Browsing by Subject "Chemical kinetics"

Now showing 1 - 5 of 5

Kinetics and dynamics of adsorption on single crystal semiconductor and metal surfaces
(2001-08) Reeves, Christopher Thomas; Mullins, C. B.
Nonholonomic Hamiltonian method for reacting molecular dynamics
(2017-09-13) Bass, Joseph Louis; Fahrenthold, Eric P.; Longoria, Raul G; Nichols, Steven P; Sepehrnoori, Kamy; Taleff, Eric M
Macroscale, mesoscale, and ab initio models of reacting shock physics are based, in their most general forms, on rate law descriptions of the chemical processes of interest. Reacting molecular dynamics simulations, by contrast, typically employ potential functions (holonomic Hamiltonian methods) to model chemical reactions. An alternative approach to reacting molecular dynamics models the bonding-debonding process using a rate law, resulting in a nonholonomic Hamiltonian formulation. In previous work at macro and meso scales, discrete nonholonomic Hamiltonian methods have been applied to develop very general models of shock impact and fragmentation process. In this dissertation a similar nonholonomic modeling methodology is used, at the molecular scale, to explicitly model transient chemical processes. Note that the chemistry problem is much more difficult, since both dissociation (fragmentation) and the formation of new molecules must be modeled. The result is the first general reacting molecular dynamics formulation which explicitly models chemical kinetics. Simulation results using this method show good agreement with experiment, for energy release and detonation products in two widely used explosives (HMX and RDX). The reacting molecular dynamics simulation results are used to propose reaction mechanisms and species concentration based kinetics models suitable for use in meso and macro scale shock to detonation simulations. Computational modeling of energetic materials is capable of estimating molecular behavior under conditions not amenable to direct experimental measurement. Further development of RMD methods may help to provide a better understanding of energetic material behavior. This in turn may help to develop improved insensitive high energy density materials.
On the representation of model inadequacy : a stochastic operator approach
(2016-05) Morrison, Rebecca Elizabeth; Moser, Robert deLancey; Oden, John Tinsley; Ghattas, Omar; Henkelman, Graeme; Oliver, Todd A; Simmons, Christopher S
Mathematical models of physical systems are subject to many sources of uncertainty such as measurement errors and uncertain initial and boundary conditions. After accounting for these uncertainties, it is often revealed that there remains some discrepancy between the model output and the observations; if so, the model is said to be inadequate. In practice, the inadequate model may be the best that is available or tractable, and so despite its inadequacy the model may be used to make predictions of unobserved quantities. In this case, a representation of the inadequacy is necessary, so the impact of the observed discrepancy can be determined. We investigate this problem in the context of chemical kinetics and propose a new technique to account for model inadequacy that is both probabilistic and physically meaningful. Chemical reactions are generally modeled by a set of nonlinear ordinary differential equations (ODEs) for the concentrations of the species and temperature. In this work, a stochastic inadequacy operator S is introduced which includes three parts. The first is represented by a random matrix which is embedded within the ODEs of the concentrations. The matrix is required to satisfy several physical constraints, and its most general form exhibits some useful properties, such as having only non-positive eigenvalues. The second is a smaller but specific set of nonlinear terms that also modifies the species’ concentrations, and the third is an operator that properly accounts for changes to the energy equation due to the previous changes. The entries of S are governed by probability distributions, which in turn are characterized by a set of hyperparameters. The model parameters and hyperparameters are calibrated using high-dimensional hierarchical Bayesian inference, with data from a range of initial conditions. This allows the use of the inadequacy operator on a wide range of scenarios, rather than correcting any particular realization of the model with a corresponding data set. We apply the method to typical problems in chemical kinetics including the reaction mechanisms of hydrogen and methane combustion. We also study how the inadequacy representation affects an unobserved quantity of interest— the flamespeed of a one-dimensional hydrogen laminar flame.
Parametric uncertainty and sensitivity methods for reacting flows
(2014-05) Braman, Kalen Elvin; Raman, Venkat
A Bayesian framework for quantification of uncertainties has been used to quantify the uncertainty introduced by chemistry models. This framework adopts a probabilistic view to describe the state of knowledge of the chemistry model parameters and simulation results. Given experimental data, this method updates the model parameters' values and uncertainties and propagates that parametric uncertainty into simulations. This study focuses on syngas, a combination in various ratios of H2 and CO, which is the product of coal gasification. Coal gasification promises to reduce emissions by replacing the burning of coal with the less polluting burning of syngas. Despite the simplicity of syngas chemistry models, they nonetheless fail to accurately predict burning rates at high pressure. Three syngas models have been calibrated using laminar flame speed measurements. After calibration the resulting uncertainty in the parameters is propagated forward into the simulation of laminar flame speeds. The model evidence is then used to compare candidate models. Sensitivity studies, in addition to Bayesian methods, can be used to assess chemistry models. Sensitivity studies provide a measure of how responsive target quantities of interest (QoIs) are to changes in the parameters. The adjoint equations have been derived for laminar, incompressible, variable density reacting flow and applied to hydrogen flame simulations. From the adjoint solution, the sensitivity of the QoI to the chemistry model parameters has been calculated. The results indicate the most sensitive parameters for flame tip temperature and NOx emission. Such information can be used in the development of new experiments by pointing out which are the critical chemistry model parameters. Finally, a broader goal for chemistry model development is set through the adjoint methodology. A new quantity, termed field sensitivity, is introduced to guide chemistry model development. Field sensitivity describes how information of perturbations in flowfields propagates to specified QoIs. The field sensitivity, mathematically shown as equivalent to finding the adjoint of the primal governing equations, is obtained for laminar hydrogen flame simulations using three different chemistry models. Results show that even when the primal solution is sufficiently close for the three mechanisms, the field sensitivity can vary.
Silicate surface chemistry and dissolution kinetics in dilute aqueous systems
(2004) Choi, Wan-joo; Bennett, Philip C.
Organic acids from the biosphere are important reactants in a number of weathering environments. Organic acids accelerate silicate dissolution, increase silicate solubility, mobilize aluminum and silica, and alter the equilibrium between the solution and precipitated secondary phases. The chemical dynamics of the weathering environment was examined by investigating the interaction between mineral surfaces and organic/inorganic electrolyte solutions. Organic acids, analogs of microbially generated siderophores, were examined for their effects on aluminosilicate dissolution kinetics at multiple temperatures in various electrolyte solutions. Mineral surface titrations were performed for six mineral samples: quartz, gibbsite, feldspars, microcline, andalusite, and kyanite. Mineral powder/distilled water mixture samples were titrated by 0.1 N HCl in the basic pH region, and by 0.1 N NaOH in the acidic pH region. UV-difference spectroscopic analysis was performed on dissolved silica-organic acid mixtures to characterize solution complexes. Mineral dissolution experiments were performed using temperature controlled, continuous-flow mixed reactors. For inorganic dissolution experiment, the solution ionic strength was controlled using LiCl while solution pH was adjusted using either dilute HCl or LiOH solutions. Reagent grade citric acid, tropolone, and 3,4-dihydroxybenzoic acid were used for organic ligand dissolution experiments. A constant flow rate was maintained by using a peristaltic proportioning pump. The mineral surface titration results revealed important surface properties that can be critical to interpreting dissolution kinetics in natural environments. One of the most important results would be that the amount of active surface sites can vary in different solution pH conditions which have been normally assumed to be fixed numbers based on surface area measurements. The UV-difference spectroscopy result shows that some siderophores form stable solution complexes with silica as well as Al and Fe. The results imply that dissolution of aluminosilicate minerals can be significantly enhanced in natural environments by bacterial siderophores, as suggested by previous researchers. Dissolution results in inorganic electrolyte solutions showed that the net effect of solution ionic strength on the aluminosilicate dissolution reactions is a decrease in the overall dissolution rates, opposite to the effect of ionic strength on quartz dissolution. When the solution ionic species interact with feldspar surfaces, the mechanism of lowering the dissolution rates may be by inhibiting the ion exchange reaction. However, when the solution ionic species do not interact with the silicate surfaces or no ion-exchangeable species are available on the mineral surface, the mechanism of lowering the dissolution rate may be attributed to the effect of activity changes in the neutral species in the solution. Microcline dissolution increased in organic ligand solutions relative to inorganic electrolyte solutions while andalusite and kyanite dissolution rates decreased in organic ligand solutions. The increased dissolution rate of microcline suggests that feldspar dissolution may be a SN2 mechanism. The decreased or unaffected dissolution rates from kyanite and andalusite suggest that the mechanism for these minerals may be a SN1 mechanism. The effect of organic ligands on dissolution rate was greater in pH 5 solutions than in pH 3. This result suggests that the dominant reaction mechanism in the pH 3 region is proton-promoted, while it is ligand-promoted in the pH 5 region. Lower activation energies in organic ligand solutions suggest that: (1) the metal-organic complex is more stable at lower temperatures; or (2) the dominant reaction mechanism at a high temperature region may be proton-promoted and is ligand-promoted at a lower temperature region.