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微流控系统中混合及汽液相变传热的数值模拟
其他题名Numerical Simulation of Vapor-liquid Phase Change Heat Transfer and Micromixing in Microfluidic Systems
毛文彬
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
学位名称硕士
学位专业工程热物理
关键词数值模拟 微混合 流动沸腾 汽液相变 微通道 Numerical Simulation Micromixing Flow Boiling Vapor-liquid Phase Change Microchannels
摘要In this thesis, under the background of microfluidic systems, numerical studies have been performed to investigate the micromixing process and the flow boiling heat transfer process in microchannels. Mixing in micron scale is a challenging issue due to the presence of laminar flow. In this paper, a new micromixer was proposed based on the characteristics of liquids mixing in micron scale. Effective mixing could be achieved using pulsating velocities in the form of square-wave or sine-wave at the entrances of micromixers. The dimensionless equations and boundary conditions were established, thus the micromixing physics and the effect of parameters could be performed using computational fluid dynamics (CFD) simulation. For the square-wave pulsating micromixer, numerical simulation demonstrated that good mixing performance could be achieved when the duty cycle of pulsing equals 0.5 and phase delay equals 180 degrees. It is shown that such micromixer possesses chaotic advection inside and pulsating flow could significantly increase the interface area of two fluids, leading to the rapid mixing. For the sine-wave pulsating micromixer, the mixing effect is superior to the square-wave pulsating micromixer. The mixing effect is dependent on the characteristics of meniscus-shape mixing interfaces in the microchannel. The Strouhal number (pulsating frequency) controls the size of mixing interfaces or the number of segments. Suitable Strouhal number yields four to eight mixing interfaces in the microchannel. Pulsating amplitude controls the symmetry of mixing interfaces. When the optimal pulsating amplitude causes symmetrical mixing interfaces in x-direction, good mixing can be achieved. The optimal pulsating amplitude increases with increasing in Strouhal numbers and decreasing in Reynolds numbers. The Reynolds number (Peclet number) affects the residence time in the mixing channel and the diffusion rate of mixing. Since the pulsating micromixing is advection dominant, the mixing efficiency has a slight decrease as the Reynolds number increases. As a result, the pulsating micromixer has a wide application scope of Reynolds number. Considering the investigation of flow boiling in microchannel, the Volume of Fluid method is used to simulate the evolution of two-phase flow patterns, from bubbly flow to slug flow. Comparing with the experiment results, a good agreement of flow patterns is observed. Based on the analysis of temperature and flow fields around the bubbles, it is found that the growth of vapor bubbles could influence the distribution of temperature and flow fields, thereby affecting the heat transfer coefficient. The heat transfer coefficient and the total heat transfer rate increase during the evolution of flow patterns. The increase in heat flow is comprised of two parts: the latent heat flow caused by vapor-liquid phase change and the enhanced convective heat transfer effect caused by bubble growth and movement. In the bubbly flow regime, the ratio of latent heat flow is small, while in the slug flow regime, the ratio of latent heat flow is increasing and the heat transfer of thin film evaporation cannot be neglected.
其他摘要本文在微流控系统的背景下,对微尺度下混合以及微通道内流动沸腾传热过程进行了深入细致的研究。 在微尺度下,由于流动处于层流状态,因此混合是一个巨大的难题。本文针对微尺度混合的特点,研究了一种脉动流微混合器。这种微混合器利用入口速度方波型或正弦波型脉动实现微通道内流体的有效混合。针对物理问题建立了通用的无量纲方程和边界条件,利用计算流体动力学方法研究了微混合器的特性和脉动参数的影响。结果表明对于方波型脉动混合器,在脉动方波占空比为0.5,相位差为180度时,能达到较好的混合效果。微混合器中流动具有混沌对流特征,可极大地增加界面面积,从而实现快速混合。 对于正弦波脉动微混合器,其混合效果比方波型好。通道内存在月牙形的脉动形状,它决定了混合的效果。St数(频率)控制着通道内脉动形状的大小,即流体分段的数目。St数并不是越大越好,而是要取合适值以使得通道内月牙形区域为4~8个。脉动幅值a控制着通道内脉动形状的对称性,当a取最佳值使得脉动形状关于x轴对称时,混合效果最好。最佳脉动幅值一般随St的增加而增加,随Re数的增加而减小。Re数(Pe数)影响流体在通道内滞留的时间,从而影响扩散混合的速率。由于脉动流微混合以对流作用占主导,故随着Re的增加,混合效果只有轻微的下降,说明脉动流微混合器具有较广的Re数适用范围。 在微通道流动沸腾的研究中,采用VOF方法对沸腾流型由泡状流向塞状流过渡的全过程进行了数值模拟,并与实验结果进行了对照,结果表明汽泡流型变化的过程与实验较为接近。通过对汽泡周围温度场和流场的分析,表明汽泡的成长会影响温度场和流场的分布,从而影响通道的传热系数。沸腾流型由泡状流向塞状流转变的过程中,通道的传热系数不断增大,总热流也不断增大。热流的增加主要由汽液界面的相变潜热热流和汽泡引起的周围流体对流传热的强化效应两部分组成。在泡状流阶段,相变潜热的热流比例很小;在弹状流阶段,相变潜热热流比例逐渐增大,壁面处薄液膜的蒸发传热效应不能忽略。
页数80
语种中文
文献类型学位论文
条目标识符http://ir.giec.ac.cn/handle/344007/5840
专题中国科学院广州能源研究所
推荐引用方式
GB/T 7714
毛文彬. 微流控系统中混合及汽液相变传热的数值模拟[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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