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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Manufacturing in space: Fluid dynamics numerical analysis

Numerical computations were performed for natural convection in circular enclosures under various conditions of acceleration. It was found that subcritical acceleration vectors applied in the direction of the temperature gradient will lead to an eventual state of rest regardless of the initial state of motion. Supercritical acceleration vectors will lead to the same steady state condition of motion regardless of the initial state of motion. Convection velocities were computed for acceleration vectors at various angles of the initial temperature gradient. The results for Rayleigh numbers of 1000 or less were found to closely follow Weinbaum's first order theory. Higher Rayleigh number results were shown to depart significantly from the first order theory. Supercritical behavior was confirmed for Rayleigh numbers greater than the known supercritical value of 9216. Response times were determined to provide an indication of the time required to change states of motion for the various cases considered.

Robertson, S. J.↗
DOE OSTI2024

Transport from electron-scale turbulence in toroidal magnetic confinement devices

Plasma transport driven by turbulence ultimately determines the energy confinement performance of controlled fusion devices regardless of their confinement schemes and configurations. A large variety of plasma instabilities have been proposed for driving turbulence responsible for anomalous plasma transport beyond classical/neoclassical transport due to collisions. Although ion-scale turbulence usually dominates due to its large eddy size and saturation level, electron-scale turbulence has been recognized to be important in regions where ion-scale turbulence is suppressed (e.g., in internal transport barrier and in spherical tokamak H-mode plasmas) or is close to marginality. Electron-scale turbulence has been shown to nonlinearly interact with ion-scale turbulence, which modifies the dynamics of both and affects the resulting plasma transport, particularly when ion-scale instability is weakly driven. In this review paper, we focus on electron-scale turbulence that is believed to operate in magnetically confinement fusion devices and aim to provide a review of theoretical, numerical, and experimental developments in understanding electron-scale turbulence and its role in driving anomalous plasma turbulence. In particular, we focus on the electrostatic electron temperature gradient (ETG) mode which is the most widely recognized plasma instability underlying electron-scale turbulence observed in magnetically confined plasmas. Finally, we note that there are other less studied instabilities that might be responsible for observed electron-scale turbulence, most notably ubiquitous mode, and short-wavelength ion temperature gradient (SWITG) mode, which will be briefly touched on in this review.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1999

The Role of Velocity Redistribution in Enhancing the Intensity of the He II 304 A Line in the Quiet Sun Spectrum

We present observational evidence of the effect of small scale ("microturbulent") velocities in enhancing the intensity of the He II lambda304 line with respect to other transition region emission lines, a process we call "velocity redistribution". We first show results from the 1991 and 1993 flights of SERTS (Solar EUV Rocket Telescope and Spectrograph). The spectral resolution of the SERTS instrument was sufficient to infer that, at the spatial resolution of 5", the line profile is nearly gaussian both in the quiet Sun and in active regions. We were then able to determine, for the quiet Sun, a lower limit for the amplitude of non-thermal motions in the region of formation of the 304 A line of the order of 10 km/s. We estimated that, in the presence of the steep temperature gradients of the solar Transition Region (TR), velocities of this magnitude can significantly enhance the intensity of that line, thus at least helping to bridge the gap between calculated and observed values. We also estimated the functional dependence of such an enhancement on the relevant parameters (non-thermal velocities, temperature gradient, and pressure). We then present results from a coordinated campaign, using SOHO/CDS and H-alpha spectroheliograms from Coimbra Observatory, aimed at determining the relationship between regions of enhanced helium emission and chromospheric velocity fields and transition region emission in the quiescent atmosphere. Using these data, we examined the behavior of the He II lambda304 line in the quiet Sun supergranular network and compared it with other TR lines, in particular with O III lambda600. We also examined the association of 304 A emission with the so-called "coarse dark mottle", chromospheric structures seen in H-alpha red wing images and associated with spicules. We found that all these observations are consistent with the velocity redistribution picture.

Andretta, Vincenzo↗
NASA NTRS2003

Numerical Calculation of the Morphology of a Solid/Liquid Interface Near an Insoluble Particle

A numerical mathematical model capable of accurately describing the evolution of the shape of the solid/liquid interface in the proximity of a foreign particle is presented in this paper. The model accounts for the influence of the temperature gradient and the Gibbs-Thomson and disjoining pressure effects. It shows that for the systems characterized by k(sub P) < k(sub L) the disjoining pressure causes the interface curvature to change its sign in the close-contact particle/interface region. It also shows that the increase of the temperature gradient diminishes the effect of the disjoining pressure. Calculated critical solidification velocities for the pushing/engulfment transition are compared with experimental measurements performed in microgravity conditions.

Catalina, Adrian V.↗
NASA NTRS2004

Analytical Solution for the Critical Velocity of Pushing/Engulfment Transition

The distribution of ceramic particles in a metal matrix composite material depends primarily on the interaction of the particles with the solid/liquid interface during the solidification process. A numerical model that describes the evolution of the shape of the solid/liquid interface in the proximity of a foreign particle will presented in this paper. The model accounts for the influence of the temperature gradient and the Gibbs-Thomson and disjoining pressure effects. It shows that for the systems characterized by k(sub p) < k(sub L) the disjoining pressure causes the interface curvature to change its sign in the close-contact particle/interface region. It also shows that the increase of the temperature gradient diminishes the effect of the disjoining pressure. The analysis of the numerical results obtained for a large range of processing conditions and materials parameters has led to the development of an analytical solution for the critical velocity of pushing/engulfinent transition. The theoretical results will be discussed and compared with the experimental measurements performed under microgravity conditions.

Catalina, Adrian V.↗
NASA NTRS1984

Maximization of Growth Rates During Czochralski Pulling

It was suggested from theory(1-4) that silicon can be grown from the melt at rates far exceeding the current state of the art. Previous theoretical and experimental investigations which predict maximum rates of pulling during Czochralski growth are reviewed. Several experimental methods are proposed to modify the temperature distribution in a growing crystal to achieve higher rates of pulling. A physical model of a Czochralski crystal of germanium in contact with its melt was used to quantitatively determine, by direct measurement of the axial temperature distribution in the solid, the increase in axial temperature gradients effected by an inverted conical heat reflector located above the melt and coaxially about the physical model. Preliminary results indicate that this is an effective method of increasing the thermal resistance between the hot melt and crucible wall and a growing crystal. Under these conditions the enhancement of the interfacial temperature gradients permit a commensurate increase in the rate of crystal pulling.

Wargo, M. J.↗
DOE OSTI2016

Temperature mediates continental-scale diversity of microbes in forest soils

Climate warming is increasingly leading to marked changes in plant and animal biodiversity, but it remains unclear how temperatures affect microbial biodiversity, particularly in terrestrial soils. Here we show that, in accordance with metabolic theory of ecology, taxonomic and phylogenetic diversity of soil bacteria, fungi and nitrogen fixers are all better predicted by variation in environmental temperature than pH. However, the rates of diversity turnover across the global temperature gradients are substantially lower than those recorded for trees and animals, suggesting that the diversity of plant, animal and soil microbial communities show differential responses to climate change. To the best of our knowledge, this is the first study demonstrating that the diversity of different microbial groups has significantly lower rates of turnover across temperature gradients than other major taxa, which has important implications for assessing the effects of human-caused changes in climate, land use and other factors.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Nuclear Fuel Based on Uranium-Zirconium Carbonitride

The results of Investigations of the nitrogen and uranium pressure in the system U–Zr–C–N in the temperature range 1873–2073 K and the constructed section of the U–Zr–C–N phase diagram at 1873 K are presented. The behavior of the solid fission products in carbonitride fuel, the interaction of the fuel with a pyrolytic carbon coating, and the behavior of micro fuel elements based on such fuel during high temperature firing and in the presence of a temperature gradient are examined. The conditions for the preparation of fuel compositions of the type uranium carbonitride in a carbide-silicon matrix and some of its physicochemical and mechanical properties are determined. It is noted that there is promise in using fuel compositions and micro fuel elements based on uranium-zirconium carbonitride for high-temperature applications.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Radiation of Gas Layer over Hot Surface

A method is presented for evaluation the radiation flux produced by a gas layer near a heated surface, where the gas temperature depends on a distance from the surface. This method refers to small temperature gradients and operates with an effective radiation temperature for each frequency, as well as with the width of the gas absorption band. These parameters are determined by the absorption spectrum of atoms or gas molecules, and also by the shape of the spectral line for the radiative transition between certain states of atomic particles of a gas. The possibilities of this method are demonstrated by examples of emission of photons from the solar photosphere, as well as emission of CO{sub 2} molecules in the atmospheres of the Earth and Venus.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Discovery of a Blind Geothermal System in Southern Gabbs Valley, Western Nevada, through Application of the Play Fairway Analysis at Multiple Scales

The Great Basin region is capable of generating much greater amounts of geothermal energy than currently produced. Most geothermal resources in this region are blind, and thus favorable characteristics for geothermal activity must be synthesized and methodologies developed to discover new commercial-grade systems. The geothermal play fairway concept involves integration of multiple parameters indicative of geothermal activity to identify promising areas for new development. In the Nevada play fairway project, geologic, geochemical, and geophysical parameters were initially synthesized to produce a new geothermal potential map of 96,000 km2. Southern Gabbs Valley in western Nevada is a particularly promising site selected for detailed study. It contains favorable structural settings, including Quaternary fault intersections and a displacement transfer zone. Geologic, geophysical, and geochemical techniques were employed to define the most likely sites for high permeability and select targets for temperature-gradient holes. Permeability models were revised to reflect results of detailed analyses and generate new detailed play fairway maps. The most promising site lies in an area of multiple fault intersections in a broader displacement transfer zone directly north of the Petrified Springs dextral fault, as revealed by gravity, magnetic, and MT data. A 2-m temperature anomaly and warm temperature gradient wells (~120oC at ~150 m depth) confirm the presence of a geothermal system and provide initial validation of the play fairway methodology. The system is blind, with no surface hot springs, fumaroles, or paleo-geothermal deposits. Lessons learned in the detailed studies include: 1) initially identified sites commonly include multiple favorable settings at a finer scale; 2) promising sites in Cenozoic basins cannot be recognized without detailed geophysical surveys; and 3) play fairway analysis is critical at multiple scales.

15 GEOTHERMAL ENERGY↗
DOE OSTI2023

Linear instabilities in the hot-ion regime in a high-field spherical tokamak

Abstract The present operating high-field compact spherical tokamak ST40 is an important step toward an ST-based fusion reactor (Gryaznevich and Asunta 2017 Fusion Eng. Des. 123 177; McNamara 2023 Nucl. Fusion 63 054002). Temperature and density profiles and their uncertainties in recent hot ion ST40 plasmas with central ion temperatures exceeding 8.6 keV, have been inferred using an integrated analysis of several diagnostics including line-of-sight integration and volume average measurements, as well as limited profile information from a charge-exchange-recombination spectrometer. A linear gyrokinetic stability analysis has been carried out to identify the most unstable micro-instabilities in these hot ion plasmas. In one of these plasmas, it is found that linear growth rates of both ion- and electron-scale ( k θ ρ s ⩾ 0.2 where k θ is the poloidal wavenumber and ρ s is the ion gyro-radius with sound speed) modes decrease from the edge toward the core of the plasma (i.e. from ρ = 0.8 to 0.3 where ρ is the square root of the normalized magnetic flux), while the change of linear growth rates at k θ ρ s < 0.2 is non-monotonic. In particular, at ρ = 0.3 no unstable mode was found at k θ ρ s > 5 , and about an order of magnitude reduction in the maximum linear growth rate in the ion-scale wavenumber range of 0.2 ⩽ k θ ρ s < 1 is seen at ρ = 0.3 compared with the ρ = 0.8 location. It is found through parametric scans that while the ion temperature gradient (ITG) mode/trapped electron mode (TEM) and kinetic ballooning mode (KBM)/kinetic shear Alfvén (KSA) mode are more important in the plasma core, ubiquitous mode (UM) is found to be the important ion-scale instability at ρ ⩾ 0.6 . At ρ = 0.8, two instabilities are found at the electron scale, with one being the typical electrostatic electron temperature gradient (ETG) mode at ρ e scale and beyond and the other being an unidentified low-real-frequency instability (at the intermediate scale between electron and ion gyroradii) which can propagate in the ion direction and is driven by ETG. The existence of UM at the ion scale and the low-real-frequency instability and ETG mode at the electron scale at ρ = 0.8 is supported by an extensive linear gyrokinetic stability analysis of another ST40 hot ion plasma in a wider parametric range, taking into account the uncertainties in the experimental profiles.

ubiquitous mode↗
NASA NTRS1986

Analytical and numerical studies of the thermocapillary flow in a uniformly floating zone

The microgravity environment of an orbiting vehicle permits crystal growth experiments in the presence of greatly reduced buoyant convection in the liquid melt. Crystals grown in ground-based laboratories do not achieve their potential properties because of dopant variations caused by flow in the melt. The floating zone crystal growing system is widely used to produce crystals of silicon and other materials. However, in this system the temperature gradient on the free sidewall surface of the melt is the source of a thermocapillary flow which does not disappear in the low-gravity environment. The idea of using a uniform rotation of the floating zone system to confine the thermocapillary flow to the melt sidewall leaving the interior of the melt passive is examined. A cylinder of fluid with an axial temperature gradient imposed on the cylindrical sidewall is considered. A half zone and the linearized, axisymmetric flow in the absence of crystal growth is examined. Rotation is found to confine the linear thermocapillary flow. A simplified model is extended to a full zone and both linear and nonlinear thermocapillary flows are studied theoretically. Analytical and numerical methods are used for the linear flows and numerical methods for the nonlinear flows. It was found that the linear flows in the full zone have more complicated and thicker boundary layer structures than in the half zone, and that these flows are also confined by the rotation. However, for the simplified model considered and for realistic values for silicon, the thermocapillary flow is not linear. The fully nonlinear flow is strong and unsteady (a weak oscillation is present) and it penetrates the interior. Some non-rotating flow results are also presented. Since silicon as a large value of thermal conductivity, one would expect the temperature fields to be determined by conduction alone. This is true for the linear and weakly nonlinear flows, but for the stronger nonlinear flow the results show that temperature advection is also important. Uniform rotation may still be a means of confining the flow and the results obtained define the procedure to be used to examine this hypothesis.

Fowlis, W. W.↗
DOE OSTI2024

Accelerating CO2 Storage Site Characterization through a New Understanding of Favorable Formation Properties and the Impact of Core-Scale Heterogeneities

CO2 sequestration in deep geologic formations can permanently reduce atmospheric CO2 emissions and help to abate climate change. Target formations must undergo a time- and resource-intensive site evaluation process, assessing storage capacity, environmental safety, and suitability for CO2 trapping via reactive transport models based on data from a limited number of core samples. As such, simulations are often simplified and omit heterogeneities in formation properties that may be significant but are not well understood. To facilitate more rapid site assessment, this work first defines the aquifer properties of favorable storage formations through the analysis of promising and active storage sites. Data show quartz is the most prevalent formation mineral with carbonate minerals, highly reactive with injected CO2, present in over 75% of formations. Porosity and permeability data are highly clustered at 10–30% and 10–1000 mD. Field-scale reactive transport simulations are then constructed and used to analyze CO2 trapping efficiency. The models consider porosity and carbonate mineral heterogeneity as well as the impacts of typical temperature gradients. Simulated sequestration efficiencies are compared to results from a comparable homogenous model to understand the implications of aquifer non-uniformities. The results show a lower sequestration efficiency in the homogeneous model during the injection phase. During the post-injection phase, the homogenization of porosity and carbonate mineralogy results in a higher sequestration efficiency. Incorporating the temperature gradient also increases the sequestration efficiency. Importantly, the maximum deviation between the homogeneous and heterogeneous simulations at the end of the 50-year study period is only ~10%. Larger impacts may be incurred for properties outside the defined, promising ranges suggested here.

Environmental Sciences & Ecology↗
NASA NTRS1987

Direct numerical simulation of buoyantly driven turbulence

Numerical simulations of homogeneous turbulence subject to buoyant forcing were performed. The presence of a mean temperature gradient combined with a gravitational field results in a forcing term in the momentum equations. The development of the turbulence was studied and compared to the decay of similar fields in the absence of gravity. In the buoyantly driven field, the vorticity is preferentially aligned with the intermediate eigenvector of the strain-rate tensor and the local temperature gradient is more likely to be aligned with the most compressive eigenvector. These relationships are qualitatively similar to those observed in previous shear flow results studied by Ashurst (1987). A tensor diffusivity model for passive scalar transport developed from shear flow results in Rogers, Moin, and Reynolds (1986) also predicts this buoyant scalar transport, indicating that the relationship between the scalar flux and the Reynolds stress is similar in both flows.

Ashurst, William T.↗
NASA NTRS1974

On the distribution of He+ in the plasmasphere from observations of resonantly scattered He II 304-A radiation

The intensity of the He II 304-A radiation in the night sky was measured from a sounding rocket launched from White Sands Missile Range on June 9, 1972. The data obtained on this flight are compared with predictions of theoretical models of the He(+) altitude distribution within the plasmasphere. The observations are not consistent with an isothermal equilibrium model of the helium ion in the plasmasphere, confirming earlier results. The data are best fit by a diffussive equilibrium model of exospheric He(+) having a plasma temperature gradient of 0.5 K per km and a number density at 1600 km of 200,000 He(+) ions per cu cm. In this model the ion densities are calculated without allowance for thermal diffusion. A similar model that allows for thermal diffusion and that has a He(+) number density at 1600 km of 1500 per cu cm and a temperature gradient of 1.2 K per km yields a less satisfactory fit to the observations but is more consistent with in situ satellite observations.

Paresce, F.↗
NASA NTRS1992

Eclipse maps of the accretion disk in the classical nova V Persei

The surface brightness distribution in the accretion disk of V Per is reconstructed using a maximum entropy technique. The resulting brightness temperature gradient is much less steep than that from a standard steady state, optically thick disk for all reasonable mass accretion rates. A model including a rim which obscures part of the disk does not resolve this discrepancy. However, if the inner disk is disrupted out to about 0.15-0.25R sub L1 (for example by a magnetic field), the brightness temperature gradient can be very close to T sub b varies as R exp -3/4 (1-b(Rin/R)exp 1/2) law. The value of Rin needed to match this steady state distribution depends on the distance to the system, its size and brightness, and the rotation rate of the white dwarf.

Wood, Janet H.↗
DOE OSTI2018

Formation of the Conducting Filament in TaO x -Resistive Switching Devices by Thermal-Gradient-Induced Cation Accumulation

Here, the distribution of tantalum and oxygen ions in electroformed and/or switched TaO x -based resistive switching devices has been assessed by high-angle annular dark-field microscopy, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. The experiments have been performed in the plan-view geometry on the cross-bar devices producing elemental distribution maps in the direction perpendicular to the electric field. The maps revealed an accumulation of +20% Ta in the inner part of the filament with a 3.5% Ta-depleted ring around it. The diameter of the entire structure was approximately 100 nm. The distribution of oxygen was uniform with changes, if any, below the detection limit of 5%. We interpret the elemental segregation as due to diffusion driven by the temperature gradient, which in turn is induced by the spontaneous current constriction associated with the negative differential resistance-type I–V characteristics of the as-fabricated metal/oxide/metal structures. A finite-element model was used to evaluate the distribution of temperature in the devices and correlated with the elemental maps. In addition, a fine-scale (~5 nm) intensity contrast was observed within the filament and interpreted as due phase separation of the functional oxide in the two-phase composition region. Understanding the temperature-gradient-induced phenomena is central to the engineering of oxide memory cells.

36 MATERIALS SCIENCE↗
NASA NTRS1974

Physical properties of solar chromospheric plages. II - Chromospheric plage models

Chromospheric models of plages are proposed to explain profiles of the Ca II H, K, 8498-A, 8542-A, and 8662-A lines. These models are consistent with boundary conditions imposed by the photosphere and the Lyman continuum. It is found that increasing emission in these lines is consistent with a picture of increasing temperature gradient in the low chromosphere and the resulting increase in pressure and electron density at similar line optical depths. With this picture it is shown how the distribution of chromospheric parameters across the solar disk can be empirically determined directly from Ca II filtergrams. It is also proposed that the high-density aspects of solar activity are produced by steep temperature gradients in the low chromosphere and thus by the enhanced heating mechanisms that steepen these gradients.

Shine, R. A.↗