Non-Equilibrium Reaction Rates in the Macroscopic Chemistry Method for DSMC Calculations

The Direct Simulation Monte Carlo (DSMC) method is used to simulate the flow of rarefied gases. In the Macroscopic Chemistry Method (MCM) for DSMC, chemical reaction rates calculated from local macroscopic flow properties are enforced in each cell. Unlike the standard total collision energy (TCE) chemistry model for DSMC, the new method is not restricted to an Arrhenius form of the reaction rate coefficient, nor is it restricted to a collision cross-section which yields a simple power-law viscosity. For reaction rates of interest in aerospace applications, chemically reacting collisions are generally infrequent events and, as such, local equilibrium conditions are established before a significant number of chemical reactions occur. Hence, the reaction rates which have been used in MCM have been calculated from the reaction rate data which are expected to be correct only for conditions of thermal equilibrium. Here we consider artificially high reaction rates so that the fraction of reacting collisions is not small and propose a simple method of estimating the rates of chemical reactions which can be used in the Macroscopic Chemistry Method in both equilibrium and non-equilibrium conditions. Two tests are presented: (1) The dissociation rates under conditions of thermal non-equilibrium are determined from a zero-dimensional Monte-Carlo sampling procedure which simulates ‘intra-modal’ non-equilibrium; that is, equilibrium distributions in each of the translational, rotational and vibrational modes but with different temperatures for each mode; (2) The 2-D hypersonic flow of molecular oxygen over a vertical plate at Mach 30 is calculated. In both cases the new method produces results in close agreement with those given by the standard TCE model in the same highly nonequilibrium conditions. We conclude that the general method of estimating the non-equilibrium reaction rate is a simple means by which information contained within non-equilibrium distribution functions predicted by the DSMC method can be included in the Macroscopic Chemistry Method.

Numerical modelling of double diffusion driven reactive flow transport in deformable fluid-saturated porous media with particular consideration of temperature-dependent chemical reaction rates

Numerical methods ave used to solve double diffusion driven reactive flow transport problems in deformable fluid-saturated porous media. in particular, thp temperature dependent reaction rate in the non-equilibrium chemical reactions is considered. A general numerical solution method, which is a combination of the finite difference method in FLAG and the finite element method in FIDAP, to solve the fully coupled problem involving material deformation, pore-fluid flow, heat transfer and species transport/chemical reactions in deformable fluid-saturated porous media has been developed The coupled problem is divided into two subproblems which are solved interactively until the convergence requirement is met. Owing to the approximate nature of the numerical method, if is essential to justify the numerical solutions through some kind of theoretical analysis. This has been highlighted in this paper The related numerical results, which are justified by the theoretical analysis, have demonstrated that the proposed solution method is useful for and applicable to a wide range of fully coupled problems in the field of science and engineering.

Nuclear astrophysics: nucleosynthesis in the Universe

Nuclear astrophysics is a relatively young science; it is about half a century old. It is a multidisciplinary subject, since it combines nuclear physics with astrophysics and observations in astronomy. It also addresses fundamental issues in astrobiology through the formation of elements, in particular those required for a carbon-based life. In this paper, a rapid overview of nucleosynthesis is given, mainly from the point of view of nuclear physics. A short historical introduction is followed by the definition of the relevant nuclear parameters, such as nuclear reaction cross sections, astrophysical S-factors, the energy range defined by the Gamow peak and reaction rates. The different astrophysical scenarios that are the sites of nucleosynthesis, and different processes, cycles and chains that are responsible for the building of complex nuclei from the elementary hydrogen nuclei are then briefly described. Received 28 February 2012, accepted 5 April 2012, first published online 9 May 2012

Coulomb dissociation of N-20,N-21

Neutron-rich light nuclei and their reactions play an important role in the creation of chemical elements. Here, data from a Coulomb dissociation experiment on N-20,N-21 are reported. Relativistic N-20,N-21 ions impinged on a lead target and the Coulomb dissociation cross section was determined in a kinematically complete experiment. Using the detailed balance theorem, the N-19(n,gamma)N-20 and N-20(n,gamma)N-21 excitation functions and thermonuclear reaction rates have been determined. The N-19(n,gamma)N-20 rate is up to a factor of 5 higher at T < 1 GK with respect to previous theoretical calculations, leading to a 10% decrease in the predicted fluorine abundance.

The (9)Be((8)Li, (9)Be)(8)Li elastic-transfer reaction

Angular distributions for the (9)Be((8)Li, (9)Be) (8)Li elastic-transfer reaction have been measured with a 27-MeV (8)Li radioactive nuclear beam. Spectroscopic factors for the <(9)Be vertical bar(8)Li + p > bound system were obtained from the comparison between the experimental differential cross sections and finite-range distorted-wave Born approximation calculations made with the code FRESCO. The spectroscopic factors so obtained are compared with shell-model calculations and other experimental values. Using the present value for the spectroscopic factors, cross sections and reaction rates for the (8)Li(p,gamma) (9)Be direct proton-capture reaction of astrophysical interest were calculated in the framework of the potential model.

Prediction of absolute rate coefficients and product branching ratios for the C(P-3) plus allene reaction system

Complex chemical reactions in the gas phase can be decomposed into a network of elementary (e.g., unimolecular and bimolecular) steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the C(P-3)+allene (C3H4) reaction, for which molecular beam experiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (mu-VTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initio potential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible. (C) 2002 American Institute of Physics.

State-to-state reaction probabilities within the quantum transition state framework

Rigorous quantum dynamics calculations of reaction rates and initial state-selected reaction probabilities of polyatomic reactions can be efficiently performed within the quantum transition state concept employing flux correlation functions and wave packet propagation utilizing the multi-configurational time-dependent Hartree approach. Here, analytical formulas and a numerical scheme extending this approach to the calculation of state-to-state reaction probabilities are presented. The formulas derived facilitate the use of three different dividing surfaces: two dividing surfaces located in the product and reactant asymptotic region facilitate full state resolution while a third dividing surface placed in the transition state region can be used to define an additional flux operator. The eigenstates of the corresponding thermal flux operator then correspond to vibrational states of the activated complex. Transforming these states to reactant and product coordinates and propagating them into the respective asymptotic region, the full scattering matrix can be obtained. To illustrate the new approach, test calculations study the D + H2(ν, j) → HD(ν′, j′) + H reaction for J = 0.

Astrophysical S factors for fusion reactions involving C, O, Ne, and Mg isotopes

Using the Sao Paulo potential and the barrier penetration formalism we have calculated the astrophysical factor S(E) for 946 fusion reactions involving stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass energies E varying from 2 to approximate to 18-30 MeV (covering the range below and above the Coulomb barrier). We have parameterized the energy dependence, S(E), by an accurate universal 9-parameter analytic expression and present tables of fit parameters for all the reactions. We also discuss the reduced 3-parameter version of our fit which is highly accurate at energies below the Coulomb barrier, and outline the procedure for calculating the reaction rates. The results can be easily converted to thermonuclear or pycnonuclear reaction rates to simulate various nuclear burning phenomena, in particular, stellar burning at high temperatures and nucleosynthesis in high density environments. (C) 2010 Elsevier Inc. All rights reserved

The reaction of methyl radical with nitrogen atom on the triplet potential energy surface: A CCSD(T)/CBS characterization

CCSD(T)/cc-pVnZ (n = D, T, Q) calculations followed by extrapolations to the CBS limit are used to characterize stationary states of species participating in the N((4)S) + CH(3) (2A ``) reaction on the triplet PES. A mechanistic model is investigated and reaction rates are computed for every step and the overall reaction. Our best CBS estimate (1.93 x 10(10) cm(3) molecule(1) s(1)) for the overall rate constant leading to the formation of H(2)CN + H compares well with the experimental values (8.5 x 10 (11) and 1.3 x 10(10) cm(3) molecule(1) s(1)), thus reducing significantly the discrepancy of a previous theoretical result (9.1 x 10(12) cm(3) molecule(1) s(1)). (C) 2008 Elsevier B.V. All rights reserved.

Hydrolysis rates of TMOS catalyzed by oxalic acid and stimulated by ultrasound

A simple calorimetric method was employed to study the kinetics of the hydrolysis of the solventless TMOS-water mixtures, under ultrasound stimulation, as a function of the concentration of oxalic acid. The reaction rates were obtained, in relative units, from the measured thermal peak of the reaction as a non-separated function of both the sonication time and the instantaneous temperature of the medium. For concentrations of oxalic acid below 0.01 M, polycondensation reaction starts before complete hydrolysis. For concentrations of oxalic acid above 0.01 M, hydrolysis is complete and, in addition, the inverse of the time, as measured from the starting of ultrasound action until the maximum hydrolysis heat release, was found to be a reasonable relative measure of the average hydrolysis rate constant. The average hydrolysis rate constant was found to be proportional to the square root of the molar concentration of the oxalic acid. This result is in agreement with the literature if we assume small dissociation degree for the catalyst in such a solventless alkoxyde-water medium.

The Li-8(p, alpha)He-5 reaction at low energies, and Be-9 spectroscopy around the proton threshold

We present a direct measurement of the low-energy Li-8(p, alpha)He-5 cross section, using a radioactive Li-8 beam impinging on a thick target. With four beam energies, we cover the energy range between E-c.m. = 0.2 and 2.1 MeV. An R-matrix analysis of the data is performed and suggests the existence of two broad overlapping resonances (5/2(+) at E-c.m. = 1.69 MeV and 7/2(+) at E-c.m. = 1.76 MeV). At low energies our data are sensitive to the properties of a subthreshold state (E-x = 16.67 MeV) and of two resonances above threshold. These resonances were observed in previous experiments. The R-matrix fit confirms spin assignments, and provides partial widths. We propose a new Li-8(p, alpha)He-5 reaction rate and briefly discuss its influence in nuclear astrophysics. DOI: 10.1103/PhysRevC.86.064321

Reversibility Iteration Method to Understand Reaction Networks and to Solve Micro-Kinetics in Heterogeneous Catalysis

Solving microkinetics of catalytic systems, which bridges microscopic processes and macroscopic reaction rates, is currently vital for understanding catalysis in silico. However, traditional microkinetic solvers possess several drawbacks that make the process slow and unreliable for complicated catalytic systems. In this paper, a new approach, the so-called reversibility iteration method (RIM), is developed to solve microkinetics for catalytic systems. Using the chemical potential notation we previously proposed to simplify the kinetic framework, the catalytic systems can be analytically illustrated to be logically equivalent to the electric circuit, and the reaction rate and coverage can be calculated by updating the values of reversibilities. Compared to the traditional modified Newton iteration method (NIM), our method is not sensitive to the initial guess of the solution and typically requires fewer iteration steps. Moreover, the method does not require arbitrary-precision arithmetic and has a higher probability of successfully solving the system. These features make it ∼1000 times faster than the modified Newton iteration method for the systems we tested. Moreover, the derived concept and the mathematical framework presented in this work may provide new insight into catalytic reaction networks.

Effect of temperature on the electro-oxidation of ethanol on platinum

We present in this work an experimental investigation of the effect of temperature (from 25 to 180 ºC) in the electro-oxidation of ethanol on platinum in two different phosphoric acid concentrations. We observed that the onset potential for ethanol electro-oxidation shifts to lower values and the reaction rates increase as temperature is increased for both electrolytes. The results were rationalized in terms of the effect of temperature on the adsorption of reaction intermediates, poisons, and anions. The formation of oxygenated species at high potentials, mainly in the more diluted electrolyte, also contributes to increase the electro-oxidation reaction rate.