【作者】 连文磊；

【导师】 宣益民；

【作者基本信息】 南京理工大学 ， 工程热物理， 2010， 博士

【摘要】 温度敏感型磁流体的饱和磁化强度会随温度的升高而显著减小,因此其热磁对流现象十分明显：处于外磁场中和非等温状态下的温度敏感型磁流体内部会产生磁力的不平衡状态,驱动流体做宏观对流运动。通过调节外磁场和温度场的协同作用,可以实现对这种流体运动的控制。如果将热磁对流控制在密闭回路中,同时保持稳定的外磁场和温度场,温度敏感型磁流体会在回路中持续稳定地循环流动,实现能量的传递。由于无须外加机械驱动部件,热磁对流回路是一种能量自主传递系统,在热能工程领域有一定的应用前景,对温度敏感型磁流体热磁对流回路特性的研究具有重要的意义。但热磁对流回路的相关研究报道还是比较少,仍有一些问题亟待解决。本学位论文研究主要包括以下几个方面的研究内容：(1)热磁对流回路内的流场可视化研究和流速测量为深入研究热磁对流回路中流体的流动与传热特征,需要对回路中磁流体流场进行测量,这个问题至今没有很好解决。本文利用粒子图像测速技术(PIV),针对磁流体可见光透过率小的特点,采用非经典的光路设置,实现了回路中磁流体的流场可视化,研究了热磁对流回路的流场特性,对充分发展段的流体进行了不同工况下的速度测量,定量分析了热磁对流的强度及其随主要影响因素的变化关系。对流速的测量有利于深入研究热磁对流回路的运行特性。(2)温度敏感型磁流体热磁对流回路运行特性的实验研究搭建了温度敏感型磁流体为工质的小型热磁对流回路实验系统,利用NdFeB永磁体提供外磁场,同时对回路的不同部分施加热源和冷源,使流体内部产生温度差异,发现回路中形成循环不断的热磁对流。分析了热源位置,永磁体位置,加热功率,冷却段温度等因素对回路流动和换热特性的影响,探索回路流动与传热的基本特征,研究了外加磁场和流体温度场的优化协同方式。(3)温敏型能量自主传递装置的设计方法在流体动力学、传热学和电磁学的一些重要理论基础上,结合磁流体的基本特性建立了包含磁、热和流动之间耦合特性的描述热磁对流的宏观数学模型。根据此数学模型,对温度敏感型磁流体热磁对流回路的运行状况进行数值模拟,建立了温敏能量自主传递系统的设计方法。将计算结果和实验结果进行对比,验证了理论模型和方法的合理性。(4)多热磁泵串联或并联回路的运行特性研究在合适的外磁场和温度场的作用下,回路中的磁流体持续循环流动,驱动力来自于外磁场和温度场的协同作用,它相当于一个“泵”为流体提供动力。迄今为止,热磁对流回路的相关研究针对的是一个热磁泵的系统。在前述的理论模型基础上,建立了多个热磁泵串联的回路模型和多泵并联回路模型,数值模拟了这些多泵系统的工作特点,并与单泵的回路系统进行了对比,以研究多泵系统中流体的流动与换热特性以及热磁动力的相互影响。(5)探索热磁对流在热能工程领域的应用前景。利用热磁对流原理建立了完整的小型热磁对流回路散热装置,通过对磁场的屏蔽减小了对外界的不良影响。研究了使用不同结构的散热回路针对模拟发热体的散热特点,结合散热能力和回路结构分析它们的应用价值和前景。更多还原

【Abstract】 A temperature-sensitive magnetic fluid means that its magnetization remarkably decreases with the increase in temperature, so that strong thermomagnetic convection can be seen to take place in the fluid:When this type of magnetic fluid experiences a temperature variation in the presence of an external magnetic field, a magnetic force will arise to drive the magnetic fluid flow. This field-induced flow may be controlled by the synergy function between the external magnetic field and the temperature difference in the fluid. If the thermomagnetic convection takes place in a loop shape channel, a continuous circulation flow can be maintained inside the loop, transferring heat from heat source to the heat sink. Since no mechanical moving part is needed in the whole device, it has application potentials in the field of thermal engineering and is worthy of deep research. However, the research efforts to the thermomagnetic convection loop are relatively sparse, and there were still some problems to be resolved. This paper is focused on the following aspects:(1) Visualization of the fluid inside the thermomagnetic convection loop and measurements of the flow velocityThe flow measurement of the magnetic fluid inside a thermomagnetic convection loop is still a challenge, which is important for deep research of the flow and heat transfer feature of the loop. In this paper, a particle image velocimetry (PIV) system has been put up, and due to the small transparency of the fluid to light a non- conventional light path was introduced to realize the visualization of the magnetic fluid inside the loop. The flow patterns of the magnetic fluid in the loop were observed and the velocity of fully-developed flow under main factors was measured. The measurement of velocity is helpful to further research of the operation characteristics of the thermomagnetic convection loop.(2) Experimental investigation on operational characteristics of the thermomagnetic convection loop using temperature-sensitive magnetic fluidA small thermomagnetic convection loop using a temperature-sensitive magnetic fluid was made. A permanent NdFeB magnet was used in the device to produce magnetic field. Heat sources and a heat sink were arranged at different parts of the loop, resulting in temperature difference in the fluid. It can be concluded that a circulation flow is formed inside the loop due to the thermomagnetic effect. The influences of heat source position, position of magnet, power of heat source and temperature of heat sink on the flow and heat transfer feature of the loop were discussed, in order to get insight into the mechanism of the operation characteristics of the loop and find out fine synergy between the magnetic field and thermal field.(3) Design method of automatic energy transport device using temperature-sensitive magnetic fluidBased on the knowledge of fluid dynamics, heat transfer, electromagnetics and the feature of magnetic fluid, a model for describing thermomagnetic convection of a temperature-sensitive magnetic fluid is established, which includes the coupling of the three fundamental phenomena, i.e., magnetic, thermal, and fluid dynamic features. The flow and heat transfer features of the fluid in the thermomagnetic convection loop were numerically simulated by using the CFD software. The design method for the temperature-sensitive automatic energy transport device is proven to be credible by the approving coincidence between the numerical values and experimental data.(4) Investigation on the operation features of thermomagnetic convection loop containing series-wound and shunt-wound multi thermomagnetic pumpsWhen the thermomagnetic convection of a temperature-sensitive magnetic fluid takes place in a loop, a continuous circulation of the fluid may be maintained due to the magnetic force resulting from the synergy of external magnetic field and the temperature gradient in the fluid, which plays as a thermomagnetic "pump". Researches on such thermomagnetic convection cooling loop have been limited to the "single-pump" situation. In this paper, mathematical models of series-wound and shunt-wound "multi-pump" thermomagnetic convection loops were established based on the above-mentioned mathematical model. The operation feature of these "multi-pump" systems were numerically investigated, which were then compared with those in the conventional "single-pump" loop, in order to find out the flow and heat transport features of these "multi-pump" systems and the influence between these thermomagnetic "pumps"(5) To explore the application potentials of the thermomagnetic convection loop in the field of thermal engineeringSmall heat transport devices are created based on the thermomagnetic effect. The magnetic field was shielded to minimize the field intensity in the surrounding area. The cooling performances of the devices with different strctures were investigated, and their application potentials were discussed by considering the cooling capacity and the configuration.更多还原