The Solutocapillary Convection And Its Coupled Effect With The Thermocapillary Convection In An Annular Pool

Solutocapillary effect is the important factor for many industrial processes such as alloy casting and solidification,the growth of semiconductor crystal and welding etc.There are also very wide application fields for the coupled solutocapillary and thermocapillary effect.In order to obtain the high-quality binary crystals and alloys,it is very necessary to study the coupled effects among the solutocapillary force,thermocapillary force and buoyancy force of the fluid in the crucible,analyze the basic characteristics and destabilization mechanism of the flow,and explore the technique for controling fluid flow.At present,there are less researches on solutocapillary convection and its coupled effect with the thermocapillary convection.There are lack of quantitative analyses for the coupled effect of capillary and buoyancy forces in many research results.In this thesis,the basic characteristic of pure solutocapillary convection in an annular liquid pool was investigated by both numerical simulation and experiment.Furthermore,the characteristics of the convection driven by the coupled effects of solutocapillary force,thermocapillary force and buoyancy force were discussed.The critical conditions and the flow pattern transition,and the destabilization mechanism are revealed.The main contents and results are as follows:(1)Pure solutocapillary flow in an annular pool subjected to a radial solutal gradient was studied by numerical simulation.Results show that the fluid flow is axisymmetric steady flow when the solutocapillary Reynolds number is small.Small flow cells are embedded in the basic flow field.With the increase of solutocapillary Reynolds number,the number of small flow cells increases,and the fluid flow is enhanced.When the solutocapillary Reynolds number exceeds a critical value,the axisymmetric steady flow transits orderly to two-dimensional periodic oscillation flow and three-dimensional oscillatory flows such as standing wave type and hydrosolutal wave(HSW)type.When the direction of radial concentration gradient is reversed,the flow become more stable.The flow pattern evolution sequences are the same under the opposite radial concentration gradients.Under the normal gravity,the buoyancy force enhances the fluid flow.When the solutocapillary Reynolds number is over the critical value,the concentration fluctuation on the free surface is in the form of the HSW.(2)Numerical simulation has been performed regarding the coupled thermal-solutal capillary convection in the annular pool subjected to the combined radial temperature and concentration gradients.Results indicate that when thermal capillary Reynolds number is small,three types of two-dimensional steady flow appear with the variation of capillary ratio and the thermal capillary Reynolds number,i.e.,the single cell flow,the flow with coexisted clockwise and counter clockwise cell,and the flow with two cells rotating in the opposite direction.With the increase of the thermal capillary Reynolds number,axisymmetric steady flow bifurcates orderly to the three dimensional steady flow,two-dimensional or three-dimensional periodic oscillatory flow.When capillary ratio varies from 0 to-1,the critical thermal capillary Reynolds numbers increase and then decrease with capillary ratio.The critical frequency and wave number are also dependent on capillary ratio.Within the calculated range of the capillary ratio,two kinds of three-dimensional periodical oscillation flows occur,i.e.the flows in the form of vibrating spokes and traveling waves.For the traveling wave,temperature and concentration fluctuations are respectively indicated as the hydrothermal waves(HTW)and hydrosolutal(HSW).(3)For the special case that the capillary force ratio is-1,the travelling wave(TW),coexisted radial propagating wave and stationary wave(CTSW),and the vibrating spoke pattern(VSP)appear orderly when the thermocapillary Reynolds numbers exceed certain critical values.For the TW pattern,the thermocapillary and the solutocapillary effects are in the comparable magnitude.The steady state flow and the periodical oscillatory flow coexist at the same time in the liquid layer for the CTSW flow.Both of them are organized in the time phase and space distribution.The surface flow pattern is in the form of orderly "petals" for VSP flow,which is disorder in the time phase.(4)Numerical simulation has been carried out for the coupled thermal-solutal capillary-buoyancy convection.Four kinds of two-dimensional steady flow appear when the thermocapillary Reynolds number is small,i.e.the single cell rolling along the counter clockwise direction,two distributed cells in left and right sides,two distributed cells upper and lower,and one rolling along the clockwise direction.With the further increase of the thermocapillary Reynolds number,the two-dimensional steady flow transits to the three-dimensional steady flow,two-dimensional or three-dimensional periodical oscillation flow orderly.When the capillary ratio varies from 0 to-0.8,the first critical thermocapillary Reynolds number,which is the bifurcation point from the two-dimensional steady flow to three-dimensional steady flow,increases gradually.However,the second critical value,which is the bifurcation point from steady flow to periodical oscillation flow,increases at first,and then decreases.The first critical value is similar under normal gravity and zero gravity conditions,while the second one is dependent on the capillary ratio.The flow pattern of the three-dimensional periodical oscillation flow is also associated with capillary ratio.Within the calculated range of capillary ratio,two types of flow patterns occur,i.e.the vibrating spokes and traveling waves.Moreover,both the HTW and HSW can be found.(5)When the capillary force is around-0.3,a phenomenon of reverse transition occurs for the coupled thermal and solutal capillary-buoyancy convection.The time-dependent flow state reverses to three-dimensional steady flow state.When the capillary ratio is-1,the thermal and solutal buoyancy forces are unbalanced in the liquid pool.Two-dimensional axisymmetric steady flow arises if there is any radial temperature difference.Once the flow destabilizes,it transits to the axisymmetric periodical oscillatory flow.When the thermocapillary Reynolds number is further increased,the flow bifurcates to the periodical oscillatory flow with a standing wave formation.At a fixed thermocapillary Reynolds number,the periodical oscillatory flow,the subcritical unsteady flow and the three-dimensional steady flow occur in the liquid pool with the increases of the aspect ratio.(6)The characteristic of the surface concentration fluctuation for the pure solutocapillary convection in the annular pool has been observed experimentally.Results show that the sequence of the flow transition is similar to that from numerical simulation.For the working fluid with the initial concentration of 26.27wt%,the experimental and numerical results are in good agreement.The critical solutocapillary Reynolds number increases with not only the initial concentration of the mixture,but also the depth of the liquid pool.