Nancy J. Brown

A MOLECULAR DYNAMICS STUDY OF THE REACTION: H2 + OH -> H2O + H

Classical trajectory calculations have been performed to determine the influence of translational temperature, H2 vibrational energy, H2 rotational energy, OH vibrational energy, and OH rotational energy on the reaction H2+OH→H2O+H. The potential energy surface was a modification of the Schatz–Elgersma analytical fit to the Walsh–Dunning surface. Reactivity increases with translational temperature, and is most strongly influenced by it. Rotational excitation of either or both molecules suppresses reactivity. Vibrational excitation of H2 enhances reactivity, and vibrational excitation of OH has no effect. A thermal rate coefficient was computed for the reaction at 1200 and 2000 K. The computed value compares favorably with the experiment at 2000 K, while the agreement at 1200 K is less satisfactory. The agreement between theory and experiment at both temperatures indicates that the potential surface is a reasonable representation of the HHOH potential energy surface.

Ignition by excimer laser photolysis of ozone

Abstract We have ignited mixtures of hydrogen, oxygen, and ozone in closed cells with 248 nm radiation from a KrF excimer laser. Ozone, the only significant absorber in this system, absorbs a single photon and produces oxygen atoms which initiate combustion. A discretized, time-dependent Beers law model is used to demonstrate that the radical concentration immediately after photolysis is a function of laser power, ozone concentration, focal length, and separation between the lens and reaction cell. Spark schlieren photographs are used to visualize the ignition events and identify the ignition sites. The effects of equivalence ratio, pressure, and the initial gas temperature on the minimum ozone concentration needed to produce ignition are presented, and only the initial temperature has a significant effect. Modeling studies of the ignition process aid in the interpretation of the experimental results, and show that the ignition we observe is not due solely to thermal effects, but is strongly dependent on the number and type of radicals present initially after photolysis. Ignition using other hydrocarbons as fuels was also demonstrated.

Characterization of the selective reduction of NO by NH3

LBL-12215 Submitted for presentation at the Western States Section of the Combustion Institute, Pullman, WA, April 13-14, 1981 CHARACTERIZATION OF THE SELECTIVE REDUCTION OF NO BY NH D. Lucas and N.J. Brown April 1981 TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Diu is ion, Ext. 6782 Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48

Low-pressure hydrogen/oxygen flame studies

Abstract Composition and temperature profiles are reported for lean, near stoichiometric and rich, low-pressure hydrogen/oxygen flames. A discussion of the consistency checks used to assure the soundness of the analysis and various methods to estimate radical concentrations is presented. The mechanism of water formation in the various flame systems is discussed.

Reactive and inelastic scattering of H2+D2 using a London‐type potential energy surface

Collisions between hydrogen and deuterium molecules are studied using quasiclassical dynamical trajectory calculations with the intermolecular field specified by a London‐type potential‐energy surface. The occurrence of chemical exchange reactions to form two HD molecules is found to be extremely sensitive to the initial relative orientations of the reactant molecules. Nonreactive collisions are examined for energy transfer processes in terms of average final state energy distribution.

Valence bond model potential energy surface for H4

Potential energy surfaces for the H4 system are derived using the valence bond procedure. An ab initio evaluation of the valence bond energy expression is described and some of its numerical properties are given. Next, four semiempirical evaluations of the valence bond energy are defined and parametrized to yield reasonable agreement with various ab initio calculations of H4 energies. Characteristics of these four H4 surfaces are described by means of tabulated energy minima and equipotential contour maps for selected geometrical arrangements of the four nuclei.

Intra‐ and intermolecular energy transfer in H2+OH collisions

We have used the method of quasiclassical dynamics to investigate intra‐ and intermolecular energy transfer in H2+OH collisions. Energy transfer has been investigated as function of translational temperature, rotational energy, and vibrational energy. The energy transfer mechanism is complex with ten types of energy transfer possible, and evidence was found for all types. There is much more exchange between the translational degree of freedom and the H2 vibrational degree of freedom than there is between translation and OH vibration. Translational energy is transferred to the rotational degrees of freedom of each molecule, and this occurs more readily for OH than H2. Both molecules exhibited intramolecular energy transfer from vibration to rotation, and this was a major pathway for vibrational deactivation. Evidence was also found for the intermolecular transfer of energy from vibration to the rotational and vibrational degrees of freedom of the other molecule.

Comparison of reactive and inelastic scattering of H2+D2 using four semiempirical potential energy surfaces

Collisions between hydrogen and deuterium molecules are examined using quasiclassical dynamical trajectory calculations with the intermolecular field specified by four semiempirical potential energy surfaces. Three of the surfaces are calculated within the valence bond model with semiempirical evaluation of the integrals, and the fourth is the London type. Various degrees of agreement are observed between these four surfaces and ab initio results. The trajectory calculations are performed at high system energies to permit the possibility of reactions. In addition to nonreactive collisions, four reaction paths are found on each surface with the product species 2H+D2, H2+2D, HD+H+D, and 2HD. The results are analyzed to determine the effect of surface properties on reaction probabilities, average final state properties of the molecules and average final state energy distributions. Dynamical results are found to be strongly dependent on surface characteristics.

The importance of thermodynamics to the modeling of nitrogen combustion chemistry

Abstract Modeling calculations have been performed to illustrate the effect of using five commonly accepted data bases of thermochemical properties on predictions of temporal species profiles. The thermochemical properties are those used for the determination of equilibrium constants employed in the calculation of reverse rate coefficients for a chemical mechanism where forward rate coefficients are specified. The modeling study was performed for hydrogen/oxygen/argon/nitrogen-comoound mixtures where the nitrogen compound was either NO or NH 3 . The mixtures reacted isothermally at 1600 K and isobarically at 1 atmosphere, and a single kinetic mechanism for which forward rate coefficients were specified was used throughout. Mixtures of equivalence ratios of 0.625, 1.0, and 1.6 were considered. Modifications in sources of thermodynamic data have been substantial since 1971 for some species. Among the data bases, thermochemical properties varied greatly for the species NH, NH 2 , NNH, and HO 2 , and those for other species important in the mechanism had variations of less than 10%. The thermochemical property variations among the data bases in NH, NH 2 , and NNH have substantial effects upon the temporal species profiles for nitrogenous species. Although this result is not surprising, unfortunately, it is often overlooked when modeling results are compared. This effect is most pronounced for rich combustion, and varies directly with equivalence ratio. Use of different data bases had little effect on the H/O species profiles. Radical species profiles (with the exception of HO 2 ) tend to be influenced strongly by their own thermochemical properties. Computed profiles also were shown to be independent of algorithm (HCT or CHEMKIN) and thermodynamic property fitting procedure between 1000 and 2000 K.

Mercury emissions from a modified in-situ oil shale retort

Commercial oil-shale production has the potential to release significant amounts of mercury to the atmosphere. Two techniques to measure mercury in oil-shale-retort offgas, Zeeman atomic absorption spectroscopy and gold-bead amalgamation collection and analysis, are discussed and compared. A technique for speciating between organic and atomic forms of Hg is also discussed. The measured mercury emission rates and speciation results are presented.