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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Mechanisms of anode power deposition in a low pressure free burning arc

Anode power deposition is a dominant power loss mechanism for arc jets and MPD thrusters. In this study, a free burning arc experiment was operated at pressures and current densities similar to those in arc jets and MPD thrusters in an attempt to identify the physics controlling this loss mechanism. Use of a free burning arc allowed for the isolation of independent variables controlling anode power deposition and provided a convenient and flexible way to cover a broad range of currents, anode surface pressures, and applied magnetic field strengths and orientations using an argon gas. Test results showed that anode power deposition decreased with increasing anode surface pressure up to 6.7 Pa (0.05 torr) and then became insensitive to pressure. Anode power increased with increasing arc current while the electron number density near the anode surface increased linearity. Anode power also increased with increasing applied magnetic field strength due to an increasing anode fall voltage. Applied magnetic field orientation had an effect only at high currents and low anode surface pressures, where anode power decreased when applied field lines intercepted the anode surface. The results demonstrated that anode power deposition was dominated by the current carrying electrons and that the anode fall voltage was the largest contributor. Furthermore, the results showed that anode power deposition can be reduced by operating at increased anode pressures, reduced arc currents, and applied magnetic field strengths and with magnetic field lines intercepting the anode.

Soulas, George C.↗
NASA NTRS1980

An ICR study of an association reaction at low pressure

The reactions of the gaseous ion CH3+ with HCN are investigated, using the ion cyclotron resonance technique. The pressure range of the experiment was 1 x 10 to the -6 to 3 x 10 to the -4 Torr with a rate coefficient of 2 x 10 to the -10 cu cm/sec. Good agreement was found between rate coefficients determined in the trapping mode at the low end of the pressure range and the drift mode at the high end of the pressure range. It is shown that the association reaction of CH3 with HCN follows second order kinetics throughout the entire pressure range of the experiment. The reaction is interpreted in terms of both a saturated three-body and a radiative association mechanism. Some explanations for the disagreement of the rate coefficient with that reported previously are also given.

Mcewan, M. J.↗
NASA NTRS2000

Low Pressure Greenhouse Concepts for Mars

A project was initiated to begin testing some environmental limits for managing plant growth systems. These limits will help determine some of the concepts for building plant enclosures for use on Mars. In particular, this study focuses on the effects of reduced atmospheric pressures. Structural design is considered as it relates to the biological processes that would occur within that structure. The design must be closely tied to the functionality of the biological system and has a few primary concerns that need to be tested to resolve the question as to the path of the design. Early tests indicate that plants can survive and grow at low (greater than 76 mb) pressure.

Fowler, Philip A.↗
NASA NTRS1979

Electrotransport purification of thorium under low pressure conditions

It was demonstrated that the ultrapurification of refractory metals by electrotransport refining could best be accomplished in the noncontaminating environment of an orbiting low density materials laboratory such as the Molecular Shield Device. Refining experiments were performed at 2 x 10 to the -12th Torr which resulted in the preparation of small quantities of the World's purest thorium metal. The microgravity occurring in orbit was simulated electromagnetically and shown to be advantageous in eliminating grain-sliding caused by the plastic deformation of the sample at high temperature. The electrotransport sample assembly was tested in several environments including simulated solar irradiation, coldness and darkness and under various pressure conditions. Ultrapure single crystals of alpha thorium were also prepared and characterized. Laboratory electronics for the experiment were developed and a totally automatic control system was used to heat the specimens.

Schmidt, F. A.↗
NASA NTRS1999

Characterization of Showerhead Performance At Low Pressure

The overall objective of this work is to characterize the flow to rough showerheads by deriving pressure drop versus velocity correlations to at can be then used in reactor scale simulations where the showerhead is approximated as a porous medium. At relatively low Reynolds numbers (less than 1-10 based on the hole length scale) and in the absence of slip flow, Darcy's Law, grad P = mu U/k, can be used to express the relation between the pressure drop and velocity where @mu@ is the fluid viscosity and it is the permeability that can be theoretically predicted as k= e R^2 /8, where e is the porosity. However, at sufficiently small hole diameters and decreased pressures (less than 5 Torr), the Knudsen number based on showerhead tube radius increases, and the flow may be in a transition regime. Different expressions have been proposed to account for this effect in the permeability by expressing k as a function of either pressure or Knudsen number. But at even higher Knudsen numbers, the pressure drop - velocity dependence is non-linear, and Darcy's Law no longer holds such that a permeability cannot be defined. The direct simulation Monte Carlo method is used along side conventional CFD techniques to determine the extent to which the CFD technique is appropriate and helps to derive correlations for the more rarefied cases of interest in these showerhead flows.

Hash, David B.↗
NASA NTRS2009

Low-Pressure, Field-Ionizing Mass Spectrometer

A small mass spectrometer utilizing a miniature field ionization source is now undergoing development. It is designed for use in a variety of applications in which there are requirements for a lightweight, low-power-consumption instrument that can analyze the masses of a wide variety of molecules and ions. The device can operate without need for a high-vacuum, carrier-gas feed radioactive ionizing source, or thermal ionizer. This mass spectrometer can operate either in the natural vacuum of outer space or on Earth at any ambient pressure below 50 torr (below about 6.7 kPa) - a partial vacuum that can easily be reached by use of a small sampling pump. This mass spectrometer also has a large dynamic range - from singly charged small gas ions to deoxyribonucleic acid (DNA) fragments larger than 104 atomic mass units - with sensitivity adequate for detecting some molecules and ions at relative abundances of less than one part per billion. This instrument (see figure) includes a field ionizer integrated with a rotating-field mass spectrometer (RFMS). The field ionizer effects ionization of a type characterized as "soft" in the art because it does not fragment molecules or initiate avalanche arcing. What makes the "soft" ionization mode possible is that the distance between the ionizing electrodes is less than mean free path for ions at the maximum anticipated operating pressure, so that the ionizer always operates on the non-breakdown side of the applicable Paschen curve (a standard plot of breakdown potential on the ordinate and pressure electrode separation on the abscissa). The field ionizer in this instrument is fabricated by micromachining a submicron-thick membrane out of an electrically nonconductive substrate, coating the membrane on both sides to form electrodes, then micromachining small holes through the electrodes and membrane. Because of the submicron electrode separation, even a potential of only 1 V applied between the electrodes gives rise to an electric field with a strength of in excess of a megavolt per meter strong enough to ionize any gas molecules passing through the holes. An accelerator grid and an electrostatic deflector focus the ions from the field ionizer into the rotating-field cell of the RFMS. The potentials applied to the electrodes of the cell to generate the rotating electric field typically range from 1 to 13 V. The ions travel in well-defined helices within this cell, after which they are collected in a Faraday cup. The mass of most of the molecules reaching the Faraday cup decreases with increasing frequency of rotation of the electric field in the cell. Therefore, the frequency of rotation of the electric field is made to vary in order to scan through a desired range of ion masses: For example, lightweight gas molecules are scanned at frequencies in the megahertz range, while DNA and other large organic molecules are scanned at kilohertz frequencies.

Hartley, Frank↗
NASA NTRS1998

Prediction of Transitional Flows in the Low Pressure Turbine

Current turbulence models tend to give too early and too short a length of flow transition to turbulence, and hence fail to predict flow separation induced by the adverse pressure gradients and streamline flow curvatures. Our discussion will focus on the development and validation of transition models. The baseline data for model comparisons are the T3 series, which include a range of free-stream turbulence intensity and cover zero-pressure gradient to aft-loaded turbine pressure gradient flows. The method will be based on the conditioned N-S equations and a transport equation for the intermittency factor. First, several of the most popular 2-equation models in predicting flow transition are examined: k-e [Launder-Sharina], k-w [Wilcox], Lien-Leschiziner and SST [Menter] models. All models fail to predict the onset and the length of transition, even for the simplest flat plate with zero-pressure gradient(T3A). Although the predicted onset position of transition can be varied by providing different inlet turbulent energy dissipation rates, the appropriate inlet conditions for turbulence quantities should be adjusted to match the decay of the free-stream turbulence. Arguably, one may adjust the low-Reynolds-number part of the model to predict transition. This approach has so far not been very successful. However, we have found that the low-Reynolds-number model of Launder and Sharma [1974], which is an improved version of Jones and Launder [1972] gave the best overall performance. The Launder and Sharma model was designed to capture flow re-laminarization (a reverse of flow transition), but tends to give rise to a too early and too fast transition in comparison with the physical transition. The three test cases were for flows with zero pressure gradient but with different free-stream turbulent intensities. The same can be said about the model when considering flows subject to pressure gradient(T3C1). To capture the effects of transition using existing turbulence models, one approach is to make use of the concept of the intermittency to predict the flow transition. It was originally based on the intermittency distribution of Narasimha [1957], and then gradually evolved into a transport equation for the intermittency factor. Gostelow and associates [1994,1995] have made some improvements to Narasimha's method in an attempt to account for both favorable and adverse pressure gradients. Their approach is based on a linear, explicit combination of laminar and turbulent solutions. This approach fails to predict the overshoot of the skin friction on a flat plate near the end of transition zone, even though the length of transition is well predicted. The major flaw of Gostelow's approach is that it assumes the non-turbulent part being the laminar solution and the turbulent part being the turbulent solution and they do not interact across the transitional region. The technique in condition averaging the flow equations in intermittent flows was first introduced by Libby [1975] and Dopazo [1977] and further refined by Dick and associates [1988, 1996]. This approach employs two sets of transport equations for the non-turbulent part and the other for the turbulent part. The advantage of this approach is that it allows the interaction of non-turbulent and turbulent velocities through the introduction of additional source terms in the continuity and momentum equations for the non-turbulent and turbulent velocities. However, the strong coupling of the two sets of equations has caused some numerical difficulties, which requires special attention. The prediction of the skin friction can be improved by this approach via the implicit coupling of non-turbulent and turbulent velocity flelds. Another improvement of the interrmittency model can be further made by allowing the intermittency to vary in the cross-stream direction. This is one step prior to testing any proposal for the transport equation for the intermittency factor. Instead of solving the transport equation for the intermittency factor, the distribution for the intermittency factor is prescribed by Klebanoff's empirical formula [1955]. The skin friction is very well predicted by this new modification, including the overshoot of the profile near the end of the transition zone. The outcome of this study is very encouraging since it indicates that the proper description of the intermittency distribution is the key to the success of the model prediction. This study will be used to guide us on the modelling of the intermittency transport equation.

Huang, George↗
NASA NTRS1991

Long-Lived, Replaceable Low-Pressure Seals

Simple units operate by metal-spring and elastomeric compression. Ring-shaped gasket has rectangular cross section from which paired cantilever spring arms extend in K-shape. In unassembled state, arms hold elastomeric jackets lightly but securely. Assembled between flanges, arms and jackets become compressed and form airtight seal on both flanges. Suitable for use on hatches and windows; combines advantages of low leakage, long life, and easy replacement.

Weddendorf, Bruce↗
NASA NTRS1976

Low pressure radon diffusion - A laboratory study and its implications for lunar venting

Results of a study of radon migration through columns of fine particulate materials, at total pressures of 0.02-0.2 torr, are reported. Materials studied were: NBS Glass Spheres (SRM 1003), Emerson & Cuming Eccospheres (IG-101), activated coconut charcoal, Lipaci obsidian, and W-1 Standard Diabase. Rates of diffusion were used to derive heats of adsorption for radon on the materials tested. The most reliable values found clustered around 8-9 kcal/mole. These high heats of adsorption, if typical for most materials, combined with low percentages of radon emanation by lunar soils found by other researchers, imply that random walk diffusion will not be an important mechanism for redistributing the radon and the radon daughters produced in the lunar regolith. In particular, since random walk migration is not a sufficient mechanism to account for localized high concentrations of radon-222 and its daughter polonium-210 observed by the Apollo 15 and 16 command modules, an alternative mechanism is proposed, in which radon would be swept to the surface by other gases during intermittent venting events.

Friesen, L. J.↗
NASA NTRS2007

High Pressure Low NOx Emissions Research: Recent Progress at NASA Glenn Research Center

In collaboration with U.S. aircraft engine companies, NASA Glenn Research Center has contributed to the advancement of low emissions combustion systems. For the High Speed Research Program (HSR), a 90% reduction in nitrogen oxides (NOx) emissions (relative to the then-current state of the art) has been demonstrated in sector rig testing at General Electric Aircraft Engines (GEAE). For the Advanced Subsonic Technology Program (AST), a 50% reduction in NOx emissions relative to the 1996 International Civil Aviation Organization (ICAO) standards has been at demonstrated in sector rigs at both GEAE and Pratt & Whitney (P&W). During the Ultra Efficient Engine Technology Program (UEET), a 70% reduction in NOx emissions, relative to the 1996 ICAO standards, was achieved in sector rig testing at Glenn in the world class Advanced Subsonic Combustion Rig (ASCR) and at contractor facilities. Low NOx combustor development continues under the Fundamental Aeronautics Program. To achieve these reductions, experimental and analytical research has been conducted to advance the understanding of emissions formation in combustion processes. Lean direct injection (LDI) concept development uses advanced laser-based non-intrusive diagnostics and analytical work to complement the emissions measurements and to provide guidance for concept improvement. This paper describes emissions results from flametube tests of a 9- injection-point LDI fuel/air mixer tested at inlet pressures up to 5500 kPa. Sample results from CFD and laser diagnostics are also discussed.

Chi-Ming, Lee↗
NASA NTRS2008

NASA Glenn High Pressure Low NOx Emissions Research

In collaboration with U.S. aircraft engine companies, NASA Glenn Research Center has contributed to the advancement of low emissions combustion systems. For the High Speed Research Program (HSR), a 90% reduction in nitrogen oxides (NOx) emissions (relative to the then-current state of the art) has been demonstrated in sector rig testing at General Electric Aircraft Engines (GEAE). For the Advanced Subsonic Technology Program (AST), a 50% reduction in NOx emissions relative to the 1996 International Civil Aviation Organization (ICAO) standards has been demonstrated in sector rigs at both GEAE and Pratt & Whitney (P&W). During the Ultra Efficient Engine Technology Program (UEET), a 70% reduction in NOx emissions, relative to the 1996 ICAO standards, was achieved in sector rig testing at Glenn in the world class Advanced Subsonic Combustion Rig (ASCR) and at contractor facilities. Low NOx combustor development continues under the Fundamental Aeronautics Program. To achieve these reductions, experimental and analytical research has been conducted to advance the understanding of emissions formation in combustion processes. Lean direct injection (LDI) concept development uses advanced laser-based non-intrusive diagnostics and analytical work to complement the emissions measurements and to provide guidance for concept improvement. This paper describes emissions results from flametube tests of a 9-injection-point LDI fuel/air mixer tested at inlet pressures up to 5500 kPa. Sample results from CFD and laser diagnostics are also discussed.

Tacina, Kathleen M.↗
NASA NTRS1964

Sensitive low-pressure relief valve has positive seating against leakage

A pilot-operated relief valve which provides positive seating against leakage in cryogenic systems is described. The principal advantage is that the pilot poppet is unaffected by variations in control pressures in the pilot cavity, and results in a more accurate sensing of inlet pressure conditions.

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