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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

THE PHYSICAL ENVIRONMENT OF THE MASSIVE STAR-FORMING REGION W42

We present an analysis of multi-wavelength observations from various data sets and Galactic plane surveys to study the star-formation process in the W42 complex. A bipolar appearance of the W42 complex is evident due to the ionizing feedback from the O5–O6 type star in a medium that is highly inhomogeneous. The Very Large Telescope/NACO adaptive-optics K and L{sup ′} images (resolutions ∼0.″2–0.″1) resolved this ionizing source into multiple point-like sources below ∼5000 AU scale. The position angle ∼15° of the W42 molecular cloud is consistent with the H-band starlight mean polarization angle, which in turn is close to the Galactic magnetic field, suggesting the influence of the Galactic field on the evolution of the W42 molecular cloud. Herschel sub-millimeter data analysis reveals three clumps located along the waist axis of the bipolar nebula, with the peak column densities of ∼(3–5) × 10{sup 22} cm{sup −2} corresponding to visual extinctions of A{sub V} ∼ 32–53.5 mag. The Herschel temperature map traces a temperature gradient in W42, revealing regions of 20 K, 25 K, and 30–36 K. Herschel maps reveal embedded filaments (length ∼1–3 pc) that appear to be radially pointed to the denser clump associated with the O5–O6 star, forming a hub-filament system. A total of 512 candidate young stellar objects (YSOs) are identified in the complex, ∼40% of which are present in clusters distributed mainly within the molecular cloud, including the Herschel filaments. Our data sets suggest that the YSO clusters, including the massive stars, are located at the junction of the filaments, similar to those seen in the Rosette Molecular Cloud.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Intrinsic dissipation in a nano-mechanical resonator

We investigate the effect of size on intrinsic dissipation in nano-structures. We use molecular dynamics simulation and study dissipation under two different modes of deformation: stretching and bending mode. In the case of stretching deformation (with uniform strain field), dissipation takes place due to Akhiezer mechanism. For bending deformation, in addition to the Akhiezer mechanism, the spatial temperature gradient also plays a role in the process of entropy generation. Interestingly, we find that the bending modes have a higher Q factor in comparison with the stretching deformation (under the same frequency of operation). Furthermore, with the decrease in size, the difference in Q factor between the bending and stretching deformation becomes more pronounced. The lower dissipation for the case of bending deformation is explained to be due to the surface scattering of phonons. A simple model, for phonon dynamics under an oscillating strain field, is considered to explain the observed variation in dissipation rate. We also studied the scaling of Q factor with initial tension, in a beam under flexure. We develop a continuum theory to explain the observed results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Numerical modeling of heat transfer in molten silicon during directional solidification process

Numerical investigation is performed for some of the thermal and fluid flow properties of silicon melt during directional solidification by numerical modeling. Dimensionless numbers are extremely useful to understand the heat and mass transfer of fluid flow on Si melt and control the flow patterns during crystal growth processes. The average grain size of whole crystal would increase when the melt flow is laminar. In the silicon growth process, the melt flow is mainly driven by the buoyancy force resulting from the horizontal temperature gradient. The thermal and flow pattern influences the quality of the crystal through the convective heat and mass transport. The computations are carried out in a 2D axisymmetric model using the finite-element technique. The buoyancy effect is observed in the melt domain for a constant Rayleigh number and for different Prandtl numbers. The convective heat flux and Reynolds numbers are studied in the five parallel horizontal cross section of melt silicon region. And also, velocity field is simulated for whole melt domain with limited thermal boundaries. The results indicate that buoyancy forces have a dramatic effect on the most of melt region except central part.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Influence of Growth Rate on Eutectic Spacing, Microhardness, and Ultimate Tensile Strength in the Directionally Solidified Al-Cu-Ni Eutectic Alloy

In this work, the mechanical properties of the Al-Cu-Ni eutectic alloy, Al-32.5 wt pct Cu-1 wt pct Ni, were investigated in terms of the dependency on growth rates. The Al-Cu-Ni eutectic alloy was directionally solidified at a constant temperature gradient, G = 4.93 K mm{sup −1}, with a wide range of growth rates (V = 9.25 to 2056.68 µm s{sup −1}) using a Bridgman-type directional solidification furnace. The eutectic spacing (λ), microhardness (HV{sub T}), and ultimate tensile strength (σ{sub UTS}) were measured with standard techniques. The dependences of λ, HV{sub T}, and σ{sub UTS} on V for directionally solidified Al-Cu-Ni eutectic alloy were experimentally obtained using regression analysis. The results obtained in the present work were compared with the similar experimental results in the literature. Finally, the elastic energy dependence on growth rates for Al-32.5 wt pct Cu-1 wt pct N alloy was determined from their nominal stress–strain plots.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Shear-flow trapped-ion-mode interaction revisited. II. Intermittent transport associated with low-frequency zonal flow dynamics

We address the mechanisms underlying low-frequency zonal flow generation in turbulent system and the associated intermittent regime of ion-temperature-gradient (ITG) turbulence. This model is in connection with the recent observation of quasi periodic zonal flow oscillation at a frequency close to 2 kHz, at the low-high transition, observed in the ASDEX Upgrade [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)] and EAST tokamak [Xu et al., Phys. Rev. Lett 107, 125001 (2011)]. Turbulent bursts caused by the coupling of Kelvin-Helmholtz (KH) driven shear flows with trapped ion modes (TIMs) were investigated by means of reduced gyrokinetic simulations. It was found that ITG turbulence can be regulated by low-frequency meso-scale zonal flows driven by resonant collisionless trapped ion modes (CTIMs), through parametric-type scattering, a process in competition with the usual KH instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

German experiences in local fatigue monitoring

The ageing management of nuclear power plants (NPP) has gained an increasing importance in the last years. The reasons are mainly due to the international context of extending period of plants operation. Moreover, new scientific discoveries, such as the corrosive influence of the medium on the fatigue process (environmentally assisted fatigue - EAF) play an important role and influence the code development (ASME, EAF code cases). The fatigue damage process takes a central position in ageing mechanisms of components. It must be ensured through appropriate evidence that facilities are being operated under allowable boundary conditions. In the design phase of NPP, fatigue analyses are still based on theoretical considerations and empirical values, which are summarized in the design transient catalogue, necessary for licensing. These analyses aim at proving the admissibility of the loads in terms of stress and fatigue usage. These analyses will also provide the fatigue-relevant positions in the NPP and give a basis for future design improvements and optimization of operating modes. The design transients are in practice conservatively correlated with the real transients occurring during operation. Uncertainties reveal very conservative assumptions regarding forecast temperatures, temperature gradients and frequencies of events. During operation of the plant, it has to be recurrently proved, that the plant is being operated under designed boundary conditions. Moreover, operating signals are constantly acquired to enable a fatigue evaluation. For example, in Germany fatigue evaluation is based on decades of experience and regulatory requirements. The rule KTA 3201.4 [1] establishes the rules for qualified fatigue monitoring. The rule DIN 25475-3 [2] on fatigue monitoring systems is available in draft version. Experience shows that some significant differences occur between the design transients and the real occurred transients during plant operation. The reasons for it are the various manual control options and also different operating modes. It is clear that showing the covering of real loads by design loads, requires a relatively complex and well-qualified detection process. The difficulty of this task is increased due to the lack of data or incomplete information and the exclusive reliance on existing operation plant data. The strategy of employing local fatigue monitoring is a straightforward solution enabling the direct measurement of loads on the fatigue-sensitive zones. Nowadays a direct derivation of the complete stress tensor at the fatigue-relevant locations is enabled thanks to the recorded local loads and combination with finite element (FE) analyses. So, additionally to the recorded temperature curves, a representation of the time evolution of the six stress components for each monitored component is possible. This allows the application of the simplified elasto-plastic fatigue check according to design codes. The fatigue level can be realistically analyzed with a suitable cycle-counting method. Furthermore, the knowledge of the time evolution of the stresses and strains enables to take into account an environmental factor to include the corrosive fluid influence in the calculations. Without local recording, it is impossible to calculate realistic fatigue usage. AREVA offers the AREVA fatigue concept (AFC) and the new fatigue monitoring system integrated (FAMOSi), necessary tools to monitor local fatigue and to provide realistic assessment. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2014

The residual zonal flow in tokamak plasmas toroidally rotating at arbitrary velocity

Zonal flows, initially driven by ion-temperature-gradient turbulence, may evolve due to the neoclassic polarization in a collisionless tokamak plasma. In our previous work [D. Zhou, Nucl. Fusion 54, 042002 (2014)], the residual zonal flow in a tokamak plasma rotating toroidally at sonic speed is found to have the same form as that of a static plasma. In the present work, the form of the residual zonal flow is presented for tokamak plasmas rotating toroidally at arbitrary velocity. The gyro-kinetic equation is analytically solved for low speed rotation to give the expression of residual zonal flows, and the expression is then generalized for cases with arbitrary rotating velocity through interpolation. The zonal flow level decreases as the rotating velocity increases. The numerical evaluation is in good agreement with the former simulation result for high aspect ratio tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Penetration of lower hybrid current drive waves in tokamaks

Lower hybrid (LH) ray propagation in toroidal plasma is shown to be controlled by combination of the azimuthal spectrum launched by the antenna, the poloidal variation of the magnetic field, and the scattering of the waves by the drift wave fluctuations. The width of the poloidal and radial radio frequency wave spectrum increases rapidly as the rays penetrate into higher density and scatter from the drift waves. The electron temperature gradient (ETG) spectrum is particularly effective in scattering the LH waves due to its comparable wavelengths and phase velocities. ETG turbulence is also driven by the radial gradient of the electron current profile giving rise to an anomalous viscosity spreading the LH driven plasma currents. The LH wave scattering is derived from a Fokker-Planck equation for the distribution of the ray trajectories with diffusivities derived from the drift wave fluctuations. The condition for chaotic diffusion for the rays is derived. The evolution of the poloidal and radial mode number spectrum of the lower hybrid waves are both on the antenna spectrum and the spectrum of the drift waves. Antennas launching higher poloidal mode number spectra drive off-axis current density profiles producing negative central shear [RS] plasmas with improved thermal confinement from ETG transport. Core plasma current drive requires antennas with low azimuthal mode spectra peaked at m = 0 azimuthal mode numbers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Preliminary results from direct-drive cryogenic target implosion experiments on SGIII prototype laser facility

Since ignition target design with layered deuterium and triterium ice had been proposed several decades ago, much effort was devoted to fabricate and implode cryogenic targets. Until recently, direct-drive cryogenic target implosion experiment was carried out on SGIII prototype laser facility. The target consisted of a plastic capsule supported by fill tube. Cryogenic helium gas was used to cool the capsule to a few degrees below the deuterium triple point. The resulting deuterium ice layer was characterized by optical shadowgraph and smoothed by applied temperature gradient. Eight laser beams with total energy of 7 kJ were used to directly drive the implosion. On the path of laser light to the capsule, there were 500 nm sealing film and helium gas of mm length. X-ray pinhole images were analyzed to confirm that the sealing film, and helium gas had little effect on aiming accuracy but caused some loss of laser energy especially when condensation on the sealing film was observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

A horizontal vane radiometer: Experiment, theory, and simulation

The existence of two motive forces on a Crookes radiometer has complicated the investigation of either force independently. The thermal creep shear force in particular has been subject to differing interpretations of the direction in which it acts and its order of magnitude. In this article, we provide a horizontal vane radiometer design which isolates the thermal creep shear force. The horizontal vane radiometer is explored through experiment, kinetic theory, and the Direct Simulation Monte Carlo (DSMC) method. The qualitative agreement between the three methods of investigation is good except for a dependence of the force on the width of the vane even when the temperature gradient is narrower than the vane which is present in the DSMC method results but not in the theory. The experimental results qualitatively resemble the theory in this regard. The quantitative agreement between the three methods of investigation is better than an order of magnitude in the cases examined. The theory is closer to the experimental values for narrow vanes and the simulations are closer to the experimental values for the wide vanes. We find that the thermal creep force acts from the hot side to the cold side of the vane. We also find the peak in the radiometer’s angular speed as a function of pressure is explained as much by the behavior of the drag force as by the behavior of the thermal creep force.

42 ENGINEERING↗
DOE OSTI2012

FIRST KECK NULLING OBSERVATIONS OF A YOUNG STELLAR OBJECT: PROBING THE CIRCUMSTELLAR ENVIRONMENT OF THE HERBIG Ae STAR MWC 325

We present the first N-band nulling plus K- and L-band V{sup 2} observations of a young stellar object, MWC 325, taken with the 85 m baseline Keck Interferometer. The Keck nuller was designed for the study of faint dust signatures associated with debris disks, but it also has a unique capability for studying the temperature and density distribution of denser disks found around young stellar objects. Interferometric observations of MWC 325 at K, L, and N encompass a factor of five in spectral range and thus, especially when spectrally dispersed within each band, enable characterization of the structure of the inner disk regions where planets form. Fitting our observations with geometric models such as a uniform disk or a Gaussian disk show that the apparent size increases monotonically with wavelength in the 2-12 {mu}m wavelength region, confirming the widely held assumption based on radiative transfer models, now with spatially resolved measurements over a broad wavelength range, that disks are extended with a temperature gradient. The effective size is a factor of about 1.4 and 2.2 larger in the L band and N band, respectively, compared to that in the K band. The existing interferometric measurements and the spectral energy distribution can be reproduced by a flat disk or a weakly shadowed nearly flat disk model, with only slight flaring in the outer regions of the disk, consisting of representative 'sub-micron' (0.1 {mu}m) and 'micron' (2 {mu}m) grains of a 50:50 ratio of silicate and graphite. This is in marked contrast to the disks previously found in other Herbig Ae/Be stars, suggesting a wide variety in the disk properties among Herbig Ae/Be stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Thermal spin-transfer torque in magnetic tunnel junctions (invited)

The thermal spin-transfer torque (TSTT) is an effect to switch the magnetic free layer in a magnetic tunnel junction by a temperature gradient only. We present ab initio calculations of the TSTT. In particular, we discuss the influence of magnetic layer composition by considering Fe{sub x}Co{sub 1–x} alloys. Further, we compare the TSTT to the bias voltage driven STT and discuss the requirements for a possible thermal switching. For example, only for very thin barriers of 3 monolayers MgO, a thermal switching is imaginable. However, even for such a thin barrier, the TSTT is still too small for switching at the moment and further optimization is needed. In particular, the TSTT strongly depends on the composition of the ferromagnetic layer. In our current study, it turns out that at the chosen thickness of the ferromagnetic layer, pure Fe gives the highest thermal spin-transfer torque.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Sedimentary Geothermal Resources in Nevada, Utah, Colorado, and Texas

The objectives of this project were to (1) perform a literature review of sedimentary geothermal resources, (2) identify data sources and develop data-collection methodologies that characterize selected resources, (3) screen sedimentary basins and formations for sedimentary geothermal potential, and (4) evaluate the technical feasibility of one or more selected locations. Numerous publications have characterized geothermal resources within sedimentary basins. A literature search reviewed publications describing resources located in Colorado, Louisiana, Nevada, Texas, Utah, and Wyoming. The most attractive resources have high temperature gradients, low drilling costs, and reservoir permeabilities greater than 10 millidarcies (mD). Prospects in Colorado, Nevada, Texas, and Utah exhibit attractive characteristics and were chosen for further analysis. Sedimentary resources in Nevada and Utah are most attractive, followed by tested resources in Texas and untested resources in Colorado. The identified resources in Wyoming and Louisiana had lower geothermal gradients and were not evaluated. Reservoir modeling and techno-economic analysis were performed at Marys River Basin–North in Nevada. Geothermal energy production at this location is expected to have a levelized cost of energy (LCOE) ranging between 10 and 20 cents/kWh. Additional work may result in lower LCOE estimates at this location and at other attractive prospects in these three regions. The Great Basin carbonate and alluvial aquifer system of eastern Nevada and western Utah includes a lower carbonate aquifer unit, which has the potential for hosting both conduction- and convection-dominated geothermal systems. Mapping has identified lateral thickness variability expressed as a roughly 120–150-km-wide central corridor, which hosts the thickest and most continuous formations and extends from near Las Vegas north to the Idaho border. Purchase and analysis of privately held legacy seismic data could potentially compensate for the lack of sufficient data documenting measured depth to the lower carbonate aquifer unit. Multiple orogenies, extension episodes, and intrusive events have deformed and displaced the target formations of the lower carbonate aquifer unit. This structural complexity and potential for dipping reservoirs emphasizes the need for detailed geological, geophysical, and reservoir modeling. Heat flow within three Colorado sedimentary basins reviewed as part of this study was calculated in targeted studies by the Colorado Geologic Survey and Colorado School of Mines. These calculations are based on bottom-hole temperature data sets with significant limitations and some variability but produce values consistently higher than the global continental average of 65 mW/m2 for all three basins. Heat flow in the Raton Basin is the highest; however, permeability measurements from specific sedimentary formations with high heat flow have not been obtained. Promising formations for sedimentary geothermal systems were found in all three regions studied—Nevada-Utah, Colorado, and Texas. The next steps for developing sedimentary geothermal resources vary due to differences in available data and resource uncertainties. Additional scopes of work are recommended for identified basins in these three regions.

15 GEOTHERMAL ENERGY↗
DOE OSTI2012

Theoretical investigation of the impact of grain boundaries and fission gases on UO 2 thermal conductivity

Thermal conductivity is one of the most important metrics of nuclear fuel performance. Therefore, it is crucial to understand the impact of microstructure features on thermal conductivity, especially since the microstructure evolves with burn-up or time in the reactor. For example, UO 2 fuels are polycrystalline and for high-burnup fuels the outer parts of the pellet experience grain sub-division leading to a very fine grain structure. This is known to impact important physical properties such as thermal conductivity as fission gas release. In a previous study, we calculated the effect of different types of {Sigma}5 grain boundaries on UO 2 thermal conductivity and predicted the corresponding Kapitza resistances, i.e. the resistance of the grain boundary in relation to the bulk thermal resistance. There have been reports of pseudoanisotropic effects for the thermal conductivity in cubic polycrystalline materials, as obtained from molecular dynamics simulations, which means that the conductivity appears to be a function of the crystallographic direction of the temperature gradient. However, materials with cubic symmetry should have isotropic thermal conductivity. For this reason it is necessary to determine the cause of this apparent anisotropy and in this report we investigate this effect in context of our earlier simulations of UO 2 Kapitza resistances. Another source of thermal resistance comes from fission products and fission gases. Xe is the main fission gas and when generated in sufficient quantity it dissolves from the lattice and forms gas bubbles inside the crystalline structure. We have performed studies of how Xe atoms dissolved in the UO 2 matrix or precipitated as bubbles impact thermal conductivity, both in bulk UO 2 and in the presence of grain boundaries.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Evidence and modeling of turbulence bifurcation in L-mode confinement transitions on Alcator C-Mod

Analysis and modeling of rotation reversal hysteresis experiments show that a single turbulent bifurcation is responsible for the Linear to Saturated Ohmic Confinement (LOC/SOC) transition and concomitant intrinsic rotation reversal on Alcator C-Mod. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite the profiles of density and temperature, which are indistinguishable within measurement uncertainty. Elements of this bifurcation are also shown to persist for auxiliary heated L-modes. The deactivation of subdominant (in the linear growth rate and contribution to heat transport) ion temperature gradient and trapped electron mode instabilities is identified as the only possible change in turbulence within a reduced quasilinear transport model across the reversal, which is consistent with the measured profiles and inferred heat and particle fluxes. Experimental constraints on a possible change from strong to weak turbulence, outside the description of the quasilinear model, are also discussed. These results indicate an explanation for the LOC/SOC transition that provides a mechanism for the hysteresis through the dynamics of subdominant modes and changes in their relative populations and does not involve a change in the most linearly unstable ion-scale drift-wave instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Collisionless microtearing modes in hot tokamaks: Effect of trapped electrons

Collisionless microtearing modes have recently been found linearly unstable in sharp temperature gradient regions of large aspect ratio tokamaks. The magnetic drift resonance of passing electrons has been found to be sufficient to destabilise these modes above a threshold plasma β. A global gyrokinetic study, including both passing electrons as well as trapped electrons, shows that the non-adiabatic contribution of the trapped electrons provides a resonant destabilization, especially at large toroidal mode numbers, for a given aspect ratio. The global 2D mode structures show important changes to the destabilising electrostatic potential. The β threshold for the onset of the instability is found to be generally downshifted by the inclusion of trapped electrons. A scan in the aspect ratio of the tokamak configuration, from medium to large but finite values, clearly indicates a significant destabilizing contribution from trapped electrons at small aspect ratio, with a diminishing role at larger aspect ratios.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

SOFIA/FORCAST OBSERVATIONS OF WARM DUST IN S106: A FRAGMENTED ENVIRONMENT

We present mid-IR (19–37 μm) imaging observations of S106 from SOFIA/FORCAST, complemented with IR observations from Spitzer/IRAC (3.6–8.0 μm), IRTF/MIRLIN (11.3 and 12.5 μm), and Herschel/PACS (70 and 160 μm). We use these observations, observations in the literature, and radiation transfer modeling to study the heating and composition of the warm (∼100 K) dust in the region. The dust is heated radiatively by the source S106 IR, with little contributions from grain–electron collisions and Lyα radiation. The dust luminosity is ≳(9.02 ± 1.01) × 10{sup 4} L{sub ⊙}, consistent with heating by a mid- to late-type O star. We find a temperature gradient (∼75–107 K) in the lobes, which is consistent with a dusty equatorial geometry around S106 IR. Furthermore, the SOFIA observations resolve several cool (∼65–70 K) lanes and pockets of warmer (∼75–90 K) dust in the ionization shadow, indicating that the environment is fragmented. We model the dust mass as a composition of amorphous silicates, amorphous carbon, big grains, very small grains, and polycyclic aromatic hydrocarbons. We present the relative abundances of each grain component for several locations in S106.

79 ASTRONOMY AND ASTROPHYSICS↗