Numerical Study On Oscillatory Solutocapillary Convection Under Microgravity

In practical applications,floating zone method is intensively used to produce high quality crystals.Liquid bridge is a scientific idealized model for studying the flow in the floating zone.In the process of producing crystals by the floating zone method under microgravity,some impurities may be mixed in raw materials and the nonuniform distribution of impurity concentration on the free surface of liquid bridge will drive the solutocapillary convection.Stripy defects are formed inside or on final products when the convection becomes oscillatory,which can seriously affect the quality of crystals.Therefore,the study on flow characteristics and oscillatory mechanism of solutocapillary convection in the floating zone are helpful to improve the process of crystal growth and the quality of crystals.In the present work,a numerical simulation has been conducted to investigate flow characteristics and surface deformation of the steady solutocapillary convection with different concentration differences.The concentration distribution,flow structure,and oscillatory characteristics of velocity and free surface for the oscillatory convection are also mainly investigated by taking the dynamic deformation of surface into account.In addition,we also discuss the effect of aspect ratio on flow characteristics for the oscillatory solutocapillary convection.The continuity equation,Navier-Stokes equations,and concentration diffusion equation are solved on uniform staggered grids to study the flow in liquid bridges.The deformation of free surface is captured by an improved level set method with reinitialization and mass conservation.From results of the numerical calculation,main conclusions can be drawn,1.For the steady solutocapillary convection,a pair of opposite and axisymmetric vortices are generated on both sides in the liquid bridge.The concentration of solute is slowly transported to the inner and the surface under return flow,making the concentration distribution in the liquid bridge more uniform.The free surface bulges in near the upper and lower disks and bulges out near the intermediate height of the liquid bridge with a small concentration difference.2.For the oscillatory solutocapillary convection,distributions of vortices and concentration contours become asymmetrical.The flow at the corner has a significant influence on the overall flow of the liquid bridge.The radial velocity oscillates more sharply than the axial velocity at the high-concentration corner,and this situation is just opposite near the intermediate height.The oscillation of velocity near the free surface is more intense than that inside the liquid bridge.The oscillation of transverse displacement of free surface at the corner is more intense than that near the middle part.3.Within a certain height range of the liquid bridge,the lower the height,the smaller the amplitude of oscillations for concentration and velocity.It indicates that the solutocapillary convection becomes more stable.There is a complex relationship between the onset time of concentration and velocity oscillations and the aspect ratio.