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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Surface diffusion activation energy determination using ion beam microtexturing

The activation energy for impurity atom (adatom) surface diffusion can be determined from the temperature dependence of the spacing of sputter cones. These cones are formed on the surface during sputtering while simultaneously adding impurities. The impurities form clusters by means of surface diffusion, and these clusters in turn initiate cone formation. Values are given for the surface diffusion activation energies for various materials on polycrystalline Cu, Al, Pb, Au, and Ni. The values for different impurity species on each of these substrates are approximately independent of impurity species within the experimental uncertainty, suggesting the absence of strong chemical bonding effects on the diffusion.

Rossnagel, S. M.↗
NASA NTRS1992

Orbital Transfer Vehicle Engine Technology High Velocity Ratio Diffusing Crossover

High speed, high efficiency head rise multistage pumps require continuous passage diffusing crossovers to effectively convey the pumped fluid from the exit of one impeller to the inlet of the next impeller. On Rocketdyne's Orbital Transfer Vehicle (OTV), the MK49-F, a three stage high pressure liquid hydrogen turbopump, utilizes a 6.23 velocity ratio diffusing crossover. This velocity ratio approaches the diffusion limits for stable and efficient flow over the operating conditions required by the OTV system. The design of the high velocity ratio diffusing crossover was based on advanced analytical techniques anchored by previous tests of stationary two-dimensional diffusers with steady flow. To secure the design and the analytical techniques, tests were required with the unsteady whirling characteristics produced by an impeller. A tester was designed and fabricated using a 2.85 times scale model of the MK49-F turbopumps first stage, including the inducer, impeller, and the diffusing crossover. Water and air tests were completed to evaluate the large scale turbulence, non-uniform velocity, and non-steady velocity on the pump and crossover head and efficiency. Suction performance tests from 80 percent to 124 percent of design flow were completed in water to assess these pump characteristics. Pump and diffuser performance from the water and air tests were compared with the actual MK49-F test data in liquid hydrogen.

Lariviere, Brian W.↗
NASA NTRS2001

An Analytical Model for Non-Uniform Magnetic Field Effects on Two-Dimensional Laminar Jet Diffusion Flames

In 1846, Michael Faraday found that permanent magnets could cause candle flames to deform into equatorial disks. He believed that the change in flame shape was caused by the presence of charged particles within the flames interacting with the magnetic fields. Later researchers found that the interaction between the flame ions and the magnetic fields were much too small to cause the flame deflection. Through a force analysis, von Engel and Cozens showed that the change in the flame shape could be attributed to the diamagnetic flame gases in the paramagnetic atmosphere. Paramagnetism occurs in materials composed of atoms with permanent magnetic dipole moments. In the presence of magnetic field gradients, the atoms align with the magnetic field and are drawn into the direction of increasing magnetic field. Diamagnetism occurs when atoms have no net magnetic dipole moment. In the presence of magnetic gradient fields, diamagnetic substances are repelled towards areas of decreasing magnetism. Oxygen is an example of a paramagnetic substance. Nitrogen, carbon monoxide and dioxide, and most hydrocarbon fuels are examples of diamagnetic substances. In order to evaluate the usefulness of these magnets in altering flame behavior, a study has been undertaken to develop an analytical model to describe the change in the flame length of a laminar diffusion jet in the presence of a nonuniform magnetic field.

Calvert, M. E.↗
DOE OSTI2018

Spectroscopic and Computational Analysis of Chemical Pollutants Interactions on Steel - 18409

There is limited understanding of the scientific interaction of the chemical contaminants to DOE facility materials which can alter the stability of the materials, affecting the general safety. The resulting studies will have a direct impact towards developing an enhanced cost-effective decontamination method and understanding the negative effects caused by the chemical waste on the structural integrity of the solid surface in contact. First order kinematic models provide a better fit to the experimental data, implying that one layer of Hg adsorbs to the steel surface. Energy dispersive X-ray analysis confirmed that stainless steel samples are mainly composed of Fe, Ni, and Cr, and have a low carbon content. Objective: To determine the physiochemical interactions of Mercury (Hg) and Cesium (Cs) to steel and concrete using spectroscopic, adsorption and computational studies. To develop a multiscale simulation that shows the effects of adsorption and diffusion of alkali metals on the mechanical performance of steel alloys used in nuclear power plants. Approach: Molecular Dynamics (MD) simulations were used to compute the stress gradient generated in steel due to Hg adsorption. Finite element models were developed for simulating the effects of temperature and pressure in a diffusion analysis. Based on experimental data, one layer of Hg was used for the MD simulations. Material properties at the atomic level were determined from the MD simulations The stress gradient from the MD simulations is used as an input parameter in the FE simulation. Conclusions and Future Work: Computational methods such as MD and FE simulations were used to determine the effects of mercury adsorption in stainless steel samples. The adsorption process induces a stress gradient in the facility materials, which can affect the mechanical performance of facility materials under service. Tensile stresses help the diffusion process while compressive have the opposite effect. Temperature has a positive effect in the mass diffusion process. Future work will evaluate the effects of temperature on the material properties at the molecular level using both experimental and MD methods. Future work will evaluate the mechanical performance of facility materials at a wide range of environmental variables.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2025

Discrete generative diffusion models without stochastic differential equations: A tensor network approach

Diffusion models (DMs) are a class of generative machine learning methods that sample a target distribution by transforming samples of a trivial (often Gaussian) distribution using a learned stochastic differential equation. In standard DMs, this is done by learning a “score function” that reverses the effect of adding diffusive noise to the distribution of interest. Here we consider the generalisation of DMs to lattice systems with discrete degrees of freedom, and where noise is added via Markov chain jump dynamics. We show how to use tensor networks (TNs) to efficiently define and sample such “discrete diffusion models” (DDMs) without explicitly having to solve a stochastic differential equation. We show the following: (i) by parametrising the data and evolution operators as TNs, the denoising dynamics can be represented exactly; (ii) the auto-regressive nature of TNs allows to generate samples efficiently and without bias; (iii) for sampling Boltzmann-like distributions, TNs allow to construct an efficient learning scheme that integrates well with Monte Carlo. We illustrate this approach to study the equilibrium of two models with non-trivial thermodynamics, the d = 1 constrained Fredkin chain and the d = 2 Ising model. Published by the American Physical Society 2025

Causer, Luke (ORCID:0000000194243473)↗
NASA NTRS1995

Numerical solution of fluid flow and heat tranfer problems with surface radiation

This paper presents a numerical scheme, based on the finite element method, to solve strongly coupled fluid flow and heat transfer problems. The surface radiation effect for gray, diffuse and isothermal surfaces is considered. A procedure for obtaining the view factors between the radiating surfaces is discussed. The overall solution strategy is verified by comparing the available results with those obtained using this approach. An analysis of a thermosyphon is undertaken and the effect of considering the surface radiation is clearly explained.

Ahuja, S.↗
DOE OSTI2025

Experimental Characterization of Hydrogen Diffusion in Shale Rocks for Geologic Storage Applications

As global energy systems undergo a transition to cleaner alternatives, geologic hydrogen storage has emerged as a promising solution for large-scale energy storage. A critical factor in determining the feasibility of this approach is the effectiveness of caprock formations, such as shale, in preventing hydrogen migration. This study investigates the diffusion behavior of hydrogen through shale to assess its suitability as a caprock for geologic hydrogen storage. Using a novel double-seal core holder design and a through-diffusion apparatus, hydrogen diffusion was measured through shale rock from the Eagle Ford and Wolfcamp Formations under dry conditions. These measurements were complemented by microstructural and mineralogical analyses using low-pressure nitrogen adsorption and X-ray diffraction. The effective diffusion coefficient of hydrogen in these shale caprocks ranged from 2.51 × 10 –8 to 9.85 × 10 –8 m 2 /s. Notably, we observed that the diffusion behavior was more related to the pore network structure and could not be attributed to differences in the total pore volume between shale types alone. Here, to further understand the role of pore network complexity, a fractal pore model was developed to correlate tortuosity with the fractal dimension of the pore structure (a measure of pore network complexity). The proposed model closely matched tortuosity values obtained from diffusion experiments, outperforming existing theoretical tortuosity–porosity correlations. These findings provide key quantitative parameters needed to assess the feasibility of geologic hydrogen storage as well as insights that can be applied to hydrogen storage in a range of geologic formations.

08 HYDROGEN↗
NASA NTRS1994

Experimental investigation of the Marangoni effect on the stability of a double-diffusive layer

Stability experiments were carried out in 4-cm-thick, salt-stratified fluid layer by heating from below and cooling from above. The bottom boundary was rigid while the top was either free or rigid. The initial solute Rayleigh number varied from 2.5 x 10(exp 6) to 4.6 x 10(exp 7). For the rigid-free case, at initial solute Rayleigh numbers R(sub s) greater than 5.4 x 10(exp 6), thermal Marangoni instabilities were observed to onset along the free surface at a relatively low thermal Rayleigh number, R(sub t). The convection was very weak, and it had almost no effect on the concentration and temperature distributions. Double-diffusive instabilities along the top free surface were observed to onset at a higher R(sub t), with much stronger convection causing changes in the concentration and temperature distributions near the top. At a yet higher R(sub t), double-diffusive convection was observed to onset along the bottom boundary. Fluid motion in the layer then evolved into fully developed thermal convection of a homogeneous fluid without any further increase in the imposed Delta T. For layers with R(sub s) less than 5.4 x 10(exp 6), Marangoni and double-diffusive instabilities onset simultaneously along the free surface first, while double-diffusive instabilities along the bottom wall onset at a higher R(sub t).

Tanny, Josef↗
NASA NTRS2013

Modeling Analysis for NASA GRC Vacuum Facility 5 Upgrade

A model of the VF5 test facility at NASA Glenn Research Center was developed using the direct simulation Monte Carlo Hypersonic Aerothermodynamics Particle (HAP) code. The model results were compared to several cold flow and thruster hot fire cases. The main uncertainty in the model is the determination of the effective sticking coefficient -- which sets the pumping effectiveness of the cryopanels and oil diffusion pumps including baffle transmission. An effective sticking coefficient of 0.25 was found to provide generally good agreement with the experimental chamber pressure data. The model, which assumes a cold diffuse inflow, also fared satisfactorily in predicting the pressure distribution during thruster operation. The model was used to assess other chamber configurations to improve the local effective pumping speed near the thruster. A new configuration of the existing cryopumps is found to show more than 2x improvement over the current baseline configuration.

cryopumping↗
DOE OSTI2025

Computing Nonequilibrium Responses with Score-Shifted Stochastic Differential Equations

Using equilibrium fluctuations to understand the response of a physical system to an externally imposed perturbation is the basis for linear response theory, which is widely used to interpret experiments and shed light on microscopic dynamics. For nonequilibrium systems, perturbations cannot be interpreted simply by monitoring fluctuations in a conjugate observable and general response results rely on path ensemble averaging. Furthermore, these techniques do not apply to perturbations that affect the diffusion tensor in a stochastic system. Here, we introduce an “effective” physical process that represents the diffusion perturbed dynamics and enables accurate calculations of responses to a change in the diffusion. Interestingly, the effective dynamics contain an additional drift involving the instantaneous “score” of the system, and we leverage score matching algorithms to carry out nonequilibrium response calculations on systems for which the exact stationary distribution is unknown.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1988

Weak rotating flow disturbances in a centrifugal compressor with a vaneless diffuser

A theory is presented to predict the occurrence of weak rotating waves in a centrifugal compression system with a vaneless diffuser. As in a previous study of axial systems, an undisturbed performance characteristic is assumed known. Following an inviscid analysis of the diffuser flow, conditions for a neutral rotating disturbance are found. The solution is shown to have two branches; one with fast rotation, the other with very slow rotation. The slow branch includes a dense set of resonant solutions. The resonance is a feature of the diffuser flow, and therefore such disturbances must be expected at the various resonant flow coefficients regardless of the compressor characteristic. Slow solutions seem limited to flow coefficients less than about 0.3, where third and fourth harmonics appear. Fast waves seem limited to a first harmonic. These fast and slow waves are described, and effects of diffuser-wall convergence, backward blade angles, and partial recovery of exit velocity head are assessed.

Moore, F. K.↗
DOE OSTI2019

Ab Initio Calculation of Equilibrium Isotopic Fractionations of Potassium and Rubidium in Minerals and Water

Here, we used first-principle approaches to calculate the equilibrium isotopic fractionation factors of potassium (K) and rubidium (Rb) in a variety of minerals of geological relevance (orthoclase, albite, muscovite, illite, sylvite, and phlogopite). We also used molecular dynamics simulations to calculate the equilibrium isotopic fractionation factors of K in water. Our results indicate that K and Rb form bonds of similar strengths and that the ratio between the equilibrium fractionations of K and Rb is approximately 3–4. Under low-temperature conditions relevant to weathering of continents or alteration of seafloor basalts (~25 °C), the K isotopic fractionation between solvated K+ and illite (a proxy for K-bearing clays) is +0.24‰, exceeding the current analytical precision, so equilibrium isotopic fractionation can induce measurable isotopic fractionations for this system at low temperature. These findings, however, cannot easily explain why the δ 41 K value of seawater is shifted by +0.6‰ relative to igneous rocks. Our results indicate that part of the observed fractionation is most likely due to kinetic effects. The narrow range of mean force constants for K and Rb in silicate minerals suggests that phase equilibrium is unlikely to create large K and Rb isotopic fractionations at magmatic temperatures (at least in silicate systems). Kinetic effects associated with diffusion can, however, produce large K and Rb isotopic fractionations in igneous rocks.

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

Improvement of stability of Nb 3 Sn superconductors by introducing high specific heat substances

High-J c Nb 3 Sn conductors have low stability against perturbations, which accounts for the slow training rates of high-field Nb 3 Sn magnets. While it is known that adding substances with high specific heat (C) into Nb 3 Sn wires can increase their overall specific heat and thus improve their stability, there has not been a practical method that is compatible with the fabrication of long-length conductors. In this work, we put forward a scheme to introduce such substances to distributed-barrier Nb 3 Sn wires, which adds minimum difficulty to the wire manufacturing process. Multifilamentary wires using a mixture of Cu and high-C Gd 2 O 3 powders have been successfully fabricated along this line. Measurements showed that addition of Gd 2 O 3 had no negative effects on residual resitivity ratio or non-Cu J c , and that flux jumps were remarkably reduced, and minimum quench energy values at 4.2 K, 14 T were increased by a factor of three, indicating that stability was significantly improved. We also discussed the influences of the positioning of high-C substances and their thermal diffusivity on their effectiveness in reducing the superconductor temperature rise against perturbations. Based on these results, we proposed an optimized conductor architecture to maximize the effectiveness of this approach.

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Structure and dynamics of ILs-based gel polymer electrolytes and its enhanced conductive properties with the incorporation of Al 2 O 3 nanofibers

Here, this work reports the enhanced mobility of ions in ionic liquid (IL)-based gel polymer electrolytes (GPEs) with the incorporation of Al 2 O 3 nanofibers. A combination of PVDF-HFP, EMIMTFSI and LiTFSI with 3 wt% Al 2 O 3 nanofibers has been prepared through solution casting technique. The room temperature ionic conductivity of PVDF-HFP: ILs electrolyte (45:55, weight ratio of 0.82) (GPE) is found to be 2.7 × 10 –5 S cm –1 , which increases up to 7.8 × 10 –5 S cm –1 in Al 2 O 3 containing GPE (Al-GPE). Pulsed field gradient (PFG) NMR results validate the increased ionic conductivity observed in Al-GPE. We found that the diffusivity of Li + , TFSI – and EMIM + increases when Al 2 O 3 nanofibers are well-distributed in the GPE matrix. The surface morphology and the amorphicity of GPEs are examined through SEM and XRD analyses. Lastly, the local structure of Al 2 O 3 fibers and the molecular-level interactions of ions with polymer, and their effect on the diffusivity of ions are established through solid-state NMR detecting 27 Al, 1 H, 13 C, 19 F nuclei including 2D 13 C{ 1 H} HETCOR NMR experiments. The 13 C DPMAS and CPMAS experiments highlight the dynamic heterogeneity associated with the ions that are embedded in the rigid and the mobile phase of GPEs. While some of the ionic species strongly interact with polymer chains in the rigid environment, the majority of them reside in the mobile phase and contribute to the overall increased conductivity. Most importantly, Al 2 O 3 nanofibers significantly affect the dynamics of ionic species that are present in the mobile phase between the polymer chains.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Kinetic energy dependence of carrier diffusion in a GaAs epilayer studied by wavelength selective PL imaging

Photoluminescence (PL) imaging has been shown to be an efficient technique for investigating carrier diffusion in semiconductors. In the past, the measurement was typically carried out by measuring at one wavelength (e.g., at the band gap) or simply the whole emission band. At room temperature in a semiconductor like GaAs, the band-to-band PL emission may occur in a spectral range over 200 meV, vastly exceeding the average thermal energy of about 26 meV. To investigate the potential dependence of the carrier diffusion on the carrier kinetic energy, we performed wavelength selective PL imaging on a GaAs double hetero-structure in a spectral range from about 70 meV above to 50 meV below the bandgap, extracting the carrier diffusion lengths at different PL wavelengths by fitting the imaging data to a theoretical model. The results clearly show that the locally generated carriers of different kinetic energies mostly diffuse together, maintaining the same thermal distribution throughout the diffusion process. Potential effects related to carrier density, self-absorption, lateral wave-guiding, and local heating are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗