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At least 109 records · Page 6

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NASA NTRS1982

Wake interference for a heated oscillating cylinder

Penney and Jefferson (1966) have studied heat transfer from an oscillating, horizontal wire. The present investigation has the objective to determine the governing parameters which indicate when interaction between an oscillating circular cylinder and its wake will reduce the apparent heat transfer coefficient in quasi-steady conditions, taking into account, if possible, also the determination of the approximate magnitude of the reduction. A definition is provided of a nondimensional vertical mass flux, representing the induced flow due to heating of the stagnant fluid. It is hypothesized that the flux is related to a natural convection parameter which describes the heating of the wake. For oscillation of a circular cylinder in air under the conditions studied, it is found that the application of a cross-flow correlation in a quasi-steady, transient analysis predicts heat transfer parameters in close agreement with experiment over a certain range, provided interaction with the heated wake is avoided.

Mceligot, D. M.↗

NASA NTRS1982

Gravitationally induced convection during directional solidification of off-eutectic Mn-Bi alloys

The effects of thermal and solute gradient, gravity induced convection during vertical directional solidification, on longitudinal macrosegregation of Bi and Mn rich off-eutectic starting compositions, has been studied as a function of composition, growth velocity and gravity vector orientation. Since the morphology of these alloys is characterized by an aligned, rodlike permanent magnet composite when grown cooperatively, the magnetic properties were used to measure composition segregation and the transition from dendritic to composite growth. Severe macrosegregation was observed in all cases studied and the degree of convection inferred by modeling the observed composition segregation using a stagnant film approach. Morphological stability was found to follow a constitutional supercooling-type law for both Bi and Mn rich compositions.

Pirich, R. G.↗

NASA NTRS1983

Photothermal degradation of ethylene/vinylacetate copolymer

Photothermal degradation studies were conducted on a 'stabilized' formulation of ethylene/vinyl acetate copolymer (EVA) in the temperature range 25-105 C under three different oxygen environments (in open air, with limited access to O2, and in a dark closed stagnant oven). These studies were performed in order to evaluate the utility of EVA as an encapsulation material for photovoltaic modules. Results showed that at low temperature (25 C), slow photooxidation of the polymer occurred via electronic energy transfer involving the UV absorber incorporated in the polymer. However, no changes in the physical properties of the bulk polymer were detected up to 1500 hours of irradiation. At elevated temperatures, leaching and evaporation of the additives occurred, which ultimately resulted in the chemical crosslinking of the copolymer and the formation of volatile photoproducts such as acetic acid.

Liang, R. H.↗

NASA NTRS1984

Mapping of glacial landforms from Seasat radar images

Glacial landforms in the drumlin drift belt of Ireland and the Alaska Range can be identified and mapped from Seasat synthetic-aperture radar (SAR) images. Drumlins cover 60 percent of the Ireland scene. The width/length ratio of individual drumlins can be measured on the SAR images, allowing regional differences in drumlin shape to be mapped. This cannot be done with corresponding Landsat multispectral scanner (MSS) images because of lower spatial resolution and because of shadowing effects that vary seasonally. The Alaska scene shows the extent and nature of morphological features such as medial and lateral moraines, stagnant ice, and fluted ground moraine in glaciated valleys. Perception of these features on corresponding Landsat MSS images is limited by seasonal diffrences in solar illumination. Because SAR is not affected by such differences or by cloud cover, it is particularly well suited for monitoring glacial movement. The disadvantage of distorted high-relief features on Seasat SAR images can be reduced in future SAR systems by modifying the radar illumination geometry.

Ford, J. P.↗

NASA NTRS1985

Determination of convective diffusion heat/mass transfer rates to burner rig test targets comparable in size to cross-stream jet diameter

Two sets of experiments have been performed to be able to predict the convective diffusion heat/mass transfer rates to a cylindrical target whose height and diameter are comparable to, but less than, the diameter of the circular cross-stream jet, thereby simulating the same geometric configuration as a typical burner rig test specimen located in the cross-stream of the combustor exit nozzle. The first set exploits the naphthalene sublimation technique to determine the heat/mass transfer coefficient under isothermal conditions for various flow rates (Reynolds numbers). The second set, conducted at various combustion temperatures and Reynolds numbers, utilized the temperature variation along the surface of the above-mentioned target under steady-state conditions to estimate the effect of cooling (dilution) due to the entrainment of stagnant room temperature air. The experimental information obtained is used to predict high temperature, high velocity corrosive salt vapor deposition rates in burner rigs on collectors that are geometrically the same. The agreement with preliminary data obtained from Na2SO4 vapor deposition experiments is found to be excellent.

Gokoglu, S. A.↗

NASA NTRS1986

Meteorite concentration mechanisms in Antarctica

The location of most Antarctic meteorite finds is on stagnant, highly ablative surfaces known as blue ice. The role of blue ice as transporter, concentrator, and preserver of specimens from the time of fall until find is discussed.

Annexstad, J. O.↗

NASA NTRS1986

Determination of convective diffusion heat/mass transfer rates to burner rig test targets comparable in size to cross-stream jet diameter

Two sets of experiments have been performed to be able to predict the convective diffusion heat/mass transfer rates to a cylindrical target whose height and diameter are comparable to, but less than, the diameter of the circular cross-stream jet, thereby simulating the same geometric configuration as a typical burner rig test specimen located in the cross-stream of the combustor exit nozzlle. The first set exploits the naphthalene sublimation technique to detetermine the heat/mass transfer coefficient under isothermal conditions for various flow rates (Reynolds numbers). The second set, conducted at various combustion temperatures and Reynolds numbers, utilized the temperature variation along the surface of the above-mentioned target under steady-state conditions to estimate the effect of cooling (dilution) due to the entrainment of stagnant room temperature air. The experimental information obtained is used to predict high temperature, high velocity corrosive salt vapor deposition rates in burner rigs on collectors that are geometrically the same. The agreement with preliminary data obtained from Na2S04 vapor deposition experiments is found to be excellent.

Gokoglu, S. A.↗

NASA NTRS1986

Advanced high temperature static strain sensor development

An examination was made into various techniques to be used to measure static strain in gas turbine liners at temperatures up to 1150 K (1600 F). The methods evaluated included thin film and wire resistive devices, optical fibers, surface acoustic waves, the laser speckle technique with a heterodyne readout, optical surface image and reflective approaches and capacitive devices. A preliminary experimental program to develop a thin film capacitive device was dropped because calculations showed that it would be too sensitive to thermal gradients. In a final evaluation program, the laser speckle technique appeared to work well up to 1150 K when it was used through a relatively stagnant air path. The surface guided acoustic wave approach appeared to be interesting but to require too much development effort for the funds available. Efforts to develop a FeCrAl resistive strain gage system were only partially successful and this part of the effort was finally reduced to a characterization study of the properties of the 25 micron diameter FeCrAl (Kanthal A-1) wire. It was concluded that this particular alloy was not suitable for use as the resistive element in a strain gage above about 1000 K.

Hulse, C. O.↗

NASA NTRS1987

On the 3-dimensional structure of plasmoids

The hypothesis that the IMF penetrates plasmoids causing them to be three- rather than two-dimensional is tested by comparing observations of By within plasmoids and related tail structures to upstream IMF By data. The magnetic topologies that result from the mergings of closed plasma sheet flux tubes and open tail lobe flux tubes at a near-earth neutral line, and merging near the tail flanks are described and studied. The particle signals and isotropic electron distributions are examined. It is observed that the IMF By penetrates plasmoids and that their structure is three-dimensional. In the three-dimensional model of plasmoids the reconnected plasma sheet field lines form a magnetic flux-ropelike structure. The three-dimensional model is utilized to analyze stagnant, slowly moving and earthward moving structures.

Hughes, W. J.↗

NASA NTRS1987

Ion energetics in the inner coma of Comet Halley

The cometary plasma in the magnetic barrier just outside the diamagnetic cavity which surrounds the nucleus of Comet Halley is virtually stagnant. The outflowing neutral gas exerts an outward ion-neutral drag force on this plasma, which balances the inward magnetic pressure gradient force in the vicinity of the contact surface. The cometary ions are frictionally heated due to the relative motion of the ion and neutral gases. The ion flow velocity must have a few km/s nonradial component in order to explain the ion temperatures measured by the ion mass spectrometer on Giotto.

Cravens, T. E.↗

NASA NTRS1988

Numerical simulations of thermal-chemical instabilities at the core-mantle boundary

Numerical simulations of thermal-chemical instabilities in the D-double-prime layer at the core-mantle boundary are presented which show that strong lateral heterogeneities in the composition and density fields can be initiated and maintained dynamically if there is continuous replenishment of material from subduced slabs coming from the upper mantle. These chemical instabilities have a tendency to migrate laterally and may help to support core-mantle boundary topography with short and long wavelengths. The thermal-chemical flows produce a relatively stagnant D-double-prime layer with strong lateral and temporal variations in basal heat flux, which gives rise to thermal core-mantle interactions influencing the geodynamo.

Hansen, Ulrich↗

NASA NTRS1989

Three component laser Doppler measurements in an axisymmetric jet

A three-component laser Doppler anemometer (LDA) was used to acquire a detailed set of three-dimensional mean and fluctuating velocity measurements in a low-speed air jet entering a stagnant ambient, over the first 20 jet exit diameters along the jet trajectory. These data are physically consistent with previous measurements in axisymmetric jets. The relative difficulty of obtaining three-dimensional and two-dimensional LDA data is briefly discussed.

Kuhlmann, John M.↗

NASA NTRS1990

Three component velocity measurements in an axisymmetric jet using LDA

A commercial three-component laser Doppler anemometer (LDA) has been used to acquire a detailed set of three-dimensional mean and fluctuating velocity measurements in a low-speed air jet entering a stagnant ambient, over the first 15 jet exit diameters along the jet trajectory. These data are consistent with previous measurements in axisymmetric, turbulent jets. Mean velocity and Reynolds stress data approach a self-preserving behavior by x/D equal to about 15. However, the RMS turbulence fluctuations were not self-preserving at this axial location, as expected based upon previous experimental studies. These data confirm the ability to obtain reliable three-dimensional velocity data using the present three-dimensional LDA system.

Kuhlman, John M.↗

NASA NTRS1991

Liquid-propellant droplet vaporization and combustion in high pressure environments

In order to correct the deficiencies of existing models for high-pressure droplet vaporization and combustion, a fundamental investigation into this matter is essential. The objective of this research are: (1) to acquire basic understanding of physical and chemical mechanisms involved in the vaporization and combustion of isolated liquid-propellant droplets in both stagnant and forced-convective environments; (2) to establish droplet vaporization and combustion correlations for the study of liquid-propellant spray combustion and two-phase flowfields in rocket motors; and (3) to investigate the dynamic responses of multicomponent droplet vaporization and combustion to ambient flow oscillations.

Yang, Vigor↗

NASA NTRS1990

Plasma sheet convection and the stability of the magnetotail

Particle simulations are used to investigate the effects of plasma sheet convection into regions of increasing tail lobe magnetic field strength on the stability of a magnetotail equilibrium. The self-consistent treatment of convection first drives B(z) to zero on axis in the region of the strongest lobe field B(x) and then causes the equilibrium to break due to the rapid growth of tearing modes driven by the induced temperature anisotropy. The net result is the evolution of the inward-convecting plasma sheet into a slowly moving or stagnant plasmoid. The time scale for this process is much more rapid than that associated with particle drift losses across the tail. These results support the suggestion that steady convection within the plasma sheet may not be possible.

Pritchett, P. L.↗

NASA NTRS1991

Troposphere-stratosphere interactions in a one-dimensional model of Jovian photochemistry

The case of a chemically unreactive species flowing downward through the stratosphere and troposphere with a constant flux is presently treated by a one-dimensional Jovian atmosphere model which encompasses the coupling between a rapidly mixed troposphere and a stagnant stratosphere, so that the contrast between the peak stratosphere and tropopause concentrations is reflective of the variation between the lower stratosphere and upper troposphere eddy diffusion coefficients. Numerical simulations of unreactive CO and C2H6 species indicate that upper troposphere abundances may possess a substantial photochemical contribution. The implications of these modeling results for higher spectral resolution observations are noted.

Landry, Bridget↗

NASA NTRS1990

Keyhole and weld shapes for plasma arc welding under normal and zero gravity

A first order study of the interfacial (keyhole) shape between a penetrating argon plasma arc jet and a stationary liquid metal weld pool is presented. The interface is determined using the Young-Laplace equation by assuming that the plasma jet behaves as a one-dimensional ideal gas flow and by neglecting flow within the weld pool. The solution for the keyhole shape allows an approximate determination of the liquid-solid metal phase boundary location based on the assumption that the liquid melt is a stagnant thermal boundary layer. Parametric studies examine the effect of plasma mass flow rate, initial plasma enthalpy, liquid metal surface tension, and jet shear on weldment shape under both normal and zero gravity. Among the more important findings of this study is that keyhole and weld geometries are minimally affected by gravity, suggesting that data gathered under gravity can be used in planning in-space welding.

Keanini, R. G.↗

NASA NTRS1991

Thermal and solutal conditions at the tips of a directional dendritic growth front

The line-of-sight averaged, time-dependent dendrite tip concentrations for the diffusion dominated vertical directional solidification of a metal model (ammonium chloride and water) were obtained by extrapolating exponentially fit diffusion layer profiles measured using a laser interferometer. The tip concentrations were shown to increase linearly with time throughout the diffusion dominated growth process for an initially stagnant dendritic array. The process was terminated for the cases chosen by convective breakdown suffered when the conditionally stable diffusion layer exceeded the critical Rayleigh criteria. The transient tip concentrations were determined to significantly exceed the values predicted for steady state, thus producing much larger constitutional undercoolings. This has ramifications for growth speeds, arm spacings and the dendritic structure itself.

Mccay, T. D.↗