Search NASA⌕ Search

SEARCH · Search NASA

Results for “temperature gradient”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Thermodiffusion in concentrated ferrofluids: A review and current experimental and numerical results on non-magnetic thermodiffusion

Ferrofluids are colloidal suspensions consisting of magnetic nanoparticles dispersed in a carrier liquid. Their thermodiffusive behaviour is rather strong compared to molecular binary mixtures, leading to a Soret coefficient (S{sub T}) of 0.16 K{sup −1}. Former experiments with dilute magnetic fluids have been done with thermogravitational columns or horizontal thermodiffusion cells by different research groups. Considering the horizontal thermodiffusion cell, a former analytical approach has been used to solve the phenomenological diffusion equation in one dimension assuming a constant concentration gradient over the cell's height. The current experimental work is based on the horizontal separation cell and emphasises the comparison of the concentration development in different concentrated magnetic fluids and at different temperature gradients. The ferrofluid investigated is the kerosene-based EMG905 (Ferrotec) to be compared with the APG513A (Ferrotec), both containing magnetite nanoparticles. The experiments prove that the separation process linearly depends on the temperature gradient and that a constant concentration gradient develops in the setup due to the separation. Analytical one dimensional and numerical three dimensional approaches to solve the diffusion equation are derived to be compared with the solution used so far for dilute fluids to see if formerly made assumptions also hold for higher concentrated fluids. Both, the analytical and numerical solutions, either in a phenomenological or a thermodynamic description, are able to reproduce the separation signal gained from the experiments. The Soret coefficient can then be determined to 0.184 K{sup −1} in the analytical case and 0.29 K{sup −1} in the numerical case. Former theoretical approaches for dilute magnetic fluids underestimate the strength of the separation in the case of a concentrated ferrofluid.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Electron-beam activated thermal sputtering of thermoelectric materials

Thermoelectricity and Seebeck effect have long been observed and validated in bulk materials. With the development of advanced tools of materials characterization, here we report the first observation of such an effect in the nanometer scale: in situ directional sputtering of several thermoelectric materials inside electron microscopes. The temperature gradient introduced by the electron beam creates a voltage-drop across the samples, which enhances spontaneous sputtering of specimen ions. The sputtering occurs along a preferential direction determined by the direction of the temperature gradient. A large number of nanoparticles form and accumulate away from the beam location as a result. The sputtering and re-crystallization are found to occur at temperatures far below the melting points of bulk materials. The sputtering occurs even when a liquid nitrogen cooling holder is used to keep the overall temperature at -170 deg. C. This unique phenomenon that occurred in the nanometer scale may provide useful clues to understanding the mechanism of thermoelectric effect.

36 MATERIALS SCIENCE↗
NASA NTRS1985

Differential photometry of magnetic faculae

New observations extending the differential continuum photometry technique to the near-infrared are presented which demonstrate that the main factor determining the facular visibility near the solar disk center is the difference in H(-) opacity (and thus in penetration depth) between the continuum passbands. It is shown how the difference signal obtained depends on the temperature gradient difference between the two faculae and photosphere. The measured facular temperature gradient is compared with that given by empirical facular models based on Fraunhofer line observations, and with theoretical predictions. An explanation for the enhanced visibility of faculae against granular noise in the difference signal is suggested.

Foukal, P.↗
NASA NTRS1988

Effect of vertical magnetic field on convection and segregation in vertical Bridgman crystal growth

A previous finite-element analysis of vertical Bridgman growth for dilute and nondilute alloys is extended to include the effect of a vertically-aligned magnetic field in the limit of zero magnetic Reynolds number. Calculations are presented for growth of a dilute gallium-germanium alloy in a vertically stabilized Bridgman-Stockbarger system and in a furnace with a uniform temperature gradient imposed along the ampoule. Steady cellular convection driven by radial temperature gradients causes good axial and radial mixing in both systems without a magnetic field. A weak magnetic field decreases the intensity of convection and the effectiveness of solute mixing. The radial nonuniformity is greatest for an intermediate field strength. Stronger fields suppress flow recirculation completely, and lead to uniform solute segregation across the crystal and to diffusion-controlled axial segregation.

Kim, Do Hyun↗
DOE OSTI2014

Transitions to improved confinement regimes induced by changes in heating in zero-dimensional models for tokamak plasmas

It is shown that rapid substantial changes in heating rate can induce transitions to improved energy confinement regimes in zero-dimensional models for tokamak plasma phenomenology. We examine for the first time the effect of step changes in heating rate in the models of Kim and Diamond [Phys. Rev. Lett. 90, 185006 (2003)] and Malkov and Diamond [Phys. Plasmas 16, 012504 (2009)], which nonlinearly couple the evolving temperature gradient, micro-turbulence, and a mesoscale flow; and in the extension of Zhu et al. [Phys. Plasmas 20, 042302 (2013)], which couples to a second mesoscale flow component. The temperature gradient rises, as does the confinement time defined by analogy with the fusion context, while micro-turbulence is suppressed. This outcome is robust against variation of heating rise time and against introduction of an additional variable into the model. It is also demonstrated that oscillating changes in heating rate can drive the level of micro-turbulence through a period-doubling path to chaos, where the amplitude of the oscillatory component of the heating rate is the control parameter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

The snake geothermal drilling project. Innovative approaches to geothermal exploration

The goal of our project was to test innovative technologies using existing and new data, and to ground-truth these technologies using slim-hole core technology. The slim-hole core allowed us to understand subsurface stratigraphy and alteration in detail, and to correlate lithologies observed in core with surface based geophysical studies. Compiled data included geologic maps, volcanic vent distribution, structural maps, existing well logs and temperature gradient logs, groundwater temperatures, and geophysical surveys (resistivity, magnetics, gravity). New data included high-resolution gravity and magnetic surveys, high-resolution seismic surveys, three slimhole test wells, borehole wireline logs, lithology logs, water chemistry, alteration mineralogy, fracture distribution, and new thermal gradient measurements.

15 GEOTHERMAL ENERGY↗
DOE OSTI2014

Thermo-optical piston in gases

The new steady state of the gas – thermo-optical pressure difference is considered. This condition occurs in the gas that is in a closed capillary in the field of resonant laser radiation and a temperature gradient. The pressure difference at the ends of the capillary is determined by the interaction of three fluxes – thermal creep, the light-induced drift and Poiseuille flux. Laser radiation and the temperature gradient play the role of thermo-optical piston (TOP) which compresses the gas in different ends of the capillary. The problem is solved based on the linearized Boltzmann kinetic equations that take into account the induced and spontaneous transitions in atoms or molecules. Expressions for the kinetic coefficients defining TOP-effect are obtained in the case of a nearly free-molecular regime. Numerical estimates of the TOP-effect are given for sodium vapor.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1974

Geophysical, geochemical, and geological investigations of the Dunes geothermal system, Imperial Valley, California

The Dunes anomaly is a water-dominated geothermal system in the alluvium of the Salton Trough, lacking any surface expression. It was discovered by shallow-temperature gradient measurements. A 612-meter-deep test well encountered several temperature-gradient reversals, with a maximum of 105 C at 114 meters. The program involves surface geophysics, including electrical, gravity, and seismic methods, down-hole geophysics and petrophysics of core samples, isotopic and chemical studies of water samples, and petrological and geochemical studies of the cores and cuttings. The aim is (1) to determine the source and temperature history of the brines, (2) to understand the interaction between the brines and rocks, and (3) to determine the areal extent, nature, origin, and history of the geothermal system. These studies are designed to provide better definition of exploration targets for hidden geothermal anomalies and to contribute to improved techniques of exploration and resource assessment.

Elders, W. A.↗
DOE OSTI2015

Control of thermal gradient using thermoelectric coolers for study of thermal effects

Thermoelectric coolers based on the Peltier effect have been utilized to control temperature gradient to study thermal effects in both bulk and thin film samples. The temperature gradient is controlled by two coolers and the polarity of the thermal gradient can be reversed by reversing an electric driven voltage. With appropriate controlled thermal gradient using this technique, the Nernst and the Seebeck effects can be measured in both bulk and thin film samples free of spurious contributions. In an arbitrary direction of thermal gradient, the Seebeck and the Nernst components can be decomposed from the measured signal based on the symmetry of the effects in a magnetic field.

47 OTHER INSTRUMENTATION↗
DOE OSTI2012

Electron profile stiffness and critical gradient studies

Electron profile stiffness was studied in DIII-D L-mode discharges by systematically varying the heat flux in a narrow region with electron cyclotron heating and measuring the local change produced in {nabla}T{sub e}. Electron stiffness was found to slowly increase with toroidal rotation velocity. A critical inverse temperature gradient scale length 1/L{sub C} {approx} 3 m{sup -1} was identified at {rho}=0.6 and found to be independent of rotation. Both the heat pulse diffusivity and the power balance diffusivity, the latter determined by integrating the measured dependence of the heat pulse diffusivity on -{nabla}T{sub e}, were fit reasonably well by a model containing a critical inverse temperature gradient scale length and varying linearly with 1/L{sub T} above the threshold.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2010

Improved Cloud Condensation Nucleus Spectrometer

An improved thermal-gradient cloud condensation nucleus spectrometer (CCNS) has been designed to provide several enhancements over prior thermal- gradient counters, including fast response and high-sensitivity detection covering a wide range of supersaturations. CCNSs are used in laboratory research on the relationships among aerosols, supersaturation of air, and the formation of clouds. The operational characteristics of prior counters are such that it takes long times to determine aerosol critical supersaturations. Hence, there is a need for a CCNS capable of rapid scanning through a wide range of supersaturations. The present improved CCNS satisfies this need. The improved thermal-gradient CCNS (see Figure 1) incorporates the following notable features: a) The main chamber is bounded on the top and bottom by parallel thick copper plates, which are joined by a thermally conductive vertical wall on one side and a thermally nonconductive wall on the opposite side. b) To establish a temperature gradient needed to establish a supersaturation gradient, water at two different regulated temperatures is pumped through tubes along the edges of the copper plates at the thermally-nonconductive-wall side. Figure 2 presents an example of temperature and supersaturation gradients for one combination of regulated temperatures at the thermally-nonconductive-wall edges of the copper plates. c) To enable measurement of the temperature gradient, ten thermocouples are cemented to the external surfaces of the copper plates (five on the top plate and five on the bottom plate), spaced at equal intervals along the width axis of the main chamber near the outlet end. d) Pieces of filter paper or cotton felt are cemented onto the interior surfaces of the copper plates and, prior to each experimental run, are saturated with water to establish a supersaturation field inside the main chamber. e) A flow of monodisperse aerosol and a dilution flow of humid air are introduced into the main chamber at the inlet end. The inlet assembly is designed to offer improved (relative to prior such assemblies) laminar-flow performance within the main chamber. Dry aerosols are subjected to activation and growth in the supersaturation field. f) After aerosol activation, at the outlet end of the main chamber, a polished stainless-steel probe is used to sample droplets into a laser particle counter. The probe features an improved design for efficient sampling. The counter has six channels with size bins in the range of 0.5- to 5.0-micron diameter. g) To enable efficient sampling, the probe is scanned along the width axis of the main chamber (thereby effecting scanning along the temperature gradient and thereby, further, effecting scanning along the supersaturation gradient) by means of a computer-controlled translation stage.

Leu, Ming-Taun↗
DOE OSTI2013

Nonequilibrium thermal effects on exciton time correlations in coupled semiconductor quantum dots

Theoretical guides to test 'macroscopic realism' in solid-state systems under quantum control are highly desirable. Here, we report on the evolution of a Leggett-Garg inequality (LGI), a combination of two-time correlations, in an out-of-equilibrium set up consisting of two interacting excitons confined in separate semiconductor quantum dots which are coupled to independent baths at different temperatures (T{sub 1} ≠ T{sub 2}). In a Markovian steady-state situation we found a rich variety of dynamical behaviors in different sectors of the average temperature (T{sub M} = (T{sub 1}+T{sub 2})/2) vs. coupling strength to the reservoirs (Γ) space parameter. For high T{sub M} and Γ values the LGI is not violated, as expected. However, by decreasing T{sub M} or Γ a sector of parameters appears where the LGI is violated at thermal equilibrium (T{sub 1} = T{sub 2}) and the violation starts decreasing when the system is moved out of the equilibrium. Surprisingly, at even lower T{sub M} values, for any Γ, there is an enhancement of the LGI violation by exposing the system to a temperature gradient, i.e. quantum correlations increase in a nonequilibrium thermal situation. Results on LGI violations in a steady-state regime are compared with other non-locality-dominated quantum correlation measurements, such as concurrence and quantum discord, between the two excitons under similar temperature gradients.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Initial Results From the Newly Upgraded LTX- β

LTX-β, the upgraded Lithium Tokamak Experiment, recently began its first campaign with a goal to study the transport properties of gradient-free electron temperature profile equilibria with increased toroidal field and neutral beam injection. The temperature-gradient-free equilibria in LTX were enabled by the lithium plasma-facing surface. To make the lithium surface conditions more consistent for LTX-β, a new, faster lithium evaporator system has been developed. The toroidal magnetic field has been nearly doubled to ≥0.3 T. Auxiliary heating and fueling with a high flux (35 A, 20 kV) neutral beam has begun. The beam provides core fueling without cold edge neutrals, to test whether flat temperature profiles can be sustained, and makes possible core ion temperature measurements via charge exchange recombination spectroscopy. Neutral beam fueling has been demonstrated with increases in line-integrated density. The coupling of the beam matches well with NUBEAM predictions for shine through. Improved performance has been observed following lithium evaporative coatings and an increase in field strength.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

First implementation of gyrokinetic exact linearized Landau collision operator and comparison with models

Gyrokinetic simulations are fundamental to understanding and predicting turbulent transport in magnetically confined fusion plasmas. Previous simulations have used model collision operators with approximate field-particle terms of unknown accuracy and/or have neglected collisional finite Larmor radius (FLR) effects. We have implemented the linearized Fokker–Planck collision operator with exact field-particle terms and full FLR effects in a gyrokinetic code (GENE). The new operator, referred to as “exact” in this paper, allows the accuracy of model collision operators to be assessed. The conservative Landau form is implemented because its symmetry underlies the conservation laws and the H-theorem, and enables numerical methods to preserve this conservation, independent of resolution. The implementation utilizes the finite-volume method recently employed to discretize the Sugama collision model in GENE, allowing direct comparison between the two operators. Results show that the Sugama model appears accurate for the growth rates of trapped electron modes (TEMs) driven only by density gradients, but appreciably underestimates the growth rates as the collisionality and electron temperature gradient increase. The TEM turbulent fluxes near the nonlinear threshold using the exact operator are similar to the Sugama model for the n e = d ln T e /d ln n e = 0 case, but substantially larger than the Sugama model for the n e = 1 case. The FLR effects reduce the growth rates increasingly with wavenumber deepening a “valley” at the intermediate binormal wavenumber as the unstable mode extends from the TEM regime to the electron temperature gradient instability regime. Application to the Hinton–Rosenbluth problem shows that zonal flows decay faster as the radial wavenumber increases and the exact operator yields weaker decay rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

Modeling the curing process of thick-section autoclave cured composites

Temperature gradients are significant during cure of large area, thick-section composites. Such temperature gradients result in nonuniformly cured parts with high void contents, poor ply compaction, and variations in the fiber/resin distribution. A model was developed to determine the temperature distribution in thick-section autoclave cured composites. Using the model, long with temperature measurements obtained from the thick-section composites, the effects of various processing parameters on the thermal response of the composites were examined. A one-dimensional heat transfer model was constructed for the composite-tool assembly. The governing differential equations and associated boundary conditions describing one-dimensional unsteady heat-conduction in the composite, tool plate, and pressure plate are given. Solution of the thermal model was obtained using an implicit finite difference technique.

Loos, A. C.↗
DOE OSTI2015

Enhanced stabilisation of trapped electron modes by collisional energy scattering in tokamaks

The collisional stabilisation via energy scattering and pitch-angle scattering of micro-instabilities in tokamak plasmas is investigated by means of gyrokinetic simulations with a special emphasis on the often neglected energy scattering operator. It is shown that in the linear regime energy scattering has a negligible effect on Ion Temperature Gradient (ITG) modes but enhances the stabilisation of Trapped Electron Modes (TEM) in presence of nonzero ion temperature and density gradients. This stabilisation is sensitive to the model used for the energy restoring term in the collision operator. The contributions of parallel and drift motion to the total growth rate in velocity space are used to characterize the complex stabilisation mechanisms behind pitch-angle and energy scattering for a range of relevant parameters such as the magnetic shear, the collisionality, the logarithmic density gradient, and the logarithmic ion temperature gradient. It is shown that depending on these parameters, energy scattering stabilisation of TEM can be either due to a decrease of the contribution from drifting trapped electrons or to an increase of the contribution from the parallel motion of passing electrons. Finally, for a standard ITG/TEM case, the effect of energy scattering on the nonlinear heat and particle fluxes is investigated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Quantitative comparison of electron temperature fluctuations to nonlinear gyrokinetic simulations in C-Mod Ohmic L-mode discharges

Long wavelength turbulent electron temperature fluctuations (k{sub y}ρ{sub s} < 0.3) are measured in the outer core region (r/a > 0.8) of Ohmic L-mode plasmas at Alcator C-Mod [E. S. Marmar et al., Nucl. Fusion 49, 104014 (2009)] with a correlation electron cyclotron emission diagnostic. The relative amplitude and frequency spectrum of the fluctuations are compared quantitatively with nonlinear gyrokinetic simulations using the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] in two different confinement regimes: linear Ohmic confinement (LOC) regime and saturated Ohmic confinement (SOC) regime. When comparing experiment with nonlinear simulations, it is found that local, electrostatic ion-scale simulations (k{sub y}ρ{sub s} ≲ 1.7) performed at r/a ∼ 0.85 reproduce the experimental ion heat flux levels, electron temperature fluctuation levels, and frequency spectra within experimental error bars. In contrast, the electron heat flux is robustly under-predicted and cannot be recovered by using scans of the simulation inputs within error bars or by using global simulations. If both the ion heat flux and the measured temperature fluctuations are attributed predominantly to long-wavelength turbulence, then under-prediction of electron heat flux strongly suggests that electron scale turbulence is important for transport in C-Mod Ohmic L-mode discharges. In addition, no evidence is found from linear or nonlinear simulations for a clear transition from trapped electron mode to ion temperature gradient turbulence across the LOC/SOC transition, and also there is no evidence in these Ohmic L-mode plasmas of the “Transport Shortfall” [C. Holland et al., Phys. Plasmas 16, 052301 (2009)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗