Investigation Of Flow And Heat Transfer Of Magnetic Fluid By Multiscale Method

Magnetic fluid is a colloidal suspension by suspended ferromagnetic nanoparticles into carrier liquid (such as water, kerosene), and the surfactant is used to keep the suspension stable. Magnetic fluid is a functional fluid which not only possesses the flowability of common Newtonian fluids, but also the magnetic features being similar to those of the bulk magnetic materials. Since the magnetic fluid exhibits some unique characteristics under the influence of external magnetic fields such as magneto-viscous effect, magneto-thermal effect and magneto-optical effect, they have found many applications in mechanical engineering, bioengineering, and thermal engineering.To investigate the heat and mass characteristics of magnetic fluid, many numerical models have been presented. The traditional methods are the Brownian Dynamic which is on nanoparticle scale and solving NS equations on macroscale. However, the calculated space and time are limited on nanometer scale by the Brownian Dynamic by the means of the huge number of particles. The macroscale method of solving NS equations is not limited of huge number of simulated particles but it regard the magnetic fluid as a single component flow and the micro character of magnetic particles can't be presented. The lattice Boltzmann method as an effective method which based on microscopic model and mesoscopic kinetic equation has been used to simulated magnetic flow. This method not only gets rid of the limitation of huge number of simulated particles, but also can present the multicomponent characteristic of magnetic fluid. The lattice Boltzmann method has demonstrated its superior performance for calculated microscale problem. However, the lattice Boltzmann method would use huge calculated time for the macroscale problem, and compared with the microscale method it can't describe the microstructure of magnetic particles. Therefore, the establishment of a microscopic-mesoscopic or mesoscopic-macroscopic multiscale coupling method will be very meaningful. In the paper, we will describe various multiscale methods based on lattice Boltzmann method to investigate the characters of magnetic fluid in the present of external magnetic field. The subjects in the thesis are focused on the following aspects. (1) Analytical investigation of the annular system based on the thermo magnetic effect of a temperature-sensitive magnetic fluid system with the no uniform mesh of thermal lattice Boltzmann methodThe magnetization of magnetic fluid is decreased with the increase of temperature. Based on such thermo magnetic effect, a circulatory system can be designed in which no pump is used. To investigate the circulatory system, a new thermal lattice Boltzmann method with no uniform mesh grids has been proposed in the paper, by which the flow and energy transfer progress with curly boundary can be calculated. For validating the present method, the convection of air in an annular cavity is simulated. The result shows that it is accordantly with both the experimental and classical calculation results. With the present method, the annular system based on the thermomagnetic effect of a temperature-sensitive magnetic fluid was investigated. The influences of difference in temperature between heating section and cooling section and the intensity of magnetic field as well as the position of magnetic were studied.(2) Anisotropic diffusion of magnetic fluid based on Brownian Dynamic and Lattice Boltzmann hybrid hierarchical multiscale methodThe microstructure of magnetic particles changed with the external magnetic field and the volume fraction, which will influence the diffusion process of heat transfer. Therefore, firstly we use the Brownian Dynamic (BD) to get the microstructure of magnetic particles for different conditions. Then, the thermal diffusion coefficients can be calculated accordingly to the microstructure. According the thus thermal diffusion coefficients, the parameters in the lattice Boltzmann method can be obtained, such as the relaxation time, the time and spatial steps. It should be noticed that the parameters used in the lattice-Boltzmann method is according to the microstructure which is changed with external magnetic field. For the chainlike structure of magnetic particles, the thermal diffusion process presents an anisotropic characteristic. Therefore, an anisotropic thermal diffusion lattice Boltzmann method is presented. To validate the model an anisotropic diffusion process was simulated. Then the thermal diffusion process of magnetic fluid under the influence of magnetic field is investigated. The thermal diffusion coefficients of the magnetic fluid in the present of magnetic field is calculated by the BD, and the macro thermal diffusion process is investigated by the anisotropic lattice Boltzmann method.(3) Simulation of Flow and Heat Transfer of magnetic fluid based on single component lattice Boltzmann and multicomponent lattice Boltzmann hybrid concurrent multiscale methodMagnetic fluid is special suspended fluid that each will have different understanding on different scales. Considering such scale characteristics, we present a single component lattice Boltzmann and multicomponent lattice Boltzmann hybrid method. In the hybrid method, the fine mesh grids are used with multicomponent lattice Boltzmann method on the regions where the physical parameters change dramatically, the coarse grids are used on the other region with single component lattice Boltzmann method. To ensure the continuity of the physical information (physical parameters) in the hybrid region, the principles of mass and momentum conservations are obeyed. Compared with the pure multicomponent lattice Boltzmann method, the hybrid method will increase calculate efficiency obviously. By the present hybrid method the flow and heat transfer of magnetic fluid flow through a heated wire under external magnetic field is investigated. The strength of magnetic field and the magnetic direction on the influence of heat and mass transfer have been analyzed. And the effects of gradient field are also studied.(4) The investigation of Flow and Heat Transfer of magnetic fluid by experimentsTo validate the present hybrid method, an experimental system is designed. In the experimental the heat and flow transfer process of Fe3O4 magnetic fluid flow through a heated wire of platinum is studied. Compared with the experimental results, the accuracy of the hybrid method is validated and the mechanism of heat and flow transfer of magnetic fluid is investigated again.(5) The exploration and application of magnetic fluid based on the thermomagnetic effectSince the magnetization of magnetic fluid is decreased with the increase of temperature, a new. engine system based on the thermomagnetic effect of a temperature-sensitive magnetic fluid is introduced. The system essentially consists of a permanent NdFeB magnet, a kerosene-based magnetic fluid and a rotor. The rotor was driven by the thermal convection of the magnetic fluid in the presence of a homogeneous external magnetic field. A digital camera was used to record the rotation speed of the rotor for investigating the performance of the engine system under varying conditions such as the heat load, the heat sink temperature, and the magnetic field distribution. The results illustrate that the rotation speed of the rotor increases with the input heat load, or as the heat sink temperature decreases. It also can be seen that the performance of the motor is considerably influenced by the imposed magnetic field. Therefore, the performance of such an engine can be controlled conveniently by changing the external magnetic field and the temperature distribution in the fluid or each of them.