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At least 775 records · Page 43

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NASA NTRS1975

Diffusion effects on the line intensities of He I and He II in the solar transition region

A heuristic treatment of diffusion in the solar chromosphere-corona transition region is developed. It is shown that diffusion becomes increasingly important with steeper temperature gradients, in active and quiet regions relative to coronal holes, and with increasing excitation potential. Numerical calculations are made for the resonance lines of He I and He II and show that diffusion can enhance these lines. Thus the helium lines may appear relatively weak in coronal holes due to a weakening of the enhancement mechanism. Most transition region lines will be less affected by diffusion than He I or He II.

Shine, R.↗

NASA NTRS1975

The effect of gas diffusion on the flow coefficient for a ventilated cavity

The results of an experimental investigation into the effect of gas diffusion on the volume flow-rate of gas needed to sustain a ventilated cavity are presented. Gas diffusion was found to have a significant effect on the ventilated flow rate required to sustain a cavity of a given size. An analysis for the gas diffusion effect was conducted based on a mathematical model of diffusion proposed by Brennen. The results compare favorably with experimental data. Also, an empirical scaling relationship is proposed for ventilated cavity flows.

Billet, M. L.↗

NASA NTRS1976

The absorption of trapped particles by the inner satellites of Jupiter and the radial diffusion coefficient of particle transport

The process of trapped particle absorption by the inner Jovian satellites is considered in detail taking into account both the particle and satellite motions in a magnetic dipole field which is displaced from the center of the planet and tilted with respect to the planetary rotation axis. An expression is derived for computing the sweeping time at a given satellite, defined as the time required for the satellite to sweep up a given fraction of the trapped particles within its sweeping region. By making use of the sweeping time and the radial diffusion equation of particle transport approximate expressions for the diffusion coefficient are derived. Measurements obtained by Pioneer 10 are then used to obtain estimates of the diffusion coefficient at the orbits of Io and Europa. We find that the diffusion coefficient is a function of energy and magnetic latitude for electrons in the energy range 0.7-14 MeV.

Mogro-Campero, A.↗

NASA NTRS1977

Reaction diffusion in the nickel-chromium-aluminum and cobalt-chromium-aluminum systems

The effects of MCrAl coating-substrate interdiffusion on oxidation life and the general mutliphase, multicomponent diffusion problem were examined. Semi-infinite diffusion couples that had sources representing coatings and sinks representing gas turbine alloys were annealed at 1,000, 1,095, 1,150, or 1,205 C for as long as 500 hours. The source and sink aluminum and chromium contents and the base metal (cobalt or nickel) determined the parabolic diffusion rate constants of the couples and predicted finite coating lives. The beta source strength concept provided a method (1) for correlating beta recession rate constants with composition; (2) for determining reliable average total, diffusion, and constitutional activation energies; and (3) for calculating interdiffusion coefficients.

Levine, S. R.↗

NASA NTRS1977

Performance of high-area-ratio annular dump diffuser using suction-stabilized-vortex flow control

A short annular dump diffuser having a geometry conductive to formation of suction stabilized toroidal vortices in the region of abrupt area change was tested. The overall diffuser area ratio was 4.0 and the length to inlet height ratio was 2.0. Performance data were obtained at near ambient temperature and pressure for inlet Mach numbers of 0.18 and 0.30 with suction rates ranging from 0 to 18 percent of total inlet mass flowrate. Results show that the exit velocity profile could be readily biased toward either wall by adjustment of inner and outer wall suction rates. Symmetric exit velocity profiles were inherently unstable with a tendency to revert to a hub or tip bias. Diffuser effectiveness was increased from about 38 percent without suction to over 85 percent at a total suction rate of 10 to 12 percent. At the same time diffuser total pressure loss was reduced from 3.1 percent to 1.1 percent at an inlet Mach number of 0.3.

Juhasz, A. J.↗

NASA NTRS1977

Measurement of insulation integrity of IUE camera tube facsimiles by partial discharges method and diffusion of gases through various silicone rubbers

Several dummy tubes imitating the IUE Camera System design were encapsulated with Solithane 2, Conathane EN-11, Green and Black Hysols and SMRD 432. Various flaws were purposefully placed in some of these. Partial discharge testing in vacuum under direct voltage conditions was carried once a week for 12 weeks, 15 kv dc being applied during normal working hours for 40 hours duration per week. None of the units showed much damage during this time judging by the P.D. energy histograms. A more complete mathematical presentation is given on diffusion and permeation than previously. Measurements of diffusion constants for various silicone rubbers are carried out by the Time-Lag method and compared to other determinations in the literature. Calculations of the time required for diffusion through a thick wall are demonstrated in the long time approximation and for dimensions pertaining to void and wall sizes of a delamination problem in the LANDSAT-C vidicon tubes. An actual delaminated LANDSAT-C tube and some facsimiles are immersed in vacuum for long periods and tested for catastrophic breakdown due to diffusion of gas, by application of high voltage.

Bever, R. S.↗

NASA NTRS1977

Variations in eddy diffusion and associated transport processes

Variation in the eddy diffusion coefficient constitutes a momentum source. This variation near the turbopause affects both the composition and the wind and temperature fields of the thermosphere. Two types of calculations are carried out, for horizontal winds considered self-consistently in a three-dimensional dynamic model and horizontal winds artificially suppressed to simulate a one-dimensional model. Momentum transfer associated with variations in eddy diffusion coefficients represents the source function in the analysis presented. It is shown that for long-term variations in the thermosphere, the compositional dependence on variations in eddy diffusion coefficients can be entirely different in one and three dimensional models with horizontal winds included. Horizontal winds can greatly reduce the effects of eddy diffusion in general and the ratio between He and O amplitudes in particular.

Mayr, H. G.↗

NASA NTRS1976

An investigation of impurity redistribution in diffused layer on /n+/-p silicon solar cells parameters

Recent calculations by Wang et al. (1974) have shown that the short-circuit current for silicon solar cells is sensitive to the impurity distribution in the diffused and base region. In the present work, these theoretical results were verified by experimentally investigating the impurity profile and sheet resistance of the diffused layers of (n+)-p silicon solar cells fabricated by normal diffusion processes and at subsequently various drive-in times. Impurity profiles of the n+ diffused layer, measured by differential Hall and resistivity technique, resemble the erfc distribution. The junction depth is larger as drive-in time increases. Measured short-circuit current is in agreement with calculated ones. Open-circuit voltage is independent of drive-in times, and its calculated values are consistent with capacitance-voltage measurements.

Wang, E. Y.↗

NASA NTRS1977

Consequences of using nonlinear particle trajectories to compute spatial diffusion coefficients

In a study of cosmic ray propagation in interstellar and interplanetary space, a perturbed orbit resonant scattering theory for pitch angle diffusion in a slab model of magnetostatic turbulence is slightly generalized and used to compute the diffusion coefficient for spatial propagation parallel to the mean magnetic field. This diffusion coefficient has been useful for describing the solar modulation of the galactic cosmic rays, and for explaining the diffusive phase in solar flares in which the initial anisotropy of the particle distribution decays to isotropy.

Goldstein, M. L.↗

NASA NTRS1977

Elevated-temperature flow strength, creep resistance and diffusion welding characteristics of Ti-gAl-2Nb-1Ta-0.8Mo

A study of the flow strength, creep resistance and diffusion welding characteristics of the titanium alloy Ti-6Al-2Nb-1Ta-0.8Mo was conducted. Two mill-processed forms of this alloy were examined. The forged material was essentially processed above the beta transus while the rolled form was subjected to considerable work below the beta transus. Between 1150 and 1250 K, the forged material was stronger and more creep resistant than the rolled alloy. Both forms exhibit superplastic characteristics in this temperature range. Strain measurements during diffusion welding experiments at 1200 K reveal that weld interfaces have no measurable effect on the overall creep deformation. Significant deformation appears to be necessary to produce a quality diffusion weld between superplastic materials. A 'soft' interlayer inserted between faying surfaces would seemingly allow manufacture of quality diffusion welds with little overall deformation.

Whittenberger, J. D.↗

NASA NTRS1977

Comments on the origins of the diffuse interstellar bands

Most work in recent years on the unidentified diffuse interstellar bands has been concentrated on the hypothesis that they are produced by some process in solid grains. We review that hypothesis and the arguments which support it, and find that diffuse band formation by certain classes of molecules should not yet be ruled out. Since several relevant observations have been made in the last four years, the observational data on the bands are reviewed. It would appear that these data are as consistent with molecular origins for the bands as with grain origins. Types of molecular transitions which give rise to 'diffuse' bands are described. These transitions can occur in relatively simple molecules composed of cosmically abundant elements and can produce broad, unresolvable profiles without destroying the molecule. New observations are suggested which could produce decisive evidence with which to determine whether the diffuse bands originate in grains, molecules, or both.

Smith, W. H.↗

NASA NTRS1977

Effect of microstructure on Na/+/ diffusion in beta alumina

The effect of microstructural features, such as grain size and orientation, on sodium ion diffusion in polycrystalline beta alumina is investigated; the ionic solid is suitable for use in high-energy and power density batteries. Two types of hot-pressed polycrystalline samples are studied, one with grains highly oriented in the direction normal to the pressing direction, the other with significantly less preferred orientation normal to the pressing direction. Results of the experimental assessments indicate that grain size and diffusion path tortuosity have little effect on diffusion kinetics, while grain boundary type and grain aspect ratio are important factors in determining the diffusion rate.

Sarian, S.↗

NASA NTRS1978

Effects of particle drift on cosmic ray transport. II - Analytical solution to the modulation problem with no latitudinal diffusion

An analytical solution to a model of the modulation of galactic cosmic rays in the presence of particle drifts is presented and discussed. The solution assumes an energy-independent radial diffusion coefficient proportional to distance and no latitudinal diffusion, and includes energy-independent particle drift velocities similar to those expected in a Parker spiral magnetic field with an equatorial current sheet. The solutions clearly demonstrate the large effects of drifts on the modulated cosmic-ray intensity. For values of the radial diffusion coefficient and particle drift velocity which are plausible for 1-GV-rigidity protons, the logarithmic radial gradient in the inner solar system is reduced by more than a factor of 5 over the value calculated in the absence of drifts. It is found that even for much smaller values of particle drift velocity and radial diffusion coefficient, such as might be expected for protons with energies of the order of 10 MeV, the effects of the drifts can be substantial.

Isenberg, P. A.↗

NASA NTRS1978

The effect of nonlinear diffusive heat transport in a simple climate model

The effects of nonlinear diffusive heat transport in North's (1975) model is reexamined. By inserting the nonlinearity into the original energy balance equation rather than the spectrally transformed linear algebraic equations, and including one more mode in the spectral expansion, it is found that nonlinear diffusion does give a significantly different sensitivity for states near the current climate. For linear diffusion, an increase of 0.1% of the solar constant from the present value is sufficient to melt all the ice on the earth, whereas an increase of about five times as large is required for nonlinear diffusion. This difference in sensitivity shows the importance of dynamical modeling in simple climate models.

Lin, C. A.↗

NASA NTRS1978

Partially averaged field approach to cosmic ray diffusion

A new nonlinear technique is used to derive the kinetic equation for particles interacting with turbulent fluctuations. Difficulties associated with quasi-linear theory are avoided. The new method evaluates the effects of the fluctuations along particle orbits which themselves include the effects of a statistically averaged subset of the possible configuration of the turbulence. As an illustration, the pitch-angle diffusion coefficient is calculated for particles interacting with 'slab model' magnetic turbulence, i.e., magnetic fluctuations linearly polarized transverse to a mean magnetic field. The pitch-angle diffusion coefficient is determined in the vicinity of 90-deg pitch angles where quasi-linear theory breaks down. The spatial diffusion coefficient parallel to a mean magnetic field is evaluated by use of the calculated pitch-angle diffusion coefficient. It is suggested that the partially averaged field method is not limited to small amplitude fluctuating fields and hence is not a perturbation theory.

Jones, F. C.↗

NASA NTRS1978

Nonneutral and quasi-neutral diffusion of weakly ionized multiconstituent plasma

A theory is developed for quasi-neutral and nonneutral ambipolar diffusion in a weakly ionized multiconstituent plasma. Magnetic field effects are disregarded. The effects of negative ions and various positive and negative ion diffusion coefficients are identified. It is found that the presence of negative ions leads to enhanced electron diffusion and a retarded diffusion of positive ions. In some cases electrons may be forcibly propelled from the area of perturbations in ionic concentrations. The ambipolar electric field is observed to cause negative ions to flow in a countergradient fashion where the gradient is small. Electrons tend toward a more homogeneous spatial distribution in the nonneutral case than in the quasi-neutral case.

Hill, R. J.↗

NASA NTRS1978

Design and subscale tests of a diffuser system for a Mach 4 scramjet test facility

A fixed-geometry diffuser system was designed for use in Mach 4 free-jet tests of a hydrogen-burning modular scramjet engine. The scramjet engine has a rectangular cross section that blocks up to 33 percent of the existing tunnel nozzle exit area and swept leading edges of the scramjet produce an asymmetric downward flow that sharply increases when the scramjet inlet unstarts. Effects of these conditions on the operation of the diffuser system are not accurately predictable. An experimental investigation has therefore been conducted in unheated air using a subscale model of the tunnel-scramjet-diffuser system to substantiate the design. Test results showed that the preliminary design had to be modified to obtain an acceptable configuration of the nozzle exit, test cabin, and diffuser that would provide shock-free flow at the scramjet inlet for simulated Mach 4 flight at altitudes from 16,764 to 20,422 meters.

Andrews, E. H., Jr.↗

NASA NTRS1978

Infrared colors and the diffuse interstellar bands

Broad-band infrared photometric measurements have been gathered for 105 stars which exhibit diffuse interstellar bands in their spectra. All normal stars obey a single reddening law, and a value of R equal to 3.08 + or - 0.15 is derived. This value is consistent with other recent determinations of R. The diffuse band indicators, the central depth of the 4430-A feature and the equivalent widths of the 5780-A and 6284-A features, show as large a scatter with the infrared color excesses as they do with E(B - V). No single-valued relation between the color excesses and the diffuse band strengths appears to exist. This casts doubt on whether dust grains which produce the visual and infrared extinctions are the carriers for the diffuse interstellar features.

Sneden, C.↗