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At least 73 records · Page 4

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NASA NTRSDate not supplied

Opposed-flow Spreading Flames: Effect of Sub-atmospheric Pressure on Spread and Burning Rates

Flame spread over solid fuels is a canonical problem in fire science, due to its direct implications on material flammability and importance in fire development. Flames in a microgravity environment can behave very differently than on Earth, posing additional risks for spaceflight life safety. Sub-atmospheric pressures in ground-based experiments have been proposed to approximately replicate the burning behavior of solid fuels in reduced gravity conditions because of similar effects on heat and mass transfer mechanisms from the flame to the solid. In opposed-flow flame spread, the solid fuel is heated by the flame ahead of its leading edge, and this process is strongly affected by the ambient conditions. In this work, we consider flames spreading over flat thin acrylic samples exposed to a forced flow of 20 cm/s, and pressures between 30 and 100 kPa. When radiation losses and chemical kinetic effects are neglected, experimental and theoretical results suggest an independence of the spread rate on pressure. However, the spread rate shows a decreasing trend with lower pressure in the range 30-60 kPa. The mass burning rate, calculated from the samples weight measured before and after the experiments, shows a similar trend. The flame length, on the other hand, shows a non-monotonic behavior, reaching a maximum at 60 kPa. Additionally, gas emissions measured during the experiments are used to estimate the heat release rate of the spreading flames. The comparison of the heat release rate obtained from the measured emissions and the estimated mass burning rates, suggests that chemical kinetics is not driving the decrease in flame spread observed at low pressures, whereas radiation losses could be responsible in analogy to what has been observed in micro and partial gravity studies. These results could provide more information to guide future Earth-based material flammability testing for spacecraft applications.

fire↗

NASA NTRS1983

Hermetic edge sealing of photovoltaic modules

The feasibility of using an electrostatic bonding (ESB) and ultrasonic welding process to produce hermetic edge seals on terrestrial solar cell modules was investigated. The fabrication sequence is to attach an aluminum foil "gasket' to the perimeter of a glass sheet. A cell circuit is next encapsulated inside the gasket, and its aluminum foil back cover is seam welded ultrasonically to the gasket. An ESB process for sealing aluminum to glass was developed in an ambient air atmosphere, which eliminates the requirement for a vacuum or pressure vessel. An ultrasonic seam welding process was also developed which did not degrade the quality of the ESB seal. Good quality welds with minimal deformation were produced. The effectiveness of the above described sealing techniques was tested by constructing 400 sq cm (8 x 8 s64 sq in) sample modules, and then subjecting them to nondestructive fine and gross leak tests. The gross leak tests identified several different causes of leaks which were then eliminated by modifying the assembly process.

Nowlan, M. J.↗

NASA NTRS1988

A compact imaging spectrometer for studies of space vehicle induced environment emissions

On the basis of spectral measurements made from the Space Shuttle and on models of the possible Space Station external environment, it appears likely that, even at the planned altitudes of Space Station, photon emissions will be induced. These emissions will occur to some degree throughout the UV-visible-IR spectrum. The emissions arise from a combination of processes including gas phase collisions between relatively energetic ambient and surface emitted or re-emitted atoms or molecules, where the surface raises some species to excited energy states. At present it is not possible to model these processes or the anticipated intensity levels with accuracy, as a number of fundamental parameters needed for such calculations are still poorly known or unknown. However, it is possible that certain spectral line and band features will exceed the desired goal that contaminant emissions not exceed the natural zodiacal background. However, in the near infrared and infrared, it appears that this level will be exceeded to a significant degree. Therefore it will be necessary to monitor emission levels in the vicinity of Space Station, both in order to establish the levels and to better model the environment. In this note, we briefly describe a small spectrometer that is suitable for monitoring the spectrum from 1200A to less than or approximately 12,000A. This instrument uses focal plane array detectors to image this full spectral range simultaneously. The spectral resolution is 4 to 12A, depending on the portion of the wavelength range.

Torr, Marsha R.↗

NASA NTRS1981

The formation and growth of sulfate aerosols in the stratosphere

The critical compositions, critical radii, and nucleation rates of sulfuric acid droplets formed through homogeneous heteromolecular nucleation process in the stratosphere under a variety of ambient conditions are calculated. Results are plotted as a function of H2SO4 concentration and H2O concentration so that characteristics of sulfate aerosols can be quickly estimated whenever environmental conditions are known. The effect of the uncertainty in the saturated vapor pressure of H2SO4 on the estimated properties of sulfate aerosols is elucidated. Plottings are presented that illustrate the relative importance of new particle formation by homogeneous nucleation vs growth by heteromolecular condensation on pre-existing nuclei in the stratosphere. The effects of ambient conditions, such as the concentrations of H2SO4 and H2O, the average stratospheric particle radius, and the number concentration of pre-existing nuclei on the relative importance of these two competing processes are discussed.

Yue, G. K.↗

NASA NTRS1988

Compact imaging spectrometer for induced emissions

On the basis of spectral measurements made from the Space Shuttle and on models of the possible Space Station external environment, it appears likely that, even at the planned altitudes of Space Station, photon emissions will be induced. These emissions will occur to some degree throughout the ultraviolet-visible-infrared spectrum. The emissions arise from a combination of processes including gas phase collisions between relatively energetic ambient and surface emitted or re-emitted atoms or molecules, where the surface raises some species to excited energy states. At the present time it is not possible to model these processes or the anticipated intensity levels with any accuracy, as a number of fundamental parameters needed for such calculations are still poorly known or unknown. However, it is possible that certain spectral line and band features will exceed the desired goal that concomitant emissions not exceed the natural zodiacal background. Also, in the near infrared and infrared, it appears that this level will be exceeded to a significant degree. Therefore it will be necessary to monitor emission levels in the vicinity of Space Station, both in order to establish the levels and to better model the environment. A small spectrometer is briefly described which is suitable for monitoring the spectrum from 1200 A to less than or equal to 12,000 A. The instrument uses focal plane array detectors to image this full spectral range simultaneously. The spectral resolution is 4 to 12 A, depending on the portion of the wavelength range.

Torr, Douglas G.↗

NASA NTRS1983

The expansion of a plasma into a vacuum - Basic phenomena and processes and applications to space plasma physics

In this review attention is called to basic phenomena and physical processes involved in the expansion of a plasma into a vacuum, or the expansion of a plasma into a more tenuous plasma, in particular the fact that upon the expansion, ions are accelerated and reach energies well above their thermal energy. Also, in the process of the expansion a rarefaction wave propagates into the ambient plasma, an ion front moves into the expansion volume, and discontinuities in plasma parameters occur. The physical processes which cause the above phenomena are discussed, and their possible application is suggested for the case of the distribution of ions and electrons (hence plasma potential and electric fields) in the wake region behind artificial and natural obstacles moving supersonically in a rarefied space plasma. To illustrate this, some in situ results are reexamined. Directions for future work in this area via the utilization of the Space Shuttle and laboratory work are also mentioned.

Wright, K. H., Jr.↗

NASA NTRS2022

Satellite Precipitation Measurements: What Have We Learnt About Cloud-Precipitation Processes From Space?

Precipitation is one of the fundamental elements that define global and regional climatology. Precipitation systems consist of a broad spectrum of three-dimensional structures in which microphysical processes interact with macro-scale processes in the cloud system and the ambient environment that prescribe the evolution of the system. Since 1970s, satellite observations of precipitation have been a fundamental tool in quantifying this complex interaction. They have first quantified the frequency and intensity of global precipitation, including remote areas over open oceans and polar regions, thus providing today’s precipitation climatology. More recently, satellite observations of cloud and precipitation have been exploited for understanding the physical mechanisms governing precipitation systems. A subset of these studies also provided observation-based metrics to probe physical processes operating in cloud-precipitation systems and to apply them as diagnostic measures for evaluating the representation of the processes in numerical models for better projections of future climate. In this chapter, we first review the theoretical basis of precipitation remote sensing from space and describe how it is practically applied in satellite missions. In the first part of the chapter, an historical overview of the satellite missions is described, summarising the instruments and retrieval algorithms developed in the missions. In the second part, we introduce a set of studies discussing the fundamental mechanisms behind precipitation formation, highlighting what we have learnt to date on cloud-precipitation processes from satellite observations.

Precipitation↗

NASA NTRS1988

Calculation of space station infrared irradiance from atmosphere-induced emissions

The excitation mechanisms and radiance estimates over the 1 to 10 micron region for CO2(v), H2O(v), CO(v), OH(v), NO2(2B-2A) and N2(B3 pi - A3 sigma) are discussed. The infrared irradiance of the Space Station at an altitude of 460 km was estimated. The surface material was presumed to be non-carbonaceous and inert. The determined number densities of the various gases relevant to the Space Station from both ambient and outgassing sources are presented. A model for the production of and emission from the infrared active molecules was constructed that considers two classes of production processes: (1) gas phase excitation of molecules in the near Station environment by collision with ambient flux; and (2) surface processes that lead to molecular excitation. A composite spectrum of all major emitting species shows that the observed irradiance is non-uniform over the 1 to 8 micron region. The comparison of predicted irradiance with the zodiacal background indicates that the Space Station infrared background from atmosphere-induced emissions may be a problem of consequence.

Fraser, M. E.↗

NASA NTRS1983

Optical contamination on the Atmosphere Explorer-E satellite

Atmospheric optical emission measurements by the Visible Airglow Experiment (VAE) on board the Atmosphere Explorer (AE-C, D and E) satellites have been analyzed and found to be contaminated at low altitudes. The contamination maximizes in the forward direction along the spacecraft velocity and is sensitive to the composition and density of the ambient atmosphere. Analysis at two different wavelengths suggests that the contamination is likely to have a diffuse band spectrum which is brighter toward the red. Some unknown processes which involve satellite surface materials and the incoming ambient particles are believed to be responsible for the contamination. A simulation model is presented here to account for the observed angular dependence.

Yee, J. H.↗

NASA NTRS1987

Simulations of auroral plasma processes - Electric fields, waves and particles

Plasma processes driven by current sheets of finite thicknesses in an ambient magnetized plasma are studied using a 2 1/2 dimensional particle-in-cell code, and similarities are found between simulated plasma processes and those observed in the auroral plasma. Current sheets are shown to be bounded by large perpendicular electric fields occurring near their edges above the conducting boundary. Shaped potential structures form when the current sheets are narrow, and when the current sheets are wide, potential structures develop a significant parallel potential drop such that the electrons are accelerated upwards. Downward parallel electric fields of variable strength are noted in the downward current region, and double layer formation is seen in both narrow and wide current sheets. High frequency oscillations near the electron plasma frequency and its harmonic are seen, and low frequency waves are observed.

Singh, Nagendra↗

NASA NTRS2004

Processing Robustness for A Phenylethynyl Terminated Polyimide Composite

The processability of a phenylethynyl terminated imide resin matrix (designated as PETI-5) composite is investigated. Unidirectional prepregs are made by coating an N-methylpyrrolidone solution of the amide acid oligomer (designated as PETAA-5/NMP) onto unsized IM7 fibers. Two batches of prepregs are used: one is made by NASA in-house, and the other is from an industrial source. The composite processing robustness is investigated with respect to the prepreg shelf life, the effect of B-staging conditions, and the optimal processing window. Prepreg rheology and open hole compression (OHC) strengths are found not to be affected by prolonged (i.e., up to 60 days) ambient storage. Rheological measurements indicate that the PETAA-5/NMP processability is only slightly affected over a wide range of B-stage temperatures from 250 deg C to 300 deg C. The OHC strength values are statistically indistinguishable among laminates consolidated using various B-staging conditions. An optimal processing window is established by means of the response surface methodology. IM7/PETAA-5/NMP prepreg is more sensitive to consolidation temperature than to pressure. A good consolidation is achievable at 371 deg C (700 deg F)/100 Psi, which yields an RT OHC strength of 62 Ksi. However, processability declines dramatically at temperatures below 350 deg C (662 deg F), as evidenced by the OHC strength values. The processability of the IM7/LARC(TM) PETI-5 prepreg was found to be robust.

Hou, Tan-Hung↗

NASA NTRS1975

High-performance vessels from an aromatic polyamide fiber/epoxy composite

The investigation reported is concerned with the development of a filament-wound, ultralightweight composite vessel for the containment of cryogenic propellants and pressurant gases. The vessels are to be used within an operational temperature range from -253 C to ambient temperature. The fiber used for the process is Kevlar 49. The design and the fabrication of specimens and vessels are discussed along with the results obtained in a testing program.

Chiao, T. T.↗

NASA NTRS1976

A two-dimensional finite difference solution for the transient thermal behavior of tubular solar collector

A numerical procedure was established using the finite-difference technique in the determination of the time-varying temperature distribution of a tubular solar collector under changing solar radiancy and ambient temperature. Three types of spatial discretization processes were considered and compared for their accuracy of computations and for selection of the shortest computer time and cost. The stability criteria of this technique was analyzed in detail to give the critical time increment to ensure stable computations. The results of the numerical analysis were in good agreement with the analytical solution previously reported. The numerical method proved to be a powerful tool in the investigation of the collector sensitivity to two different flow patterns and several flow control mechanisms.

Lansing, F. L.↗

NASA NTRS1980

A two-dimensional finite-difference solution for the transient thermal behavior of a tubular solar collector

A numerical procedure was established using the finite-difference technique in the determination of the time-varying temperature distribution of a tubular solar collector under changing solar radiancy and ambient temperature. Three types of spatial discretization processes were considered and compared for their accuracy of computations and for selection of the shortest computer time and cost. The stability criteria of this technique were analyzed in detail to give the critical time increment to ensure stable computations. The results of the numerical analysis were in good agreement with the analytical solution previously reported. The numerical method proved to be a powerful tool in the investigation of the collector sensitivity to two different flow patterns and several flow control mechanisms.

Lansing, F. L.↗

NASA NTRS1981

Electrical behavior of a Shuttle Electrodynamic Tether System /SETS/

Many novel electrodynamic and space plasma physics experiments can be done from the space shuttle using a gravity gradient stabilized subsatellite tethered 10-30 km above the Orbiter via a long, conducting wire which is insulated from the ionospheric plasma. This system, called the Shuttle Electrodynamic Tether System (SETS), is described in the present paper with emphasis upon the various electrical processes which determine its coupling to the ambient plasma. The three most important physical effects include sheath formation and electron collection by the subsatellite, the generation of a large emf through the orbital motion of the tether across geomagnetic field lines, and the active ejection of electrons from the Orbiter into the surrounding ionosphere. An electrical circuit analogy is presented for SETS together with a brief outline of possible areas related to the artificial generation of magnetohydrodynamic waves.

Banks, P. M.↗

NASA NTRS1991

Brominated graphitized carbon fibers

Low cost, high break elongation graphitized carbon fibers having low degree of graphitization are inert to bromine at room or higher temperatures, but are brominated at -7 to 20 C, and then debrominated at ambient. Repetition of this bromination-debromination process can bring the bromine content to 18 percent. Electrical conductivity of the brominated fibers is three times of the before-bromination value.

Hung, Ching-Cheh↗

NASA NTRS1992

Geology and distribution of impact craters on Venus - What are they telling us?

Data on the distribution of impact craters on Venus are analyzed in relation to the geologic history of Venus and the effects of high ambient pressures and temperatures on the cratering process itself. The cratering record of Venus is discussed in terms of size, number, spatial distribution, and state of preservation of craters. The morphology of Venusian craters is examined as a function of the diameter, and features unique to these craters are discussed. Attention is also given to several craters that illustrate important features of Venusian crater; surface 'splotches' that are closely related in origin to impact craters; and implications of the distribution of severely modified craters for Venus' recent geologic history.

Schaber, G. G.↗

NASA NTRS1995

Solid surface combustion experiment flame spread in a quiescent, microgravity environment implications of spread rate and flame structure

A unique environment in which flame spreading, a phenomenon of fundamental, scientific interest, has importance to fire safety is that of spacecraft in which the gravitational acceleration is low compared with that of the Earth, i.e., microgravity. Experiments aboard eight Space Shuttle missions between October 1990 and February 1995 were conducted using the Solid Surface Combustion Experiment (SSCE) payload apparatus in an effort to determine the mechanisms of gas-phase flame spread over solid fuel surfaces in the absence of any buoyancy induced or externally imposed oxidizer flow. The overall SSCE effort began in December of 1984. The SSCE apparatus consists of a sealed container, approximately 0.039 cu m, that is filled with a specified O2/N2 mixture at a prescribed pressure. Five of the experiments used a thin cellulosic fuel, ashless filter paper, 3 cm wide x 10 cm long, 0.00825 cm half-thickness, ignited in five different ambient conditions. Three of the experiments, the most recent, used thick polymethylmethacrylate (PMMA) samples 0.635 cm wide x 2 cm long, 0.32 cm half-thickness. Three experiments, STS 41, 40 and 43, were designed to evaluate the effect of ambient pressure on flame spread over the thin cellulosic fuel while flights STS 50 and 47 were at the same pressure as two of the earlier flights but at a lower oxygen concentration in order to evaluate the effect of ambient oxygen level on the flame spread process at microgravity. For the PMMA flights, two experiments, STS 54 and 63, were at the same pressure but different oxygen concentrations while STS 64 was at the same oxygen concentration as STS 63 but at a higher pressure. Two orthogonal views of the experiments were recorded on 16 mm cine-cameras operating at 24 frames/s. In addition to filmed images of the side view of the flames and surface view of the burning samples, solid- and gas-phase temperatures were recorded using thermocouples. The experiment is battery powered and follows an automated sequence upon activation by the Shuttle Crew. In this study we separate the SSCE data into two groups according to the fuel type: (1) thin cellulose; and (2) thick PMMA. The experimental spread rates are compared with prediction from a number of models in an effort to uncover the important physics that characterize microgravity flame spread. Both steady and unsteady solutions are employed to explore the flame evolution, especially for thick fuels. Finally, the flame structure in downward spread is compared with the microgravity flame structure and modeling results to delineate the difference between the two configurations and the influence of normal gravity.

Bundy, Matthew↗