The Instability Of Thermocapillary Convection In Liquid Bridges With Dynamic Interface And Active Control By MHD Effect

Floating zone is an important technology for growth of high-quality, high-purity single crystal materials due to its free of crucible contamination. Under microgravity environment, the buoyancy convection is gradually obscured due to the extreme weakened gravitational field. The thermocapillary convection becomes the major convection in the melt and its instability will cause the formation of some crystal defects such as microscopic imperfections and macroscopic stripes, which poses great challenge for the floating zone crystal growth. Therefore, in order to improve the quality of the produced single crystal materials, it is extreme important to investigate the behaviors of the thermocapillary flow and control its instability. In this thesis, numerical simulations are performed to investigate the instability of thermocapillary convection and the characteristics of free-surface deformation in liquid br idges with dynamic free-surface. The volume of fluid(VOF) method is adopted to track the free-surface movement. Furthermore, the impact of the external magnetic field is also investigated on the instability of thermocapillary convection and free-surface deformation.Firstly, the instability of thermocapillary convection and the dynamic behaviors of free-surface deformation are numerically investigated in small scale liquid bridges with diameter of 1cm under microgravity. When Ma=1.22×103, the thermocapillary convection exhibits a stable periodic oscillation and its oscillation period is ?=0.58 s. Thus the structure of the flow and temperature fields appears a periodic variation in the whole melten zone. When aspect ratio Ar is 1, the critical Marangoni number is 421.21 for the appearance of thermocapillary convection oscillation. When the periodic oscillation of the thermocapillary convection occurs, the azimuthal and radial velocities appear inhomogeneity along the circumferential direction. With the increment of Marangoni number, the ‘petal' and strength of the azimuthal and radial waves increase. The free-surface presents a typical narrow ‘neck-shaped' structure with convex near the cold and hot disks and concave at z=0.921 cm region. The maximum free-surface deformation ratio ? appears a periodic sinusoidal fluctuation with time and its oscillation period is identical to the oscillation period of thermocapillary convection. Moreover, the deformation ratio ? increases with the increment of Ma.Secondly, the effect of Marangoni number and aspect ratio Ar are investigated on the thermocapillary convection and free-surface deformation in large scale liquid bridges with diameter of 2cm. Under the Marangoni effect, the whole molten zone is practically occupied by convection vortexes with clockwise circulation in the left-half domain and anti-clockwise circulation in the right-half domain. When the thermocapillary convection becomes instable, the melt flow shows a three-dimensional unstable oscillation in the whole liquid bridge. With the increment of Marangoni number, the intensity and instability of thermocapillary convection gradually increase together with the amplitude and frequency of the transient temperature. Conversely, with the increment of Ar, the intensity and instability of thermocapillary convection gradually reduce together with the amplitude of the transient temperature, while the frequency decreases. In addition, the maximum free-surface deformation ratio of free-surface ? increases with the increment of Ma as well as Ar.Finally, the influence of the external transverse, axial and cusp magnetic fields are investigated on the flow field, temperature field and free-surface deformation in a large liquid bridge with diameter of 2 cm. Especially the inhibiting capability of different magnetic fields are contrastively analyzed on the flow instability and free-surface deformation. The suppressing effect of transverse magnetic field is directional on the thermocapillary flow. The inhibiting effect on thermocapillary convection is signif icant on the ?=0° plane, while relatively weak on the ?=90° plane. The axial and cusp magnetic fields have a manifest inhibiting effect on the thermocapillary convection in the whole liquid bridge, and induce a concentration of the convection vortexes near the free-surface. The axial and cusp magnetic fields also effectively suppress the flow instability both on radial and axial directions. The inhibiting capability of cusp magnetic field is the strongest on the flow instability and free-surface deformation, and the critical Hartmann number of deformation damping on the ?=0° and ?=90° planes are 98.36. The axial magnetic field is the second, and the critical Hartmann number of deformation damping on the ?=0° and ?=90° planes is 214.11 and 208.12, respectively. The transverse magnetic field is the weakest, and the critical Hartmann number of deformation damping on the ?=0° and ?=90° planes is 230.12 and 240.12, respectively.