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At least 199 records · Page 11

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NASA NTRS1976

Does Mercury have a molten core

The question of whether or not Mercury could contain a molten metallic core is investigated by studying the possible thermal evolution of a metallic core in that planet. The calculations involve the solution of the equation of heat conduction for a spherically symmetric body with internal heat sources, modifications to take account of the latent heat of fusion as well as the redistribution of radioactive heat sources as a consequence of melting, the terrestrial Fe/U ratio, and a Th/U ratio of 3.7. The temperature profile predicted by the calculations for a period of 4.6 billion years indicates that the inner 1400 km of the core would now be solid while the outer 500 km would be molten. It is emphasized that this result is a direct consequence of a discontinuity in melting temperatures at the core-mantle boundary and that although a dynamo is possible, it would have to be driven mechanically rather than by thermal convection.

Fricker, P. E.↗

NASA NTRS1976

Heat Transfer and Fluid Mechanics Institute, Meeting, 25th, University of California, Davis, Calif., June 21-23, 1976, Proceedings

Major research advances in heat transfer and fluid dynamics are outlined, with particular reference to relevant energy problems. Of significant importance are such topics as synthetic fuels in combustion, turbulence models, combustion modeling, numerical methods for interacting boundary layers, and light-scattering diagnostics for gases. The discussion covers thermal convection, two-phase flow and boiling heat transfer, turbulent flows, combustion, and aerospace heat transfer problems. Other areas discussed include compressible flows, fluid mechanics and drag, and heat exchangers. Featured topics comprise heat and salt transfer in double-diffusive systems, limits of boiling heat transfer in a liquid-filled enclosure, investigation of buoyancy-induced flow stratification in a cylindrical plenum, and digital algorithms for dynamic analysis of a heat exchanger. Individual items are announced in this issue.

Mckillop, A. A.↗

NASA NTRS1976

Metal melting, joining, and alloying under weightlessness

During the Skylab mission experiments were conducted concerning the influence of weightlessness on molten metals and the solidification process. It was found that the absence of gravity-induced thermal convection and sedimentation modified the properties of the bulk melt. A metals melting experiment was conducted to study the behavior of metal melted by an electron beam and to evaluate the feasibility of joining metals in space. An exothermic brazing experiment investigated the spreading, mixing, and capillary flow of molten braze material. The containerless melting and solidification of small samples of several metals was studied in another experiment. A description is also given of a study of the directional solidification of solid solution alloys.

Snyder, R. S.↗

NASA NTRS1977

A survey of lunar rock types and comparison of the crusts of earth and moon

The principal known types of lunar rocks are briefly reviewed, and their chemical relationships discussed. In the suite of low-KREEP highland rocks, Fe/(Fe + Mg) in the normative mafic minerals increases and the albite content of normative plagio-clase decreases as the total amount of normative plagioclase increases, the opposite of the trend predicted by the Bowen reaction principle. The distribution of compositions of rocks from terrestrial layered mafic intrusives is substantially different: here the analyses fall in several discrete clusters (anorthositic rocks, norites, granophyres and ferrogabbros, ultramafics), and the chemical trends noted above are not reproduced. It is suggested that the observed trends in lunar highland rocks could be produced by crystal fractionation in a deep global surface magma system if (1) plagiociase tended to float, upon crystallization, and (2) the magma was kept agitated and well mixed (probably by thermal convection) until crystallization was far advanced and relatively little residual liquid was left. After the crustal system solidified, but before extensive cooling had developed a thick, strong lithosphere, mantle convection was able to draw portions of the lunar anorthositic crust down into the mantle.

Wood, J. A.↗

NASA NTRS1979

Investigation of the free flow electrophoretic process. Volume 1: Executive summary

Weiss, R. A.↗

NASA NTRS1979

Investigation of the free flow electrophoretic process. Volume 2: Technical analysis

The effect of gravity on the free flow electrophoretic process was investigated. The demonstrated effects were then compared with predictions made by mathematical models. Results show that the carrier buffer flow was affected by gravity induced thermal convection and that the movement of the separating particle streams was affected by gravity induced buoyant forces. It was determined that if gravity induced buoyant forces were included in the mathematical models, then effective predictions of electrophoresis chamber separation performance were possible. The results of tests performed using various methods of electrophoresis using supportive media show that the mobility and the ability to separate were essentially independent of concentration, providing promise of being able to perform electrophoresis with higher inlet concentrations in space.

Weiss, R. A.↗

NASA NTRS1983

Magnetism and thermal evolution of the terrestrial planets

The absence in the cases of Venus and Mars of the substantial intrinsic magnetic fields of the earth and Mercury is considered, in light of thermal history calculations which suggest that, while the cores of Mercury and the earth are continuing to freeze, the cores of Venus and Mars may still be completely liquid. It is noted that completely fluid cores, lacking intrinsic heat sources, are not likely to sustain thermal convection for the age of the solar system, but cool to a subadiabatic, conductive state that cannot maintain a dynamo because of the gravitational energy release and the chemically driven convection that accompany inner core growth. The models presented include realistic pressure- and composition-dependent freezing curves for the core, and material parameters are chosen so that correct present-day values of heat outflow, upper mantle temperature and viscosity, and inner core radius, are obtained for the earth.

Stevenson, D. J.↗

NASA NTRS1983

Terrestrial whisker growth experiments which anticipate some special effects of a space station environment

The effects of the absence of gravitationally driven thermal convection on the growth of whiskers by chemical reduction of metal salts was studied. It was possible to accomplish nearly complete suppression of such convection. Suppression of the convection does indeed effect the growth but in subtle, not necessarily detrimental ways: none of the changes observed were such as to hamper efforts to produce whiskers in space. Copper whiskers grown from cuprous iodide respond most positively to the suppression of convection; therefore, they are strongly recommended for tests in the space environment. Cobalt whiskers grown from cobaltous bromide show the greatest independence from conditions of convection and applied electric fields of any material studied; therefore, this medium is highly recommended. A strong pulse of electric field forces the whiskers to stick to the growth vessel top plate, this facilitates study or "harvesting'. On the space station it is recommended that the growth vessels be mounted outside the laboratory and joined with the station by means of double vacuum valves and gas service lines.

Hobbs, H. H.↗

NASA NTRS1984

Analysis and calculation of macrosegregation in a casting ingot, exhibits C and E

A computer model which describes the solidification of a binary metal alloy in an insulated rectangular mold with a temperature gradient is presented. A numerical technique, applicable to a broad class of moving boundary problems, was implemented therein. The solidification model described is used to calculate the macrosegregation within the solidified casting by coupling the equations for liquid flow in the solid/liquid or mushy zone with the energy equation for heat flow throughout the ingot and thermal convection in the bulk liquid portion. The rate of development of the solid can be automatically calculated by the model. Numerical analysis of such solidification parameters as enthalpy and boundary layer flow is displayed. On-line user interface and software documentation are presented.

Poirier, D. R.↗

NASA NTRS1983

Viscosity of rock-ice mixtures and applications to the evolution of icy satellites

Theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix. A rheological model for rock-ice mixtures is described, establishing bounds for the range of possible viscosity enhancements provided by a suspension of silicate spheres in an ice matrix. A parametrized thermal convection model is described and used to determine a criterion for criticality, defined as the heat flow and/or silicate volume fraction for which the satellite temperature profile intercepts the melting curve of water ice. The consequences of achieving this critical state are examined, and it is shown that under certain circumstances a 'runaway' differentiation can occur in which the silicates settle to form a core and extensive melting of water ice takes place, the latent heat being supplied by the gravitational energy of differentiation. A possible application of these results to Ganymede and Callisto is described.

Friedson, A. J.↗

NASA NTRS1985

A calculation of the Rosseland mean opacity of dust grains in primordial solar system nebulae

The Rosseland mean opacity owing to grains was calculated as a function of temperature and density for nebulae having solar elemental abundances. The values of the mean opacity were evaluated with a generalized formulation allowing for anisotropic scattering. The values of the mean opacity do not depend sensitively on the choice of the particle size distribution function, provided that there are few particles having sizes in excess of several tens of microns. The results indicate that thermal convection in primordial nebulae occurs over broader ranges of altitudes at low temperatures than at high temperatures, and for size distributions for which extensive aggregation has not yet occurred.

Pollack, J. B.↗

NASA NTRS1986

Quasi-geostrophic free mode models of long-lived Jovian eddies: Forcing mechanisms and crucial observational tests

Observations of Jupiter and Saturn long-lived eddies, such as Jupiter's Great Red Spot and White Ovals, are presently compared with laboratory experiments and corresponding numerical simulations for free thermal convection in a rotating fluid that is subject to horizontal differential heating and cooling. Difficulties in determining the essential processes maintaining and dissipating stable eddies, on the basis of global energy budget studies, are discussed; such difficulties do not arise in considerations of the flow's potential vorticity budget. On Jupiter, diabatically forced and transient eddy-driven flows primarily differ in the implied role of transient eddies in transporting potential vorticity across closed geostrophic streamlines in the time mean.

Read, P. L.↗

NASA NTRS1985

Automation of analytical isotachophoresis

The basic features of automation of analytical isotachophoresis (ITP) are reviewed. Experimental setups consisting of narrow bore tubes which are self-stabilized against thermal convection are considered. Sample detection in free solution is discussed, listing the detector systems presently used or expected to be of potential use in the near future. The combination of a universal detector measuring the evolution of ITP zone structures with detector systems specific to desired components is proposed as a concept of an automated chemical analyzer based on ITP. Possible miniaturization of such an instrument by means of microlithographic techniques is discussed.

Thormann, Wolfgang↗

NASA NTRS1988

Tharsis block tectonics on Mars

The concept of block tectonics provides a framework for understanding many aspects of Tharsis and adjoining structures. This Tharsis block tectonics on Mars is manifested partly by mantle-related doming and partly by response to loading by subsequent volcanic construction. Although the origin of the volcanism from beneath Tharsis is a subject of controversy explanations have to include inhomogeneities in Martian internal structure, energy distribution, magma accumulation and motion below the lithosphere. Thermal convection can be seen as a necessary consequence for transient initial phase of Martian cooling. This produced part of the elevated topography with tensional stresses and graben systems radial to the main bulge. The linear grabens, radial to the Tharsis center, can be interpreted to indicate rift zones that define the crustal block boundaries. The load-induced stresses may then have contributed on further graben and ridge formation over an extended period of time.

Raitala, Jouko T.↗

NASA NTRS1989

Time-dependent computational studies of flames in microgravity

The research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of the NASA Microgravity Science and Applications Program is described. The primary focus was on investigating fundamental questions concerning the propagation and extinction of premixed flames in Earth gravity and in microgravity environments. The approach was to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. A combination of one-dimensional and two-dimensional simulations was used to investigate the effects of curvature and dilution on ignition and propagation of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the mechanism leading to cellular instability, and to study the effects of gravity on the transition to cellular structure. A flame in a microgravity environment can be extinguished without external losses, and the mechanism leading to cellular structure is not preferential diffusion but a thermo-diffusive instability. The simulations have also lead to a better understanding of the interactions between buoyancy forces and the processes leading to thermo-diffusive instability.

Oran, Elaine S.↗

NASA NTRS1990

Chemical heterogeneity and imperfect mixing in the solar nebula

The 'standard' accretion-disk model of the solar nebula is examined together with its basic equations, which are solved for the steady-state case. It is shown that, for this model of the solar nebula, the efficiency of mixing the products of thermochemical processing from small to large disk radii depends not on the magnitude of the eddy diffusivity but only on the ratio k between the eddy diffusivity and the eddy viscosity. In most cases, this ratio k is probably of order of unity or less, implying that most of the solar nebula was not contaminated by the consequences of thermochemical equilibria that were established at 'small' disk radii. It is noted that, while this condition is almost certainly satisfied if the physical process responsible for viscosity is thermal convection or waves, it has uncertain validity during the early phases of disk evolution where accretion-induced shear instabilities may dominate.

Stevenson, David J.↗

NASA NTRS1991

Initiating Growth Of Crystals Away From Container Walls

Nucleation controlled to obtain better crystals. In technique proposed specifically for growing large protein crystals in microgravity (where no thermal convection), small region of high supersaturation created by injection of hot concentrated solution or by use of cold probe. Crystals nucleate preferably in this small region. Also conceivable technique applied on Earth to crystallizations in melts and solutions sufficiently viscous to suppress convection to extent necessary to prevent cooling-induced nucleation in undesired sites.

Kroes, Roger L.↗

NASA NTRS1990

Microgravity laboratory simulations of planetary and stellar circulations

This paper summarizes experiments on thermal convection in a rotating differentially heated spherical shell of liquid that were carried out in the microgravity laboratory of Spacelab III. In these experiments, dielectric polarization forces were used to generate a radially-directed buoyancy force. This enabled the simulation of a number of geophysically and astrophysically important situations in which sphericity and rotation both impose strong constraints on the global scale fluid dynamics. Recent theoretical and numerical interpretations of the original data are discussed. Further laboratory experiments in the areas of geophysical and g-jitter fluid dynamics are suggested.

Hart, John E.↗