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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The reactions of cobalt, iron and nickel in SO2 atmospheres Similarities and differences

The reactions of cobalt, iron and nickel in SO2 atmospheres are reviewed and compared. A mixed oxide-sulfide product layer is observed in all cases. Cobalt and nickel exhibit similar behavior. The observed rates are near the sulfidation rates, and the reaction rate is strongly influenced by the outward diffusion of metal through an interconnected sulfide network. A continuous interconnected sulfide is not observed in the oxide-sulfide scales formed on iron, and the reaction rates are more difficult to summarize. The differences and similarities among the three metals are explained in terms of the absence of scale-gas equilibrium and the ratio of the metal diffusivity in the corresponding oxide and sulfide.

Jacobson, N. S.↗
NASA NTRS1986

Rate constants for chemical reactions in high-temperature nonequilibrium air

In the nonequilibrium atmospheric chemistry regime that will be encountered by the proposed Aeroassisted Orbital Transfer Vehicle in the upper atmosphere, where air density is too low for thermal and chemical equilibrium to be maintained, the detailed high temperature air chemistry plays a critical role in defining radiative and convective heating loads. Although vibrational and electronic temperatures remain low (less than 15,000 K), rotational and translational temperatures may reach 50,000 K. Attention is presently given to the effects of multiple temperatures on the magnitudes of various chemical reaction rate constants, for the cases of both bimolecular exchange reactions and collisional excitation and dissociation reactions.

Jaffe, R. L.↗
NASA NTRS1986

Calculated potential surfaces for the reactions: O + N2 yields NO + N and N + O2 yields NO + O

Complete Active Space SCF/Contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N + O2 reaction is exoergic by 32 kcal/mole and is computed to have an early barrier of 10.2 kcal/mole for the (sup 2)A(sup prime) surface and 18.0 kcal/mole for the (sup 4)A(sup prime) surface. The O + N2 reaction is endoergic by 75 kcal/mole. The (sup 3)A(sup double prime) surface is calculated to have a late barrier of 0.5 kcal/mole, while the (sup 3)A(sup prime) surface is calculated to have a late barrier of 14.4 kcal/mole.

Walch, Stephen P.↗
NASA NTRS1987

Reaction rate oscillations during catalytic CO oxidation: A brief overview

It is not the intent here to present a comprehensive review of the dynamic behavior of the catalytic oxidation of CO. This reaction is one of the most widely studied in the field of catalysis. A review paper by Engel and Ertl has examined the basic kinetic and mechanistic aspects, and a comprehensive paper by Razon and Schmitz was recently devoted to its dynamic behavior. Those interested in further study of the subject should consult these reviews and a number of general review papers on catalytic reaction dynamics. The goal is to present a brief overview of certain interesting aspects of the dynamic behavior of this reaction and to discuss a few questions and issues, which are still the subject of study and debate.

Tsotsis, T. T.↗
NASA NTRS1987

Investigation of the reaction of liquid hydrogen with liquid air in a pressure tube

A pressure tube should protect the FR-2 reactor from the consequences of a hydrogen-air reaction, which is conceivable in the breakdown of several safety devices of the planned cold neutron source Project FR-2/16. The magnitudes and time pattern of the pressures to be expected were investigated. In the geometry used and the ignition mechanism selected, which is comparable to the strongest ignition process conceivable in the reactor, the reaction proceeds with greater probability than combustion. The combustion is possibly smaller if local limited partial detonations are superimposed. The magnitude of the pressure was determined by the masses of the reaction partners, liquid H2 and liquid air, and determines their ratio to each other.

Karb, Erich H.↗
NASA NTRS1987

Kinetics and mechanisms of some atomic oxygen reactions

Mechanisms and kinetics of some reactions of the ground state of oxygen atoms, O(3P), are briefly summarized. Attention is given to reactions of oxygen atoms with several different types of organic and inorganic compounds such as alkanes, alkenes, alkynes, aromatics, and some oxygen, nitrogen, halogen and sulfur derivatives of these compounds. References to some recent compilations and critical evaluations of reaction rate constants are given.

Cvetanovic, R. J.↗
NASA NTRS1986

Rate constant for the reaction SO + BrO yields SO2 + Br

The rate of the radical-radical reaction SO + BrO yields SO2 + Br has been determined at 298 K in a discharge flow system near 1 torr pressure with detection of SO and BrO via collision-free sampling mass spectrometry. The rate constant was determined using two different methods: measuring the decay of SO radicals in the presence of an excess of BrO and measuring the decay of BrO radicals in excess SO. The results from the two methods are in reasonable agreement and the simple mean of the two values gives the recommended rate constant at 298 K, k = (5.7 + or - 2.0) x 10 to the -11th cu cm/s. This represents the first determination of this rate constant and it is consistent with a previously derived lower limit based on SO2 formation. Comparison is made with other radical-radical reactions involving SO or BrO. The reaction SO + BrO yields SO2 + Br is of interest for models of the upper atmosphere of the earth and provides a potential coupling between atmospheric sulfur and bromine chemistry.

Brunning, J.↗
NASA NTRS1986

Rate constant for the reaction of O(3P) with diacetylene from 210 to 423 K

The absolute rate constant for the reaction of O(3P) with diacetylene (C4H2) has been measured as a function of pressure and temperature by the flash-photolysis/resonance-fluorescence method. At 298 K and below, no pressure dependence of the rate constant was observed, but at 423 K a moderate (factor-of-2) increase was detected in the range 3 to 75 torr Ar.Results at or near the high-pressure limit are represented by an Arrhenius expression over the temperature range 210 to 423 K. The results are compared with previous determinations, all of which employed the discharge-flow/mass-spectrometry technique. The mechanism of the reaction is considered, including both primary and secondary processes. The heats of formation of the reactants, adducts, and products for the O(3P) + C4H2 reaction are discussed and contrasted with those for O(3P) + C2H2.

Mitchell, M. B.↗
NASA NTRS1986

Kinetic study of the reaction CH (X 2Pi) + H2 yields CH2 (X 3B1) + H in the temperature range 372 to 675 K

The kinetics of the reversible reaction CH (X 2Pi) + H2 yields CH2 (X 3B1) + H at 372-675 K and total pressure 100 torr (mainly Ar) is investigated experimentally. The ground-state CH radicals are produced by photolysis of CHBr3 using 10-mJ 266-nm laser pulses (repetition rate 10 Hz) and monitored by measuring the fluorescence induced by a 429.8-nm dye laser, in the apparatus described by Berman et al. (1982) and Berman and Lin (1984). The results are presented in tables and graphs and characterized. The absolute rate constants for the forward and reverse reactions are determined, and their temperature dependence is given by Arrhenius expressions and formulas obtained in transition-state-theory calculations. The heat of formation of CH2 at 0 K is estimated (assuming that the recombination reaction CH2 + H has zero activation energy) as 92.6 + or - 0.5 kcal/mol.

Zabarnick, S.↗
NASA NTRS1987

Carbon kinetic isotope effect in the reaction of CH4 with HO

The carbon kinetic isotope effect in the CH4 + HO reaction is measured experimentally and the use of carbon isotope ratios to diagnose atmospheric methane is examined. The chemical, photolysis, and analytical experimental conditions and procedures are described. It is determined that the CH4 + HO reaction has a carbon kinetic isotope effect of 1.010 + or 0.007 for k(12)k(13) (rate constants ratio) at 297 + or - 3 K. This value is compared with the data of Rust and Stevens (1980). Causes for the poor correlation between the data at high methane conversions are discussed. It is supposed that the difference between the k(12) and k(13) values is due to a difference in the activation energy of the two reactions.

Davidson, J. A.↗
NASA NTRS1986

Systematic development of reduced reaction mechanisms for dynamic modeling

A method for systematically developing a reduced chemical reaction mechanism for dynamic modeling of chemically reactive flows is presented. The method is based on the postulate that if a reduced reaction mechanism faithfully describes the time evolution of both thermal and chain reaction processes characteristic of a more complete mechanism, then the reduced mechanism will describe the chemical processes in a chemically reacting flow with approximately the same degree of accuracy. Here this postulate is tested by producing a series of mechanisms of reduced accuracy, which are derived from a full detailed mechanism for methane-oxygen combustion. These mechanisms were then tested in a series of reactive flow calculations in which a large-amplitude sinusoidal perturbation is applied to a system that is initially quiescent and whose temperature is high enough to start ignition processes. Comparison of the results for systems with and without convective flow show that this approach produces reduced mechanisms that are useful for calculations of explosions and detonations. Extensions and applicability to flames are discussed.

Frenklach, M.↗
NASA NTRS1987

Calculated potential surfaces for the reactions - O + N2 - NO + N and N + O2 - NO + O

Complete active space SCF/contracted CI calculations using large Gaussian basis sets are presented for selected portions of the potential surfaces for reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N + O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A-prime surface and 18.0 kcal/mol for the 4A-prime surface. The O + N2 reaction is endoergic by 75 kcal/mol. The 3A-double prime surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A-prime surface has a late barrier of 14.4 kcal/mol relative to NO + N. These results are significant for determining the physical and chemical conditions which aeroassisted orbital transfer vehicles will encounter while transferring between high and low altitude earth orbits.

Walch, Stephen P.↗
NASA NTRS1987

Rate constant for the reaction of atomic oxygen with phosphine at 298 K

The rate constant for the reaction of atomic oxygen with phosphine has been measured at 298 K using flash photolysis combined with time-resolved detection of O(3P) via resonance fluorescence. Atomic oxygen was produced by flash photolysis of N2O or NO highly diluted in argon. The results were shown to be independent of (PH3), (O), total pressure and the source of O(3P). The mean value of all the experiments is k1 = (3.6 + or -0.8) x 10 to the -11th cu cm/s (1 sigma). Two previous measurements of k1 differed by more than an order of magnitude, and the results support the higher value obtained in a discharge flow-mass spectrometry study. A comparison with rate data for other atomic and free radical reactions with phosphine is presented, and the role of these reactions in the aeronomy or photochemistry of Jupiter and Saturn is briefly considered.

Stief, L. J.↗
NASA NTRS1987

The carbon dioxide chaperon efficiency for the reaction H + O2 + M yields HO2 + M from ignition delay times behind reflected shock waves

Ignition delay times for stoichiometric hydrogen-oxygen in argon with and without carbon dioxide were measured behind reflected shock waves. A 20-reaction kinetic mechanism models the measured hydrogen-oxygen delay times over the temperature range 950 to 1300 K. The chaperon efficiency for carbon dioxide determined for the hydrogen-oxygen carbon dioxide mixture was 7.0. This value is in agreement with literature values but much less than a recent value obtained from flow tube experiments. Delay times measured behind a reflected shock wave were about 20% longer than those measured behind incident shock waves. The kinetic mechanism successfully modeled the high-pressure data of Skinner and the hydrogen-air data of Stack. It is suggested that the lowest temperature points for the hydrogen-air data of Slack are unreliable and that the 0.27-atm data may illustrate a case where vibrational relaxation of nitrogen is important. The reaction pathway HO2 yields H2O2 yields OH yields H was required to model the high-pressure data of Skinner. The successful modeling of the stoichiometric hydrogen-air data demonstrates the appropriateness of deriving kinetic models from data for gas mixtures highly diluted with argon. The technique of reducing a detailed kinetic mechanism to only the important reactions for a limited range of experimental data may render the mechanism useless for other test conditions.

Brabbs, Theodore A.↗
NASA NTRS1988

An analytical study of the hydrogen-air reaction mechanism with application to scramjet combustion

A chemical kinetic mechanism for the combustion of hydrogen has been assembled and optimized by comparing the observed behavior as determined in shock tube and flame studies with that predicted by the mechanism. The reactions contained in the mechanism reflect the current state of knowledge of the chemistry of the hydrogen/air system, and the assigned rate coefficients are consistent with accepted values. It was determined that the mechanism is capable of satisfactorily reproducing the experimental results for a range of conditions relevant to scramjet combustion. Calculations made with the reaction mechanism for representative scramjet combustor conditions at Mach 8, 16, and 25 showed that chemical kinetic effects can be important and that combustor models which use nonequilibrium chemistry should be used in preference to models that assume equilibrium chemistry. For the conditions examined the results also showed the importance of including the HO2 chemistry in the mechanism. For Mach numbers less than 16, the studies suggest that an ignition source will most likely be required to overcome slow ignition chemistry. At Mach 25, the initial temperature and pressure was high enough that ignition was rapid and the presence of an ignition source did not significantly affect reaction rates.

Jachimowski, Casimir J.↗
NASA NTRS1987

Kinetics of the reactions of SH radicals with NO2 and O2

The SH + NO2 reaction rate constant was measured by monitoring SH concentrations with long-path optical absorption at 324 nm in a laser photolysis system. At 298 K, the rate constant was found to be (4.8 + or - 1.0) x 10 to the -11th cu cm/molecule per sec. An upper limit of 4 x 10 to the -19th cu cm/molecule per sec was inferred for the SH + O2 reaction, indicating that the atmospheric oxidation of the SH radical may occur via reaction with species such as NO2 and O3.

Stachnik, R. A.↗
NASA NTRS1987

Effects of diatomic reagent alignment on the A + BC reaction

A computational study is reported on the A + BC - AB + C bimolecular exchange reaction in which BC is aligned with respect to the approach direction of atom A so that the initial rotational angular momentum vector of BC is either parallel (or equivalently antiparallel) or perpendicular to the initial velocity vector of A. The calculations employ a modification of the extended LEPS potential, which permits straightforward generation of noncollinear minimum energy reaction paths. The calculations clearly demonstrate that diatomic reagent alignment can markedly affect the nature of reaction product early partitioning; they also demonstrate that diatomic reagent alignment affects reactive cross sections.

Pattengill, M. D.↗
NASA NTRS1987

Reaction of chlorine nitrate with hydrogen chloride and water at Antarctic stratospheric temperatures

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO2) with H2O and HCl on surfacers that simulate polar stratospheric clouds are studied at temperatures relevant to the Antarctic stratosphere. The gaseous products of the resulting reactions, HOCl, Cl2O, and Cl2, could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the additional formation of condensed-phase HNO3 could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.

Tolbert, Margaret A.↗