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 fioatmg 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 m 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.

Impulsive motions of the floating zone

A set of problems concerning the behaviour of a suddenly disturbed ideal floating zone is considered. Mathematical techniques of asymptotic expansions arc used to solve these problems. It is seen that many already available solutions, most of them concerning liquids enclosed in cavities, will be regarded as starting approximations which are valid except in the proximity of the free surface which laterally bounds the floating zone. In particular, the problem of the linear spin-up of an initially cylindrical floating zone is considered in some detail. The presence of a recircuiating fluid pattern near the free surface is detected. This configuration is attributed to the interplay between Coriolis forces and the azimuthal component of the viscous forces.

A regular perturbation approach to surface tension driven flows

Surface tension induced convection in a liquid bridge held between two parallel, coaxial, solid disks is considered. The surface tension gradient is produced by a small temperature gradient parallel Co the undisturbed surface. The study is performed by using a mathematical regular perturbation approach based on a small parameter, e, which measures the deviation of the imposed temperature field from its mean value. The first order velocity field is given by a Stokes-type problem (viscous terms are dominant) with relatively simple boundary conditions. The first order temperature field is that imposed from the end disks on a liquid bridge immersed in a non-conductive fluid. Radiative effects are supposed to be negligible. The second order temperature field, which accounts for convective effects, is split into three components, one due to the bulk motion, and the other two to the distortion of the free surface. The relative importance of these components in terms of the heat transfer to or from the end disks is assessed

Laboratorio en el espacio - Laboratorio en la tierra

Este trabajo presenta ciertos resultados recientes que están relacionados con el equilibrio dinámico de puentes líquidos mantenidos entre discos solidos paralelos y coaxiales. Estos estudios tienen por objeto analizar los experimentos realizados a bordo del spacelab en diciembre de 1983. Se ha desarrollado un programa muy activo en tierra para simular, tanto numérica como experimentalmente, los efectos de la gravedad reducida en configuraciones capilares. La simulación con la técnica de plateau permite obtener en muchos casos resultados extremadamente precisos, siempre que se ponga un gran cuidado en gobernar la densidad del liquido soporte. Aunque las aplicaciones comerciales de estas investigaciones están lejos, su contenido científico justifica trabajar mas todavía. En el caso particular que consideramos, el puente liquido es la configuración mas sencilla, voluminosa y gobernable limitada por una superficie libre no plana, y esto es en sí mismo científicamente interesante. La competencia comercial con desarrollos paralelos en tierra será desfavorable para los procesos espaciales mientras no aumenten las oportunidades de vuelo.

Floating liquid zones

The aim of our experiment on the Spacelab Dl Mission was to study the stability of long liquid columns under microgravity. Nominal configuration was a cylindrical liquid bridge anchored at the edges of two equal solid discs, 35 mm in diameter. Mechanical stimuli were applied through the discs and the liquid outer shape recorded for analysis. Nominal experiment procedures [1] were similar to those proposed for Spacelab-1 (1983), where by wetting problems allowed only partial success [2]. The same Fluid Physics Module, but with corrected end discs and a manually operated syringe for liquid injection, was used.

Condensation in ducts

Phase changing flows are being considered for thermal management in space platforms. The resulting flow patterns are very complicated and extremely sensitive to gravity action. Concerning fluid flow in ducts, the available evidence indicates that although the pressure loss does not depend too much on the fluid flow pattern,the heat transfer (and resulting phase change) does. A simple exercise to illustrate this point is presented in this paper. It deals with condensing flow in straight circular cross-sectional ducts. Two extreme configurations are considered here, one corresponds to a stratified flow and the other to an annular flow. Both types of flow patterns have been extensively considered in the past and from this point of view almost nothing is new in the paper, but past results look conflictive and this could be due to the limitations and computational intricacies of the models used. Thus the problem has been reformulated from the onset and the results are presented as the evolution of the vapor quality (vapor to total mass flow rate) along the duct, in typical cases. The results presented here indicate that within the validity of the present models and the assumed ranges of mass flow rate, duct diameter, thermal conditions and fluid characteristics,the length of the ducts required to achieve complete condensation under zero gravity are an order of magnitude larger than in horizontal tubes under normal terrestrial conditions.

Aerodinámica

La Aerodinámica estudia las fuerzas que un viento relativo ejerce sobre un obstáculo sumergido en él. Desde este punto de vista tan general las aplicaciones de la Aerodinámica no se reducen exclusivamente al estudio de las fuerzas sobre aviones más o menos clásicos sino que trascienden a otros muchos campos por el efecto de arrastre que es característico de ciencias y técnicas avanzadas. La conferencia, dentro de un ciclo sobre Ingeniería Aeroespacial, se centra en la aerodinámica de objetos volantes producidos por el hombre, pero es de temer que el interés del conferenciante por otras aplicaciones se deje ver demasiado. El tema es de una gran extensión y, sobre todo, una gran complicación. Se ha repetido hasta la saciedad que para su estudio es más útil la experimentación que el raciocinio (Sir George Cayley (1733-1857).

Spacecraft thermal control design data

This paper shows the actual state of a compilation work on Thermal Control Design Data being done at Madrid (Lamf-ETSIA) under several ESTEC contracts, introducing a Handbook already issued, its additions and updatings.

Floating zone stability

The present state of de preparation of an experiment on floating liquid zones to be performed in the first Spacelab flight is presented. In this experiment,a liquid bridge is to be placed between two parallel coaxial disks (in the Fluid Physics Module)and subjected to very precise disturbances in order to check the theoretical predictions about its stability limits and behavior under mechanical inputs: stretching of the zone, filling or removing the liquid,axial vibration, rotation, disalignment, etc. Several aspects of the research are introduced:1) Relevance of the study. 2) Theoretical predictions of the liquid behavior regarding the floating-zone stability limits and the expected response to vibrational and rotational disturbances. 3) Ground support experiments using the Plateau technique or the small scale simulation. 4) Instrumental aspects of the experimentation: the Fluid Physics Module utilization and post-flight data analysis.5)Research program for future flights.

A new boundary condition solved with B.I.E.M.

Among the classical operators of mathematical physics the Laplacian plays an important role due to the number of different situations that can be modelled by it. Because of this a great effort has been made by mathematicians as well as by engineers to master its properties till the point that nearly everything has been said about them from a qualitative viewpoint. Quantitative results have also been obtained through the use of the new numerical techniques sustained by the computer. Finite element methods and boundary techniques have been successfully applied to engineering problems as can be seen in the technical literature (for instance [ l ] , [2], [3] . Boundary techniques are especially advantageous in those cases in which the main interest is concentrated on what is happening at the boundary. This situation is very usual in potential problems due to the properties of harmonic functions. In this paper we intend to show how a boundary condition different from the classical, but physically sound, is introduced without any violence in the discretization frame of the Boundary Integral Equation Method. The idea will be developed in the context of heat conduction in axisymmetric problems but it is hoped that its extension to other situations is straightforward. After the presentation of the method several examples will show the capabilities of modelling a physical problem.

Conservation laws for liquid bridges

Conservation laws for an inviscid liquid bridge set into motion by conservative forces are given in integral form. These laws provide useful information on the overall motion of the bridge in the presence of unexpected or uncontrolled disturbances and could, in addition, be monitored in a computational solution of the problem as an accuracy check. Many of the resulting conservation laws are familiar to fluiddynamicists. Nevertheless, a systematic approach providing an exhaustive list of these laws reveals the existence of new conserved properties hardly deducible in the classical way. Although the present analysis concerns the case of axial, and constant, gravity it can be applied, with minor refinements, when the gravity field varies with time in both direction and intensity.

Some physical and chemical proceddes in fluids

The ability to reproduce reduced gravity conditions for long periods is one of the reasons why the orbiting laboratory is so attractive. In this paper several fluid dynamics problem areas are reviewed in which zero-gravity conditions are of great importance. Although emphasis is placed on space processing, there are some older problems also in which gravity masks the phenomcna, impeding a reasonably simple approach to the solution. Three problems are considered: Thermal convection under reduced gravity. The dumping effect ofsurface gravity waves at the outset of convection induced by surface tractions is discussed in particular. The existence of convection is of concern for some satellite thermal control techniques presently used, and for most of the proposed manufacturing processes. Whereas convection should be normally avoided, problems related to the containerless stirring ofa melt constitute an exception. Secondly, gravity and chemical reactions. Although chemical reactions are independent of gravity because of the small mass of the molecules and atoms involved, in many cases the reaction rate dcpends on the arrival of the species to the reaction zone. When the arrival process is buoyancy-controlled, the net specd of the reaction will be affected by the gravity. Thirdly, two-phase flows under reduced gravity provkle interesting problems from boiling heat transfer to degasslng of melts. This part of the paper deals only with the measurement of sound veiocity in a liquid containing bubbles. It is suggested that such measurements should be mude under reduced gravity to provide reliable residís.

Fluid-physics-module experiments

Some floating-liquid-zone experiments performed under reduced-gravity conditions are reviewed. Several types of instabilities are discussed, together with the relevant parameters controlling them. It is shown that the bounding values of these parameters could be increased, by orders of magnitude in several instances, by selecting appropriate liquids. Two of the many problems that a Fluid-Physics Module, devised to perform experiments on floating zones in a space laboratory, would involve are discussed: namely (i) procedures for disturbing the zoneunder controlled conditions, and (ii) visualisation of the inner flow pattern. Several topics connected with the nonisothermal nature and the phase-changes of floating zones are presented. In particular, a mode of propagation through the liquid zone for disturbances which could appear in the melting solid/liquid interface is suggested. Although most research on floating liquid zones is aimed at improving the crystal-growth process, some additional applications are suggested.