The Instability Of Two-Layer Fluids Thermocapillary-Buoyant Convection And Active Control By External Magnetic Field

Thermocapillary convection is driven by surface tension gradient due to a non-uniform temperaturedistribution at the fluid interface, which is also called Marangoni convection. This flow phenomenonwidely exists in many engineering applications including crystal growth, metal casting, and filmcoating process. The instability of thermocapillary convection can cause adverse effects, i.e.,imperfections in crystal growth. In this thesis, the appearance and evaluation ofthermocapillary–buoyant convection instability in an annular two-layer fluids system and the effect ofmagnetic field on the convection instability were investigated. Meanwhile, the influence of externalmagnetic field on the heat and mass transfer in Cz method crystal growth process was investigated,also. The detailed content and results as follows:Firstly, the appearance and evaluation of thermocapillary–buoyant convection instability in anannular two-layer fluids system and the effect of magnetic field on it were investigated by numericalsimulation. The oscillation of thermocapillary convection instability initiates around the inner wallregion, and propagates along the direction of temperature gradient. The aspect ratio has obvious effecton thermocapillary convection, in which the azimutahl wave number increases at first and thendecreases as the aspect ratio decreases, but the average oscillatoty amplitude of azimuthal velocity isstrengthened gradually. When aspect ratio is0.5, the critical Ma number of themocapillary-buoyantconvection instability is2.33x103. With the gravitational acceleration increasing, the energy ofdisturbance from surface tension is reduced, and the azimuthal wave number decreases. The rotationof annular pool can suppress the thermocapillary convection instability effectively, and the oscillatoryconvection becomes steady convection. All the magnetic fields can suppress the thermocapillaryconvection, in which the electromagnetic damping of thermocapillary convection instability underaxial magnetic field is the most advantageous, that of cusp magnetic field is the second, and thehorizontal magnetic field results in the weakest damping.Secondly, the influence of external magnetic field on the heat and mass transfer and segregation ofimpurity in Cz method crystal growth process was investigated by numerical simulation. Under theinfluence of axial and cusp magnetic fields, with magnetic field intensity increasing the temperaturegradient on the interface of melt and crystal decreases, the oxygen concentration increases gradually,and the convective pattern becomes more regular and, the damping effect of cusp magnetic field isstronger than that of axial magnetic field. When rotating magnetic field is applied to, the rotations ofcrystal, crucible and fluid are in the same direction, a better crystal quality can be obtained. Theexternal magnetic field can improve the impurity distribution in crystal. The gradient of impuritydistribution mainly locates at the direction of θ=π/2and θ=3π/2of crystal under horizontalmagnetic field, but that of under cusp magnetic field locates at the margin of crystal. The applicationof cusp magnetic field is better for the control of impurity segregation.Developed Level set method was applied to the simulation of interface characterstics of two-layerfluids, and the flow characteristics of thermocapillary-buoyant convection with a deformable interfacewas investigated. It is found that, under the impact of Marangoni effect on the interface bulges outnear the hot wall and bulges in near the cold wall. The deformability of free surface is increased with Ma number and aspect ratio increasing, but it is decreased as the magnetic field intensity andgravitational acceleration increase. The contact condition of interface with the end walls is importantfor the prediction of thermocapillary convection characteristics.Lastly, the temperature field of thermacapillary-buoyant convection in a rectangular cavity wasinvestigated experimentally by Laser shear interference method, and the three-dimensionaltemperature field was reconstructed by algebraic reconstruction technique method. The constructedtemperature field is in good agreement with numerical simulation result.