Study On Thermocapillary Convection For High Prandtl Number Fluid In Deep Space Environment

In the fields of national defense,communications and aerospace,the high quality semiconductor component is indispensable important carrier for the control system.The floating zone method is the main method of making this semiconductor material.With continuously exploration for the space environment by human beings,it is found that thermocapillary convection is the main cause of generated micron dimension impurities stripe inside single crystals by floating zone method under the deep space environment which is difficult or no access to obtain on the earth.Therefore,the exact studies for the thermocapillary convection in crystal growth process by the floating zone method and inhibition methods are greatly important.Then,the studies for effects of applied magnetic field,vibration,surface coating and shear airflow on thermocapillary convection and oscillation mechanism in deep space environment have become a frontier topic in the field of microgravity fluid mechanics.In this dissertation,numerical and experimental investigations for thermocapillary convection in the liquid bridge of half floating zone just launch under above engineering and theoretical backgrounds.However,the simplified model mostly has been adopted in previous numerical studies without considering synchronous dynamic deformation of free surface of liquid bridge.The free surface is assumed to be a non-deformation straight line(cylindrical surface)or is a non-deformation curve(curved surface)determined by the Young-Laplace equation.It certainly influences the accuracy of results according to above practices.Then,it is difficult to reflect coupling characteristics between the internal flow and the response deformation of interface for liquid bridge.Aiming at above existing problems,the area compensation method is innovatively used to compensate the non-conservation of mass in level set method and the improved level set method is realized to track any tiny deformation of two-phase free surface for liquid bridge.In a word,the researches are carried out from the following several aspects:1.Aiming at buoyancy-thermalcapillary convection system for liquid bridge under the microgravity environment,the competition model between the buoyancy convection and the thermalcapillary convection is proposed.The study finds that three kinds of flow stages are presented in the buoyancy-thermalcapillary convection.In the flow stage dominated by the buoyancy convection.The surface presents reverse "S"-shape,and the entrained peripheral flow of buoyancy convection shrinks.In the balanced transitional stage of thermalcapillary convection and the buoyancy convection,the free surface shape changes from the"2"-shape into the distorted "M"-shape.In the flow stage dominated by the thermocapillary convection,the thermocapillary convection occupies and swallows up the buoyancy convection.The free surface shape changes from the distorted "M"-shape into the "S"-shape.The results show that there are perturbations on the free surface in changing process of the each flow states.This discovery can enrich the theory of interface instability and provide necessary theoretical basis for the interface oscillation mechanism of buoyancy-thermalcapillary convection in the liquid bridge.2.Aiming at ambiguous oscillation mechanism of thermocapillary convection and transitional process in the present,the model of oscillatory thermalcapillary convection in the liquid bridge is established.The oscillation mechanism of thermalcapillary convection is also proposed froon m a brand new angle.Results show that two cell flows periodically encroach each other in the stage of oscillatory thermocapillary convection.Their vortexes core paths are step changing.They move asymmetrically on the lateral direction and present a reciprocating vertical migration.The temperature firstly oscillates at hot corner.The essence of oscillation mechanism for thermocapillary convection is proposed and oscillatory thermocapillary convection essentially originates from the coupling effect of oscillatory temperature,velocity and free surface.The thermocapillary convection initiates oscillation at the hot corner.According to oscillation morphologies of velocity and interface at the intermediate height,the study finds two kinds of oscillation morphologies at the intermediate height:the stable oscillation morphology(including low frequency oscillation stage and the high-frequency oscillation stage)and the large amplitude pulsation morphology.3.Aiming at inhibitory effect of horizontal vibration on thermalcapillary convection and the surface flow,the mathematical model for liquid bridge with the horizontal vibration was established.Researches show that vortex core of cell flow periodically oscillates around an equilibrium position in longitudinally and horizontally.Amplitude of vortex core in the initial stage is larger than that in the stable stage.The response amplitude of free surface is the maximum near the intermediate height of liquid bridge.The horizontal vibration causes waggle of internal cell flow and excites periodic exchange flow across the center axis of liquid bridge.At the same time,the vortex core moves towards the cold disk and the center,which intensifies the radial convection.Researches propose that the horizontal vibration can reduce the thermal activity at the hot corner and inhibit the surface flow.The horizontal vibration can excite the transverse vortex flow structure in the three-dimensional isothermal liquid bridge.Further,the acceleration direction of horizontal vibration can change the rotation direction of transverse vortex and affect the velocity of surface flow.4.Aiming at the inhibitory effect of uniform magnetic field on the thermalcapillary convection of silicon oil based ferrofluid,the mathematical model of liquid bridge with the uniform magnetic field is established.The transverse magnetic field generates the longitudinal lorentz force which directly inhibits the longitudinal velocity of internal fluid.The transverse magnetic field doesn't inhibit the thermocapillary convection and transfers the influence of thermocapillary convection from the interior to free surface.The axial magnetic field generates radial lorentz force which directly inhibits the transverse velocity of internal fluid,and it blocks radial channels between the bulk returning flow and the surface flow.Researches show that the stagnated zone appears within the liquid bridge.The axial uniform magnetic reduces the convective heat transfer through inhibiting thermocapiliary convection and promotes the development of heat conduction model.5.Aiming at the influence of shear airflow on surface flow characteristic and the internal flow pattern of liquid bridge,the mathematical model of liquid bridge with shear airflow is established.The study shows the following.(1)The formation of cell flow is due to the shear airflow in isothermal liquid bridge.The structure of cell flow is small and is tightly close to the free surface.The flow direction of cell flow is depending on the direction of shear airflow.The position of vortex core changes with accelerated shear airflow.On the other hand,the shear airflow changes free surface shape.The downward shear airflow suppresses the convex region of“S”-shape free surface.The convex region is sunken and laterally extended to the gas side,which increases the risk of fracture of liquid bridge.The upward shear airflow lifts the convex region of“S”-shaped free surface.The free surface is extended to the gas side and evolves into "M"-shape.(2)The flow condition is relatively complicated in the non-isothermal liquid bridge with the shear airflow because of synergies and antagonism between the thermalcapillary force and shear force.When the shear airflow is introduced from the top of liquid bridge,the cell flow is at the hot corner near the free surface which is dominated by thermocapillary convection.However,the shear airflow accelerates the surface flow as the increasing velocity of shear airflow and the reverse cell flow finally appears near the bottom of liquid bridge.The free surface shape is still the "S"-shape.When the shear airflow is introduced from the bottom of liquid bridge,the directions of two kinds of surface flow are opposite because they are respectively dominated by thermocapillary convection and the shear force.Eventually,the free surface shape is determined on the competitive consequences between the thermalcapillary force and the shear force.The free surface shape presents the"S"-shape at first,and then it gradually evolves into the "M"-shape.6.Aiming at the effect of shear airflow on the surface deformation,the experiment platform of liquid bridge is independently designed and set up.It can meet all kinds of experiments for liquid bridge(including experiments of isothermal and non-isothermal liquid bridge with the shear airflow).The programs is developed in this dissertation,which can realize independent recognition from the optical image to configuration data for interface of liquid bridge.Researches propose the theory formula of laterally variable quantity for maximum displacement point on the interface under the changing shear airflow.Experimental results show the following.When the direction of shear airflow is the same as that of gravity,the convex region of free surface moves down and the curvature of concave region increases strongly.When the direction of shear airflow is opposite to that of gravity,the convex region of free surface moves up.The risk of fracture of liquid bridge is increased.The free surface deformation presents certain sine law for the liquid bridge with V<1(such as F=0.802 and V=0.899);When the volume ratio is greater than 1(such as V=1.071),the convex region is dragged up by the upward shear airflow and the free surface shape is the multimodal structure.For the liquid bridge with large height-diameter ratio(such as ?=1.4),the free surface shape has no great change.The free surface deformation increases as the decrease of height-diameter ratio(such as ?=1.2).