Reaction Time Scale Research Articles

Molecular simulation on viscous effects for microscale combustion in reactive shock-bubble interaction

Molecular simulations of reacting shock-bubble interaction were performed to study viscous effects on the supersonic premixed combustion, using the Direct Simulation Monte Carlo (DSMC) method. Different bubble diameters that are comparable to the thickness of the normal shock wave (Mach=2) were taken into account. The evolution of bubble morphology indicates that viscous effects (diffusion and dissipation) leads to the disappearance of some typical phenomena, including the shock focusing, positive viscosity deposition and wake-like cross-section. Meanwhile, the viscous dissipation is mainly responsible for the enstrophy degeneration, which is enhanced by reducing the bubble scale and heat release in combustion. The observed combustion process, which is prominent near the bubble core, is more evident with a larger bubble diameter and unlikely to occur with the smallest bubble diameter. The corresponding reason is that the strong viscous diffusion reduces the collision events between fuel and oxygen molecules. In addition, a time-scale analysis was conducted to illustrate the viscous diffusion in the hydrodynamic and chemical reaction time-scales with different bubble diameters. As a consequence, a changing dimensionless Damkohler number was presented to understand the dominant diffusion effect in microscale supersonic premixed combustion.

Regulation of reaction fluxes via enzyme sequestration and co-clustering.

Experimental observations suggest that cells change the intracellular localization of key enzymes to regulate the reaction fluxes in enzymatic networks. In particular, cells appear to use sequestration and co-clustering of enzymes as spatial regulation strategies. These strategies should be equally useful to achieve rapid flux regulation in synthetic biomolecular systems. Here, we leverage a theoretical model to analyse the capacity of enzyme sequestration and co-clustering to control the reaction flux in a branch of a reaction-diffusion network. We find that in both cases, the response of the system is determined by two dimensionless parameters, the ratio of total activities of the competing enzymes and the ratio of diffusion to reaction timescales. Using these dependencies, we determine the parameter range for which sequestration and co-clustering can yield a biologically significant regulatory effect. Based on the known kinetic parameters of enzymes, we conclude that sequestration and co-clustering represent a viable regulation strategy for a large fraction of metabolic enzymes, and suggest design principles for reaction flux regulation in natural or synthetic systems.

A steady-state precipitation model for flowsheet simulation and its application

Precipitation of sparingly soluble salts is an important process to produce solid products as color pigments or catalysts. High levels of supersaturation are obtained in this process during the mixing step. Consequently, reaction timescales are fast and mixing often influences the product particle size distribution (PSD).In this work, we present a generic model for flowsheet simulation of steady-state precipitation of sparingly soluble salts. Model validation is performed by experiments involving barium sulfate precipitation from aqueous solutions in a confined imping jet mixer (CIJM). Furthermore, experimental and simulative methods are applied to investigate the influence of recycle streams on the product PSD. The experiments show an increase in the average particle size for high recycle flows which is well-predicted by the model. We, furthermore, demonstrate by simulations that recycled particles influence the supersaturation buildup in the CIJM only if a critical particle density is exceeded.

How coherent structures dominate the residence time in a bubble wake: An experimental example

Timescales and residence times in reactive multiphase flows are essential for product selectivity. For instance when a gas species is consumed, e.g., by a competitive consecutive reaction with moderate reaction kinetics where reaction timescales are comparable to relevant mixing timescales. To point out the importance of the details of the fluid flow, we analyze experimental velocity data from a Taylor bubble wake by means of Lagrangian methods. By adjusting the channel diameter in which the Taylor bubble rises, and thus the rise velocity, we obtain three different wake regimes. Remarkably, the residence times of passive particles advected in the bubble wake’s velocity field show a peak for intermediate rise velocities. This observation seems unintuitive at first glance because one expects a faster removal of passive tracers for a faster overall flow rate. However, the details of the flow topology analyzed using Finite Time Lyapunov Exponent (FTLE) fields and Lagrangian Coherent Structures (LCS) reveal the existence of a coherent vortical pattern in the bubble wake which explains the long residence times. The increased residence times within the vortical structure in combination with the close bubble interface acting as a constant gas species source could enhance side product generation of a hypothetical competitive consecutive reaction, where the first reaction with the gas species forms the desired product and the second the side product.

Mechanisms and competition of halide substitution and hydrolysis in reactions of N2O5 with seawater.

SN2-type halide substitution and hydrolysis are two of the most ubiquitous reactions in chemistry. The interplay between these processes is fundamental in atmospheric chemistry through reactions of N2O5 and seawater. N2O5 plays a major role in regulating levels of O3, OH, NO x , and CH4. While the reactions of N2O5 and seawater are of central importance, little is known about their mechanisms. Of interest is the activation of Cl in seawater by the formation of gaseous ClNO2, which occurs despite the fact that hydrolysis (to HNO3) is energetically more favorable. We determine key features of the reaction landscape that account for this behavior in a theoretical study of the cluster N2O5/Cl-/H2O. This was carried out using ab initio molecular dynamics to determine reaction pathways, structures, and time scales. While hydrolysis of N2O5 occurs in the absence of Cl-, results here reveal that a low-lying pathway featuring halide substitution intermediates enhances hydrolysis.

Effect of Atomization Quality on Lean Blow-Out Limits and Acoustic Oscillations in a Swirl Stabilized Burner

ABSTRACT The present experimental work highlights the influence of atomization quality on lean blow-out (LBO) limits and acoustic oscillations in a swirl stabilized burner with simplex atomizer. With decrease in the initial spray droplet diameter, the LBO limit shifts toward lower equivalence ratios. Reduction in droplet size also strongly influences the mode of LBO from diffusion flame to premixed lifted flame. Correlations have been developed for the LBO limit, involving mainly the time scales for evaporation, reaction and residence times for the fuel drops, as well as the gas flow. Delay in evaporation causes vapor accumulation before combustion and hence it influences both acoustic oscillations and LBO limit. The frequency of acoustic oscillations locks-in with the quarter wave frequency of the combustor duct for all initial droplet diameters considered. The amplitude of acoustic oscillations decreases with decrease in the initial droplet size.

A simulation of free radicals induced oxidation of dopamine in aqueous solution

Understanding the basic chemistry between highly reactive free radicals and dopamine is an important step in characterizing the antioxidative activity of catecholamine neurotransmitters. In this work, we simulated the reactions between dopamine and hydroxyl, peroxyl and methoxy radicals in aqueous solution by employing first principle molecular dynamics based on density functional theory and the BLYP functional. The simulations provide mechanistic insight into the reaction mechanisms but underestimate reaction timescales. The failure of the BLYP functional to address the formal hydrogen atom transfer barriers between dopamine and free radicals is attributed to the self-interaction error.

Correlation of combustor lean blowout performance to supercritical pyrolysis products

Studies to assess correlations between fuel pyrolysis products and fuel-to-air equivalence ratio at combustor lean blowout (LBO) were conducted. Eighteen fuels, including conventional and alternative fuel formulations and blends, single compounds and binary surrogates of hydrocarbon compounds, were studied. The pyrolysis tests consisted of thermally cracking the fuels in a flow reactor at supercritical conditions of 4.14 MPa and up to 625 °C, at varying residence times. The liquid and gaseous pyrolysis products were analyzed via gas chromatography to correlate yields to the fuel LBO data acquired in a previous study on a single-nozzle swirl-stabilized combustor. Focus was given to correlate LBO with primary decomposition products, since it is hypothesized that fuel combustion characteristics, particularly ignition and flame extinction, are related to the rate and type of products formed upon initial thermal decomposition of the parent fuel. Therefore, only the range of conversions for which low molecular weight products had a near-constant formation rate was considered. Results show that the highly branched fuels primarily decomposed to branched C4 and methane, while fuels with high concentration of linear alkanes show greater yields of C2-C4 alkanes and 1-alkenes. Regression analyses of the pyrolysis gas and LBO data show very strong correlations (R2 > 0.90) for several pyrolysis products for the conventional and alternative fuels, with ethane, ethylene and iso-butylene yields providing the best correlations to LBO. The calculated LBOs based on regression parameters and concentrations of the pyrolysis gases were within 2% of the measured LBO for most fuels tested. These initial results suggest that despite the vast differences in fuel reaction timescales in the flow reactor (∼0.2–11 s) and combustor environments (<0.5 ms), fuel pyrolysis species in a flow reactor environment are relevant and may be used to predict fuel combustion behavior. If further validated, this method provides a capability to identify fuel components that can improve combustion performance via their primary pyrolytic product slates, potentially providing a more cost-effective alternative to combustion testing for determination of fuel combustion performance metrics. Results of the pyrolysis tests of all fuels, and future efforts are discussed.

A general stirred-flow model for time-dependent adsorption and desorption of heavy metal in soils

Quantitative understanding of kinetics and mechanisms heavy metal adsorption-desorption and transport processes, batch and stirred-flow experiments were carried out with vanadium (V) and molybdenum (Mo) on soils at different reaction conditions and time scale. Batch experiments indicated that adsorption of V and Mo on soils was highly nonlinear and time-dependent, where V and Mo retention showed typical biphasic reaction kinetics. The stirred-flow experiments showed that both V and Mo adsorption consisted of a fast initial reaction, as indicated by the fact that effluent solute concentration was close to zero for the first few minutes. A stirred-flow multi-reaction model (MRM) which accounts for slow as well as fast reactions of the reversible and irreversible type was developed to describe V and Mo adsorption and desorption processes on soils. Based on model simulations and experimental tracer breakthrough curves (BTCs) results, we concluded that the proposed model is valid for stirred-flow conditions. The strong retardation and slow release behaviors of V and Mo from stirred-flow experiments were successfully described using the proposed stirred-flow MRM where retardation and irreversible reactions were necessary. Our work provides a general stirred-flow model which is capable of describing reactive and non-reactive solutes.

Emission of light charged particles and fission fragments from Hf⁎ and Pt⁎ nuclei

Using dynamical cluster-decay model, the time scale of fission and light charged particles emitted in the decay of Hf⁎ and Pt⁎ nuclei is estimated. The decay analysis is carried out for isotopes and isobars of 170,174Hf⁎ and 170,174Pt⁎ nuclei. The structural information from the excited Hf⁎ and Pt⁎ nuclei is extracted using the fragmentation potential and preformation probability, which in turn accordingly influence the cross section yields. The cross sections of evaporation residue and fission-fragments are calculated by optimizing the neck length ΔR with in the frame work of dynamical cluster-decay model (DCM). The neck length ΔR provides significant information about the shape elongation of decaying fragments and in turn provides an insight of the reaction time scale. Applying momentum conservation, the velocity and time scale of fission fragments is calculated for 170Hf⁎ nucleus at temperature T=2.011MeV, which comes out of the order of (0.7–1)×10−21 s and the same for light charged particle is 0.5×10−21 s.

Diffusive–stochastic–viscoelastic model for specific adhesion of viscoelastic solids via molecular bonds

A diffusive–stochastic–viscoelastic model is proposed for the specific adhesion of viscoelastic solids via stochastically formed molecular bonds. In this model, it is assumed that molecular-level behaviors, including the diffusion of mobile adhesion molecules and stochastic reaction between adhesion molecules and binding sites, are Markovian stochastic processes, while the mesoscopic deformation of the viscoelastic media is governed by continuum mechanics. Systematic Monte Carlo simulations of this model are used to investigate how competition between the time scales of molecular diffusion, reaction, and deformation creep of the solids may influence the lifetime and dynamic strength of their adhesion. The results reveal that there exists an optimal characteristic time for molecular diffusion corresponding to the longest lifetime and greatest adhesion strength, which is in good agreement with experimentally observed characteristic time scales of molecular diffusion in cell membranes. In addition, the results show that the viscosity of the media can significantly increase the lifetime and dynamic strength, since deformation creep and stress relaxation can effectively reduce the concentration of interfacial stress and increase the rebinding probability of molecular bonds.

Kinetic Modeling of pH‐Dependent Molybdenum(VI) Adsorption and Desorption on Iron Oxide‐Coated Sand

Core Ideas Molybdenum(VI) sorption and kinetic rate are significantly reduced by increasing pH. A Diffuse Layer Model simulated protonated monodentate complex was dominant at low pH. A kinetic two‐site model successfully described Mo(VI) adsorption–desorption behavior. Quantitative understanding of retention kinetics controlling molybdenum [Mo(VI)] on soil constituents such as iron oxide binding sites is crucial for accurately evaluating environmental risk of ground and surface water contamination. In this study, kinetic batch experiments were performed to investigate Mo(VI) adsorption–desorption behavior on iron oxide‐coated sand (IOCS) at different reaction conditions and time scale. Batch results indicated that adsorption of Mo(VI) on IOCS was highly nonlinear and kinetic, where both sorption and the extent of kinetics decreased with increasing solution pH. A two‐site model based on the heterogeneity of binding sites and variations of solution chemistry was developed to describe Mo(VI) adsorption and desorption on IOCS. The approach integrates kinetic nonlinear sorption into the surface complexation Diffuse Layer Model (DLM). Our results indicated that kinetic rate of non‐protonated monodentate complexes was much higher than that of protonated monodentate complexes. The kinetic model was capable of describing the experimental data with limited number model fitting parameters.

Fragility and aging behavior of SixSe1-x glasses and liquids.

The composition dependence of the fragility of SixSe1-x liquids with 0.05 ≤ x ≤ 0.33 is determined using the calorimetric method and is found to be rather similar to that characteristic of their Ge analogues. In addition, the nature and the time scale of the structural relaxation of the Si25Se75 glass during aging at 40 K below Tg are measured using Raman spectroscopy. The structural relaxation in this glass, which belongs to the so-called intermediate phase, involves progressive conversion of the doubly edge-shared SiSe4/2 tetrahedra E2 into singly edge-shared E1 and corner-shared E0 tetrahedra upon lowering of temperature. This tetrahedral speciation can be expressed in the form of the reaction 2 E2 → E0 + E1. The time scale of this tetrahedral conversion reaction corresponds well with that of shear relaxation. This result is inconsistent with the claim made previously in the literature that intermediate phase compositions do not undergo aging. Moreover, when taken together, the fragility and the structural relaxation results suggest that the constraint counting scheme typically adopted in the literature for edge- vs. corner-shared tetrahedra in chalcogenide networks may need to be revised. A rigid-polytope based constraint counting approach is shown to be more consistent with the experimental results.

Time scale based analysis of in-situ crystal formation in droplet undergoing rapid dehydration

The surface structure of crystalline particles affects the functionality of the particles in drug delivery. Prediction of the final structure of particles that crystallize easily within the spray drying process is of interests for many applications. A theoretical framework was developed for the prediction of crystal structure precipitating on the surface of the particle. This model was based on the dimensionless Damkohler number (Da), to be an indicator of final particle morphology. Timescales of evaporation and reaction were required for calculation of the Damkohler number. The modified evaporation time scale was estimated based on the time that is available for the crystal to precipitate after supersaturation. The reaction time scale was estimated based on the time scale for induction time. Mannitol was produced under different processing conditions in order to validate the theoretical model. Results showed for the high Damkohler numbers, the surface structure of the particle was rough, while smaller Damkohler numbers led to relatively smooth particle surfaces. Additionally, although the beta polymorph was dominant in all of the experiments, alpha polymorph was precipitated in the experiments with a large Damkohler number. The theoretical framework developed will be a useful predictive tool to guide the manipulation of particle crystallization in spray dryers.

Gas-phase autoignition of diesel/gasoline blends over wide temperature and pressure in heated shock tube and rapid compression machine

Diesel/gasoline blended fuel, also named as dieseline, has attracted worldwide interests in varied advanced engine combustion modes dominated by mixing time scale and chemical reaction time scale. However, the research on ignition properties and chemical reaction mechanism of diesel/gasoline blends over wide operating conditions is still lacking. In current study, ignition delay time measurement of diesel/gasoline blends with different diesel proportion (30%, 50%, and 70% by volume) were conducted in a heated shock tube and a heated rapid compression machine (RCM) covering a broad range of temperatures between 636 and 1317 K at pressures between 6 and 20 bar for gas-phase dieseline/O2/N2 mixture at varying equivalence ratios from 0.5 to 1.5. The experimental results revealed different dependence of ignition delay time on pressure, equivalence ratio and composition at different temperatures and varying negative-coefficient (NTC) behaviors were also observed at intermediate-temperature regime. The increase in proportion of diesel led to shorter ignition delay times under all conditions, though the ignition data at high temperature only showed marginal discrepancies. Besides, a crossover of ignition data occurred at the highest temperature for varying equivalence ratios. A tertiary surrogate for diesel and a tertiary surrogate for gasoline were chosen to calculate conditions after reflected shock in shock tube and compression in RCM. Then, simulations of ignition delay time for diesel/gasoline blends were conducted using a detailed model. Finally, kinetic analyses were performed to gain insight into the ignition characteristic of diesel/gasoline blends.

High-speed tracking of fast chemical precipitations.

Heterogeneous reactions taking place in the aqueous phase bear significant importance both in applied and fundamental research. Among others, producing solid catalysts, crystallizing biomorphs or pharmaceutically relevant polymorphs, and yielding bottom-up synthesised precipitate structures are prominent examples. To achieve a better control on product properties, reaction kinetics and mechanisms must be taken into account especially in dynamic systems where transport processes are coupled to chemistry. Since the characteristic time scale of numerous precipitation reactions falls below 1 s within the relevant concentration range, unique experimental protocols are needed. Herein we present a high-speed camera supported experimental procedure capable of determining such characteristic time scales in the range of 10 ms to 1 s. The method is validated both experimentally and by performing 3D hydrodynamic simulations.

Examination of Reaction Times of Athletes of Kyrgyz National Team during Preparation Period for 2016 Summer Olympic Games

This research has been carried out in order to determine the reaction levels of Kyrgyzstan national team athletes during the preparation period before 2016 summer Olympic Games. 9 Greco-Roman style wrestlers (24±4.5), 10 freestyle wrestlers (22,10±3,21), 8 judoists (23,6±1.89), 8 male athletes (22,29±2.87) and 3 female athletes (21.67±2.08), totally 35 male and 3 female national athletes have been included to the study. The reaction times of the athletes against sound and light have been determined by the Newtest 1000 brand multi-choice reaction time scale. Statistical evaluation of the study has been carried out by SPSS 21.0 computer package program and One Way ANOVA test has been used to determine the difference between the groups. The differences on P 0,05). Greco-Roman style wrestlers’ reaction times have been found significantly higher than male and female athletes (p 0,05). No statistically significant difference was found among the left hand reaction times of the branches (p>0,05). When the obtained results are examined, it has been observed that the right hand reaction times of the athletics are better than the wrestling and judo which are the branches of the fight sports.

Operator splitting for stiff chemical reaction systems

AbstractChemical reaction systems often result in high‐dimensional, stiff differential equations. Solving these differential equations is computational demanding. The computational effort can be reduced by the usage of an operator splitting scheme. The most frequently used splitting schemes are the Lie‐Trotter splitting [1] and the Strang splitting [2]. However, the usage of a splitting scheme results in an additional splitting error. The Lie‐Trotter splitting is a first order scheme, and the Strang splitting is a second order scheme. If the timescale of the fastest chemical reaction is smaller than the splitting time step, the Strang splitting suffers from order reduction. Therefore, the Lie‐Trotter splitting and the Strang splitting are first order schemes for reasonable step sizes. The Richardson extrapolation of the Lie‐Trotter splitting is a second order scheme. We show that the Richardson extrapolation does not suffer from order reduction for a splitting between a fast chemical source term and a slow transport term.

“Shuttle” in Polysulfide Shuttle: Friend or Foe?

The polysulfide shuttle effect, where sulfur species reach the negative electrode surface and undergo chemical reduction, is believed to be a major bottleneck in lithium–sulfur chemistry. The importance of this phenomenon is often judged based on phenomenological arguments, which do not account for mesoscale complexations. This work presents a comprehensive investigation of the coupled interactions arising from speciation, concentrated electrolyte solution, and reaction time scales. Polysulfide transport consists of diffusion and migration, which determines the net flux. This study demonstrates that the polysulfide shuttle effect can be bounded between reaction-limited and shuttle-limited regimes, depending on the operational extremes. At high sulfur to electrolyte ratio (i.e. lean electrolyte condition), the polysulfide shuttle effect may not necessarily attribute to the cell performance limitation, believed contrarily otherwise.

Ring-opening attachment as an explanation for the long lifetime of the octafluorooxolane anion.

Octafluorooxalane, C4F8O, has recently attracted attention as a possible replacement of SF6 in high voltage insulation, and its reactivity with respect to free-electron attachment was investigated by mass spectrometry. The most intense signal peaks at 0.9 eV and corresponds to the parent anion, C4F8O-; fragments stemming from complex breakup reactions are detected starting above ∼1.6 eV. Since parent anions in free-electron attachment are normally associated with threshold attachment or an embedding environment allowing excess energy deposition, this observation is highly unusual. Based on density functional calculations, it was nevertheless interpreted as attachment followed by intermolecular-vibrational-relaxation. Here, electron-attachment to octafluorooxalane is studied computationally. First, the electron affinity (EA) is characterized using density functionals and ab initio methods. Moreover, the negative vertical EA is estimated by extrapolating electron binding energies computed in the vicinity of C4F8O- to the geometry of neutral octafluorooxalane. Then, alternative explanations for the 0.9 eV peak are considered. Specifically, a ring-opening reaction that yields a distonic isomer of C4F8O- is identified. Our analysis reveals that the chain isomer possesses many conformers, all of which are considerably more stable than the ring isomer, and that the time scale for the unimolecular ring opening reaction is significantly faster than 1 μs. Thus, at the experimental energy, the ring isomer of C4F8O- is predicted to convert practically completely into the chain isomer, and we argue that the long lifetime and the peak position are effectively determined by the properties of the ring-opening transition state.