自然对流换热的强化与控制随着微电子和微加工技术的快速发展受到越来越多的关注和重视，但现有技术主要集中于自然对流换热表面的结构设计和优化。然而，对自然对流产生条件的分析表明，非保守体积力的驱动是自然对流形成的原因，通过主动控制流体所受体积力形态，也可以实现自然对流换热的强化和控制。本文将易于调节和控制的磁场体积力引入到对流换热的强化与控制上，以场协同理论为指导，通过理论和数值分析研究了磁场作用下对流换热的基本规律，提出了通过梯度磁场实现对流换热强化和控制的基本方法和原则。 分别针对顺磁性和逆磁性流体，推导了热磁对流的磁浮升力。通过对磁浮升力与重力浮升力的类比，提出磁加速度的概念。与重力加速度相比，磁加速度是一个依赖于空间位置的变量，其大小和方向随外磁场的磁路布置发生改变。由磁加速度的定义出发，提出了理想磁场模型和实现微重力环境所要求的磁场强度分布。分析了理想磁场作用下自然对流换热强化与控制的基本原理，指出通过合理的空间磁场布局可使自然对流换热过程得到有效强化或控制。对理想磁场作用下封闭腔内的自然对流换热进行了分析，指出在一定磁加速度大小和方向下，可实现微重力环境，磁致纯导热过程，磁致Benard对流以及其它自然对流流动形态的改变。 分析了典型永磁体和超导磁体工作空间内的磁场强度和磁加速度的分布规律，可为梯度磁场的选型和设计提供理论依据。针对永磁梯度磁场作用下封闭腔内的自然对流换热，提出通过不同磁场布置产生不同磁加速度的方法。将四极磁场引入对流换热强化和控制，得到离心力形态的Kelvin磁体积力，自然对流流态呈现出有别于重力场中的结构。鉴于需通风场合一般存在与环境空气之间的氧气浓度差异，提出了一种超导磁体氧气浓度差磁致通风方式。以场协同原理为指导，分析了通道内对流换热强化所应遵循的基本原则，据此提出了四极磁场作用下有源纵向涡强化换热技术。分别对层流流动/热入口段及充分发展段得到了四极磁场作用下对流换热的速度场和温度场，以及反映动力学特性和换热特性的阻力系数和Nu的变化规律。场协同分析表明，纵向涡的形成可改善速度场和温度场的协同状况，使对流换热过程得到强化，而流动/热充分发展段对流换热的纵向涡强化效果要优于入口段。基于理论分析的结果，进行了磁场作用下自然对流换热的强化实验，实验结果与数值模拟结果基本吻合。

With the rapid development of the micro-electronics and micro-processing technology, the enhancement and control of the natural convection heat transfer are receiving increased attentions. However, the currently existing techniques focus mainly on the design and optimization of the heat transfer surface for natural convection. Analyses on condition for generating the natural convection show that the natural convection results from the non-conservative body force, so, its enhancement and control can also be achieved by active control of the body force acting on the fluid. In the present research, the easy-to-adjust-and-control magnetic body force is introduced to the convection heat transfer and the natural convection heat transfer under magnetic field is theoretically and numerically studied under the guidance of Field Coordination Principle. The methods and rules for realizing the convection heat transfer enhancement and control by application of magnetic gradient field are presented. The magnetic buoyancy forces for paramagnetic and diamagnetic fluids are deduced. On the analogy of the magnetic and gravitational buoyancy forces, the concept of the magnetic acceleration is proposed. Unlike the gravitational acceleration, the magnetic acceleration is a location-dependent variable and its magnitude and direction vary with the magnetic circuit configuration. From the definition for the magnetic acceleration, the ideal magnetic field model and the magnetic field intensity profile needed for creating the micro-gravity environment are presented. Basic principle for the enhancement and control of natural convection heat transfer under ideal magnetic field is analyzed. It is found that the effective convection heat transfer enhancement and control can be accomplished by appropriate arrangement for magnetic field. The natural convection in enclosure under ideal magnetic field is analyzed, the results support that the micro-gravity environment, magnetically induced conduction and magnetically induced Benard convection can be achieved by using a certain magnetic acceleration. The profiles of the magnetic field intensity and magnetic acceleration in the concerned rooms of typical permanent magnet and superconducting magnet systems are presented, which may be helpful for the selection and design of the magnetic gradient field. For the natural convection in enclosure under permanent magnetic gradient field, the way to generate different magnetic accelerations by different magnetic field arrangements is developed. By introducing the magnetic quadrupole field to the convection heat transfer, the centrifugal-form magnetic Kelvin body force is achieved. In view of the existence of the oxygen concentration difference between the ventilation room and ambient air, technique for magnetically induced air ventilation by superconducting magnet is developed. Under the guidance of Field Coordination Principle, the basic rule that the convection heat transfer enhancement in channel should obey is analyzed and the active longitudinal vortices enhancement for convection heat transfer by application of the magnetic quadrupole field is developed. For the laminar hydraulically/thermally developing and fully developed flows, the velocity and temperature fields as well as the friction coefficients and Nusselt numbers for the convection heat transfer under magnetic quadrupole field are presented. Field coordination analysis shows that the formation of the longitudinal vortices can improve the coordination between velocity and temperature fields, consequently is able to enhance the convection heat transfer. The longitudinal vortices enhancement is more effective for the fully developed flow than for the developing flow. Experiments are conducted on natural convection under magnetic gradient field and a reasonable agreement is observed between the experiment and calculation.