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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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Thermal diffusive conductivity of dilute superfluid (He-3)-(He-4) mixtures

A new technique is described for measuring the effective thermal conductivity of superfluid (He-3)-(He-4) mixtures. The results confirm the large departures from hydrodynamic theory reported previously. The analysis suggests that the measured conductivity is the finite thermal diffusive conductivity of superfluid helium in the absence of counterflow. This conductivity need not diverge in the limit of small He-3 concentration.

Chui, T. C. P.↗
NASA NTRS1990

Laboratory measurements and modeling of microwave absorption by ammonia in gas mixtures applicable to giant planet atmospheres

Accurate knowledge of the microwave absorption behavior of ammonia is critical to the correct interpretation of radio astronomical and radio occultation data from the giant planets. New cavity resonator techniques developed at the Stanford Center for Radar Astronomy have allowed accurate laboratory measurements of the microwave absorptivity and refractivity spectra of gas mixtures containing trace amounts of ammonia. A parameterized version of the modified Ben-Reuven formalism of Berge and Bulkis was optimized to fit the new data. The new formalism produced by this method predicts ammonia absorptivity much more accurately than previous formalism over a significant range of conditions.

Spilker, T. R.↗
NASA NTRS1990

Coarsening in binary solid-liquid mixtures

A theory of Ostwald ripening has been developed for a solid-liquid mixture cosisting of a low volume fraction array of spherical solid particles in a liquid wherein the coarsening process proceeds via the transport of both heat and mass. It is found that the simultaneous transport of heat and mass during ripening does not alter the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large decrease in the rate constant can be expected with small additions of solute to a pure metal. Possible extensions of this theory to the analogous problem of ripening in isothermal ternary alloys are also discussed.

Voorhees, P. W.↗
NASA NTRS1989

Nitriding kinetics of Si-SiC powder mixtures as simulations of reaction bonded Si3N4-SiC composites

The nitriding kinetics of Si and Si plus SiC powder mixtures were studied to simulate the fabrication of RBSN-SiC ceramic matrix composites. Very clean, assynthesized, and solvent-exposed powders were studied; C-rich and Si-rich SiC 0.04-0.05 micron diameter powders were mixed in varying concentrations with SiH4-derived 0.2-0.3 micron diameter Si powder. Complete nitridation is achieved with C-rich SiC powders in 140 min at 1250 C, and in the centers of Si-rich SiC powders in 15 min. The effects on the incubation periods, fast reaction periods, and slow reaction periods that characterize these nitriding processes were studied to explain unusual reverse reaction gradients and other effects of contamination.

Lightfoot, A.↗
NASA NTRS1990

Long-wave morphological instabilities in the directional solidification of a dilute binary mixture

Long-wave instabilities in a directionally-solidified binary mixture may occur in several limits. Sivashinsky (1983) identified a small-segregation-coefficient limit and obtained a weakly nonlinear evolution equation governing subcritical two-dimensional bifurcation. Brattkus and Davis (1988) identified a near-absolute-stability limit and obtained a strongly nonlinear evolution equation governing supercritical two-dimensional bifurcation. The present investigation identifies a third strongly nonlinear evolution equation, arising in the small-segregation-coefficient, large-surface-energy limit. This equation links both of the former and describes the change from the sub- to super-critical bifurcations. This study sets the previous long-wave analyses into a logical framework.

Riley, D. S.↗
NASA NTRS1990

Observations of columnal recombination in the ionization tracks of energetic heavy nuclei in an argon-methane gas mixture

Measurements of ionization signals resulting from the passage of energetic heavy nuclei through a gas mixture consisting of 95 mol percent Ar plus 5 mol percent CH4, at an absolute pressure of 3 atm are presented. The measurements take place under a uniform electric field perpendicular to the ionization track. The signals were compared to the calculated energy losses, with an assumption of proportionality between energy loss rate and ionization rate. Significant deviations from proportionality are found for energy loss rate grater than about 3000 MeV sq cm/g, while fractional deviations are found to be proportional to the energy loss rate (dE/dx) exp m, where m is equal to about two. These results are attributed to the columnal recombination.

Wiedenbeck, Mark E.↗
NASA NTRS1990

Diagnostic evaluations of microwave generated helium and nitrogen plasma mixtures

The goal of this work is to continue the development to fundamentally understand the plasma processes as applied to spacecraft propulsion. The diagnostic experiments used are calorimetric, dimensional, and spectroscopic measurements using the TM 011 and TM 012 modes in the resonance cavity. These experimental techniques are highly important in furthering the understanding of plasma phenomena and of designing rocket thrusters. Several experimental results are included using nitrogen and helium gas mixtures.

Haraburda, Scott S.↗
NASA NTRS1990

Linear mixing rule in screened binary ionic mixtures

The validity of the linear mixing rule is examined for the following two cases (1) when the response of the electron gas is taken into account in the effective ionic interaction and (2) when finite-temperature effects are included in the dielectric response of the electrons, i.e., when the ions interact with both temperature- and density-dependent screened Coulomb potentials. It is found that the linear mixing rule remains valid when the electron response is taken into account in the interionic potential at any density, even though the departure from linearity can reach a few percent for the asymmetric mixtures in the region of weak degeneracy for the electron gas. A physical explanation of this behavior is proposed which is based on a simple additional length scale.

Chabrier, G.↗
NASA NTRS1990

Pressure drop in fully developed, duct flow of dispersed liquid-vapor mixture at zero gravity

The dynamics of steady, fully developed dispersed liquid-vapor flow in a straight duct at 0-g is simulated by flowing water containing n-butyl benzoate droplets. Water and benzoate are immiscible and have identical density at room temperature. The theoretical basis of the simulation is given. Experiments showed that, for a fixed combined flow rate of water and benzoate, the frictional pressure drop is unaffected by large changes in the volume fraction of benzoate drops and their size distribution. Measured power spectra of the static wall pressure fluctuations induced by the turbulent water-benzoate flow also revealed that their dynamics is essentially unaltered by the presence of the droplets. These experimental findings, together with the theoretical analysis, led to the conclusion that the pressure drop in fully developed, dispersed liquid-vapor flow in straight ducts of constant cross section at 0-g is identical to that due to liquid flowing alone at the same total volumetric flow rate of the liquid-vapor mixture and, therefore, can be readily determined.

Sridhar, K. R.↗
NASA NTRS1990

A mixture neural net for multispectral imaging spectrometer processing

Each spatial region viewed by an imaging spectrometer contains various elements in a mixture. The elements present and the amount of each are to be determined. A neural net solution is considered. Initial optical neural net hardware is described. The first simulations on the component requirements of a neural net are considered. The pseudoinverse solution is shown to not suffice, i.e. a neural net solution is required.

Casasent, David↗
NASA NTRS1990

Modeling the use of a binary mixture as a control scheme for two-phase thermal systems

Two-phase thermal loops using mechanical pumps, capillary pumps, or a combination of the two have been chosen as the main heat transfer systems for the space station. For these systems to operate optimally, the flow rate in the loop should be controlled in response to the vapor/liquid ratio leaving the evaporator. By substituting a mixture of two non-azeotropic fluids in place of the single fluid normally used in these systems, it may be possible to monitor the temperature of the exiting vapor and determine the vapor/liquid ratio. The flow rate would then be adjusted to maximize the load capability with minimum energy input. A FLUINT model was developed to study the system dynamics of a hybrid capillary pumped loop using this type of control and was found to be stable under all the test conditions.

Benner, S. M.↗
NASA NTRS1991

Laser induced spark ignition of methane-oxygen mixtures

The use of laser-induced spark ignition in liquid-propellant rocket engines is investigated by studying the ignition of mixtures of methane and oxygen. Specific attention is given to minimum ignition energy, spark efficiency, the effect of ignition energy on flame kernels, the effect of laser wavelength, and comparisons with electrode-spark ignition. A pulsed frequency-doubled Nd:YAG laser is tested at atmospheric pressure and 296 K under both laminar-premixed and turbulent-incompletely-mixed conditions. Laser sparks of 10 and 40 mJ and an electrode spark of 6 mJ are measured for flame-kernel radius as a function of time with pulsed laser shadowgraphy. Initially, the flame-kernel size is similar to those predicted by the Taylor spherical-blast-wave model, and subsequent growth is characterized by rapid acceleration. The growth rate is significantly affected by the effect of incomplete fuel-oxidizer mixing.

Lee, T. W.↗
NASA NTRS1991

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

The effects of grain size and shape on the reflectance spectra of mineral mixtures are investigated to improve a reflectance model called the isograin model, whose prototype was proposed by M. Kinoshita in 1985. The sample powder was assumed to consist of an infinite number of layers, each of which has the same thickness with the grain size d.

Hiroi, T.↗
NASA NTRS1989

Mixture block coding with progressive transmission in packet video. Appendix 1: Item 2

Video transmission will become an important part of future multimedia communication because of dramatically increasing user demand for video, and rapid evolution of coding algorithm and VLSI technology. Video transmission will be part of the broadband-integrated services digital network (B-ISDN). Asynchronous transfer mode (ATM) is a viable candidate for implementation of B-ISDN due to its inherent flexibility, service independency, and high performance. According to the characteristics of ATM, the information has to be coded into discrete cells which travel independently in the packet switching network. A practical realization of an ATM video codec called Mixture Block Coding with Progressive Transmission (MBCPT) is presented. This variable bit rate coding algorithm shows how a constant quality performance can be obtained according to user demand. Interactions between codec and network are emphasized including packetization, service synchronization, flow control, and error recovery. Finally, some simulation results based on MBCPT coding with error recovery are presented.

Chen, Yun-Chung↗
NASA NTRS1991

Oxidation-chlorination of binary Ni-Cr alloys in flowing Ar-O2-Cl2 gas mixtures at 1200 K

Nickel-chromium alloys are resistant to oxidation because of the selective oxidation of chromium to form a protective Cr2O3 scale. In chlorine-containing environments, volatile corrosion products can also be formed. The mixed oxidation-chlorination of Ni-4.5Cr, Ni-13.8Cr, and Ni-26.5Cr (by weight) alloys in Ar-O2-Cl2 gas mixtures is investigated using thermogravimetric analysis and atmospheric-pressure-sampling mass spectrometry, followed by examination of the corrosion products using scanning electron microscopy and X-ray diffraction analysis. The overall kinetics of the corrosion are affected by the relative amounts of oxides and chlorides formed and the composition of the oxide corrosion products.

Mcnallan, M. J.↗
NASA NTRS1992

On the mixture flow problem in lubrication of hydrodynamic bearings - Small solid volume fraction

The lubrication problem of infinitely long slider bearings with a mixture of fluid and particulate solid at small volume fraction level is studied. Closed-form analytical solutions for pressure and shear stress are obtained for a class of solid aggregates. The results reduce to those of pure fluid in the limiting case. A parametric study of the bearing performance with particulate solid is presented.

Khonsari, M. M.↗
NASA NTRS1992

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

An 'isograin model' of reflectance from particulate material is presented. The model treats reflectance as a series of grain interactions, incorporating optical constants of each material and two parameters that model the activity of scattering and absorption in the vertical and the horizontal directions, respectively. Mixing ratios of components in mixture are treated as variables and are calculated using least-squares fit. Grain size and shape have a significant effect on the accuracy of predicting mixing ratios but can be modeled by introducing the concept of effective grain size. The model calculates spectra in good agreement with measured spectra and models reflectance within 1 percent and mixing ratios within 5 wt pct.

Hiroi, T.↗
NASA NTRS1992

Observations of soot during droplet combustion at low gravity - Heptane and heptane/monochloroalkane mixtures

Experimental observations of the combustion of sooting fuel droplets, performed in a drop tower to create a low gravity environment, are reported. Free n-heptane droplets and suspended droplets of heptane, monochloroalkanes, and mixtures of monochloro-octane and heptane were studied. Initial droplet diameters ranged from 0.4 to 1.1. mm. The results suggest that soot may influence droplet vaporization rates. Spherical symmetry of the flame allowed for extended observation of soot agglomerates inside the droplet flame. Effects of slight convective flows were also observed, both through variations of natural convection around the suspended droplets and through variations in the drift velocities of the unsupported droplets. Slight convective flows around the suspended droplets reduced flame luminosity as well as soot accumulation inside the flame and increased droplet vaporization rates. Mixing monochloro-octane with n-heptane demonstrated the effectiveness of n-heptane in reducing soot emissions from the flames of the chlorinated fuels. Finally, trends of initial droplet diameter with burning rate were observed and may be linked to the effect of droplet size upon soot formation inside the flame.

Jackson, G. S.↗