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Results for “Effective diffusivity”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Dynamic response of a pulsed Burke-Schumann diffusion flame

Turbulent flames are often envisioned as an ensemble of random vortices interacting with the combustion process. A better understanding of the vortex-flame interactions therefore would be useful in improving the modeling of turbulent diffusion flames. Substantial simplification may be made by investigating controlled interactions in a laminar flame, as opposed to random interactions in a turbulent flame. The general goals of the research project are to improve our understanding of (1) the influence of buoyancy on co-flow diffusion flames and (2) the effects of buoyancy on vortex-flame interactions in co-flow diffusion flames. As a first step toward objective (2), we conducted a joint experimental and numerical investigation of the vortex-flame interaction. Vortices were produced by mechanically pulsing the fuel flow at a low frequency, e.g., 10 Hz. Experiments were conducted using a nonflickering Burke-Schumann flame in both microgravity (mu-g) and normal gravity (1g) as a means of varying the buoyant force without modification of the pressure (i.e., density). The effects of buoyant convection may then be determined by a comparison of the mu-g and 1g results. The mu-g results may also reveal the important mechanisms which are masked or overwhelmed by buoyant convection in 1g. A numerical investigation was conducted using a validated, time-accurate numerical code to study the underlying physics during the flame interaction and to assist the interpretation of the experimental results.

Sheu, Jyh-Cherng↗
NASA NTRS1988

Earth radiation pressure effects on satellites

A diffuse-earth radiation force model is presented, which includes a latitudinally varying representation of the shortwave and longwave radiation of the terrestrial sphere. Applications to various earth satellites indicate that this force, in particular the shortwave component, can materially affect the recovery of estimated parameters. Earth radiation pressure cannot explain the anomalous deceleration of LAGEOS, but can produce significant along track accelerations on satellites with highly eccentric orbits. Analyses of GEOS-1 tracking data confirm this result.

Knocke, P. C.↗
NASA NTRS1990

Low-frequency angular broadening and diffuse interstellar plasma turbulence

The effect of turbulence in the interplanetary medium (IPM) on measurements of a low-frequency interferometer is discussed. It is concluded that scattering by IPM will limit the effective sensitivity of a low-frequency interferometer to far above its theoretical value. If the limitations imposed by the IPM can be overcome, a low-frequency interferometer could be used to search for turbulence near supernova remnants, which has been hypothesized to play an important role in the acceleration of the cosmic rays.

Spangler, Steven R.↗
NASA NTRS1999

Synthesis of Fullerenes in Low Pressure Benzene/Oxygen Diffusion Flames

The interest in fullerenes is strongly increasing since their discovery by Kroto et al. in 1985 as products of the evaporation of carbon into inert gas at low pressure. Due to their all carbon closed-shell structure, fullerenes have many exceptional physical and chemical properties and a large potential for applications such as superconductors, sensors, catalysts, optical and electronic devices, polymers, high energy fuels, and biological and medical materials. This list is still growing, because the research on fullerenes is still at an early stage. Fullerenes can be formed not only in a system containing only carbon and an inert gas, but also in premixed hydrocarbon flames under reduced pressure and fuel rich conditions. The highest yields of fullerenes in flames are obtained under conditions of substantial soot formation. There is a need for more information on the yields of fullerenes under different conditions in order to understand the mechanisms of their formation and to enable the design of practical combustion systems for large-scale fullerene production. Little work has been reported on the formation of fullerenes in diffusion flames. In order to explore the yields of fullerenes and the effect of low pressure in diffusion flames, therefore we constructed and used a low pressure diffusion flame burner in this study.

Hebgen, Peter↗
NASA NTRS1975

Modulation and diffusion theory of cosmic rays

Papers given on the modulation of galactic cosmic rays by the solar wind at the 14th International Cosmic Ray Conference are reviewed. Some of the topics treated in this review are Pioneer and Helios radial gradient measurement, heliolatitude effects in modulation models, diffusion (scattering) theory - including pitch angle diffusion diagrams - solar flare particle transport theory and cosmic ray fluctuations at neutron monitor energies.

Forman, M. A.↗
NASA NTRS1977

The effects of thermospheric winds and chemistry in the diurnal variations of thermospheric species

The reported investigation considers on the basis of a theoretical model, the diurnal variations of the thermospheric composition (H, He, O, O2, and Ar) in terms of thermal expansion with diffusive equilibrium and transport effects associated with thermospheric winds, chemistry, and exospheric flow. The theoretical results are compared with satellite composition data which indicate that the fundamental diurnal tide can be reasonably well understood. It is found that winds are only important for molecular oxygen below 180 km, while thermal expansion due to the larger mass is relatively more important for O2 than for O. Distinct from O, photodissociation and in particular photoionization of O2 are very significant for molecular oxygen.

Mayr, H. G.↗
NASA NTRS1990

Parametric evaluation of the aerodynamic performance of an annular combustor-diffuser system

This study experimentally determined the effects of dominant geometric and flow parameters on the aerodynamic performance of an annular combustor-diffuser system. For each parametric configuration, data were obtained at 13 different flow rates through the inner annulus, outer annulus, and combustor dome. The parametric effects investigated included the dump gap dimension, flow splits, prediffuser inlet velocity profile, and combustor channel height. During these measurements, the average prediffuser inlet Mach number was 0.305. The measured static pressure recovery coefficient and the total pressure loss coefficient in the prediffuser as well as the overall system are presented in this paper. The study found that the performance of the combustor-diffuser system improves with increasing dome flow rate or dump gap dimension.

Srinivasan, R.↗
NASA NTRS1988

The role of the turbulent Prandtl number in turbine blade heat transfer prediction

Improvement of the prediction of external (gas-to-blade) heat transfer coefficients in gas turbine engines was undertaken. The effects of improved eddy diffusivity of heat modeling in the turbulence model was specifically investigated. A two-dimensional boundary STAN5, was selected and modified by incorporating several different turbulent Prandtl number models. Results indicated that slight effects were attributable to the modified turbulence model. Boundary layer character appeared to be much more significant.

Whitaker, Kevin W.↗
NASA NTRS1990

The role of the turbulent Prandtl number in turbine blade heat transfer prediction

Improvement of the prediction of external (gas-to-blade) heat transfer coefficients in gas turbine engines was undertaken. The effects of improved eddy diffusivity of heat modeling in the turbulence model was specifically investigated. A two-dimensional boundary STAN5, was selected and modified by incorporating several different turbulent Prandtl number models. Results indicated that slight effects were attributable to the modified turbulence model. Boundary layer character appeared to be much more significant.

Whitaker, Kevin W.↗
NASA NTRS1986

Simulation of the thermospheric tides by use of the NCAR thermospheric general circulation model

Numerical calculations of the thermospheric tidal winds and temperatures at equinox are presented. The calculations were made using the National Center for Atmospheric Research Thermospheric General Circulation Model (TGCM), which includes the effects of viscosity, conductivity, diffusion, ion drag, winds, and temperature gradients. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion-neutral momentum coupling. The semidiurnal tidal calculations also include the effects of upward propagating waves generated by heating in the lower atmosphere. This semidiurnal propagating component is modeled by use of the classical tidal perturbations as lower boundary conditions. The model is tuned by adjusting the propagating tidal forcing term until calculated semidiurnal wind and temperature fields best approximate incoherent scatter observations. The tidal TGCM results are consistent with previous theoretical work and successfully reproduce high altitude temperature and meridional velocity data, but they give significantly lower magnitudes for velocities and temperatures near 160 km than are seen by observations.

Fensen, C. G.↗
NASA NTRS1976

On the flux and the energy spectrum of interstellar ions in the solar system

The flux density of ions created by ionization of interstellar neutral particles in the solar system and picked up by the solar wind is calculated as a function of the neutral particles. For atomic hydrogen the flux density is estimated to exceed 10,000/sq cm/sec over the distance range from a few to nearly 100 AU. The velocity space distribution of the interstellar ions is calculated under the assumption of no significant energy diffusion but with inclusion of adiabatic effects as well as a possible strong pitch angle diffusion. The energy spectrum is highly nonthermal and much broader than that of the solar wind ions; interstellar protons are easily distinguishable from solar wind protons by their location in velocity space. If charge exchange is an important contributor to the ionization of hydrogen, the observed local intensity of interstellar protons should exhibit time variations correlated with the density changes of the solar wind stream structure.

Vasyliunas, V. M.↗
NASA NTRS1978

A core origin for group IVA iron meteorites - A reply to Moren and Goldstein

Because uncertainties in experimental data are large, one has considerable latitude in choosing the input parameters needed to calculate iron meteorite cooling rates. The best way to test input parameters is by examining their ability to yield the observed properties of the meteorites. Our phase diagram yields fits to kamacite profiles that are superior to those based on the Moren-Goldstein phase diagram. Our method of allowing for the effect of P on the Ni diffusion coefficient takes into account the enhancement in this effect with decreasing temperature; Moren and Goldstein use a relationship derived for a temperature of 1100 C, well outside the 700-350 C range where kamacite growth occurs. Use of our input parameters yields cooling rates in IVA irons that are independent of composition, consistent with a core origin. Since the fractionation of siderophiles in group IVA also indicates a core origin, we conclude that this is the correct model for this group.

Willis, J.↗
NASA NTRS1981

Effect of short wavelength illumination on the characteristic bulk diffusion length in ribbon silicon solar cells

The influence of short wavelength light on the characteristic bulk minority carrier diffusion length of the ribbon silicon photovoltaic cell has been investigated. We have measured the intensity and wavelength dependence of the diffusion length in an EFG ribbon cell, and compared it with a standard Czochralski grown silicon cell. While the various short wavelength illuminations have shown no influence on the diffusion length in the CZ cell, the diffusion lengths in the ribbon cell exhibit a strong dependence on the volume generation rate as well as on the wavelength of the superimposed lights. We have concluded that the trap-filling phenomenon at various depths in the bulk neutral region of the cell is consistent with the experimental observation.

Ho, C. T.↗
NASA NTRS1986

Thermophoretically augmented mass transfer rates to solid walls across laminar boundary layers

Predictions of mass transfer (heavy vapor and small particle deposition) rates to solid walls, including the effects of thermal (Soret) diffusion ('thermophoresis' for small particles), are made by numerically solving the two-dimensional self-similar forced convection laminar boundary-layer equations with variable properties, covering the particle size range from vapor molecules up to the size threshold for inertial (dynamical nonequilibrium) effects. The effect of thermophoresis is predicted to be particularly important for submicron particle deposition on highly cooled solid surfaces, with corresponding enhancement factors at atmospheric conditions being over a thousand-fold at T(w)/T(e) equal to about 0.6. As a consequence of this mass transfer mechanism, the particle size dependence of the mass transfer coefficient to a cooled wall will be much weaker than for the corresponding case of isothermal capture by Brownian-convective diffusion.

Gokoglu, S. A.↗
NASA NTRS2014

Mass Dependency of Isotope Fractionation of Gases Under Thermal Gradient and Its Possible Implications for Planetary Atmosphere Escaping Process

Physical processes that unmix elements/isotopes of gas molecules involve phase changes, diffusion (chemical or thermal), effusion and gravitational settling. Some of those play significant roles for the evolution of chemical and isotopic compositions of gases in planetary bodies which lead to better understanding of surface paleoclimatic conditions, e.g. gas bubbles in Antarctic ice, and planetary evolution, e.g. the solar-wind erosion induced gas escaping from exosphere on terrestrial planets.. A mass dependent relationship is always expected for the kinetic isotope fractionations during these simple physical processes, according to the kinetic theory of gases by Chapman, Enskog and others [3-5]. For O-bearing (O16, -O17, -O18) molecules the alpha O-17/ alpha O-18 is expected at 0.5 to 0.515, and for S-bearing (S32,-S33. -S34, -S36) molecules, the alpha S-33/ αlpha S-34 is expected at 0.5 to 0.508, where alpha is the isotope fractionation factor associated with unmixing processes. Thus, one isotope pair is generally proxied to yield all the information for the physical history of the gases. However, we recently] reported the violation of mass law for isotope fractionation among isotope pairs of multiple isotope system during gas diffusion or convection under thermal gradient (Thermal Gradient Induced Non-Mass Dependent effect, TGI-NMD). The mechanism(s) that is responsible to such striking observation remains unanswered. In our past studies, we investigated polyatomic molecules, O2 and SF6, and we suggested that nuclear spin effect could be responsible to the observed NMD effect in a way of changing diffusion coefficients of certain molecules, owing to the fact of negligible delta S-36 anomaly for SF6.. On the other hand, our results also showed that for both diffusion and convection under thermal gradient, this NMD effect is increased by lower gas pressure, bigger temperature gradient and lower average temperature, which indicate that the nuclear spin effect may not be the significant contributor as the energies involved in the hyperfine effect are much smaller than those with molecular collisions, especially under convective conditions.

Sun, Tao↗