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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Variations in pore structure of reaction-bonded silicon nitride /RBSN/

A discussion is presented relating the observed pore structures (sizes) to the reaction mechanisms in reaction-bonded silicon nitride (alpha- and beta-Si3N4) on the basis of information available from the literature. While the techniques for reducing the residual macroporosity are quite well-developed for reaction-bonded Si3N4 (RBSN), it is important to be aware of three other orders of magnitude for porosity present in RBSN as a result of the nitriding process itself, and how these types of nitridation-induced porosity can be controlled. For ease of description, these types of nitridation-induced porosity are called micropores, nanopores, and picopores in order of their decreasing size. A scanning electron micrograph is presented, showing nanopores isolated in the unreacted Si and picopores in the alpha-matte Si3N4. The assumption that an alpha-matte growth mechanism is active explains the occurrence of nanopores and their partial filling with alpha-Si3N4, leaving behind very fine-grained alpha-matte and picopores.

Danforth, S. C.↗
NASA NTRS1978

Effective thermal conductivity of fully and partially saturated metal wicks

The purpose of the study is threefold: (1) to perform accurate experiments covering a wide range of porosities to determine the effective thermal conductivity of sintered fiber porous metals fully saturated with air, water, and water vapor; (2) to propose correlations for these cases, and to compare these correlations with existing analytical predictions, experimental correlations and data; and (3) to determine the effective thermal conductivity of porous metals partially saturated with water and water vapor, respectively. The experimental apparatus, based on the steady-state method of comparison, is designed and used for determining the effective thermal conductivity. The wicks are made of nickel 200, stainless steel 430, and copper, covering a porosity range 0.30-0.67. Based on the experimental results, a single correlation is derived for predicting the effective thermal conductivity of all wicks fully saturated with air, water, and water vapor. For two partially saturated wick specimens with both high porosity and large matrix thermal conductivity, a liquid-vapor transition zone is observed between the phases, which increases the total combined effective thermal conductivity of the specimen.

Kar, K.↗
NASA NTRS1979

Formation of porous surface layers in reaction bonded silicon nitride during processing

An effort was undertaken to determine if the formation of the generally observed layer of large porosity adjacent to the as-nitride surfaces of reaction bonded silicon nitrides could be prevented during processing. Isostatically pressed test bars were prepared from wet vibratory milled Si powder. Sintering and nitriding were each done under three different conditions:(1) bars directly exposed to the furnance atmosphere; (2) bars packed in Si powder; (3) bars packed in Si3N4 powder. Packing the bars in either Si of Si3N4 powder during sintering retarded formation of the layer of large porosity. Only packing the bars in Si prevented formation of the layer during nitridation. The strongest bars (316 MPa) were those sintered in Si and nitrided in Si3N4 despite their having a layer of large surface porosity; failure initiated at very large pores and inclusions. The alpha/beta ratio was found to be directly proportional to the oxygen content; a possible explanation for this relationship is discussed.

Shaw, N. J.↗
NASA NTRS1978

Low-speed aerodynamic performance of 50.8-centimeter-diameter noise-suppressing inlets for the Quiet, Clean, Short-haul Experimental Engine (QCSEE)

Two basic inlet concepts, a high throat Mach number (0.79) design and a low throat Mach number (0.60) design, were tested with four diffuser acoustical treatment designs that had face sheet porosity ranging from 0 to 24 percent for the high Mach number inlet and 0 to 28 percent for the low Mach number inlet. The tests were conducted in a low speed wind tunnel at free stream velocities of 0, 41, and 62 m/sec and angles of attack to 50 deg. Inlet throat Mach number was varied about the design value. Increasing the inlet diffuser face sheet porosity resulted in an increase in total pressure loss in the boundary layer for both the high and low Mach number inlet designs, however, the overall effect on inlet total pressure recovery of 0.991 at the design throat Mach number, a free stream velocity of 41 m/sec, and an angle of attack of 50 deg; Inlet flow separation at an angle of attack of 50 deg was encountered with only one inlet configuration the high Mach number design with the highest diffuser face sheet porosity (24 percent).

Abbott, J. M.↗
NASA NTRS1981

Oxidation and hot corrosion of coated and bare oxide dispersion strengthened superalloy MA-755E

Cyclic hot corrosion and oxidation of an experimental oxide dispersion strengthened (ODS) superalloy MA-755E were conducted in a hot gas stream at Mach 0.3. The response of the ODS alloy, bare or with protective coatings, was similar to that of a conventional cast alloy, IN-792, in hot corrosion at 900 C. However, during oxidation at 1100 and 1150 C the ODS alloy differed from the cast alloy by developing a greater amount of subsurface porosity. Compared with a diffused aluminide coating, an electron beam vapor deposited NiCrAlY coating offered superior oxidation protection and decreased porosity formation. In additional testing, the tendency to form porosity was associated with the large grains of recrystallized powder metallurgy alloys but was independent of the presence of an oxide dispersion.

Glasgow, T. K.↗
NASA NTRS1982

Investigation of passive shock wave-boundary layer control for transonic airfoil drag reduction

The passive drag control concept, consisting of a porous surface with a cavity beneath it, was investigated with a 12-percent-thick circular arc and a 14-percent-thick supercritical airfoil mounted on the test section bottom wall. The porous surface was positioned in the shock wave/boundary layer interaction region. The flow circulating through the porous surface, from the downstream to the upstream of the terminating shock wave location, produced a lambda shock wave system and a pressure decrease in the downstream region minimizing the flow separation. The wake impact pressure data show an appreciably drag reduction with the porous surface at transonic speeds. To determine the optimum size of porosity and cavity, tunnel tests were conducted with different airfoil porosities, cavities and flow Mach numbers. A higher drag reduction was obtained by the 2.5 percent porosity and the 1/4-inch deep cavity.

Nagamatsu, H. T.↗
NASA NTRS1982

Turbulent boundary layer over porous surfaces with different surface geometries

The turbulent boundary layer over three porous walls with different surface geometries was studied in order to investigate the individual influences of porosity and small roughness, as well as their combined effects, on turbulent boundary layer behavior. The tests were conducted in a 2 m x 2 m tunnel on a large axisymmetric model at speeds corresponding to Re(L) = 5,000,000-6,000,000. The development of the turbulent boundary layer was compared for that of sintered metal, bonded screening, and perforated sheet and then to that for the flow over a solid smooth wall and a solid, sand-roughened wall. The comparisons reveal that the effect of porosity is to shift the logarithmic region of the wall law down by a certain amount from the solid wall results and to increase the skin friction values by about 30-40%. The downward shift of the logarithmic region of the wall law and the increase of the skin friction value by the combined effects of small roughness and porosity are found to be roughly the sum of their individual effects.

Kong, F. Y.↗
NASA NTRS1982

Oxidation stability of advanced reaction-bonded Si3N4 materials

Four slip-cast, injection-molded and isostatically-pressed specimens of reaction-bonded silicon nitride (RBSN) were subjected to static oxidation tests at 900 C for 10 hours. Specimens containing 8-10% interconnected open porosity of size greater than one micron exhibited a 20-30% decrease in average room temperature four-point flexure strength, while those with 10% open porosity of magnitudes much smaller than one micron as well as those with 2-4% interconnected open porosity of about one micron did not decrease in strength after 900 C exposure. It was determined that preoxidation treatment at 1350 C prevents the 20-30% strength degradation due to internal oxidation, and evidence is presented which suggests that surface pit formation in some RBSN may result from contamination by the furnace environment rather than any intrinsic material properties.

Lindberg, L. J.↗
NASA NTRS1982

Ecological optimality in water-limited natural soil-vegetation systems. II - Tests and applications

The long-term optimal climatic climax soil-vegetation system is defined for several climates according to previous hypotheses in terms of two free parameters, effective porosity and plant water use coefficient. The free parameters are chosen by matching the predicted and observed average annual water yield. The resulting climax soil and vegetation properties are tested by comparison with independent observations of canopy density and average annual surface runoff. The climax properties are shown also to satisfy a previous hypothesis for short-term optimization of canopy density and water use coefficient. Using these hypotheses, a relationship between average evapotranspiration and optimum vegetation canopy density is derived and is compared with additional field observations. An algorithm is suggested by which the climax soil and vegetation properties can be calculated given only the climate parameters and the soil effective porosity. Sensitivity of the climax properties to the effective porosity is explored.

Eagleson, P. S.↗
NASA NTRS1986

Effect of rotopositioning on the growth and maturation of mandibular bone in immobilized Rhesus monkeys

The rates of bone formation and mineralization in the mandibular cortex of juvenile Rhesus monkeys exposed to immobilization/rotopositioning are evaluated. The monkeys were restrained in a supine position and rotated 90 deg every 30 minutes through a full 360 deg for 14 days. The microscopic distribution of mineral densities in osteonal bone and the porosity of cortical bone are studied using microradiographs, and osteon closure rates are assessed using tetracycline labeling; normal distributions of osteons of different mineral density and cortical bone porosity values are observed. It is concluded that 14 days of immobilization/rotopositioning did not cause abnormal changes in osteon mineralization, cortical porosity, and osteon closure rates.

Simmons, D. J.↗
NASA NTRS1987

A thermodynamic prediction for microporosity formation in aluminum-rich Al-Cu alloys

A computer model is used to predict the formation and degree of microporosity in a directionally solidified Al-4.5 wt pct Cu alloy, considering the interplay between solidification shrinkage and gas porosity. Macrosegregation theory is used to determine the local pressure within the interdendritic liquid. Results show interdendritic porosity for initial hydrogen contents in the 0.03-1 ppm range, and none below contents of 0.03. An increase in either the thermal gradient or the solidification rate is show to decrease the amount of interdendritic porosity.

Poirier, D. R.↗
NASA NTRS1990

From interstellar dust to comets - A unification of observational constraints

The interstellar dust model of comets is numerically worked out to satisfy simultaneously several basic constraints provided by observations of Comet Halley, and to derive the porosity of coma dust. The observational constraints are (1) the strengths of the 3.4 and 9.7 micron emission bands, (2) the shape of the 9.7 micron band, (3) the amount of silicates relative to organic materials, and (4) the mass distribution of the dust. The method used involves precise calculations of temperatures and the emission characteristics of porous aggregates of interstellar dust as a function of their mass, porosity, and distance to the sun and the wavelength. The results indicate that coma dust has a porosity in the range 0.93-0.975, i.e., a packing factor of 0.07 or less, consistent with independent observations of comet densities of 0.6 to 0.26 g/cu cm and meteor densities of less than 0.2 g/cu cm.

Greenberg, J. Mayo↗
NASA NTRS1991

Effects of processing induced defects on laminate response - Interlaminar tensile strength

Four different layup methods were used in the present study of the interlaminar tensile strength of AS4/3501-6 graphite-reinforced epoxy as a function of defects from manufacturing-induced porosity. The methods were: (1) baseline hand layup, (2) solvent wipe of prepreg for resin removal, (3) moisture-introduction between plies, and (4) a low-pressure cure cycle. Pore characterization was conducted according to ASTM D-2734. A significant reduction is noted in the out-of-plane tensile strength as a function of increasing void content; the porosity data were used in an empirical model to predict out-of-plane strength as a function of porosity.

Gurdal, Zafer↗
NASA NTRS1991

Euler study on porous transonic airfoils with a view toward multipoint design

Euler solutions for steady transonic flow (Free-stream Mach 0.63-0.8, alpha = 0-2 deg) over NACA 0012 and supercritical airfoils with solid as well as porous surfaces suggest porosity as a means to realize multipoint design for transonic airfoils. The porous surfaces extend over at least 90 percent of the chord. The porosity distribution is described by a modified sine wave with several amplitudes. Either connected or separated cavities are assumed to lie underneath the upper and lower surfaces. Applied to an NACA 0012 airfoil, porosity generally increases lift, in some instances by up to 65 percent. Porous NACA 0012 airfoils in supercritical flow yield reductions of an order of magnitude in wave drag at constant lift, compared to their solid counterpart. Making the surface of a supercritical airfoil permeable also leads to sizeable reductions in wave drag at constant lift for overspeed conditions. The discussion of the computed results addresses issues such as grid sensitivity and checks for systematic errors.

Hartwich, Peter M.↗
NASA NTRS1992

Airborne lidar and radiometric observations of PBL- and low clouds

Boundary layer- and low altitude clouds over open ocean and continent areas have been studied during several field campaigns since mid-1990 using the French airborne backscatter lidar LEANDRE in conjunction with on-board IR and visible radiometers. LEANDRE is an automatic system, and a modification of the instrumental parameters, when airborne, is computer controlled through an operator keyboard. The vertical range squared lidar signals and instrument status are displayed in real time on two dedicated monitors. The lidar is used either down- or up-looking while the aircraft is flying above or below clouds. A switching of the viewing configuration takes about a minute. The lidar measurements provide a high resolution description of cloud morphology and holes in cloud layers. The flights were conducted during various meteorological conditions on single or multilayer stratocumulus and cumulus decks. Analysis on a single shot basis of cloud top (or bottom) altitude and a plot of the corresponding histogram allows one to determine a probability density function (PDF). The preliminary results show the PDFs for cloud top are not Gaussian and symmetric about the mean value. The skewness varies with atmospheric conditions. An example of results recorded over the Atlantic ocean near Biarritz is displayed, showing: (1) the range squared lidar signals as a function of time (here 100 s corresponds to about 8 km, 60 shots are averaged on horizontal); the Planetary Boundary Layer (PBL) - up to 600 m - is observed at the beginning of the leg as well as on surface returns, giving an indication of the porosity; (2) the cloud top altitude variation between 2.4 to 2.8 km during the 150 to 320 s section; and (3) the corresponding PDF. Similar results are obtained on stratocumulus over land. Single shot measurements can be used also to determine an optical porosity at a small scale as well as a fractional cloudiness at a larger scale. A comparison of cloud top altitude retrieved from lidar and narrowbeam IR radiometer is conducted to study the scale integration problem. A good agreement within less than 100 m relies on spatial uniformity and an optically thick layer. In the presence of holes, a discrepancy is observed. This is illustrated in figure 2, displaying as a function of time (1) the lidar signals; (2) the target temperature (either clouds or sea surface) retreived from a narrowbeam IR radiometer, 17 C is the sea surface temperature on that day; and (3) the visible flux, linked to cloud albedo, measured by a pyranometer. In preparation of ASTEX, down- and up-looking measurements where conducted on stratocumulus clouds over the Atlantic Ocean near Quimper in Brittany. Depending on the flight pattern orientation with respect to the wind, the top and bottom cloud morphologies are different. Preliminary results are given on cloud morphology, cloud top PDFs, optical porosity, fractional cloudiness, and comparison of lidar and radiometric measurements.

Flamant, P. H.↗
NASA NTRS1991

Thermally driven diffusion of SO2 within the surface of Io

The presence of sulfur dioxide (SO2) on Io, together with the fact that the surface layer of Io has extremely high porosity, suggests the possibility of diffusion of this volatile within the surface, as well as exchange between the surface and an atmosphere. The former possibility is investigated through the development of a surface layer thermal model and subsequent calculations of the thermally driven diffusion flux of SO2 within the layer. The major factors affecting the diffusion process are the temperature and temperature gradient in the surface layer throughout the day, and the porosity and grain size in the surface layer. The results indicate that the net transport of SO2 in the near-surface region is downward into the subsurface, causing near-surface depletion of SO2. Near-surface depletion would result in a layer of reduced thermal inertia overlying the bulk of the surface, consistent with thermal eclipse observations of Io. For the present nominal model with 10-micron grains and a porosity of 85 percent, the peak net diurnal downward flux reaches nearly 0.008 g/sq cm per period.

Meade, Paul E.↗
NASA NTRS1993

A method for the compensation of the effects of surface cloth impressions on polar backscatter applied to porous epoxy and biaxial graphite/epoxy composites

The anisotropy of polar backscatter from graphite/epoxy composites is a potentially useful parameter for the characterization of porosity levels. However, the effects of release-cloth impressions on measured integrated polar backscatter levels are sufficient to inhibit the detection of porosity with this method. Recently, we developed a theoretical model to predict the frequency distribution of the backscattered power along the high-symmetry directions of release-cloth impressions. This study investigates experimentally the usefulness of limiting the bandwidth to regions not dominated by the scattering from the surface impressions, hence increasing the probability of detecting flaws such as porosity.

Madaras, Eric I.↗
NASA NTRS1997

Micromechanical Modeling of Woven Metal Matrix Composites

This report presents the results of an extensive micromechanical modeling effort for woven metal matrix composites. The model is employed to predict the mechanical response of 8-harness (8H) satin weave carbon/copper (C/Cu) composites. Experimental mechanical results for this novel high thermal conductivity material were recently reported by Bednarcyk et al. along with preliminary model results. The micromechanics model developed herein is based on an embedded approach. A micromechanics model for the local (micro-scale) behavior of the woven composite, the original method of cells (Aboudi), is embedded in a global (macro-scale) micromechanics model (the three-dimensional generalized method of cells (GMC-3D) (Aboudi). This approach allows representation of true repeating unit cells for woven metal matrix composites via GMC-3D, and representation of local effects, such as matrix plasticity, yarn porosity, and imperfect fiber-matrix bonding. In addition, the equations of GMC-3D were reformulated to significantly reduce the number of unknown quantities that characterize the deformation fields at the microlevel in order to make possible the analysis of actual microstructures of woven composites. The resulting micromechanical model (WCGMC) provides an intermediate level of geometric representation, versatility, and computational efficiency with respect to previous analytical and numerical models for woven composites, but surpasses all previous modeling work by allowing the mechanical response of a woven metal matrix composite, with an elastoplastic matrix, to be examined for the first time. WCGMC is employed to examine the effects of composite microstructure, porosity, residual stresses, and imperfect fiber-matrix bonding on the predicted mechanical response of 8H satin C/Cu. The previously reported experimental results are summarized, and the model predictions are compared to monotonic and cyclic tensile and shear test data. By considering appropriate levels of porosity, residual stresses, and imperfect fiber-matrix debonding, reasonably good qualitative and quantitative correlation is achieved between model and experiment.

Bednarcyk, Brett A.↗