【作者】 刘斌；

【导师】 淮秀兰； 蔡军；

【作者基本信息】 中国科学院研究生院（工程热物理研究所） ， 工程热物理， 2014， 博士

【摘要】 随着微电子技术与激光技术的迅猛发展,电子芯片与激光二极管列阵等器件的集成度越来越高,其功率热耗不断攀升,在有限空间内及时消除因功率耗散转化的废热已成为制约大功率集成电路技术与激光技术发展的主要瓶颈。面对集成微电子器件日益高涨的热流密度,传统微通道冷却技术面临巨大挑战,加强相关强化换热技术研究,对于提高微通道热沉换热能力,推动相关冷却技术发展具有重要科学意义与实际价值。基于上述背景,本文选取微通道热沉为研究对象,以水力空化强化换热为切入点,采用数值模拟、实验研究与可视化观测相结合的手段对微通道热沉耦合水力空化强化换热机理展开研究,以期查明不同尺度与结构下空化泡形成、生长及溃灭的动力学行为特性及其对传热的作用机制,掌握微通道内流体流型随空化参数的变化规律以及流型转换对传热强化的影响规律,全方位多角度揭示液体空化强化微通道传热微观机理,为其实际应用奠定坚实的科学理论基础和提供重要技术支撑。论文首先从空泡动力学角度,基于VOF模型和Navier-Stokes方程,对半无限大空间静止流体中近壁面空化泡的生长、溃灭过程及其对壁面传热的影响进行了研究,成功捕捉到了微秒量级内空化泡溃灭瞬间泡壁界面的动态变化过程,获得了由于泡壁界面运动诱发的液体速度场和温度场分布。通过深入分析空化泡壁轮廓、液体流场与热边界层三者之间的内在变化关系,揭示出空化泡溃灭产生的高速微射流是其强化传热的主要微观机制。在此基础上,分析了空化泡与壁面之间的初始无量纲距离、空化泡初始半径以及初始饱和压力等因素对空化泡在单壁面附近动力学特性和传热的影响。基于单壁面研究结果,论文进一步对平行双壁面间的空化泡动力学特性及其对传热的影响进行了研究。研究结果表明：由于双壁面的限制作用,空化泡的溃灭过程呈现出了明显不同于单壁面的特征,空化泡在双壁面间的运动将经历两次溃灭过程。第一次溃灭期间,空化泡壁的收缩主要发生在双壁面的对称中心处,形成“哑铃”形状,并最终分割成两个子气泡。第二次溃灭期间,两个子气泡在分别向上、下壁面移动的过程中逐渐收缩,最终被垂直于壁面的液体射流穿透。由于空化泡在平行双壁面间的动力学行为特性与单壁面不同,因此对壁面传热的影响也不尽相同。为此,论文就空化泡初始半径、泡内初始压力以及气泡初始位置等因素对空化泡运动和传热的影响进行了分析和讨论。在二维模拟研究的基础上,论文建立了三维空化泡动力学模型,并对空化泡在三维微通道内的动力学特性及其对传热过程的影响进行了研究。研究结果显示,空化泡在三维微通道内的生长、溃灭过程与单壁面和平行双壁面存在明显的差异。由于通道四周壁面的限制,空化泡的生长主要朝着通道出口方向,形成椭球状。在溃灭阶段,空化泡壁首先沿着生长的反方向收缩,待空化泡形成扁平形状后,开始自壁面向通道中心收缩,形成“橄榄”状。由于空化泡自身的热阻较大,与空化泡壁贴近的通道壁面区域内的传热条件将恶化,而在液体射流引起局部漩涡流动的区域,传热得到强化。在此基础上,进一步分析了空化泡初始半径与初始位置等因素对空化泡在微通道内的运动和传热过程的影响。为了从实验角度对液体空化强化微通道换热进行研究,论文采用AutoCAD与三维Inventor实体建模相结合的方法,设计了内置矩形和渐缩-渐扩形限流结构的两种铜基微通道热沉,并搭建了可视化实验测试平台,分别对两种微通道热沉在各种空化数、雷诺数以及热流密度工况下的空化流动特性和传热特性进行了研究,掌握了各种因素对空化流型和传热的影响规律,阐明了两种不同空化结构的流动阻力与传热特性差异,为微通道内空化强化换热结构的优化设计奠定了理论基础。在实验研究的基础上,论文进一步探索了常规空化模型在微通道空化流动与传热模拟研究中的应用。数值模拟得到的空化流型与实验观察到的结果较为吻合,表明常规空化模型对于微通道内空化流动的模拟具有一定的适应性。但是,受制于空化模型的不足,传热模拟结果不理想,论文对其原因进行了具体分析并提出了改进的方向。更多还原

【Abstract】 With the rapid development of integrated microelectronic technology and laser technology, the integration level of laser diode arrays and electronic chips highly increases, and the power also continuously rises. Timely elimination of the waste heat in the limited space has become a major bottleneck in the development of high-power integrated circuit technology and laser technology. With the rising heat flux in integrated microelectronic devices, the traditional micro-channel cooling technology faces enormous challenges. The research of related heat transfer enhancement methods is important to improve the cooling technology, and has important scientific significance and practical value.Based on the above background, hydrodynamic cavitation enhanced heat transfer in the micro-channel heat sink has been investigated. In the present work, by combining the numerical method and visualized experiments, studies are conducted to reveal the bubble dynamics and its effects on the heat transfer. Also, the effects of flow pattern and several cavitation parameters on the heat transfer enhancement are studied. The results lay a solid foundation of scientific theory and provide important technical support for its practical application.In this paper, the cavitation bubble dynamics near a heated wall is firstly studied based on the VOF model and the Navier-Stokes equations. The change of bubble profiles happened within microseconds and has been successfully captured. The corresponding velocity fields and temperature fields were also obtained. Through an in-depth analysis about the intrinsic relationship among the bubble profiles, liquid velocity fields and temperature layer, the impinging of micro-jet in the bubble collapse is found to be the main reason for the heat transfer enhancement. On this basis, effects of non-dimensional distance between the bubble center and the wall, initial bubble radius and bubble pressure on bubble behavior and heat transfer are analyzed.Further, the bubble dynamics between parallel heated walls and its effects on heat transfer are studied. The results show that due to the limitation of the two walls the bubble behaviors show a totally different characteristic. The bubble between the two parallel walls will experience two collapse processes. In the first collapse, the bubble shrinks mainly occurred in the center direction of the double walls. Therefore, a "dumbbell" shaped bubble will be formed, and finally the bubble is split into two sub-bubbles. In the second collapse, the sub-bubbles continuously shrink and are finally penetrated by micro-jets. Because of the differences of dynamic behavior, effects of the bubble behaviors on the heat transfer are totally different. Thus, parameters of the bubble initial radius, initial pressure and initial position on the cavitation bubble motion and heat transfer are analyzed and discussed in details.On the basis of the two-dimensional simulation, a three-dimensional dynamic model of the cavitation bubble is established for the study of bubble dynamics in the micro-channel and its impact on the heat transfer. The results show that the cavitation bubble growth and collapse behaviors is different from its process near the single wall and between the two parallel walls. Due to the limitation of channel walls, the cavitation bubble grows primarily towards the channel exit direction and forms an ellipsoid. In the collapse process, cavitation bubble firstly shrinks in the opposite direction as the growth process. When the bubble contracts to a flat shape, it shrinks from the bottom and top channel wall surfaces to the center and forms an "olive" shape. Due to the thermal resistance induced by the bubble itself, in the bubble faced region heat transfer conditions are deteriorated. However, in the region of vortex liquid jet flow, heat transfer is strengthened. On this basis, the bubble initial radius, the initial bubble position and other factors on cavitation bubble behaviors and its effects on the heat transfer are further investigated.The AutoCAD and Inventor are employed for the design of visualized experiment platform. The cooper-based micro-channels contain two kinds of flow restricted structures, a rectangular shaped and a two diverging shaped respectively. The flow characteristics and heat transfer performance under the two restricted structures are studied. The effects of Reynolds number, cavitation number and heat flux on the flow patterns and heat transfer are investigated, which lay the theoretical foundation for optimization designs. On the basis of experimental studies, the paper further explores the application of cavitation model in the simulation of micro-channel cavitating flow with surface heat transfer. The flow pattern obtained by the calculation is in accordance with the experimental results. It indicates that the present model is generally applicable for the cavitation flow in micro-channels. However, subject to the imperfection of the model, heat transfer simulation results are not satisfactory. The improvement advices has been proposed based on the further analysis.更多还原