Research On Flow Characteristics Of Capillary Convection Of Binary Mixtures In A Rotating Annular Pool

The capillary convectcion induced by the surface tension gradient playes a key role in numerous physical and industrial processes.During the process of Czochralski growth of binary crystals,the temperature difference between the crucible and the crystal interface creates the thermal capillary force on the free surface.The concentration difference formed by the segregation effect at the crystal interface or insoluble impurities produces the solutal capillary force on the free surface.The rotation of the crystal and the crucible produce the centrifugal force and Coriolis force.The uneven distributions of temperature and concentration create buoyancy in the melt under normal gravity.Hence,a complex convection is created by the coupling of multiple driving forces during the growth of the binary crystal.The investigation on this complex convection is helpful to control the flow,which can improve the quality of the crystal.Howerver,the present researches on capillary convection of binary mixture are few and limited in the static system.Thus,the study of the capillary convection of binary mixture in the rotating system can not only complete the complex flow dynamics theory,but also provide theoretical guidance for the improvement of the binary crystal growth.For this purpose,a series of experiments and numerical simulations has been conducted to investigate the complex characteristics of the capillary convection of bainary mixture in a rotating annular pool.The basic flow pattern,flow stability and oscillatory flow characteristics of the capillary convection of binary mixture in the rotating annular liquid pool were explored.The flow instability mechanism and flow pattern evolution were revealed.The main contents and results are as follows:First,a visual experimental system and three-dimensional numerical simulation were used to study the thermal capillary convection affected by the Soret effect in the rotating annular pool.The effects of Taylor number and the aspect ratio on the flow stability and the oscillatory flow pattern of binary mixture were investigated.The results show that,in the rotating liquid pool,the basic flow of thermal capillary convection affected by the Soret effect is an axisymmetric steady flow formed by an anticlockwise vortex.The pool rotation hardly affects the radial flow of the basic flow,but significantly enhances the relative azimuthal flow.The critical thermal capillary Reynolds number increases with the Taylor number increases,and decreases with the aspect ratio increases.The oscillatory flow pattern strongly dependes on the Taylor number and the aspect ratio.A flow pattern in which HTWs and internal fluctuation coexist appears at high Taylor number.The internal fluctuation is generated by rotating cells contained in the basic flow.It creates a large internal temperature fluctuation in the fluid,which makes the temperature fluctuation on the free surface increase as the Taylor number increases.The quasi-periodic oscillation induced by HTWs and the single-periodic oscillation with lower frequency caused by the internal fluctuation are obtained on the free surface.Then,the thermal-solutal capillary convection in the rotating annular pool under microgravity was investigated through numerical simulations.The variation curve of the critical thermal capillary Reynolds number with the Taylor number at various capillary ratios was ploted.The flow pattern evolution,flow structure and formation mechanism of oscillatory flow patterns were determined.The results show that,when the thermal capillary Reynolds number is small,the flow state of thermal-solutal capillary convection can be divided into three types according to the capillary ratio.The effects of the rotation on the flow stability strongly dependeson the capillary ratio R_?.With the increase of the Taylor number,the critical thermal capillary Reynolds number at R_?=-1 decreases first,and then tends to be constant;The critical value at R_?=-0.5 and-0.8 increases first,then gradually decreases and finally tends to be constant;The critical thermal capillary value at R_?=-1.25 and-2 decreases first and then gradually increases.The oscillatory flow pattern is determined by the capillary ratio,the thermal capillary Reynolds number and the Taylor number.The basic flow bifurcates to traveling waves created by the radial disturbance at R_?=-1.Multiple abrupt points exist in the traveling wave flow pattern when Ta=0.The concentration fluctuations induced by the abrupt points create spoke waves inside the liquid layer and periodically change the propagation direction of the traveling wave.When R_?=-1.25,HSWs-DC flow pattern,two types of HSWs coexisting flow patterns and two groups of HSWs-DCR coexisting flow patterns appear at different Taylor numbers.The competition between solute capillary force and thermal capillary force makes the thermal-solutal capillary convection easily transits to the chaotic state at R_?=-2.The increase of the Taylor number can suppress the chaotic flow.Finally,the thermal-solutal capillary-buoyancy convection in the rotating annular liquid pool under normal gravity is studied through three-dimensional numerical simulations.The effects of capillary ratio and Taylor number on the flow structure,flow stability and flow pattern evolution were discussed.The results show that the effect of rotation on the basic flow structure and flow stability of thermal-solutal capillary-buoyancy convection is different at various capillary ratios.As the Taylor number increases,the critical thermal capillary Reynolds number at R_?=-0.5,-0.8,and-1 increases first and then decreases;The critical thermal capillary Reynolds number at R_?=-1.25 and-2 increases monotonically.The flow pattern evolution of thermal-solutal capillary-buoyant convection is determined by the capillary ratio and the Taylor number.When the Taylor number is small,the thermal-solutal capillary-buoyancy convection at R_?=-0.5 and-0.8 is dominated by the thermal capillary force,which leads to a single flow pattern transision.The increase of solutal capillary force and Taylor number make the flow pattern evolution of thermal-solutal capillary-buoyancy convection more complicated.