Experiments on the transition from the steady to the oscillatory Marangoni-convection of a floating zone under reduced gravity effect

The transition from the steady to the oscillatory Marangoni convection in a floating zone under reduced influence of the gravity is visualized by means of the already proved method of “light-cut-technique”. The Marangoni convection is caused by a negative temperature gradient from the upper to the lower copper plug, whose diameter is kept to a small magnitude of 3 mm Φ to obtain a small Bond's number. By this way the micro-gravity effect in respect to the Marangoni convection can be simulated. From the visualization and the simultaneous measurements of the local temperature the range of the Marangoni number for the transition is determined. A strong correlation exists between the observed period of the oscillation of the branching streamline between the two vortices on the plane with the axis of the floating zone on one hand and the period of the temperature oscillation on the other hand. A model conception for the physical mechanism is introduced to understand this oscillatory instability.     

Investigation of thermosolutally caused convection during -growth of BiSbTe3-mixed crystals at normal and reduced gravity

BiSbTe₃-mixed crystals have been grown at normal and reduced gravity (during the MIR'97-mission) using a -configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means of a segregation analysis of the system components and an additional Pb-dopant. The BiSbTe₃-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low number, high number) but there are also some special properties relating to the segregationally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregation analysis have shown that the mass transport in the melt at normal gravity is mainly influenced by convective mixing determined by thermally and solutally caused buoyancy forces. Numerical simulations have been performed for the real experimentally used configurations. These simulations have shown that a strong coupling of thermal and solutal effects exists and have given axial as well as radial segregation profiles being in excellent agreement with the experimental results for the vertical normal gravity grown crystals. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled component segregation has been predicted.The results of the two micro gravity grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boundary will be given and discussed in the paper. They will be compared with the results of numerical simulations of the melt flow for the real processing parameters measured during the TITUS growth processes and with experimental as well as numerical results for vertical normal gravity grown reference Samples.     

Investigation of thermosolutally caused convection during Bridgman-growth of BiSbTe3-mixed crystals at normal and reduced gravity

BiSbTe₃-mixed crystals have been grown at normal and reduced gravity (during the MIR'97-mission) using a Bridgman-configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means of a segregation analysis of the system components and an additional Pb-dopant. The BiSbTe₃-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low Prandtl number, high Schmidt number) but there are also some special properties relating to the segregationally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregation analysis have shown that the mass transport in the melt at normal gravity is mainly influenced by convective mixing determined by thermally and solutally caused buoyancy forces. Numerical simulations have been performed for the real experimentally used configurations. These simulations have shown that a strong coupling of thermal and solutal effects exists and have given axial as well as radial segregation profiles being in excellent agreement with the experimental results for the vertical normal gravity grown crystals. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled component segregation has been predicted.The results of the two micro gravity grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boundary will be given and discussed in the paper. They will be compared with the results of numerical simulations of the melt flow for the real processing parameters measured during the TITUS growth processes and with experimental as well as numerical results for vertical normal gravity grown reference Samples.     

Effect of liquid bridge form on oscillatory thermocapillary convection under 1 g and μg conditions

This study analyses the effect of temperature difference between hot and cool disk (ΔT), and non-dimensional liquid bridge volume (V/V_o) on the transition process from steady thermocapillary convection to periodic or chaotic thermocapillary convection in a liquid bridge modeled after the floating zone method under normal gravity and microgravity conditions. From normal gravity and drop shaft experiments, the difference of the regime of the steady state and the oscillatory state was clarified on the ΔT−V/V_o plane under 1 g and μg conditions. A gap or stability region was observed in the specific V/V_o range under 1 g conditions. In the gap or stable region, after the gravity changed from 1 g to μg conditions, the temperature signals showed oscillation. From these results, the critical temperature difference under the μg conditions appeared to be smaller than that under the 1 g conditions. Temperature signals were defined as 6 different types of states. The various temperature oscillatory state regimes were obtained on a ΔT−V/V_o plane under 1 g and μg conditions. Under μg conditions, in these experimental conditions, all temperature oscillatory states exhibited only the Periodic state.     

On the structure of the floating zone in melting

Floating zone melting is used in crystal growth and purification of high melting materials. The use of a reduced gravity environment will remove the constraint imposed on the length of the zone by the hydrostatic pressure. The equilibrium of the floating zone may involve, (1) Hydrostatic forces, when the zone rotates as a whole. (2) Convective driving forces, when the zone is stationary but fluid property gradients appear. (3) Hydrodynamic forces, when some parts of the zone are set into motion with respect to others. The last effects are considered in this paper. The flow pattern of a floating zone held between two discs in relative motion is complicated, and thence the solution of the problem is difficult even assuming a constant property-newtonian liquid. Nevertheless, when a small parameter appears in the problem, the complete flow field can be split into zones where simple solutions are found. To illustrate this approach, the spin-up from rest of an initially cylindrical floating zone is considered with detail. Here the small parameter is the time elapsed from the impulsive starting of motion. Since the problem which has been considered, as well as some others which can be tackled by use of similar methods, concern the viscous layer close to either plate, they can be simulated experimentally in the ground laboratory with short floating zones. Procedures to produce these zones are indicated.     

Some aspects of continuous flow electrophoresis in microgravity

Zone electrophoresis is a highly efficient method of separating biological products. It is based on the differences of mobilities of ionized particles in an electric field. During the separation complications arise due to electro-osmosis or thermal convection generated by Joule heating.This paper analyses the hydrodynamical running of a continuous flow zone electrophoresis cell and shows the influence of specific parameters on the separation.The modelling of the flow structure of the buffer solution is developed. The equations and the corresponding boundary conditions are solved by finite difference method. The model established provides knowledge of the hydrodynamical perturbations generated by electro-osmosis and thermal free convection. The case of microgravity is considered.     

Thermo-electro-hydrodynamic instabilities in a dielectric liquid under microgravity

Linear stability analysis of a dielectric fluid confined in a cylindrical annulus of infinite length is performed under microgravity conditions. A radial temperature gradient and a high alternating electric field imposed over the gap induce an effective gravity that can lead to a thermal convection even in the absence of the terrestrial gravity. The linearized governing equations are discretized using a spectral collocation method on Chebyshev polynomials to compute marginal stability curves and the critical parameters of instability. The critical parameters are independent of the Prandtl number, but they depend on the curvature of the system. The critical modes are non-axisymmetric and are made of stationary helices.     

Effects of Constant and Variable Accelerations on Crystals Grown onboard Spacecraft by the Method of Directional Crystallization

The effect of constant and time-dependent accelerations (vibrations) on the melt flow and heat and mass transfer in the process of crystal growth by the method of directional crystallization (Bridgman method) onboard spacecraft is numerically investigated. The mathematical formulation of the problem and the technique to solve it numerically are given. The time-averaged flow arising under the action of vibrations in a nonisothermal fluid is investigated. With the help of a rational choice of dimensionless similitude parameters, a generalized dependence on the intensity of melt flow is obtained for the radial segregation of dopants. This dependence is invariant with respect to the type of motive power and thermal boundary conditions in the region of very small velocities of melt flow (creeping flow), which are characteristic for microgravity conditions. The allowable levels of constant accelerations, as well as the frequency dependences of tolerable vibrations, are obtained for five typical semiconductor materials: Ge(Ga), GaAs(Te), InSb(Te), Si(P), and Si(B). It is shown that the radial segregation of dopant is much more sensitive to microaccelerations than the axial one. In the region of small velocities, the latter is determined by the duration of the transition regime, which depends on certain physical properties of the melt. New problems that resulted from the investigations performed are discussed.     

Effect of gravity on the solidification of binary alloys—A numerical simulation

The paper illustrates the influence of gravity on convection in the mushy zone of solidifying alloys by numerical solution of the governing equations for the fluid flow.Different models for the permeability are compared and the effect of convection in the liquid ahead of the solidification front is taken into account.     

Bidimensional liquid bridges in a gravity field

The analytical solution for the different shapes that a bidimensional liquid bridge in a gravity field may adopt is reviewed, criticizing the well-known limit of Heywang. The work is shown in the context of a wider-scope program to model the fluid-mechanical behaviour of molten bridges in the floating zone and related techniques of crystal growth, which has blossomed with the advent of microgravity platforms.     

The study of Marangoni convection and the solid/liquid interface shape on a germanium float zone in microgravity

The study of the instability of the float zone in microgravity is necessary in order to produce pure and homogeneous crystals. Three types of instabilities may be present in a float zone. The first two, the static and dynamic instabilities, have been investigated by many authors. The third, onset of Marangoni convection is investigated in this study. The Navier-Stokes equations and the energy equation, in cylindrical coordinates, were solved using the finite element method. These pure and homogeneous germanium crystals will find application as integral components of sensitive γ radiation measuring equipment.     

Gravitational Sensitivity of Melts at the Growth of InSb:Te Crystals by the Bridgman and Floating Zone Methods under the Conditions of Microgravity

The comparative analysis of the results of space and ground-based experiments IMET RAS on the growth of InSb:Te crystals by the Bridgman method and floating zone method (FZM) is made for the purpose of studying the influence of microgravity on the growth, structure, and properties of grown crystals, and thus the gravity sensitivity of InSb melt is demonstrated. It is shown that, under microgravity conditions, the Bridgman method makes it possible to grow InSb:Te crystals without contact with the ampoule walls, which provides for the single crystal structure, the absence of striations, and a low dislocation density. For the first time, InSb:Te monocrystals were grown with the FZM under microgravity. The anomalous behavior of the impurity core (facet effect) in these crystals correlates with the changed magnitude and direction of the quasi-stationary (residual) microaccelerations.     

Robust stationkeeping and reconfiguration of underactuated spacecraft formations

Feasibility of achieving reliable control for spacecraft formation keeping and reconfiguration without the need for thrust in either the radial or along-track direction is explored in this paper. Analysis of the linearized dynamics without along-track input indicates the presence of an uncontrollable eigenvalue at the origin. A nonlinear controller is designed to indirectly stabilize the uncontrollable modes to a stable manifold around the equilibrium. Conditions for robustness against unmatched uncertainties and disturbances are derived to establish the regions of asymptotic stabilization. The benefits of the proposed control method are also validated via numerical simulations to show that precise formation maintenance can be achieved by dealing with the issues of system nonlinearities, variations in initial conditions, and external disturbances, concurrently.     

组合发动机微细通道氦加热器设计与流动换热

氦加热器是预冷组合发动机中重要的换热器之一,其原理是利用燃气燃烧的热量提高氦气做功能力,进而提升整个循环系统运行效率.研究中首先设计了蛇形管式、瓦片式、辐射式三种微细通道氦加热器.其次,基于FLUENT15.0软件对微细通道氦加热器管内外的对流换热系数和流动阻力进行了研究.对比模拟结果和经典关联式的计算结果,确定了适用...     

空间站空空支架天线热设计与仿真分析

为研究核心舱飞行姿态、空间外热流、核心舱发动机羽流参数以及天线外表面热控涂层对空间站空空支架天线温度的综合影响,验证天线被动热控设计的有效性,进行了2种低温工况和6种高温工况的热分析。结果显示：低温工况下,通信天线惯性飞行时的最低温度低于正向飞行时的;展开臂多层表面最低温度为-85 ℃,满足温控指标。高温工况下,通信天线惯性飞行时的温度高于正向飞行时的;轨控发动机的羽流热效应大于偏航发动机的。通信天线内外表面均喷涂ACR-1温控白漆,1倍轨控发动机羽流热流密度时,最高温度为123 ℃,可满足实际使用要求。     

The Impact of Variable Accelerations on Crystal Growth onboard Spacecraft by the Floating Zone Method

The numerical investigation of the impact of time-dependent accelerations (vibrations) on the flow and heat and mass transfer in the melt is carried out for the case of modeling the crystal growth by the floating zone method under conditions of microgravity that exist onboard spacecraft. The effects of the Archimedean buoyancy force and of vibrations of the free surface of fluid are considered separately. When solving the problem of the effect of the vibrations of the free surface of fluid, the previously obtained data were used. It is shown that vibrations of the free surface have a much stronger effect on the processes under consideration than the buoyancy. Some problems that are related to the newly discovered effects are discussed. The use of vibroprotected systems and a rotating magnetic field can help solve these problems. We plan to continue our investigations in future spacecraft experiments, in particular, at the International Space Station, which is under construction at the moment.     

深度变推发动机浮动环工作适应性研究

按照我国载人航天登月需求,正在开展多次起动、10%~100%深度变推力80 k N液氧/甲烷发动机关键技术研究。氧涡轮泵作为发动机的核心组件,采用的浮动环密封变工况工作适应性将直接影响到涡轮泵工作的安全性和可靠性。依据发动机系统变推力要求,采用数值计算方法对浮动环密封进行不同推力工况、不同偏心率条件下的浮起力计算,并与一维方法计算的浮起阻力结果进行对比分析,确定全工况范围内一、二级浮动环工作时偏心率介于0.4~0.6之间,浮动环泄漏量约为2.58~39.4 g/s。浮动环在此偏心率范围内工作可靠性高,可以有效地避免浮动环碰磨、崩边,具备大范围变工况工作能力,满足涡轮泵安全性工作和发动机深度变推力工作要求,可以为涡轮泵方案论证及设计提供理论依据。     

Vibration controlled convection — preparation and perspective of the Maxus 4 experiment

A comparatively new possibility to influence convection in crystal growth melts is the application of controlled interface vibration, especially in systems with free melt surfaces like the float-zone process. The present paper concerns the development and testing of a vibrational device which will be integrated into a microgravity crystal growth facility for the growth of silicon crystals. In the case of silicon grown by the float-zone technique, time-dependent thermocapillary convection is present even under mirogravity and leads to unfavourable variations of the crystal composition profile.The developed setup can operate in the range of approximately 0.1 to 50kHz producing maximum amplitudes of 0.25μm (non resonance case) and 3.5μm (resonance case) respectively. The power consumption is below 5W, the maximum operation temperature of the device is restricted to 200°C, limited by an epoxy-based connection between vibrator and sample. The first microgravity application will be during the European Maxus 4 campaign in April 2001.     

密封舱内漂浮小球运动规律的数值模拟研究

“天宫二号”空间实验室开展了首次人机协同在轨维修技术试验,机械臂操作终端抓取漂浮小球试验为在轨试验任务之一。为研究漂浮小球与机械臂操作终端运动特性之间的匹配性,文章建立了在轨微重力环境下小球运动的物理模型与数学模型,采用动网格方法对不同小球的运动特性进行数值模拟,得到小球的运动规律,由此确定小球的设计参数范围,作为机械臂操作终端系统方案设计的依据。最终,数值模拟结果与“天宫二号”空间实验室在轨试验数据的对比初步验证了该计算结果的正确性。