Numerical Simulation Of Bubble Motions In Conducting Fluid Under Uniform-Vertical Magnetic Fields

The two-phase flow with bubbles involves in many industrial productions and technology areas such as energy, fossil oil, nuclear power and so on. In which the motions of bubbles in liquid metal and magnetic fluid play a crucial role. For instance, argon gas is injected into liquid metal to remove impurities during the continuous casting production process or gas bubbles form in foam metal production. With the development of the magnetohydrodynamics(MHD)theory which focuses on the interactions between electromagnetic field and conductive fluid, the electromagnetic technology is gradually used in material process, metallurgy and other industries. Similarly, appending an external magnetic field is an alternative way to control the movements of the bubbles without contact. Obviously, it’s significative to prone into the magnetic field functions on bubble motions in conducting fluid.In the present dissertation, the numerical simulations about bubble motions in conducting fluid are carried out by the commercial software of Fluent. A single bubble rising, two-bubble coalescence and the interaction between bubble and free surface in conducting fluid under different uniform-vertical magnetic field intensities ranging from 0 to 0.4T are systematically investigated, where bubble radiuses range from 2mm to 6mm(R*=0.2, 0.3, 0.4, 0.5 and 0.6). The effects of imposed magnetic fields on the rising velocity, instantaneous bubble shape, pressure field, coalescence time as well as the break time are determined and discussed carefully.The following conclusions are obtained. 1. Bubble is elongated along vertical direction by Lorentz force, and stronger magnetic field intensities contribute to longer bubble shape. 2. For single bubble, the imposed magnetic fields have an inhibitory effect on the rising process for bubbles with strong surface tension as well as bubbles with weak surface tension and small sizes of R*=0.2 and 0.3. But for bubbles with weak surface tension and large sizes of R*=0.4,0.5 and 0.6, the rising progress is promoted by weak magnetic field intensities whereas inhibited by strong magnetic field intensities. The critical Hartmann numbers from positive effect to negative effect and the Hartmann numbers with the maximum promoting effect increase when the radiuses of the bubble increase. 3. For the coalescence process of two bubbles, weak magnetic fields when Ha<20.12 delay the merging time for small bubbles with R*=0.2 and0.3 while advance it for large bubbles with R*=0.5 and 0.6. For bubble with R*=0.4, the coalescence time fluctuates with magnetic field intensities. When Ha>20.12, the coalescence process of two bubbles is inhibited for all sizes of bubbles involved in this work. And the stronger the magnetic field intensity is, the more difficult the bubble merging is. 4. About interaction between bubble and free surface, the result exhibits that the lifting height of free surface by bubble is increased by magnetic fields. Besides, the Lorentz force prolongs the break time of bubble. The time before bubble bursting lasts longer with the increasing of the magnetic field intensities.Finally, through comparing the effects of magnetic fields on single bubble rising process under three groups of physical properties and analyzing the flow fields and pressure distributions around the bubbles, the detail reasons and physical mechanisms under these phenomena are explained clearly.