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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Global effects of thermal conduction on two-phase media

The evolution of two-phase systems of astrophysical gases which change mass between the phases is studied to see whether a steady state is ever reached. The criterion for thermal instability in a cloudy medium is derived. The evolution of the pressure and density of the intercloud medium under the combined effects of heating and radiative cooling of the intercloud gas on the one hand and evaporation and condensation of the clouds on the other is determined. The equilibrium density to which the system evolves is determined for the case when the pressure is fixed. The theory is illustrated by the case in which the intercloud gas is heated by Compton scattering in a hard radiation field and cooled by bremsstrahlung and inverse Compton scattering.

Begelman, Mitchell C.↗
NASA NTRS1988

Case study of active array feed compensation with sidelobe control for reflector surface distortion

The feasibility of electromagnetically compensating for reflector surface distortions has been investigated. The performance characteristics (gain, sidelobe levels, etc.) of large communication antenna systems degrade as the reflector surface distorts mainly due to thermal effects from a varying solar flux. The techniques described in this report can be used to maintain the design performance characteristics independently of thermal effects on the reflector surface. With the advent of monolithic microwave integrated circuits (MMIC), a greater flexibility in array-fed reflector system design can be achieved. MMIC arrays provide independent control of amplitude and phase for each of many radiating elements of the feed array. It is assumed that the surface characteristics (x,y,z, its first and second derivatives) under distorted conditions are known.

Acosta, R. J.↗
NASA NTRS1989

Adaptive feed array compensation system for reflector antenna surface distortion

The feasibility of a closed loop adaptive feed array system for compensating reflector surface deformations has been investigated. The performance characteristics (gain, sidelobe level, pointing, etc.) of large communication antenna systems degrade as the reflector surface distorts mainly due to thermal effects from a varying solar flux. The compensating systems described in this report can be used to maintain the design performance characteristics independent of thermal effects on the reflector surface. The proposed compensating system employs the concept of conjugate field matching to adjust the feed array complex excitation coefficients.

Acosta, Roberto J.↗
NASA NTRS1972

Flight-and ground-test evaluation of pyrrone foams

Two Pyrrone materials, pure Pyrrone foam with a density of 481 kg/cu m and hollow glass microsphere-Pyrrone composite with a density of 962 kg/cu m, were tested in the Langley 20-inch hypersonic arc heated tunnel at pressure levels from 0.06 to 0.27 atm and heating rates from 1.14 to 11.4 MW/sq m. The 481-kg/cu m Pyrrone foam was also flight tested as an experiment aboard a Pacemaker test vehicle. The results of the ground tests indicated that the thermal effectiveness of the 481-kg/cu m Pyrrone foam was superior to the 962-kg/cu m glass sphere-Pyrrone composite. The 481-kg/cu m Pyrrone foam had approximately one-half the thermal effectiveness of low density phenolic nylon. The 481-kg/cu m Pyrrone foam experienced random mechanical char removal over the entire range of test conditions. Char thermal property inputs for an ablation computer program were developed from the ground test data of the 481-kg/cu m Pyrrone foam. The computer program using these developed char thermal properties, as well as the measured uncharred material properties, adequately predicted the in-depth temperature histories measured during the Pacemaker flight.

Mclain, A. G.↗
NASA NTRS1987

Fluid mass and thermal loading effects on the modal characteristics of space shuttle main engine liquid oxygen inlet splitter vanes

An analysis and evaluation of experimental modal survey test data on the variations of modal characteristics induced by pressure and thermal loading events are presented. Extensive modal survey tests were carried out on a Space Shuttle Main Engine (SSME) test article using liquid nitrogen under cryogenic temperatures and high pressures. The results suggest that an increase of pressure under constant cryogenic temperature or a decrease of temperature under high pressure induces an upward shift of frequencies of various modes of the structures.

Panossian, H. V.↗
NASA NTRS2006

An Initial Non-Equilibrium Porous-Media Model for CFD Simulation of Stirling Regenerators

The objective of this paper is to define empirical parameters for an initial thermal non-equilibrium porous-media model for use in Computational Fluid Dynamics (CFD) codes for simulation of Stirling regenerators. The two codes currently used at Glenn Research Center for Stirling modeling are Fluent and CFD-ACE. The codes porous-media models are equilibrium models, which assume solid matrix and fluid are in thermal equilibrium. This is believed to be a poor assumption for Stirling regenerators; Stirling 1-D regenerator models, used in Stirling design, use non-equilibrium regenerator models and suggest regenerator matrix and gas average temperatures can differ by several degrees at a given axial location and time during the cycle. Experimentally based information was used to define: hydrodynamic dispersion, permeability, inertial coefficient, fluid effective thermal conductivity, and fluid-solid heat transfer coefficient. Solid effective thermal conductivity was also estimated. Determination of model parameters was based on planned use in a CFD model of Infinia's Stirling Technology Demonstration Converter (TDC), which uses a random-fiber regenerator matrix. Emphasis is on use of available data to define empirical parameters needed in a thermal non-equilibrium porous media model for Stirling regenerator simulation. Such a model has not yet been implemented by the authors or their associates.

Tew, Roy C.↗
NASA NTRS1978

Thermal environment effects on strength and impact properties of boron-aluminum composites

A systematic study was conducted regarding the degradation of fracture strength and impact energy in commercial B-Al composites in both static and cyclic thermal environments. The composites used in the study contained approximately 50 vol % boron fibers, unidirectionally aligned in either a 6061 Al or 1100 Al matrix. The tensile strengths of the composites after 3000 thermal cycles as a function of upper cycle temperature are presented in graphs. The temperature at which the strengths of 6061 Al matrix, B-Al composites were significantly degraded after 3000 cycles was noticeably higher than that for the 1100 Al matrix composites. Static heating at 420 C resulted in no significant strength degradation for the 6061 Al matrix composites. In the case of 1100 matrix composites, some degradation was observed at 420 C but markedly less than in the composites cycled to 420 C.

Grimes, H. H.↗
NASA NTRS1993

Theory Of Damping In Composite Laminates

Integrated mathematical model predicts vibration-damping properties of laminated fiber/matrix composite material from micromechanical level to laminate level. Model accounts simultaneously for wider, more realistic variety of laminate configurations and physical phenomena, including asymmetrically oriented plies, anisotropic fiber damping properties, hygral and thermal effects, and coupling among hygral, thermal, and mechanical effects.

Saravanos, D. A.↗
NASA NTRS1991

Neutron, gamma ray and post-irradiation thermal annealing effects on power semiconductor switches

Experimental data showing the effects of neutrons and gamma rays on the performance characteristics of power-type NPN bipolar junction transistors (BJTs), metal-oxide-semiconductor field effect transistors (MOSFETs), and static induction transistors (SITs) are given. These three types of devices were tested at radiation levels which met or exceeded the SP-100 requirements. For the SP-100 radiation requirements, the BJTs were found to be most sensitive to neutrons, the MOSFETs were most sensitive to gamma rays, and the SITs were only slightly sensitive to neutrons. Postirradiation thermal anneals at 300 K and up to 425 K were done on these devices and the effectiveness of these anneals are also discussed.

Schwarze, G. E.↗
NASA NTRS1991

Neutron, gamma ray and post-irradiation thermal annealing effects on power semiconductor switches

The effects of neutron and gamma rays on the electrical and switching characteristics of power semiconductor switches must be known and understood by the designer of the power conditioning, control, and transmission subsystem of space nuclear power systems. The SP-100 radiation requirements at 25 m from the nuclear source are a neutron fluence of 10(exp 13) n/sq cm and a gamma dose of 0.5 Mrads. Experimental data showing the effects of neutrons and gamma rays on the performance characteristics of power-type NPN Bipolar Junction Transistors (BJTs), Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs), and Static Induction Transistors (SITs) are presented. These three types of devices were tested at radiation levels which met or exceeded the SP-100 requirements. For the SP-100 radiation requirements, the BJTs were found to be most sensitive to neutrons, the MOSFETs were most sensitive to gamma rays, and the SITs were only slightly sensitive to neutrons. Post-irradiation thermal anneals at 300 K and up to 425 K were done on these devices and the effectiveness of these anneals are also discussed.

Schwarze, G. E.↗
NASA NTRS1990

Effect of thermal cycling on stress in metallic films on ceramic substrates

The hydrogen maser is the most stable frequency standard currently available for averaging intervals of hours to weeks. A major contributor to maser frequency variations is the maser's microwave resonant cavity: by means of the cavity pulling effect, a change in the cavity's resonance frequency produces a proportional change in the maser's output frequency. To minimize variations in the cavity's dimensions, and thus in its resonance frequency, maser cavities are often constructed of a low-expansivity glass-ceramic material coated on its surface with a conductive metallic film. It was previously shown that silver films like those used in SAO maser cavities develop tensile stress when cooled to room temperature after being fired onto the cavity, and that the stress in such films relaxes with time at a rate proportional to the level of stress. Stress relaxation in maser cavity coatings can alter the shape, and hence the resonance frequency, of the cavity, resulting in a slow variation in the maser's output frequency. The possibility was investigated of reversing the initial tensile stress by precooling the coated cavity material. It was hypothesized that cooling the material well below its normal working temperature and then warming it to its normal temperature would result in a lower tensile stress or even a compressive stress. Under such a condition stress relaxation, and thus any consequent frequency drifts, might be reduced or reversed.

Mattison, Edward M.↗
NASA NTRS1986

The calibration of photographic and spectroscopic films. 1: Film batch variations of reciprocity failure in IIaO film. 2: Thermal and aging effects in relationship to reciprocity failure. 3: Shifting of reciprocity failure points as a function of thermal and aging effects

Reciprocity failure was examined for IIaO spectroscopic film. Three separate experiments were performed in order to study film batch variations, thermal and aging effects in relationship to reciprocity failure, and shifting of reciprocity failure points as a function of thermal and aging effects. The failure was examined over ranges of time between 5 and 60 seconds. The variation to illuminance was obtained by using thirty neutral density filters. A standard sensitometer device imprinted the wedge pattern on the film as exposure time was subjected to variation. The results indicate that film batch differences, temperature, and aging play an important role in reciprocity failure of IIaO spectroscopic film. A shifting of the failure points was also observed in various batches of film.

Peters, K. A.↗
NASA NTRS1973

The dynamic behavior and compliance of a stream of cavitating bubbles.

Study of the dynamic response of streams of cavitating bubbles to imposed pressure fluctuations to determine the role played by turbopump cavitation in the POGO instability of liquid rockets. Both quasi-static and more general linearized dynamic analyses are made of the perturbations to a cavitating flow through a region of reduced pressure in which the bubbles first grow and then collapse. The results, when coupled with typical bubble number density distribution functions, yield compliances which compare favorably with the existing measurements. Since the fluids involved are frequently cryogenic, a careful examination was made of the thermal effects both on the mean flow and on the perturbations. As a result, the discrepancy between theory and experiment for particular engines could be qualitatively ascribed to reductions in the compliance caused either by these thermal effects or by relatively high reduced frequencies.

Brennen, C.↗
NASA NTRS1992

Atomic oxygen effects on thermal control and mirror coatings: Evaluation of experiment A0034

Evaluation of the degradation attributed to contamination of the collector mirrors in Experiment A0034, which were exposed to an earth orbital environment while on the leading and trailing edges of the LDEF, has provided some evidence for outgassing of coatings stimulated by atomic oxygen. Effects of the natural space environment on the coatings and mirrors, including the effects of incident and re-directed atomic oxygen flux, were investigated. The effects of exposure on the coatings and mirrors were evaluated by optical, surface profiling, chemical, and scanning electron microscopy (SEM) measurements. Preliminary results of these analyses are discussed for application to both the issues of mirror and coating stability and the specific issue of atomic oxygen stimulated outgassing.

Linton, Roger C.↗
NASA NTRS1990

All-metal, compact heat exchanger for space cryocoolers

This report describes the development of a high performance, all metal compact heat exchanger. The device is designed for use in a reverse Brayton cryogenic cooler which provides five watts of refrigeration at 70 K. The heat exchanger consists of a stainless steel tube concentrically assembled within a second stainless steel tube. Approximately 300 pairs of slotted copper disks and matching annular slotted copper plates are positioned along the centerline axis of the concentric tubes. Each of the disks and plates has approximately 1200 precise slots machined by means of a special electric discharge process. Positioning of the disk and plate pairs is accomplished by means of dimples in the surface of the tubes. Mechanical and thermal connections between the tubes and plate/disk pairs are made by solder joints. The heat exchanger assembly is 9 cm in diameter by 50 cm in length and has a mass of 10 kg. The predicted thermal effectiveness is greater than 0.985 at design conditions. Pressure loss at design conditions is less than 5 kPa in both fluid passages. Tests were performed on a subassembly of plates integrally soldered to two end headers. The measured thermal effectiveness of the test article exceeded predicted levels. Pressure losses were negligibly higher than predictions.

Swift, Walter L.↗
NASA NTRS1995

Thermal Line Shift and Broadening of Ho(3+) in Y3AI5O12 and Lu3AIO12

The interaction between the active ion and the host lattice manifests itself in two distinct ways: the static interaction with the crystal field that causes the splitting of the free ion energy levels and the active interaction through the surrounding phonon system that produces temperature dependent characteristics in the optical spectrum of the ion (e.g. line broadening and line shift). The strength of the splitting depends heavily on the electronic configuration of the atom. The model give by McCumber and Sturge describes the thermal effects on line width and position above 77 K with Raman scattering of Debye model phonons. These processes predict a Lorentzian line shape. However, below 77 K the principal contributions are from crystal inhomogeneities that result in a Gaussian line shape. We have investigated the experimental Stark levels as well as the thermal effects on the line width and the position of trivalent holmium ions in both yttrium aluminum garnet, Y3Al5O12 (YAG) and lutetium aluminum garnet, Lu3Al5O12 (LuAG) crystals. We have compared the Stark levels of the (5)I(6) state, and the thermal line shift and broadening of an isolated transition (Z2 to X13) between the (5)I(8) and (5)I(6) stark levels in these crystals. This transition occurs in the near infrared region at approximately 1117 nm.

Snoke, Elizabeth R.↗
NASA NTRS1984

Holographic interferometry - Errors caused by thermal gradients in holographic plates

Thermal effects as sources of errors in holographic interferometry are illustrated on an example of reflection holography patterns obtained on graphite-epoxy specimens exposed before and after compressive loading. The numerical interpretation of the obtained holograms indicated some peculiar unexpected thickness changes in the specimen. In addition, some patterns showed random occurrence of zones of low and zero contrast. These discrepancies occurred usually in the first of the two exposures and were traced to thermal warping and thermal relaxation patterns caused by small temperature gradients that resulted from heat transfer to the photographic plates during their installation. Suggested precautions to minimize thermal effects include using thick holographic plates (photographic plates attached to glass plates) and insulated gloves during handling and allowing time to establish thermal equilibrium before the exposure.

Post, D.↗
NASA NTRS1988

Thermal cycling effects on the dimensional stability of P75 and P75-T300 (fabric) hybrid graphite/epoxy laminates

The response of cross-ply P75/934 laminates and hybrid laminates consisting of P75/934 unidirectional tape and T300/934 woven fabric were compared for temperature exposures between + and - 250 F up to a maximum of 250 cycles. The properties monitored included microcrack density, coefficient of thermal expansion (CTE), residual strain, and tensile modulus. Hybrid laminates, with stiffnesses and CTE's comparable to P75/934 cross-ply laminates, demonstrated significantly improved thermal cycling stability in one direction. The hybrid laminates also had predicted longitudinal properties comparable to low angle, off-axis P75/934 configurations, but had significantly higher predicted transverse strengths. T300 Gr/Ep plain-weave fabric laminates were shown to be much less susceptible to thermal cycling damage than T300 Gr/Ep cross-ply laminates. None of the laminates tested exhibited any significant changes in tensile modulus after thermal cycling.

Bowles, David E.↗