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At least 451 records · Page 25

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NASA NTRS1991

Chemical vapor deposition modeling: An assessment of current status

The shortcomings of earlier approaches that assumed thermochemical equilibrium and used chemical vapor deposition (CVD) phase diagrams are pointed out. Significant advancements in predictive capabilities due to recent computational developments, especially those for deposition rates controlled by gas phase mass transport, are demonstrated. The importance of using the proper boundary conditions is stressed, and the availability and reliability of gas phase and surface chemical kinetic information are emphasized as the most limiting factors. Future directions for CVD are proposed on the basis of current needs for efficient and effective progress in CVD process design and optimization.

Gokoglu, Suleyman A.↗

NASA NTRS1982

Laser system for natural gas detection. Phase 1: Laboratory feasibility studies

This project demonstrated the feasibility of using laser remote sensing technology as a tool for leak survey work in natural gas distribution systems. A laboratory device was assembled using a pair of helium neon (HeNe) lasers to measure methane. One HeNe laser emits radiation at a wavelength of 3.3922 micrometers, which corresponds to a strong absorption feature of methane, while the other emits radiation at a wavelength of 3.3911 micrometers, which corresponds to a weak absorption by methane. As a particular area is scanned for leaks, the laser is pointed at convenient topographic targets within its operating range, about 25 m. A portion of the backscattered radiation is collected by a receiver and focused onto an indium antimonide (InSb) photodetector, cooled to 77K. Methane concentrations were determined from the differential absorption at the two wavelengths for the backscattered radiation.

Grant, W. B.↗

NASA NTRS2007

Liquid Water Oceans in Ice Giants

Aptly named, ice giants such as Uranus and Neptune contain significant amounts of water. While this water cannot be present near the cloud tops, it must be abundant in the deep interior. We investigate the likelihood of a liquid water ocean existing in the hydrogen-rich region between the cloud tops and deep interior. Starting from an assumed temperature at a given upper tropospheric pressure (the photosphere), we follow a moist adiabat downward. The mixing ratio of water to hydrogen in the gas phase is small in the photosphere and increases with depth. The mixing ratio in the condensed phase is near unity in the photosphere and decreases with depth; this gives two possible outcomes. If at some pressure level the mixing ratio of water in the gas phase is equal to that in the deep interior, then that level is the cloud base. The gas below the cloud base has constant mixing ratio. Alternately, if the mixing ratio of water in the condensed phase reaches that in the deep interior, then the surface of a liquid ocean will occur. Below this ocean surface, the mixing ratio of water will be constant. A cloud base occurs when the photospheric temperature is high. For a family of ice giants with different photospheric temperatures, the cooler ice giants will have warmer cloud bases. For an ice giant with a cool enough photospheric temperature, the cloud base will exist at the critical temperature. For still cooler ice giants, ocean surfaces will result. A high mixing ratio of water in the deep interior favors a liquid ocean. We find that Neptune is both too warm (photospheric temperature too high) and too dry (mixing ratio of water in the deep interior too low) for liquid oceans to exist at present. To have a liquid ocean, Neptune s deep interior water to gas ratio would have to be higher than current models allow, and the density at 19 kbar would have to be approx. equal to 0.8 g/cu cm. Such a high density is inconsistent with gravitational data obtained during the Voyager flyby. In our model, Neptune s water cloud base occurs around 660 K and 11 kbar, and the density there is consistent with Voyager gravitational data. As Neptune cools, the probability of a liquid ocean increases. Extrasolar "hot Neptunes," which presumably migrate inward toward their parent stars, cannot harbor liquid water oceans unless they have lost almost all of the hydrogen and helium from their deep interiors.

Wiktorowicz, Sloane J.↗

NASA NTRS1993

Risk of decompression sickness in the presence of circulating microbubbles

In this study, we examined the association between microbubbles formed in the circulation from a free gas phase and symptoms of altitude decompression sickness (DCS). In a subgroup of 59 males of mean (S.D) age 31.2 (5.8) years who developed microbubbles during exposure to 26.59 kPa (4.3 psi) under simulated extravehicular activities (EVA), symptoms of DCS occurred in 24 (41 percent) individuals. Spencer grade 1 microbubbles occurred in 4 (7 percent), grade 2 in 9 (15 percent), grade 3 in 15 (25 percent), and grade 4 in 31 (53 percent) of subjects. Survival analysis using Cox proportional hazards regression showed that individuals with less than grade 3 CMB showed 2.46 times (95 percent confidence interval = 1.26 to 5.34) higher risk of symptoms. This information is crucial for defining the risk of DCS for inflight Doppler monitoring under space EVA. Altitude decompression sickness (DCS) occurs when there is acute reduction in ambient pressure. The symptoms of DCS are due to the formation of a free gas phase (in the form of gas microbubbles) in tissues during decompression. Musculo-skeletal pain of bends is the commonest form of DCS in altitude exposures. In the space flight environment, there is a risk of DCS when astronauts decompress from the normobaric shuttle pressure into the hypobaric space suit pressure (currently about 29.65 kPa (4.3 psi) for extra-vehicular activities (EVA). This risk is counterbalanced by a judicious combination of prior denitrogenation and staged decompression. Studies of DCS are limited by the duration of the test at reduced pressure. Since only a proportion of subjects tested develop symptoms, the information on DCS is generally incomplete or 'censored'. Many studies employ Doppler ultrasound monitoring of the precordial area for detecting circulating microbubbles (CMB). Although the association between CMB and bends pain is not causal, CMB are frequently monitored during decompression. In this paper, we examine the association between CMB and symptoms of DCS under simulated EVA profiles.

Kumar, K. Vasantha↗

NASA NTRS2013

Investigating Titan's Atmospheric Chemistry at Low Temperature in Support of the NASA Cassini Mission

Titan's atmosphere, composed mainly of N2 and CH4, is the siege of a complex chemistry induced by solar UV radiation and electron bombardment from Saturn's magnetosphere. This organic chemistry occurs at temperatures lower than 200 K and leads to the production of heavy molecules and subsequently solid aerosols that form the orange haze surrounding Titan. The Titan Haze Simulation (THS) experiment has been developed on the COSMIC simulation chamber at NASA Ames in order to study the different steps of Titan's atmospheric chemistry at low temperature and to provide laboratory data in support for Cassini data analysis. The chemistry is simulated by plasma in the stream of a supersonic expansion. With this unique design, the gas mixture is adiabatically cooled to Titan-like temperature (approx. 150 K) before inducing the chemistry by plasma discharge. Different gas mixtures containing N2, CH4, and the first products of the N2,-CH4 chemistry (C2H2, C2H4, C6H6...) but also heavier molecules such as PAHs or nitrogen containing PAHs can be injected. Both the gas phase and solid phase products resulting from the plasma-induced chemistry can be monitored and analyzed. Here we present the results of recent gas phase and solid phase studies that highlight the chemical growth evolution when injecting heavier hydrocarbon trace elements in the initial N2-CH4 mixture. Due to the short residence time of the gas in the plasma discharge, only the first steps of the chemistry have time to occur in a N2-CH4 discharge. However by adding acetylene and benzene to the initial N2-CH4 mixture, we can study the intermediate steps of Titan's atmospheric chemistry as well as specific chemical pathways. These results show the uniqueness of the THS experiment to help understand the first and intermediate steps of Titan fs atmospheric chemistry as well as specific chemical pathways leading to Titan fs haze formation.

Sciamma-O'Brien, Ella↗

NASA NTRS1976

Oscillatory combustion of liquid monopropellant droplets

A theoretical investigation was conducted on the open-loop combustion response of monopropellant droplets and sprays to imposed pressure oscillations. The theoretical model was solved as a perturbation analysis through first order, yielding linear response results. Unsteady gas phase effects were considered in some cases, but the bulk of the calculations assumed a quasi-steady gas phase. Calculations were conducted using properties corresponding to hydrazine decomposition. Zero-order results agreed with earlier measurements of hydrazine droplet burning in combustion gases. The droplet response was greatest (exceeding unity in some cases) for large droplets with liquid phase temperature gradients; at frequencies near the characteristic frequency of the liquid phase thermal wave. The response of a spray is less than that of its largest droplet, however, a relatively small percentage of large droplets provides a substantial response (exceeding unity in some cases).

Chanin, S. P.↗

NASA NTRS1976

Oscillatory combustion of monopropellant droplets

A theoretical study of combustion response to imposed pressure oscillations was conducted for monopropellant droplets and sprays. A first-order perturbation analysis was used, yielding the linear response. Transient gas phase phenomena were considered in some cases, but the bulk of the calculations assume a quasi-steady gas phase. The calculations employed properties representative of hydrazine decomposition. Calculated steady combustion results and response predictions for large hydrazine droplets agreed favorably with earlier measurements. Droplet and spray response factors in excess of unity were found. High response was associated with high pressures (1000-10,000 kN/sq m), large droplets, large mean liquid temperature gradients, and frequencies near the characteristic frequency of the liquid phase thermal wave. Spray response factors are lower than the response factor of the largest injected drop, however, a small percentage of large drops can still provide substantial response for the combustion process.

Chanin, S. P.↗

NASA NTRS1993

A two-dimensional model with coupled dynamics, radiation, and photochemistry. 1: Simulation of the middle atmosphere

Spatial and seasonal distribution of ozone and other trace gases are simulated using a interactive two-dimensional model of the stratosphere updated to include full gas phase chemistry. The model consists of a primitive equation dynamics module, a full radiative transfer scheme, and a comprehensive gas phase chemistry module. The circulation is derived from heating rates in the stratosphere that are calculated using model-generated ozone. In the troposphere, parameterized heating rates are adopted. The eddy momentum flux divergence in the zonal mean momentum equation is given by the eddy fluxes of potential vorticity. Eddy fluxes of potential vorticity and tracers are parameterized using a set of predetermined diffusion coefficients. The adopted values for K(sub yy), show a hemispherical asymmetry in that the values in the lower stratosphere are consistently smaller in the southern hemisphere. The asymmetry in K(sub yy) and in the parameterization of the tropospheric heating rate results in an asymmetry in the circulation giving rise to unique signatures in the trace gas distributions. The model successfully simulates the observed asymmetry in the column abundance of the springtime ozone maxima between northern and southern hemisphere. Results for other trace gases are in agreement with the gross observed features although specific differences exist.

Ko, Malcolm K. W.↗

NASA NTRS1995

VRA Modeling, phase 1

The destruction of organic contaminants in waste water for closed systems, such as that of Space Station, is crucial due to the need for recycling the waste water. A co-current upflow bubble column using oxygen as the gas phase oxidant and packed with catalyst particles consisting of a noble metal on an alumina substrate is being developed for this process. The objective of this study is to develop a plug-flow model that will predict the performance of this three phase reactor system in destroying a multicomponent mixture of organic contaminants in water. Mass balances on a series of contaminants and oxygen in both the liquid and gas phases are used to develop this model. These mass balances incorporate the gas-to-liquid and liquid-to-particle mass transfer coefficients, the catalyst effectiveness factor, and intrinsic reaction rate. To validate this model, a bench scale reactor has been tested at Michigan Technological University at elevated pressures (50-83 psig,) and a temperature range of 200 to 290 F. Feeds consisting of five dilute solutions of ethanol (approx. 10 ppm), chlorobenzene (approx. 20 ppb), formaldehyde (approx. 100 ppb), dimethyl sulfoxide (DMSO approx. 300 ppb), and urea (approx. 20 ppm) in water were tested individually with an oxygen mass flow rate of 0.009 lb/h. The results from these individual tests were used to develop the kinetic parameter inputs necessary for the computer model. The computer simulated results are compared to the experimental data obtained for all 5 components run in a mixture on the differential test column for a range of reactor contact times.

Kindt, Louis M.↗

NASA NTRS1972

Numerical solutions of reactive fluid flows during postignition transients in hybrid rocket systems.

A computational method has been developed for the study of the post-ignition transients in hybrid rocket systems. The particular system chosen consisted of a gaseous oxidizer flowing within a tube of solid fuel, resulting in heterogeneous combustion. With the appropriate assumptions, two-dimensional, time-dependent conservation equations were derived for the reacting gas phase, and for the solid phase, in a cylindrical coordinate system. These were then programmed for numerical computation, using two implicit finite-difference schemes, the Lax-Wendroff scheme for the gas phase, and the Crank-Nicolson scheme for the solid phase. Appropriate initial and boundary conditions were represented, including heat and mass conservation at the interface between gas and solid. Initially, no attempt was made to relate the recession rate at the surface to the surface temperature, or to include heat transfer by radiation. A simple case was selected for preliminary calculations, with aluminum and oxygen as fuel and oxidizer, and aluminum oxide as the product.

Hung, W. S. Y.↗

NASA NTRS2002

The effect of exercise and rest duration on the generation of venous gas bubbles at altitude

BACKGROUND: Decompression, as occurs with aviators and astronauts undergoing high altitude operations or with deep-sea divers returning to surface, can cause gas bubbles to form within the organism. Pressure changes to evoke bubble formation in vivo during depressurization are several orders of magnitude less than those required for gas phase formation in vitro in quiescent liquids. Preformed micronuclei acting as "seeds" have been proposed, dating back to the 1940's. These tissue gas micronuclei have been attributed to a minute gas phase located in hydrophobic cavities, surfactant-stabilized microbubbles, or arising from musculoskeletal activity. The lifetimes of these micronuclei have been presumed to be from a few minutes to several weeks. HYPOTHESIS: The greatest incidence of venous gas emboli (VGE) will be detected by precordial Doppler ultrasound with depressurization immediately following lower extremity exercise, with progressively reduced levels of VGE observed as the interval from exercise to depressurization lengthens. METHODS: In a blinded cross-over design, 20 individuals (15 men, 5 women) at sea level exercised by performing knee-bend squats (150 knee flexes over 10 min, 235-kcal x h(-1)) either at the beginning, middle, or end of a 2-h chair-rest period without an oxygen prebreathe. Seated subjects were then depressurized to 6.2 psia (6,706 m or 22,000 ft altitude equivalent) for 120 min with no exercise performed at altitude. RESULTS: Of the 20 subjects with VGE in the pulmonary artery, 10 demonstrated a greater incidence of bubbles with exercise performed just prior to depressurization, compared with decreasing bubble grades and incidence as the interval of rest increased prior to depressurization. No decompression illness was reported. CONCLUSIONS: There is a significant increase in decompression-induced bubble formation at 6.2 psia when lower extremity exercise is performed just prior to depressurization as compared with longer rest intervals. Analysis indicated that micronuclei half-life is on the order of an hour under these hypobaric conditions.

Controlled Clinical Trial↗

NASA NTRS1991

Lunar oxygen production by pyrolysis of regolith

Oxygen represents one of the most desirable products of lunar mining and manufacturing. Among the many processes which have been proposed for oxygen production, pyrolysis stands out as one which is uncomplicated and easy to bootstrap. Pyrolysis or vapor-phase reduction involves heating regolith to temperatures sufficient to allow partial decomposition and vaporization. Some metal oxides give up oxygen upon heating, either in the gas phase to form reduced gaseous species or in the condensed phase to form a metallic phase. Based on preliminary experiments and equilibrium calculations, the temperatures needed for pyrolysis are expected to be in the range of 2000 to 2200 K, giving total gas pressures of 0.001 to 0.1 torr. Bulk regolith can be used as a feedstock without beneficiation with concentrated solar radiation supplying most of energy needed. Further, selective condensation of metal-containing species from the gas phase may yield metallic iron and silicon as byproducts.

Senior, Constance L.↗

DOE OSTI2026

Modeling Radiolysis and Chemical Reactions during Dry Storage of Aluminum-clad Spent Nuclear Fuel

After aluminum-clad spent nuclear fuel (ASNF) is removed from the reactor, it is initially stored in spent fuel pools, which are specially designed water-filled basins that provide temporary cooling to reduce the temperature of the fuel assemblies and provide radiation shielding. ASNF continues to generate heat due to the radioactive decay of elements within the fuel, which persists for many years post-shutdown as the residual radioactive products decay into more stable elements. During the wet storage period, an oxyhydroxide layer composed of boehmite/bayerite forms on the surfaces of the aluminum cladding from exposure to water in the pools. Road-ready packaging for long-term disposition of the ASNF involves dry storage in helium backfilled DOE standard canisters (DSCs). When the ASNF is removed from water storage and dried, most of the water is removed, but some physisorbed and chemisorbed water remains in the oxyhydroxide layers. This residual water can produce hydrogen when exposed to radiation from the ASNF during dry storage. Predicting hydrogen accumulation over time in the DSCs is critical for long-term storage considerations. Previous modeling efforts have developed coupled computational fluid dynamics (CFD)-chemical models to simulate temperature, pressure, and gas phase concentrations within the DSCs. These models use the thermal field predicted by CFD as input to a radiolysis model for the gas phase and the surface oxyhydroxide layer chemistry. Given the long storage period of the DSCs and the impracticality of long-term experiments, a simulation-based approach is necessary to assess chemical evolution within the canisters. This study advances the development of a modeling framework designed to simulate the chemical evolution of spent fuel canisters. Both thermal and radiation-driven reactions are considered, with radiation kinetics quantified using G-values. Sensitivity analysis identifies key parameters influencing species composition. Reaction pathway diagrams offer insight into dominant species formation routes, enabling more effective comparisons between model predictions and experimental observations, particularly regarding the production of hydrogen. Results show that the model predicts significant hydrogen gas production with minimal oxygen generation, primarily due to hydrogen formation via boehmite pathways. These findings underscore the importance of accurately characterizing surface-bound species and radiolysis kinetics. A deeper understanding of these mechanisms is critical for evaluating the long-term safety of nuclear waste storage.

12 - MGMT OF RADIOACTIVE AND NON-RADIOACTIVE WASTE↗

DOE OSTI2024

A Reactor Scale-Up Methodology from Lab-Scale to Pilot-Scale Operations: Numerical Modeling of THFA Dehydration to DHP in Packed-Bed Reactors

This manuscript discusses developing a model-based scale-up methodology for a successful technology transfer of gas-phase catalytic reactors from a lab-scale to a pilot-scale operation. The manuscript demonstrates the methodology for gas-phase dehydration of tetrahydrofurfuryl alcohol (THFA) to dihydropyran (DHP) process over commercial Al 2 O 3 catalysts. A two-dimensional reactor model was developed using COMSOL Multiphysics 6.1 software. The model solves heat and mass transport equations in bed-scale and particle scales simultaneously. This powerful feature enables accurate prediction of the heat and mass transfer limitations in pilot-scale reactors, if any exists. Further, the model uses isothermal lab-scale experimental data to derive and validate the reaction chemistry, flow fields and boundary conditions. The model was then scaled-up to project conversion, selectivity, yield and formation rate of DHP in a pilot-scale reactor. The results highlight the complex nature of chemistry, heat, and mass transfer effects in lab-scale and pilot-scale reactors. The model results inform the possible operational limitations of the pilot-scale reactor and design strategies to improve process efficiency. Although the scale-up approach is explained through the THFA dehydration process, the methodology is applicable to any catalytic packed-bed reactor models for a successful process scale-up.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Unraveling In-Situ Formation of Surface Nickel Nitride Structures in Plasma-Assisted Catalytic Ammonia Synthesis

We report the in situ formation of Ni nitride for plasma-assisted ammonia synthesis. Both the surface nitrogen concentration and the ammonia formation rate exhibit dependence on the N 2 :H 2 feed ratio. The maximum surface nitrogen concentration occurs at a N 2 :H 2 ratio of 4:1, and the maximum catalytic activity occurs at 2:1. In contrast, the formation of gas phase radicals is less sensitive to feed composition, indicating that Ni nitride is more kinetically relevant to ammonia production than gas-phase radicals. The plasma-induced formation of Ni nitride is therefore proposed to be a critical contributor to the synergistic effects in plasma-assisted catalytic ammonia synthesis. Additionally, Ni nitride alters the surface reaction mechanism of plasma-assisted ammonia synthesis, with the rate-determining-step (RDS) shifting to surface-bound NH 3 formation rather than N 2 activation at temperatures below 373 K. These findings provide mechanistic insight that opens opportunities for optimizing the performance of plasma-assisted catalytic ammonia synthesis

Ammonia↗

NASA NTRS1974

Photoballistics of volcanic jet activity at Stromboli, Italy

Two night eruptions of the volcano Stromboli were studied through 70-mm photography. Single-camera techniques were used. Particle sphericity, constant velocity in the frame, and radial symmetry were assumed. Properties of the particulate phase found through analysis include: particle size, velocity, total number of particles ejected, angular dispersion and distribution in the jet, time variation of particle size and apparent velocity distribution, averaged volume flux, and kinetic energy carried by the condensed phase. The frequency distributions of particle size and apparent velocities are found to be approximately log normal. The properties of the gas phase were inferred from the fact that it was the transporting medium for the condensed phase. Gas velocity and time variation, volume flux of gas, dynamic pressure, mass erupted, and density were estimated. A CO2-H2O mixture is possible for the observed eruptions. The flow was subsonic. Velocity variations may be explained by an organ pipe resonance. Particle collimation may be produced by a Magnus effect.

Chouet, B.↗

NASA NTRS1989

Effect of reactor loading on atomic oxygen concentration as measured by NO chemiluminescence

It has previously been observed that the etch rate of polyethylene samples in the afterglow of an RF discharge in oxygen increases with reactor loading. This enhancement of the etch rate is attributed to reactive gas phase products of the polymer etching. In the present work, emission spectroscopy is employed to examine the species present in the gas phase during etching of polyethylene. In particular, the concentration of atomic oxygen downstream from the polyethylene samples is studied as a function of the reactor loading. It is found that the concentration of atomic oxygen increases as the reactor loading is increased. The increase of etch rate with increased reactor loading is attributed to the increase of atomic oxygen concentration in the vicinity of the sample.

Lerner, N. R.↗

NASA NTRS1998

Fiber-Supported Droplet Combustion Experiment-2

A major portion of the energy produced in the world today comes from the burning of liquid hydrocarbon fuels in the form of droplets. Understanding the fundamental physical processes involved in droplet combustion is not only important in energy production but also in propulsion, in the mitigation of combustion-generated pollution, and in the control of the fire hazards associated with handling liquid combustibles. Microgravity makes spherically symmetric combustion possible, allowing investigators to easily validate their droplet models without the complicating effects of gravity. The Fiber-Supported Droplet Combustion (FSDC-2) investigation was conducted in the Microgravity Glovebox facility of the shuttles' Spacelab during the reflight of the Microgravity Science Laboratory (MSL- 1R) on STS-94 in July 1997. FSDC-2 studied fundamental phenomena related to liquid fuel droplet combustion in air. Pure fuels and mixtures of fuels were burned as isolated single and duo droplets with and without forced air convection. FSDC-2 is sponsored by the NASA Lewis Research Center, whose researchers are working in cooperation with several investigators from industry and academia. The rate at which a droplet burns is important in many commercial applications. The classical theory of droplet burning assumes that, for an isolated, spherically symmetric, single-fuel droplet, the gas-phase combustion processes are much faster than the droplet surface regression rate and that the liquid phase is at a uniform temperature equal to the boiling point. Recent, more advanced models predict that both the liquid and gas phases are unsteady during a substantial portion of the droplet's burning history, thus affecting the instantaneous and average burning rates, and that flame radiation is a dominant mechanism that can extinguish flames in a microgravity environment. FSDC-2 has provided well-defined, symmetric droplet burning data including radiative emissions to validate these theoretical models for heptane, decane, ethanol, and methanol fuels. Since most commercial combustion systems burn droplets in a convective environment, data were obtained without and with convective flow over the burning droplet (see the following photos).

Colantonio, Renato O.↗