Three-dimensional Numerical Simulation Of Oscillatory Thermocapillary Flow In Encapsulated Liquid Bridge

Floating zone method is widely used in producing crystals. The thermocapillary convection induced by interface tension gradient becomes a prominent factor influencing the quality of materials under microgravity condition. The liquid encapsulation technology plays a very important role in reducing thermocapillary convection. The physical model of two immiscible coaxial liquid columns is researched being aimed at float zone technology of crystal growth and liquid encapsulation technology in the present paper. In order to understand the fundamental characteristics of thermocapillary convection in an encapsulated liquid bridge, the physical and mathematical models of thermocapillary-buoyancy convection in two immiscible liquid layers are established. Numerical simulation of thermocapillary-buoyancy convection in encapsulated liquid bridge is performed by finite volume method (FVM). The distributions of temperature and velocity in encapsulated liquid bridge are then obtained and effects of a series of non-dimensional parameters on the thermocapillary-buoyancy convection are analyzed. The results obtained can be confirmed that the liquid encapsulation technology can weaken the themocapillary in liquid bridge which can be applied directly to crystal growth procedure in the production of pure and high quality crystals. According to the simulation results, we can draw the following conclusions:Thermocapillary convection in an encapsulated liquid bridge is axisymmetric and steady at the small Marangoni number. When Marangoni number exceeds a critical value, the flow will undergo a transition to the three-dimensional oscillatory flow. So the critical Marangoni number is confirmed. The critical Marangoni number obtained from the numerical simulation is slightly higher than one from the experiments under normal gravity. It suggests that buoyancy can bring forward the transition of the flow pattern. When the three-dimensional oscillatory thermocapillary convection happened, oscillatory amplitude of the flow field is very great in the liquid encapsulation, but the flow is rather weak in the liquid bridge. When the thickness of liquid encapsulation decreases, thermocapillary convection of the fluid in the liquid bridge becomes weaker. The thinner the liquid encapsulation is the greater the reduction of thermocapillary convection in the liquid bridge. It hints that thermocapillary convection in liquid bridge can be much more weakened by the liquid encapsulation. The azimuthal wave number of the temperature oscillatory is slightly related to the diameter of liquid bridge and Marangoni number. When the diameter of the liquid bridge increases, the azimuthal wave number of the temperature oscillatory increases. The frequency of the oscillations increases with the increasement of Marangoni number and the inner diameter, decreases with the inceasement of the outer diameter.