Reaction Rate Approach Research Articles

An Investigation of Axisymmetric Disk Stabilized Propane-Air Flames Operating under Inlet Mixture Preheat and Stratification

ABSTRACT Turbulent, recirculating, lean propane-air flames with inlet mixture stratification and preheat have been simulated under stable and limiting burning conditions. The modeled burner setup comprises a supply tube enclosing three sequential disks producing two consecutive premixing cavities. Fuel is injected in the first cavity and is partially premixed with primary air flowing through this cavity system, resulting in a radially stratified equivalence ratio profile at the inlet of the afterbody disk flame stabilizer. Detailed velocity, turbulence, fuel-air mixing, and imaging data have been previously reported for inlet preheats from 300 to 573 K and for a range of fuel flow rates. The simulations were carried out with a finite-volume-based method, using the dynamic Smagorinsky subgrid model coupled with two combustion methodologies, a quasi-laminar reaction rate approach and the Thickened Flame Model approach. Propane oxidation was modeled with a 22-species skeletal scheme. OH* chemiluminescence distributions were also computed by post-processing quasi-steady state derived algebraic expressions, exploiting directly simulated species thus enabling comparisons with experimental images. The simulations were evaluated against velocity, turbulence, and temperature measurements as well as chemiluminescence images. These comparisons allowed for an assessment of the methodologies’ capability to reproduce important trends such as the notable extension of stable operation to ultra-lean mixtures, the appreciable effect on the near flame aerodynamic stretch and the variations in the local flame structure.

LES of Reacting Flow in a Hydrogen Jet into Supersonic Crossflow Combustor Using a New Turbulent Combustion Model

Modelling the complete flow physics and chemical kinetics of supersonic combustion is a particularly complex and daunting task that requires significant computational resources. To foster performance evaluation tools for future hypersonic vehicles, developing accurate yet computationally efficient solution methods is of great importance. In this work, a new subgrid combustion model for large eddy simulations is derived and used in a three-dimensional in-house flow solver to provide simulations of experimental scramjet ground tests. In particular, this paper introduces a hybrid model closure with the reaction-rate approach to close the filtered chemical source terms in the governing equations for species mass fractions and total energy. The model developed here makes use of a linear bridging function, depending on the segregation rate of the mixture fraction, between a resolved contribution issued from a perfectly stirred reactor (PSR) estimation, and a subgrid-scale (SGS) contribution where a closure that approximates the Lagrangian trajectory in the composition space is retained. The new model considers the effect of fluctuations of compositions and can be extended to take into account, for example, the fluctuations of temperature. The new approach is tested using a hydrogen-fueled scramjet combustor from circular injector into a Mach 2 vitiated airflow for total pressure and temperature of 0.40 MPa and 1695 K, respectively. The selected operating conditions are representative of the LAPCAT-II dual-mode ramjet/scramjet combustion. Chemistry is described using a four-step reduced mechanism. The results obtained with the present modelling proposal are compared to those issued from numerical simulations performed with the quasi-laminar chemistry or PSR approach. These results do show that, even for a highly resolved computational mesh, the effects of composition fluctuations remain significant, especially in the vicinity of the injection where the SGS fluctuations of the scalar field are non-negligible.

Large eddy simulations of premixed CH$$_{{\mathbf {4}}}$$ bluff-body flames operating close to the lean limit using quasi-global chemistry and an algebraic chemiluminescence model

The present work describes the numerical study of unconfined turbulent premixed methane/air flames stabilized on an axisymmetric conical baffle under lean and ultra-lean, close to blow-off conditions. A finite-volume-based LES method, using the dynamic Smagorinsky subgrid model in combination with two turbulent combustion methodologies, the thickened flame model and the implicit LES (ILES) laminar reaction rate approach were employed in the investigation. Methane–air oxidation was modeled with a 14-species reduced scheme. OH* chemiluminescent species levels were also evaluated by post-processing quasi-steady-state-derived algebraic expressions, exploiting directly simulated species thus enabling direct comparisons with experimental images. The quality of the simulations was appraised against experimental data for velocity, turbulence, temperature, species mass fractions, heat release profiles as well as chemiluminescence images for conditions far and close to blow-off. Both turbulent combustion models followed closely several intrinsic trends and variations of the flame front anchoring and disposition close to the burner rim shear layers and along the reverse flow region as the fuel level was reduced toward the lean limit. The interaction of the bluff-body recirculation with the adjacent toroidal reacting shear layer and the impact of combustion and heat release on the development of the turbulent velocity and species fields in the near-wake recovery zone were adequately reproduced for both mixtures. Quantitative deviations between simulations and measurements, regarding heat release and OH species, increased for the near-LBO flame with an attendant extrapolated underprediction of the blow-off event by about 8% in terms of equivalence ratio.

Subgrid Reaction-Diffusion Closure for Large Eddy Simulations Using the Linear-Eddy Model

Turbulent combustion models approximate the interaction between turbulence, molecular transport and chemical reactions. Among the many available turbulent combustion models, the present focus is the linear-eddy model (LEM) used as a subgrid combustion model for large eddy simulations. In particular this paper introduces a new LEM closure with the reaction-rate approach to close the filtered chemical source terms in the governing equations for species mass fractions and enthalpy. The new approach is tested using a non-premixed syngas flame and a bluff-body stabilized premixed flame problem. Simulation results are compared to data from a direct numerical simulation and experiments. This comparison shows that mean and rms quantities compare well with experiments and are in the range of previous simulation studies. These results are obtained with a pressure-based and unstructured computational-fluid-dynamics solver, an approach that is preferred in industry.

Tuning the glass transition of asphalt binder based on viscosity criteria under zero shear rates

With the theory of absolute reaction rate or transition-state approach together with the boundary perturbation method we calculate the possible temperature-dependent viscosity of bitumen or asphalt binder via tuning of activation volume as well as activation energy for the first time under zero shear rates. Before we present our novel approach and numerical results we firstly verify our calculations with both ambient temperature and high temperature viscosity measurements for bitumen conducted previously. Our tuning activation volume of bitumen will be useful to the current needs of intelligent compaction and application of nanotechnology to easily handling or processing asphalt binder.

Toward More Reliable Measurements of Electron-Transfer Kinetics at Nanoelectrodes: Next Approximation.

Steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) have recently been used to measure kinetics of several rapid heterogeneous electron transfer (ET) reactions. One problem with those experiments was that the dependence of the shape of the steady-state voltammogram on kinetic parameters becomes weak when the reaction rate approaches the diffusion limit. The possibility to fit the same experimental voltammogram using different combinations of the standard rate constant, transfer coefficient, and standard potential results in significant uncertainties in extracted parameter values. In this article, the reliability of the kinetic analysis was improved by obtaining steady-state voltammograms with both oxidized and reduced forms of redox species initially present in solution. Additional improvements were attained by characterizing the nanoelectrode geometry with the atomic force microscope and using water with a very low level of organic contaminants (TOC ≤ 1 ppb). This approach was used to re-evaluate the ET rate constants measured for several electroactive species, including ferrocene, ferrocenemethanol, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ferrocyanide at Pt electrodes. The obtained standard rate constants are higher than the values measured earlier at Pt and Au nanoelectrodes but comparable to those obtained in recent nanogap/SECM experiments.

Possible anomalous transitional transport of electronic fluids in hydrogen sulphide under high pressures

With the verified quantum chemistry theory of absolute reaction rate or transition state approach, we illustrate the possible enhancing superconducting transition temperature of hydrogen sulphide under high pressures via tuning of activation volume together with activation energy for the first time under zero shear rates. Before we present our novel approach and numerical results we firstly verify our preliminary results with previous Drozdov et al.’s [Nature2015, 525, 73] measurements. After some necessary verifications, we illustrate our novel numerical results via the enhancing of the onset or superconducting transition temperature for electronic liquids in hydrogen sulphide within the same lower pressure regime (compared to previous Drozdov et al.’s experimental data) by decreasing of activation volume: ∼ 10−25m3 decrement under zero shear rates. The latter might also be linked to the glass transition state for electronic fluids in hydrogen sulphide under high pressures once the shear viscosity is calculated.

The role of microemulsions in lipase‐catalyzed hydrolysis reactions

The kinetics of the p-nitrophenyl butyrate hydrolysis reaction, catalyzed by Candida rugosa lipase in the water-in-oil microemulsion cetyltrimethylammonium bromide/water/pentanol/hexane, was investigated. The results described in the present manuscript reveal two peculiar characteristics of the reaction: (i) the initial rate of hydrolysis is very fast and (ii) by decreasing the water content of the microemulsion, the reaction rate approaches the typical behavior of reactions performed in aqueous solution. In particular, for microemulsion systems with a high water content, the end points of the reactions are dictated by the shape stability of the microemulsion. For these systems, our methodological approach shows that the process follows a second-order kinetics equation, indicative of the dual role played by water, which is involved both as a component of the microemulsion, i.e., relevant for the microemulsion stability and as a reagent of the hydrolysis reaction. In contrast, for microemulsions containing a small amount of water, after the hydrolysis reaction the system seems to fall in the no existence range of the microemulsion. Accordingly, the kinetics results are more complex: in the initial stage, the reaction follows a zero-order kinetics equation, while for longer reaction times a first-order kinetics equation fits the experimental data, as would be expected for an enzymatic reaction in a homogeneous system.

Modelling of silicon oxynitridation by nitrous oxide using the reaction rate approach

Large technological progress in oxynitridation processing leads to the introduction of silicon oxynitride as ultra-thin gate oxide. On the theoretical side, few studies have been dedicated to the process modelling of oxynitridation. Such an objective is a considerable challenge regarding the various atomistic mechanisms occurring during this fabrication step. In this article, some progress performed to adapt the reaction rate approach for the modelling of oxynitride growth by a nitrous ambient are reported. The Ellis and Buhrman's approach is used for the gas phase decomposition modelling. Taking into account the mass balance of the species at the interface between the oxynitride and silicon, a minimal kinetic model describing the oxide growth has been calibrated and implemented. The influence of nitrogen on the reaction rate has been introduced in an empirical way. The oxidation kinetics predicted with this minimal model compares well with several experiments.

Silicon dry oxidation kinetics at low temperature in the nanometric range: Modeling and experiment

Kinetics of silicon dry oxidation is investigated theoretically and experimentally at low temperature in the nanometer range where the limits of the Deal and Grove model become critical. Based on a fine control of the oxidation process conditions, experiments allow the investigation of the growth kinetics of nanometric oxide layer. The theoretical model is formulated using a reaction rate approach. In this framework, the oxide thickness is estimated with the evolution of the various species during the reaction. Standard oxidation models and the reaction rate approach are confronted with these experiments. The interest of the reaction rate approach to improve silicon oxidation modeling in the nanometer range is clearly demonstrated.

Degradation of atrazine using solar driven fenton-like advanced oxidation processes

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was degraded using cobalt-peroximonosulfate (Co/PMS) advanced oxidation process (AOP). Three Co concentrations (0.00, 0.25 and 0.50 mM) and five peroximonosulfate (PMS) concentrations (0, 5, 8, 16 and 32 mM) were tested. Maximum degradation reached was 88% using dark Co/PMS in 126 minutes when 0.25 mM of cobalt and 32 mM of PMS were used. Complete atrazine degradation was achieved when the samples were irradiated by the sun under the same experimental conditions described. Tests for identification of intermediate products allowed identification and quantification of deethylatrazine in both dark and radiated conditions. Kinetic data for both processes was calculated fitting a pseudo-first order reaction rate approach to the experimental data. Having kinetic parameters enabled comparison between both conditions. It was found that the kinetic approach describes data behavior appropriately (R2 ≥ 0.95). Pseudo-kinetic constants determined for both Co/PMS processes, show k value of 10−4 for Co/PMS and a k value of 10−3 for Co/PMS/ultraviolet (UV). This means, that, with the same Co/PMS concentrations, UV light increases the reaction rate by around one order of magnitude than performing the reaction under dark conditions.

Computing the moments of high dimensional solutions of the master equation

Derived from the Markov character only, the master equation of chemical reactions is an accurate stochastic description of quite general systems in chemistry. Exact solutions of this equation are rare and the most frequently used approximative solution method is to write down the corresponding set of reaction rate equations. In many cases this approximation is not valid, or only partially so, as stochastic effects caused by the natural noise present in the full description of the problem are poorly captured. In this paper it is shown how a certain set of higher order equations can be derived. It is shown by theory and example that stochastic effects are better captured using this technique while still maintaining the computational advantages of the reaction rate approach.

The lives and deaths of positrons in the interstellar medium

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

Comparison of Surface Chemical Kinetic Models for Ablative Reentry of Graphite

A general formulation for surface chemical reactions is used with a finite element heat conduction code to compare computations of the ablated mass flux from carbon bodies experiencing conditions representative of Earth reentry. Several credible models for the surface chemical kinetics are exercised with the formulation and are compared both to each other and to test data obtained by the Passive Nosetip Technology program in the mid-1970s. Sublimation of C 5 and C 7 is shown to be a concern for surface temperatures greater than about 3900 K. The best match between measurements and the calculations is obtained with surface chemical models that use the usual CO formation reactions and the sublimation of C 1 -C 3 but that also include CN formation and the sublimation of C 5 and C 7 . For surface temperatures above 3500 K and for similar assumptions for the equilibrium vapor pressure and evaporation coefficients of the sublimated species, the net reaction rate approach and the surface site occupation approach give similar ablated mass fluxes.

APPLICATION OF CONSTANT REACTION RATE TG TO THE DETERMINATION OF KINETIC PARAMETERS BY Hi-Res TG

A simple operation mode to determine the apparent activation energy Ea is introduced. Ea can be determined with a double-curve method by using a constant reaction rate (CRR) approach of Hi-Res TG. The most appropriate mechanism function f(α) and frequency factor A are determined by a single-curve method when the activation energies provided by the two methods are in good agreement with each other. The deacetylation of EVA copolymer has been used for illustration. Advantages of the CRR are discussed.

Ordering in ternary B2 alloys

This paper focuses on the predictions of ordering schemes in ternary B2 alloys based on the Nb–Ti–Al and Fe–Al–Ni systems, and comparing these results with those derived experimentally. The ordering tie-line (OTL) approach for depicting sublattice occupancies has been used as a basis for this comparison. Predictions of ordering schemes have been provided using a simple model based on classical Bragg–Williams theory of order–disorder transformations. Thus, OTLs have been calculated using a model based on an exchange reaction rate approach to predicting equilibrium properties. Experimentally, the OTLs have been determined by measurements of the apparent sublattice compositions using atom location by channeling enhanced microanalysis (ALCHEMI). It is shown that reasonably useful predictions may be obtained from the application of a simple thermodynamic model.

Escherichia coli Diacylglycerol Kinase Is an Evolutionarily Optimized Membrane Enzyme and Catalyzes Direct Phosphoryl Transfer

In this contribution the kinetic mechanism and substrate specificity of Escherichia coli diacylglycerol kinase were examined. Steady state kinetic studies were carried out under mixed micellar conditions using a novel continuous coupled assay system. The kinetic data were consistent with a random equilibrium mechanism, implying that diacylglycerol kinase catalyzes direct phosphoryl transfer from MgATP to diacylglycerol. This was supported by failure to detect an enzyme-phosphate covalent intermediate and by the observation that the bisubstrate analog adenosine 5'-tetraphosphoryl-3-O-(1,2-dihexanoyl)-sn-glycerol inhibits the enzyme (Ki << Km,DAG). While diacylglycerol kinase's kcat/Km is modest compared with the efficiency of many water-soluble enzymes, the enzyme nevertheless appears to be an evolutionarily optimized biocatalyst in the sense that its chemical reaction rate approaches the substrate diffusion-controlled limit. The in vivo rate-limiting step of DAGK's reaction appears to be, in part, the transbilayer diffusion of diacylglycerol from the outer leaflet to the inner leaflet of the cytoplasmic membrane where DAGK's active site is located. DAGK was observed to maintain a high nucleotide substrate specificity, with most of this specificity being expressed in the form of reductions in kcat for ATP analogs.

The dynamic and the stochastic reaction rate approaches to nonradiative processes in condensed media

AbstractA comparison is made between the exact classical dynamic theory and the stochastic theory of reaction rates in dense media. The conditions of their equivalence are derived on the basis of a harmonic oscillator model. It is shown in a direct way that in the general case of a multidimensional system the approximate activated complex (transition‐state) theory is not identical to the stochastic theory at any value of the viscosity coefficient. © 1992 John Wiley &amp; Sons, Inc.

Steady-state decomposition of ammonia on the ruthenium(001) surface

Steady-state specific reaction rates for the catalytic decomposition of ammonia at pressures of 1 x 10/sup -6/ and 2 x 10/sup -6/ Torr have been measured on the Ru(001) surface at temperatures between approximately 500 and 1250 K. For temperatures above 750 K, the reaction rate approaches first order in ammonia pressure, and the apparent activation energy is 5.0 +/- 0.3 kcal x mol/sup -1/. A kinetic isotope effect is observed at these high temperatures, and the activation energy for the decomposition of deuteriated ammonia is 6.6 +/- 0.3 kcal x mol/sup -1/. This apparent activation energy is associated with the difference in activation energies between that of the cleavage of a nitrogen-hydrogen (nitrogen-deuterium) bond in chemisorbed ammonia and that of the desorption of molecularly chemisorbed ammonia. At lower temperatures, the reaction rate is independent of ammonia pressure, and the apparent activation energy is 43 +/- 3 kcal x mol/sup -1/, which is associated with the recombinative desorption of nitrogen. From transient thermal desorption mass spectrometric measurements, the activation energy of the recombinative desorption of nitrogen on Ru(001) is 44.0 +/- 0.5 kcal x mol/sup -1/, and the preexponential factor of the desorption rate coefficient is (1.3 +/- 0.6) xmore » 10/sup -3/ cm/sup 2/ x s/sup -1/. On the basis of thermal desorption measurements during the steady-state decomposition of ammonia at 2 x 10/sup -6/ Torr, nitrogen adatoms are shown to be the dominant surface species under these experimental conditions. A mechanistic model that has been introduced previously describes accurately the pressure and temperature dependence of both the measured decomposition kinetics and the steady-state coverage of nitrogen adatoms.« less

A time-domain EPR study of the SO−3 combination reaction: Radical pair CIDEP without singlet–triplet mixing

The radical ion ⋅SO−3 was studied by in situ pulsed radiolysis of aqueous alkaline sodium sulfite solutions in a pulsed EPR spectrometer. Initial radical concentrations on the order of 5×10−4 M were generated, and detailed measurements of the decay of ⋅SO−3 magnetization were performed on the microsecond time scale. It was found that the second order decay of both longitudinal and transverse magnetization lags behind the second order decay of radical population when the reaction rate approaches the relaxation rates 1/T1,2. The observations may be explained in terms of the spin-pairing requirements for formation of a singlet reaction product. The spin pairing gives rise to a ‘‘passive’’ form of radical pair mechanism (RPM) chemically induced dynamic electron polarization (CIDEP) which has previously gone unrecognized. RPM polarization therefore occurs in ⋅SO−3 encounters despite the lack of hyperfine dependent singlet–triplet mixing between radical reencounters, which is necessary for the accepted RPM mechanism. The implications of these observations for the interpretation of time-resolved EPR experiments are discussed.