Convection Heat Transfer With Magnetic Field And Its Field Coordination Analysis

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 theconcerned 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.