了脉冲热管里的两相流动是确定性混沌运动。通过比较不同工况参数（加热功率、倾角、充液比和工质）下的换热效率和混沌特征量，给出了换热效果与混沌状态的关系——混沌程度越高，换热效果越好。此外，由混沌吸引子相图和关联维可以判断脉冲热管所对应的不同动力系统和流型。最后，本文研究了利用同轴毛细管形成串联co-flow的微流控装置。这种装置可以生成高度可控的数字化流复合液滴。只需调整流量，就能改变复合液滴大小、生成频率。对这种依靠多重co-flows生成复合液滴的过程，本文进行了深入细致的数值模拟。几何参数和各种无量纲参数如Re数、We数，以及不同流体的粘性比、密度比等对生成液滴的影响，都在文中作了详细讨论。本文还进一步研究了轴对称和非对称装置中生成的复合液滴的稳定性问题。此外，我们以微尺度下的低雷诺数理论为基础，建立了单一复合液滴在长毛细管里运动的理论模型。通过联合转换圆柱坐标和双球坐标，获得了这个理论模型以流函数形式表示的精确级数解，成为进一步研究数字化流复合液滴的运动的基础。本文的研究可以为微芯片电子冷却和微流控系统的设计和运行提供科学指导。
页数	147
语种	中文
文献类型	学位论文
条目标识符	http://ir.giec.ac.cn/handle/344007/5824
专题	中国科学院广州能源研究所
推荐引用方式 GB/T 7714	宋延熙. 微纳系统中三类典型流动与传热机理及分析[D]. 广州能源研究所. 中国科学院广州能源研究所,2009.

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