Visualized Experiment Research Of Thermocapillary Convection In Non-isothermal Liquid Bridges

Producing high-quality single crystal by the floating method relies on its own surface tension completely,in which the growth of single crystal melt does not contact directly with the crucible avoiding the imperfection of crystal dislocation and the melt pollution caused by the crucible material.On the ground,the lower half of the whole floating zone is studied as a floating half-zone liquid bridge to simplify the process of producing crystal and suppress the buoyant convection in the melt.The upper disk is heated to make a certain temperature difference AT between upper and lower disks,and then the vortex-shapped flow,named as thermocapillary convection,will be generated.Thermocapillary convection,also named as Marangoni convection,is driven by the uneven distribution of the surface tension caused by the temperature difference on the gas-liquid free surface.When surface tension exceeds the viscous force of liquid,it will drive the liquid move;when the temperature difference ?T exceeds a certain critical value,the thermocapillary convection inside of liquid bridge will evolve from steady flow to oscillatory flow,which affects the crystallization process directly.In addition,in the experimental study and crystal producing on the ground,there will be buoyant convection caused by gravity in the liquid bridge.The coupling by buoyant-thermocapillary convection also can affect the quality of crystal preparation.Therefore,it is of theoretical and practical significance to study thermocapillary convection and buoyant-thermocapillary convection.In this experiment,the method of flow pattern visualization and the particle tracking velocimetry(PTV)were adopted to study the flow field of thermocapillary convection and buoyant-thermocapillary convection in the floating half-zone liquid bridge of 10cSt silicone oil(Pr=105).For the thermocapillary convection,not only the flow patterns of steady and oscillatory flow were investigated qualitatively but also the flow velocity of steady flow and the influence of temperature difference ?T on flow velocity were studied quantitatively.For the buoyant-thermocapillary convection,the influences of temperature difference ?T,height-radius ratio A,volume ratio V and the diameter of upper and lower disks D on flow pattern were discussed detailedly.The following conclusions are obtained through the systematic experimental analysis:1.For the thermocapillary steady flow,the velocity of surface flow is larger than that of sickle-shapped backflow;the resultant velocity of surface flow has a peak,while there are two peaks for the resultant velocity of sickle-shapped backflow.The flow velocities at different flow layers increase first and then decrease from the vertex center to the center axis.The higher the temperature difference is,the faster the flow velocity is,and the increasing extent in the region close to vertex center is larger than that away from vortex center.2.When ?T>(?T)?,the thermocapillary convection will shift from a two-dimensional time-independent steady flow which is symmetric along with the center axis to a three-dimensional time-dependent periodic oscillatory flow which is asymmetric along with the center axis.In the two-dimensional image of oscillatory flow,the flow pattern presents as periodic expansion or shrink of the left and right vortices;while in the three-dimensional image of oscillatory flow,the vortices on both sides flow along with the oblique axis which rotates along with the center axis conically simultaneously.3.Under the condition of gravity field,increasing temperature difference ?T can strengthen buoyancy effect.With the increasing of A ?T,the flow pattern in the liquid bridge of large volume ratio and diameter experiences three stages:a.when ?T is small,the thermocapillary vortex shrinks to the free surface and upper disk gradually;b.when ?T increases to a certain value,the buoyant vortex will generate in the center close to lower disk and squeeze the thermocapillary vortex;c.when ?T>(?T)c,the thermocapillary vortex and buoyant vortex compete with each other,and another periodic oscillation which is different from the three-dimensional oscillation of thermocapillary convection generates.4.Under the condition of gravity field,increasing the radial size of the liquid bridge system(including the diameter of upper and lower disks D and neck width dmin)can strengthen buoyancy effect.And increasing the volume ratio V of the liquid bridge will increase dmin,and then strengthen buoyancy effect,while increasing the height-radius A of the liquid bridge will decrease dmin,and then weaken buoyancy effect.