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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Numerical thermal analyses of heat exchangers for the stirling engine application

The Regenerator, Cooler and Heater for the NASA Space Power Research Engine (SPRE) have been analyzed in detail for laminar, incompressible and oscillatory flow conditions. Each component has been analyzed independently and in detail with the regenerator being modeled as two-parallel-plates channel with a solid wall. The ends of the channel are exposed to two reservoir maintained at different temperature thus providing an axial temperature gradient along the channel. The cooler and heater components have been modeled as circular pipes with isothermal walls. Two different types of thermal boundary conditions have been investigated for the cooler and heater, namely, symmetric and asymmetric temperature inflow. In symmetric temperature inflow the flow enters the channel with the same temperature in throughout the velocity cycle whereas, in asymmetric temperature inflow the flow enters with a different temperature in each half cycle. The study was conducted over a wide range of Maximum Reynolds number (RE(max) varying from 75 to 60000, Valensi number (Va) from 2.5 to 800, and relative amplitude of fluid displacement (A(sub r) from 0.357 to 1.34. A two dimensional Finite volume method based on the SIMPLE algorithm was used to solve the governing partial differential equations. Post processing programs were developed to effectively describe the heat transfer mechanism under oscillatory flows. The computer code was validated by comparing with existing analytical solutions for oscillating flows. The thermal field have been studied with the help of temperature contour and three dimensional plots. The instantaneous friction factor, wall heat flux and heat transfer coefficient have been examined. It has been concluded that in general, the frictional factor and heat transfer coefficient are higher under oscillatory flow conditions when the Valensi number is high. Also, the thermal efficiency decreases for lower A(r) values. Further, the usual steady state definition for the heat transfer coefficient does not seem to be valid.

Kannapareddy, Mohan Raj↗
DOE OSTI2025

Is There a Scalar Atmospheric Surface Layer Within a Convective Boundary Layer? Implications for Flux Measurements

Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO 2 fluxes, influencing the interpretation of eddy covariance (EC) measurements in the unstable atmospheric surface layer (ASL). Using large eddy simulations for convective boundary layer flows, we demonstrate that decreased temperature gradients across the entrainment zone increase entrainment fluxes by enhancing the entrainment velocity, amplifying the asymmetry between top‐down and bottom‐up flux contributions. These changes alter scalar flux profiles, causing flux divergence or convergence and leading to the breakdown of the constant flux layer assumption (CFLA) in the ASL. As a result, EC‐measured fluxes either underestimate or overestimate “true” surface fluxes during divergence or convergence phases, contributing to energy balance non‐closure. The varying degrees of the CFLA breakdown are a fundamental cause for the non‐closure issue. These findings highlight the underappreciated role of entrainment in interpreting EC fluxes, addressing non‐closure, and understanding site‐to‐site variability in flux measurements.

eddy covariance fluxes of scalars↗
NASA NTRS1989

Temperature and circulation in the stratospheres of the outer planets

A zonally symmetric, linear radiative-dynamical model is compared with observations of the upper tropospheres and stratospheres of the outer planets. Seasonal variation is included in the model. Friction is parameterized by linear drag (Rayleigh friction). Gas opacities are accounted for but aerosols are omitted. Horizontal temperature gradients are small on all the planets. Seasonal effects are strongest on Saturn and Neptune but are weak even in these cases, because the latitudinal gradient of radiative heating is weak. Seasonal effects on Uranus are extremely weak because the radiative time constant is longer that the orbital period. One free parameter in the model is the frictional time constant. Comparison with observed temperature perturbations over zonal currents in the troposphere shows that the frictional time constant is on the same order as the radiative time constant for all these objects. Vertical motions predicted by the model are extremely weak. They are much smaller than one scale height per orbital period, except in the immediate neighborhood of tropospheric and zonal currents.

Conrath, Barney J.↗
NASA NTRS1990

Temperature and circulation in the stratosphere of the outer planets

A zonally symmetric, linear radiative-dynamical model is compared with observations of the upper tropospheres and stratospheres of the outer planets. Seasonal variation is included in the model. Friction is parameterized by linear drag (Rayleigh friction). Gas opacities are accounted for but aerosols are omitted. Horizontal temperature gradients are small on all the planets. Seasonal effects are strongest on Saturn and Neptune but are weak even in these cases, because the latitudinal gradient of radiative heating is weak. Seasonal effects on Uranus are extremely weak because the radiative time constant is longer than the orbital period. One free parameter in the model is the frictional time constant. Comparison with observed temperature perturbations over zonal currents in the troposphere shows that the frictional time constant is on the same order as the radiative time constant for all these objects. Vertical motions predicted by the model are extremely weak. They are much smaller than one scale height per orbital period, except in the immediate neighborhood of tropospheric and zonal currents.

Conrath, Barney J.↗
NASA NTRS1977

Measurements of H-beta, He D3, and Ca/+/ 8542-A line emission in quiescent prominences

Measurements of H-beta, He D3, and Ca(+) 8542-A line emission in 18 quiescent prominences are presented which were performed with a 25-cm coronagraph/coude spectrograph system and an optical multichannel analyzer utilizing a Si-vidicon detector assembly mounted at the single-pass spectrograph port. The data obtained show that the line profiles deviate in the wings from those calculated on the basis of an isothermal atmosphere and a Gaussian absorption coefficient. Both a non-Gaussian microturbulence component and a temperature gradient from prominence-core to coronal values can successfully account for this phenomenon. The line widths from a given line of sight of prominence material are reduced on the basis of a common core emitting region with a temperature of 7500 to 11,600 K and a microturbulent velocity of 4.5 to 7.1 km/s for the isothermal model (these values are 5% to 10% less for the two-temperature model). It is noted that long-period low-amplitude oscillations in the D3 line width and intensity were observed when a specific line of sight in a quiescent prominence was followed. These oscillations are found to remain coherent for as many as 3 or 4 cycles and to be associated with prominence structural activation.

Landman, D. A.↗
NASA NTRS1990

The influence of gas velocity on surface heat pumping for the orifice pulse tube refrigerator

The basic pulse tube refrigerator produces cooling by a mechanism known as surface heat pumping. Cyclical compression and expansion of a gas within an enclosed tube produces large temperature gradients. The purpose of the tube is to act as 'nodal regenerator'. Heat is stored at node positions along the tube wall and is transported between nodes by moving gas parcels. This process gives rise to refrigeration, with hot temperatures at the closed end and cold temperatures at the open end. Unfortunately, much of the available refrigeration is not realized because the closed end of the basic pulse tube restricts gas movement - gas at the extreme temperatures does not come in contact with the heat exchangers. The orifice pulse tube overcomes this limitation by using a valve and surge volume assembly at the warm, closed end. This allows for a residual gas velocity to remain present during the heat transfer process, thereby permitting more gas to exchange heat at the heat exchangers. This paper describes the pulse tube as a nodal regenerator and the effect residual gas velocity has on the heat transfer mechanism of the orifice pulse tube.

Lee, J. M.↗
NASA NTRSDate not supplied

Cryogenic Vacuum Chamber Testing of A Conductively-Cooled, High Temperature Superconducting Rotor for a 1.4 MW Electric Machine for Aeronautics Applications

This paper presents the second ever demonstration of a superconducting rotor cooled only via conductive connection to a cryocooler, i.e., without pumped or solid cryogens. The fullscale rotor was operated statically in its intended environment – about 1e-3 torr vacuum with only conductive cooling through mechanical connections to a cryocooler contained on the rotor. Stable operation of the rotor in a representative magnetic environment was demonstrated up to its rated direct current (57.2 A) and the designed temperature limit for the superconductor (62 K). Additional electrical measurements increased confidence that the superconducting coils functioned as intended. A collection of steady state temperature distributions was measured at operating and non-operating conditions. In all but one case, the observed temperature gradient between the cold tip and superconducting coils satisfies the design but with little margin. Opportunities to reduce the gradient are identified.

Electrified aircraft propulsion↗
NASA NTRSDate not supplied

Cryogenic Vacuum Chamber Testing of A Conductively-Cooled, High Temperature Superconducting Rotor for a 1.4 MW Electric Machine for Aeronautics Applications

This paper presents the first ever demonstration of a superconducting rotor cooled conductively without a cryogenic fluid or solid. The full-scale rotor was operated statically in its intended environment – about 1e-3 torr vacuum with only conductive cooling through mechanical connections to a cryocooler contained on the rotor. Stable operation of the rotor in a representative magnetic environment was demonstrated up to its rated direct current (57.2 A) and the designed temperature limit for the superconductor (62 K). Additional electrical measurements increased confidence that the superconducting coils functioned as intended. A collection of steady state temperature distributions was measured at operating and non-operating conditions. In all but one case, the observed temperature gradient between the cold tip and superconducting coils satisfies the design but with little margin. Opportunities to reduce the gradient are identified.

Electrified aircraft propulsion↗
NASA NTRS1970

The dynamic characteristics of a turbo-rotor simulator supported on gas-lubricated foil bearings. Part 2: Operation with heating and thermal gradients

A high speed rotor, supported by gas-lubricated foil bearings, is free from self-excited whirl and displays no loss of load capacity when vibrated at frequency equal to the rotational speed. It is demonstrated that in addition to tolerance of geometrical imperfections, misalignment and foreign particles, the foil bearing performs well at elevated temperatures and accommodates appreciable temperature gradients. The foil bearing is endowed with superior wipe-wear characteristics and the flexibility of the foil accounts not only for the stability of the foil bearing, but also for its accommodation of, and compensation for, distortion, contamination, and contact.

Licht, L.↗
NASA NTRS1973

Blade dynamics analysis using NASTRAN

The complexities of turbine engine blade vibration are compounded by blade geometry, temperature gradients, and rotational speeds. Experience indicates that dynamics analysis using the finite element approach provides an effective means for predicting vibration characteristics of compressor and turbine blades whose geometry may be irregular, have curved boundaries, and be subjected to high temperatures and speeds. The NASTRAN program was chosen to help analyze the dynamics of normal modes, rotational stiffening and thermal effects on the normal modes, and forced responses. The program has produced reasonable success. This paper presents the analytical procedures and the NASTRAN results, in comparison with a conventional beam element program and laboratory data.

Kuo, P. S.↗
NASA NTRS1990

High-temperature thermal storage systems for advanced solar receivers materials selections

Advanced space power systems that use solar energy and Brayton or Stirling heat engines require thermal energy storage (TES) systems to operate continuously through periods of shade. The receiver storage units, key elements in both Brayton and Stirling systems, are designed to use the latent heat of fusion of phase-change materials (PCMs). The power systems under current consideration for near-future National Aeronautics and Space Administration space missions require working fluid temperatures in the 1100 to 1400 K range. The PCMs under current investigation that gave liquid temperatures within this range are the fluoride family of salts. However, these salts have low thermal conductivity, which causes large temperature gradients in the storage systems. Improvements can be obtained, however, with the use of thermal conductivity enhancements or metallic PCMs. In fact, if suitable containment materials can be found, the use of metallic PCMs would virtually eliminate the orbit associated temperature variations in TES systems. The high thermal conductivity and generally low volume change on melting of germanium and alloys based on silicon make them attractive for storage of thermal energy in space power systems. An approach to solving the containment problem, involving both chemical and physical compatibility, preparation of NiSi/NiSi2, and initial results for containment of germanium and NiSi/NiSi2, are presented.

Wilson, D. F.↗
NASA NTRS1996

A Model of Radiative and Conductive Energy Transfer in Planetary Regoliths

The thermal regime in planetary regoliths involves three processes: propagation of visible radiation, propagation of thermal radiation, and thermal conduction. The equations of radiative transfer and heat conduction are formulated for particulate media composed of anisotropically scattering particles. Although the equations are time dependent, only steady state problems are considered in this paper. Using the two-stream approximation, solutions are obtained for two cases: a layer of powder heated from below and an infinitely thick regolith illuminated by visible radiation. Radiative conductivity, subsurface temperature gradients, and the solid state greenhouse effect all appear intrinsically in the solutions without ad hoc additions. Although the equations are nonlinear, approximate analytic solutions that are accurate to a few percent are obtained. Analytic expressions are given for the temperature distribution, the optical and thermal radiance distributions, the hemispherical albedo, the hemispherical emissivity, and the directional emissivity. Additional applications of the new model to three problems of interest in planetary regoliths are presented by Hapke.

Hapke, Bruce↗
NASA NTRS1999

On the Dome Effect of Flux Radiometers to Radiative Forcing

Since the introduction of thermopile, pyranometers (solar, e.g., 0.3-3.0 microns) and pyrgeometers (terrestrial, e.g., 4-50 microns) have become instruments commonly used for measuring the broadband hemispherical irradiances at the surface in a long-term, monitoring mode for decades. These commercially available radiometers have been manufactured in several countries such as from the United States, Asia, and Europe, and are generally reliable and economical. These worldwide distributions of surface measurements become even more important in the era of Earth remote sensing in studying climate forcing. However, recent studies from field campaigns have pointed out that erroneous factors (e.g., temperature gradients between the filter dome and detector, emissivity of the thermopile) are responsible for the unacceptable level of uncertainty (e.g., 10-20 W/square Meter). Using a newly developed instrument of Quantum Well Infrared Photodetector (QWIP), we have characterized the brightness temperature fields of pyranometers and pyrgeometers under various sky conditions. The QWIP is based on the superlattice (GaAs/AlGaAs) technology and has a noise equivalent temperature (NE delta T) less than 0.1 K. The quality of pyranometer and pyrgeometer measurements can be improved largely by applying proper knowledge of the thermal parameters affecting the operation of the thermopile systems. For example, we show a method to determine the "dome factor" (the longwave emission divided by the longwave transmission of a pyrgeometer dome) from field measurements. The results show, and are verified independently by the QWIP, that our dome factors of 0.59 and 0.90 are much smaller than the value of 4.0 assumed by the WMO. Data correction procedure and algorithm will be presented and discussed.

Tsay, S.-C.↗
NASA NTRS2022

Volcanic Stratospheric Injections up to 160 Tg(S) Yield a Eurasian Winter Warming Indistinguishable from Internal Variability

Early observational and modeling work suggested that low-latitude volcanic eruptions, comparable to the one of Pinatubo in 1991 or Krakatau in 1883, cause substantial surface warming over the northern continents at mid-latitudes in winter. The proposed mechanism consists of the formation of an anomalously strong Equator-to-pole temperature gradient in the stratosphere due to the presence of volcanic aerosols in the tropics, which are accompanied by an acceleration of the stratospheric polar vortex, which then shifts the Northern Annular Mode into a positive phase, resulting in warming surface temperatures over Eurasia. However, a large body of research in the past decade has shown that, for eruptions such as Pinatubo or Krakatau, no such warming is seen in simulations with more recent climate models which, in general, have much finer vertical and horizontal resolution than the early ones, and which have separated the forced response from the internal variability by using large ensembles of integrations. Since the proposed physical mechanism is sound, it is then possible that eruptions comparable to those of Pinatubo or Krakatau are simply too weak, but even larger ones might indeed be capable of causing Eurasian surface warming in winter. In this study, we explore this possibility using a state-of-the-art, stratosphere-resolving climate model, forced with prescribed aerosols from the Easy Volcanic Aerosol protocol. We consider eruptions with stratospheric sulfur injections of 5, 10, 20, 40, 80, and 160 Tg(S). With 20-member ensembles, we find that with injections of 20 Tg(S) or more – roughly twice the amplitude of the Pinatubo and Krakatau eruptions – our model simulates a winter surface warming over Eurasia, which is statistically significant with a t test given our 20-member ensembles. However, the forced volcanic signal on Eurasian winter surface temperatures is very small, barely exceeding the 1σ range of internal variability for the 160 Tg(S) injection case, and much smaller for smaller eruptions. Most importantly, the number of eruptions needed to establish statistical significance is considerably larger than the number of eruptions known to have occurred in the past 2000 years.

Volcanic eruptions↗
NASA NTRS1976

Passive microwave mapping of ice thickness

Basic calculations are presented for evaluating the feasibility of a scanning microwave radiometer system for mapping the thickness of lake ice. An analytical model for the apparent brightness temperature as a function of ice thickness has been developed, and elaborated to include such variables as galactic and atmospheric noise, aspect angle, polarization, temperature gradient in the ice, the presence of transition layers such as snow, slush, and water, increased loss due to air inclusions in the ice layer, and the presence of multiple ice thicknesses within the antenna footprint. It was found that brightness temperature measurements at six or seven frequencies in the range of 0.4 to 0.7 GHz were required to obtain unambiquous thickness estimates. A number of data processing methods were examined. The effects of antenna beamwidth, scanning rate, receiver bandwidth, noise figure, and integration time were studied.

Apinis, J. J.↗
NASA NTRS1982

Solidification phenomena of binary organic mixtures

The coalescence rates and motion of liquid bubbles in binary organic mixtures were studied. Several factors such as temperature gradient, composition gradient, interfacial tension, and densities of the two phases play important roles in separation of phases of immiscible liquids. An attempt was made to study the effect of initial compositions on separation rates of well-dispersed organic mixtures at different temperatures and, ultimately, on the homogeneity of solidification of the immiscible binary organic liquids. These organic mixtures serve as models for metallic pseudo binary systems under study. Two specific systems were investigated: ethyl salicylate - diethyl glycol and succinonitrile - water.

Chang, K.↗
NASA NTRS1999

3D-Stereoscopic Analysis of Solar Active Region Loops: SoHo/EIT Observations at Temperatures of 1.5-2.5 MK - 2

In this paper we study the three-dimensional (3D) structure of hot (T(sub e) approximately equals 1.5 - 2.5 MK) loops in solar active region NOAA 7986, observed on 1996 August 30 with the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SoHO). This complements a first study on cooler (T(sub e) approximately equals 1.0 - 1.5 MK) loops of the same active region, using the same method of Dynamic Stereoscopy to reconstruct the 3D geometry. We reconstruct the 3D-coordinates x(s), y(s), z(s), the density n(sub e)(s), and temperature profile T(sub e)(s) of 35 individual loop segments (as function of the loop coordinate s) using EIT 195 A and 284 A images. The major findings are: (1) All loops are found to be in hydrostatic equilibrium, in the entire temperature regime of T(sub e) = 1.0 - 2.5 MK; (2) The analyzed loops have a height of 2-3 scale heights, and thus only segments extending over about one vertical scale height have sufficient emission measure contrast for detection; (3) The temperature gradient over the lowest scale height is of order dT/ds is approximately 1 - 4 K/km; (4) The radiative loss rate is found to exceed the conductive loss rate by about two orders or magnitude, making thermal conduction negligible to explain the temperature structure of the loops; (5) A steady-state can only be achieved when the heating rate E(sub H) matches the radiative loss rate in hydrostatic equilibrium, requiring a heat deposition length lambda(sub H) of the half density scale height lambda, predicting a scaling law with the loop base pressure, EH varies as p(sub 0 exp 2). This favors coronal heating mechanisms that operate near the loop footpoints; (6) We find a reciprocal correlation between the loop pressure p(sub 0) and loop length L, i.e. p(sub 0) varies as 1/L, implying a scaling law of the steady-state requirement with loop length, i.e. E(sub H ) varies as 1/L(exp 2). The heating rate shows no correlation with the loop-aligned magnetic field component B(sub z) at the footpoints, but is correlated with the azimuthal field B(sub phi) = Bz(RDelta Phi/L) of a twisted loop, and is thus consistent with heating mechanisms based on field-aligned currents.

Aschwanden, Markus J.↗
NASA NTRS2000

Characterization of Thermal Parameters for Improving Pyranometer and Pyrgeometer Measurements

Since the introduction of thermopile, pyranometers (solar, e.g., 0.3-3.0 micrometers) and pyrgeometers (terrestrial, e.g., 4-50 micrometers) have become instruments commonly used for measuring the broadband hemispherical irradiances at the surface in a long-term, monitoring mode for decades. These commercially available radiometers have been manufactured in several countries such as from the United States, Asia, and Europe, and are generally reliable and economical. These worldwide distributions of surface measurements become even more important in the era of Earth remote sensing in studying climate change. However, recent studies from field campaigns have pointed out that erroneous factors (e.g., temperature gradients between the filter dome and detector, emissivity of the thermopile) are responsible for the unacceptable level of uncertainty (e.g., 20 W m(exp -2)). Using a newly developed instrument of Quantum Well Infrared Photodetector (QWTP), we have characterized the brightness temperature fields of pyranometers and pyrgeometers under various sky conditions. The QWIP is based on the superlattice (GaAs/AlGaAs) technology and has a noise equivalent temperature (NEAT) less than 0.1 K. The quality of pyranometer and pyrgeometer measure- ments can be improved largely by applying proper knowledge of the thermal parameters affecting the operation of the thermopile systems. Data correction procedure and algorithm will be presented and discussed.

Tsay, Si-Chee↗