【作者】 刘江涛；

【导师】 彭晓峰；

【作者基本信息】 清华大学 ， 动力工程与工程热物理， 2008， 博士

【摘要】 微通道在现代科技微细化进程中扮演重要角色,对于认识微观结构的热质传递机理有着创新性的学术意义和应用价值。本文以微通道内液体单相对流和气液两相流动沸腾为研究对象,进行实验、数值模拟和理论研究。采用MEMS工艺加工不同截面尺寸的微通道实验段,结合流动、加热、可视化观察、测量和数据采集等装置构建微通道单相/相变一体化实验系统。对比液体单相对流实验建立数学物理模型在三维笛卡尔坐标系内进行微通道流固耦合数值模拟。分析了流动发展、耦合传热以及物性变化等重要效应对微通道单相对流的影响方式和作用大小。高热流密度下的物性变化引起流场与温度场耦合发展导致流动传热规律发生改变,对传热强化具有重要意义。单相变物性问题的针对性研究描述流动和热再发展的整体趋势与局部特征,总结变物性作用引起局部传热性能强化的现象,具体讨论物性变化引起传热强化作用的机理。分别分析入口速度、温度和热流密度对传热强化机理的影响,指出高热流密度条件下变物性机理具有非线性特征。对微通道内沸腾相变过程整体稳态与瞬态特征进行分阶段讨论。通道内部相变不均匀性显示出过渡性特点。受到流动分配、局部压力变化和热流耦合分布等因素影响,在过渡、旺盛沸腾和蒸气流阶段普遍出现振荡,局部竞争,以及气泡动力学多样性等现象。局部加热条件下长气泡上下游端分别出现蒸发和冷凝。气泡上下游气液界面相变行为具有多样性,蒸发或冷凝模态对振荡过程起主导作用。信号分析显示,在膜态蒸发/冷凝作用下气液界面的振荡频率,均显著高于相同条件下在核化/珠状冷凝作用下气液界面振荡的频率。同一气泡上下游界面振荡具有相同特征频率(频段)。更多还原

【Abstract】 Microchannel plays a critically important role in the development of modern science and technology, particularly micro/nano technology. Heat and mass transfer in microchannels has attracted worldwide researchers as one of frontiers in the energy and thermal fluid science discipline, showing great significance in both understanding fundamentals of transport phenomena at microscale and promoting development of new technologies such as micro energy systems, compact heat transfer devices/equipments, and cooling technology, etc. The present work carried out a series of experiments, simulations and theoretical investigations on liquid single-phase convection and flow boiling two phase flow in microchannels.Microchannels with different sizes were fabricated by employing MEMS technology. An integrated single-phase/phase-change experimental system was established with the microchannel test sections, flow and heating devices, visualization system, measuring and data acquisition instruments. Based on the liquid single-phase convection experiments, mathematical and physical models were constructed for the three-dimensional fluid-solid conjugated convection simulation in a microchannel. The effects of flow development, conjugated heat transfer and property variation on microchannel flow and convective heat transfer were distinguished and evaluated. Property variation at high heat flux caused coupled hydrodynamic and thermal development, altered the flow and heat transfer performance, and showed its importance on heat transfer enhancement. The overall trend and local characteristics of flow and thermal re-development were described in the single-phase property variation study. The heat transfer enhancement was discussed on the specific mechanisms of variable properties. The impacts of inlet velocity, temperature and heat flux on heat transfer enhancement were distinguished. The property variation effect induced marked non-linear feature at high heat flux.Different stages in a microchannel flow boiling process were identified and discussed. The non-uniform phase change in microchannels showed strongly transitional features. Due to the influence of flow distribution, local pressure fluctuation and conjugated heat transfer, different oscillation phenomena appeared in the stages of transition, vigorous boiling and vapor single-phase flow. Local interface competition and diversified bubble dynamics phenomena were also observed. At local heating condition, evaporation and condensation occurred at the upstream and downstream caps of an elongated bubble. Diversified phase-change modes showed dominating effect on the interface oscillation. Signal analysis explored that the film-driven oscillations of both evaporating and condensing interfaces generally operated at higher frequencies than their counterparts driven by nucleation or dropwise condensation. The upper and lower caps of one elongated bubble oscillated at the same characteristic frequency (band).更多还原