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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. Here this study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C–O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10–11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1996

A Detailed Level Kinetics Model of NO Vibrational Energy Distributions

Several contemporary problems have pointed to the desirability of a detailed level kinetics approach to modeling the distribution of vibrational energy in NO. Such a model is necessary when vibrational redistribution reactions are insufficient to maintain a Boltzmann distribution over the vibrational energy states. Recent calculations of the rate constant for the first reaction of the Zeldovich mechanism (N2 + O (goes to) NO + N) have suggested that the product NO is formed in high vibrational states. In shock layer flowfields, the product NO molecules may experience an insufficient number of collisions to establish a Boltzmann distribution over vibrational states, thus necessitating a level kinetics model. In other flows, such as expansions of high temperature air, fast, near-resonance vibrational energy exchanges with N2 and O2 may also require a level specific model for NO because of the relative rates of vibrational exchange and redistribution. The proposed report will integrate computational and experimental components to construct such a model for the NO molecule.

Sharma, Surendra P.↗
NASA NTRS2005

System Regulates the Water Contents of Fuel-Cell Streams

An assembly of devices provides for both humidification of the reactant gas streams of a fuel cell and removal of the product water (the water generated by operation of the fuel cell). The assembly includes externally-sensing forward-pressure regulators that supply reactant gases (fuel and oxygen) at variable pressures to ejector reactant pumps. The ejector supply pressures depend on the consumption flows. The ejectors develop differential pressures approximately proportional to the consumption flow rates at constant system pressure and with constant flow restriction between the mixer-outlet and suction ports of the ejectors. For removal of product water from the circulating oxygen stream, the assembly includes a water/gas separator that contains hydrophobic and hydrophilic membranes. The water separator imposes an approximately constant flow restriction, regardless of the quality of the two-phase flow that enters it from the fuel cell. The gas leaving the water separator is nearly 100 percent humid. This gas is returned to the inlet of the fuel cell along with a quantity of dry incoming oxygen, via the oxygen ejector, thereby providing some humidification.

Vasquez, Arturo↗
NASA NTRS1992

Pulmonary and leg VO2 during submaximal exercise: implications for muscular efficiency

Insights into muscle energetics during exercise (e.g., muscular efficiency) are often inferred from measurements of pulmonary gas exchange. This procedure presupposes that changes of pulmonary O2 (VO2) associated with increases of external work reflect accurately the increased muscle VO2. The present investigation addressed this issue directly by making simultaneous determinations of pulmonary and leg VO2 over a range of work rates calculated to elicit 20-90% of maximum VO2 on the basis of prior incremental (25 or 30 W/min) cycle ergometry. VO2 for both legs was calculated as the product of twice one-leg blood flow (constant-infusion thermodilution) and arteriovenous O2 content difference across the leg. Measurements were made 3-5 min after each work rate imposition to avoid incorporation of the VO2 slow component above the lactate threshold. For all 17 subjects, the slope of pulmonary VO2 (9.9 +/- 0.2 ml O2.W-1.min-1) was not different (P greater than 0.05) from that for leg VO2 (9.2 +/- 0.6 ml O2.W-1.min-1). Estimation of "delta" efficiency (i.e., delta work accomplished divided by delta energy expended, calculated from slope of VO2 vs. work rate and a caloric equivalent for O2 of 4.985 cal/ml) using pulmonary VO2 measurements (29.1 +/- 0.6%) was likewise not significantly different (P greater than 0.05) from that made using leg VO2 measurements (33.7 +/- 2.4%). These data suggest that the net VO2 cost of metabolic "support" processes outside the exercising legs changes little over a relatively broad range of exercise intensities. Thus, under the conditions of this investigation, changes of VO2 measured from expired gas reflected closely those occurring within the exercising legs.

Non-NASA Center↗
NASA NTRS1980

Kinetics of the ClO + NO2 + M reaction

The ClO + NO2 + M reaction has been studied with two techniques: Fourier transform infrared spectroscopy of the products, and flash photolysis-ultraviolet absorption to monitor the decay of ClO in excess NO2. The measured third-order rate constant is 1.5 x 10 to the -31st cm to the 6th/molecule-squared per sec at 298 K with M = N2, in good agreement with previous literature values, but the rate constant appears to decrease by up to a factor of three in the presence of increasing amounts of OClO. For the infrared studies a stoichiometric mixture of ClO and NO2 was prepared in a flow system by mixing NO with OClO; at least as much NO2 as ClONO2 was produced under a variety of experimental conditions. These two sets of results are incompatible with the assumption made in previous kinetic studies that ClONO2 is the only recombination product; other isomers such as OClONO or ClOONO are likely to be formed three to four times faster. These results imply that potential stratospheric ozone depletion due to chlorofluoromethanes may be even larger than previously thought.

Molina, M. J.↗
DOE OSTI2021

Applications of Kinetic Methods in Thermal Analysis: A Review

Determining kinetic parameters such as activation energy (Ea), pre-exponential factor (A), rate constants, and reaction order (n) via thermal analysis techniques is important to material synthesis and fabrication, industrial production, biomedicals, energy storage, catalysis, etc. Various kinetic methods based on the non-isothermal techniques have been developed to obtain the kinetic parameters. In this review, we focus on introducing various kinetic methods in non-isothermal analysis and their applications in different reactions, including thermal decomposition, solid-state phase transformation, crystallization, thermal ignition, curing process, etc.

Zhang, Xin↗
NASA NTRS1972

Electron loss coefficients for the D-region of the ionosphere from rocket measurements during the eclipses of March 1970 and November 1966.

Observation that recombination-like electron loss coefficients measured during the March 1970 eclipse coincide with those from the November 1966 eclipse and range from 2 x 10 to the minus 7th power cu cm/sec at Z sub zero + 6 km to 0.00005 cu cm/sec at Z sub zero -2 km, where Z sub zero is the altitude of the steep gradient of electron concentration. During totality, Z sub zero was 82.5 km in 1970, and 86.8 km in 1966. For full-sun conditions, Z sub zero was 84 km on both eclipse days. Attachment-like electron loss coefficients from both eclipses are in good agreement below Z sub zero, and have a nearly constant value of about 0.008 per sec. Below Z sub zero, attachment-like loss rates agree with production rates - i.e., electron concentration is proportional to electron production.

Mechtly, E. A.↗
NASA NTRS1988

Non-equilibrium of high-temperature nitrogen and oxygen

A stationary colloidal-radiative model for determination of production of electrons, excited atoms, and spectral line intensities in high-temperature atomic N and O is presented. The populations of several atomic levels and partition functions, and the intensities of several lines, are calculated in a wide range of conditions. Transport of radiation is included by coupling the rate equations for production of the electrons and excited atoms with the Holstein escape factors which are not constant, but depend on plasma conditions.

Kunc, J. A.↗
NASA NTRS1989

Collisional-radiative non-equilibrium in high-temperature helium

A stationary, nonlinear collisional-radiative model for determination of production of electrons, excited atoms and atomic line intensities for high-temperature helium is presented. The populations of several atomic levels and the intensities of spectral and continuum lines are calculated in a wide range of conditions (Te = 12,000-22,000 K, Nt = 10 to the 12 - 10 to the 20th per cu cm. Transport of radiation is included by coupling the rate equations for production of the electrons and excited atoms with the radiation escape factors which are not constant but depend on plasma conditions.

Kunc, J. A.↗
NASA NTRS1989

Radiation of hot atomic hydrogen

A nonlinear collisional-radiative model for the determination of nonequilibrium production of electrons, ions, excited atoms, and spectral and continuum line intensities in stationary partially-ionized atomic hydrogen is presented. Transport of radiation is included by coupling the rate equations for production of the electrons and excited atoms with the radiation escape factors which are not constant but depend on plasma conditions.

Kunc, J. A.↗
NASA NTRS1989

Collisional-radiative nonequilibrium in partially ionized atomic nitrogen

A nonlinear collisional-radiative model for determination of nonequilibrium production of electrons, excited atoms, and bound-bound, dielectronic and continuum line intensities in stationary partially ionized atomic nitrogen is presented. Populations of 14 atomic levels and line intensities are calculated in plasma with T(e) = 8000-15,000 K and N(t) = 10 to the 12th - 10 to the 18th/cu cm. Transport of radiation is included by coupling the rate equations of production of the electrons and excited atoms with the radiation escape factors, which are not constant but depend on plasma conditions.

Kunc, J. A.↗
DOE OSTI2021

An Experimental Kinetics Study of Isopropanol Pyrolysis and Oxidation behind Reflected Shock Waves

Isopropanol has potential as a future bio-derived fuel and is a promising substitute for ethanol in gasoline blends. Even so, little has been done in terms of high-temperature chemical kinetic speciation studies of this molecule. To this end, experiments were conducted in a shock tube using simultaneous CO and H2O laser absorption measurements. Water and CO formation during isopropanol pyrolysis was also examined at temperatures between 1127 and 2162 K at an average pressure of 1.42 atm. Species profiles were collected at temperatures between 1332 and 1728 K and at an average pressure of 1.26 atm for equivalence ratios of 0.5, 1.0, and 2.0 in highly diluted mixtures of 20% helium and 79.5% argon. Species profiles were also compared to four modern C3 alcohol mechanisms, including the impact of recent rate constant measurements. The Li et al. (2019) and Saggese et al. (2021) models both best predict CO and water production under pyrolysis conditions, while the AramcoMech 3.0 and Capriolo and Konnov models better predict the oxidation experimental profiles. Additionally, previous studies have collected ignition delay time (τign) data for isopropanol but are limited to low pressures in highly dilute mixtures. Therefore, real fuel–air experiments were conducted in a heated shock tube with isopropanol for stoichiometric and lean conditions at 10 and 25 atm between 942 and 1428 K. Comparisons to previous experimental results highlight the need for real fuel–air experiments and proper interpretation of shock-tube data. The AramcoMech 3.0 model over predicts τign values, while the Li et al. model severely under predicts τign. The models by Capriolo and Konnov and Saggese et al. show good agreement with experimental τign values. A sensitivity analysis using these two models highlights the underlying chemistry for isopropanol combustion at 25 atm. Additionally, modifying the Li et al. model with a recently measured reaction rate shows improvement in the model’s ability to predict CO and water profiles during dilute oxidation. Finally, a regression analysis was performed to quantify τign results from this study.

09 BIOMASS FUELS↗
NASA NTRS1989

The use of direct numerical simulation in the study of turbulent, chemically-reacting flows

Full turbulence simulations are used here to study the effects of chemical heat release on the large-scale structures in turbulent mixing layers. In agreement with laboratory results, it is found that the heat release lowers the rate at which the mixing layer grows and reduces the rate at which chemical products are formed. The baroclinic torque and thermal expansion in the mixing layer produce changes in the flame vortex structure that act to produce more diffuse vortices than in the constant density case, resulting in lower rotation rates of the large-scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layer are shown to result from vorticity generation by baroclinic torques. Calculations of the energy in the various wavenumbers shows that the heat release has a stabilizing effect on the growth rate of individual modes. This methodology can be applied for Reynolds numbers less than several hundred and for Damkoehler numbers less than about ten.

Riley, J. J.↗
NASA NTRS2004

Water cycle and its management for plant habitats at reduced pressures

Experimental and mathematical models were developed for describing and testing temperature and humidity parameters for plant production in bioregenerative life support systems. A factor was included for analyzing systems operating at low (10-101.3 kPa) pressure to reduce gas leakage and structural mass (e.g., inflatable greenhouses for space application). The expected close relationship between temperature and relative humidity was observed, along with the importance of heat exchanger coil temperature and air circulation rate. The presence of plants in closed habitats results in increased water flux through the system. Changes in pressure affect gas diffusion rates and surface boundary layers, and change convective transfer capabilities and water evaporation rates. A consistent observation from studies with plants at reduced pressures is increased evapotranspiration rates, even at constant vapor pressure deficits. This suggests that plant water status is a critical factor for managing low-pressure production systems. The approach suggested should help space mission planners design artificial environments in closed habitats.

NASA Center KSC↗
NASA NTRS1990

Radiation of partially ionized atomic hydrogen

A nonlinear collisional-radiative model for determination of production of electrons, positive and negative ions, excited atoms, and spectral and continuum line intensities in stationary partially ionized atomic hydrogen is presented. Transport of radiation is included by coupling the rate equations for production of the electrons, ions, and excited atoms with the radiation escape factors, which are not constant but depend on plasma conditions. It is found that the contribution of the negative ion emission to the total continuum emission can be important. Comparison of the calculated total continuum emission coefficient, including the negative ion emission, is in good agreement with experimental results.

Soon, W. H.↗
NASA NTRS1974

Theoretical nitric oxide production incidental to autoignition and combustion of several fuels homogeneously dispersed in air under some typical hypersonic flight conditions

A reaction package of 100 chemical reactions and attendant reaction rate constants defined for the autoignition and combustion of four carbonaceous fuels, CH4, CH3OH, C2H6, and C2H5OH. Definition of the package was made primarily by means of comparison between trial calculations and experimental data for the autoignition of CH4. Autoignition and combustion of each of these four fuels was calculated under three sets of conditions realistic for hypersonic flight applications, for comparison to hydrogen fuel, particularly with respect to formation of nitric oxide. Results show that, for all of the fuels including hydrogen, if NO production is a significant problem, compromise must be made between approaching equilibrium heat release and approaching equilibrium NO concentration.

Bahn, G. S.↗
DOE OSTI2020

Combustion of Silane-Hydrogen-Nitrous oxide Mixtures: Analysis of Laminar Flame Propagation and Condensed Products

The laminar burning rate, the explosion pressure, and the pressure rise coefficient have been measured for silane-nitrous oxide-argon mixtures using the spherically expanding flame technique in a constant volume combustion chamber. For these three parameters, the values obtained were higher than for hydrogen-nitrous oxide-argon and typical hydrocarbon-based mixtures. As an example, a maximum burning rate of 1800 g/m 2 s was measured at 101 kPa, whereas under similar conditions, a maximum burning rate around 950 g/m 2 s has been reported for hydrogen-nitrous oxide-argon mixtures. Here, the condensed combustion products formed in the silane-nitrous oxide-argon flames were sampled and characterized using electron micrograph, electronic diffraction, energy-dispersive spectroscopy, and X-ray powder diffraction. For all equivalence ratios, silica spherical particles with a mean diameter in the range 200-300 nm were observed. In addition, for mixtures with Φ ≥2.2, silicon nanowires were formed. X-ray diffraction experiments showed that the silicon nanowires are composed of a material referred to as “metal silicon” characterized by a cubic structure (lattice parameter: a = 5.425 Å) with the Fm-3m space group.

42 ENGINEERING↗