Study On Natural Convection And Boiling Heat Transfer Of Magnetic Fluid Under Magnetic Gravity Compensation

Convection heat transfer processes can be found in metallurgy, oil, chemical, power, electronics, aviation and aerospace, transportation, light industry and other industries in national economy. Studying the mechanism of convection heat transfer exhaustively can not only promote the development of heat transfer theory, but also guide the design of various heat exchange and transmission equipment and control convection heat transfer processes, so as to realize the rational and efficient use of energy resources. Natural convection heat transfer process and boiling heat transfer process are two typical convection heat transfer processes. Under earth normal gravity environment, the temperature difference of fluid leads to density difference and the fluid motion is caused by the buoyancy force, which means buoyancy force is the driving force of natural convection. Buoyancy force is also the driving force that makes bubbles depart from the heated wall in boiling heat transfer process. With the development of aviation and aerospace industry, the performance requirements of large spacecrafts in thermal management are growing and researches on the mechanism of convection heat transfer under microgravity are increasingly urgent. The mechanism of convection heat transfer under microgravity is different from the mechanism under earth normal gravity as the buoyancy force extremely reduces. But due to various reasons, related researches are not deep enough. For this reason we realized simulated microgravity environment based on the “Magnetic Gravity Compensation” method and magnetic fluid. Whereafter we studied natural convection and boiling heat transfer of magnetic fluid under magnetic gravity compensation.In the first part, we design a brand new method to carry out magnetic gravity compensation, which is realize by combining the magnetic fluid and two pair Helmholtz-Maxwell coils. After that, we analyze the distribution of magnetic field and the gravity compensation level of magnetic fluid respectively. At first, a finite element model and precise teslameter is used to measure the magnitude of magnetic field separately. The results show that the field distribution is linear along the vertical direction, and uniform in the center area. Also, the experiment are consistent with the numerical simulation accurately. Subsequently, by calculating the gravity compensation level of magnetic fluid, we find a uniform gravity compensation region, which consists a cylindrical area with diameter of 60 mm and height of 60 mm, which exists in the center of the external coils. And the gravity can be totally compensated while the current in Maxwell coil reaches a specific value.In the second part, conditions for generating natural convection and the effect of gradient magnetic field on natural convection have been theoretically analysed firstly. Experiments on natural convection heat transfer of magnetic fluid at different concentration under earth normal gravity and at different magnetic gravity compensation levels have been performed after designing and building the natural convection heat transfer experiment system. The results show that with the increase of the volume fraction of magnetic fluid, the Nusselt number decreases. It indicates that nano magnetic particles added in magnetic fluid suppress natural convection heat transfer. The viscosity of magnetic fluid increases under a uniform magnetic field, so the natural convection heat transfer efficiency under a uniform magnetic field is lower than that under earth normal gravity consequently; The magnetic force acting on the magnetic fluid applied by the gradient magnetic field causes the magnetic acceleration. The magnetic force will suppress natural convection of magnetic fluid when the direction of the magnetic acceleration is contrary to the direction of the gravitational acceleration. The magnetic acceleration increases with the increase of magnetic force, so does the suppression of natural convection.In the third part, experiments on bubble dynamics and boiling heat transfer of magnetic fluid under earth normal gravity and at different magnetic gravity compensation levels have been carried out after designing and building the bubble dynamics and boiling heat transfer experiment systems. The results show that bubble emerging, growth and departure exist in magnetic fluid no matter under earth normal gravity or at different magnetic gravity compensation levels. The bubble dynamics in magnetic fluid when applied a uniform magnetic field only are similar to those under earth normal gravity. The bubble departure diameter increases with the increase of magnetic gravity compensation level. But the departure frequency decreases with the increase of magnetic gravity compensation level. The bubbles become flat when rising in the magnetic fluid under earth normal gravity but become long and narrow along the direction of the magnetic field at different magnetic gravity compensation levels. The bubble rising velocity decreases with the increase of magnetic gravity compensation level.The boiling heat transfer efficiency under magnetic gravity compensation is lower than that under earth normal gravity. The viscosity of magnetic fluid increases under a uniform magnetic field and this leads to the decrease of boiling heat transfer efficiency on the heated wall. The magnetic force acting on magnetic fluid increases with the increase of magnetic gravity compensation level, so does the magnetic acceleration. As the direction of the magnetic acceleration is contrary to the direction of the gravitational acceleration, the buoyancy force acting on the bubble decreases and this causes the increase of bubble departure diameter. The boiling heat transfer efficiency decreases because the bubbles are hard to depart from the heated wall. The convection heat transfer between the heated wall and magnetic fluid decreases with the decrease of effective gravity acceleration. As a consequence, these two reasons make the boiling heat transfer efficiency decrease with the increase of magnetic gravity compensation level.In the present research, the easy-to-adjust-and-control magnetic body force is introduced to convection heat transfer and natural and boiling heat transfer laws are theoretically analyzed and experimentally investigated. A basic method to control convection heat transfer through gradient magnetic field has been proposed and can be the reference for convection heat transfer researches under microgravity.