Kinetic Scheme Research Articles

Hydrodynamic performance of manta rays swimming in staggered arranged group

To investigate the hydrodynamic performance of manta rays swimming in staggered arranged group, a morphological and kinematic model of manta rays is developed based on biological observations, and then a numerical calculation method is established for group swimming of manta rays based on the Immersed Boundary Method and the Sphere function-based Gas Kinetic Scheme (IB-SGKS). The group swimming of two manta rays with a fixed vertical spacing of 0.1 times the body thickness, and a flow direction spacing of 0—1.5 times the body length is systematically investigated. The average thrust/efficiency of the group system and each individual in the group are analyzed by combining the global three-dimensional (3D) vortex structure and the characteristic cross-section two-dimensional (2D) vortex structure. The numerical results are shown below. When the streamwise spacing between individuals is small, the propulsive performance decreases sharply compared with swimming alone; as the streamwise spacing increases, the propulsive performance of the leader manta ray is consistently better than that of swimming alone, with the maximum thrust enhanced up to 11.24% when <i>D</i><sub><i>x</i></sub> = 0.4BL, and the maximum efficiency is enhanced up to 3.58% when <i>D</i><sub><i>x</i></sub> = 0.3BL; with the increase of the streamwise spacing, in the thrust/efficiency curves of the follower manta ray appears volatility, with the maximum thrust enhanced to 48.14% when <i>D</i><sub><i>x</i></sub> = 0.4BL and the maximum efficiency reached to 12.39% when <i>D</i><sub><i>x</i></sub> = 0.5BL; the system average thrust and efficiency enhancement both reach their corresponding maximum values, specifically, 29.69% and 6.77%, when <i>D</i><sub><i>x</i></sub> = 0.4BL, which is because the tail vortex of the leading manta ray just passes through the front edge of the follower manta ray and directly acts on the tip vortex that initially falls off from the follower manta rays, thus substantially increasing their vortex energy.

Low-temperature trapping of N2 reduction reaction intermediates in nitrogenase MoFe protein-CdS quantum dot complexes.

The biological reduction of N2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E0. Illumination for sequential 1-h periods at 233K under 1 atm of N2 led to a cumulative attenuation of E0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E2) and four-electron (E4) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E6 and E8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe-Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

Visible Light Induced Photocatalytic Removal of Methylene Blue Using Cu-tunable p-type ZnO Nanoparticles.

Removal of phototoxicity and zootoxicity pollutants from the aqueous environment is of great importance to human and aquatic life. Copper-tunable p-type zinc oxide (Cu-ZnO) photocatalysts have been prepared by the chemical co-precipitation method. The structural, morphological, elemental and optical properties of the obtained catalysts were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray (EDX) analysis and ultraviolet-visible (UV-Vis) spectrophotometry. The diffraction patterns of the as-synthesized catalysts were matched with that of the hexagonal wurtzite structure for the standard ZnO nanoparticles. The photocatalytic activity of the prepared Cu-doped ZnO catalyst was evaluated using methylene blue (MB) dye under various conditions. The effect of operational parameters such as MB initial concentration, catalyst dosage, and solution pH was optimized using a face central composite design (FCCD) of the response surface methodology (RSM). The optimum photodegradation efficiency of 98.00% was found at 0.30g/L catalyst dose, 10.00mg/L initial concentration of MB and initial pH at 6.00. The degradation model was statistically remarkable with p < 0.0001% in which the MB initial concentration and solution pH were the most significant variables influencing the removal of MB over the Cu tunable p-type ZnO photocatalyst under visible light irradiation. Finally, the photocatalytic degradation of MB using the undoped and Cu-doped ZnO nanoparticles was nicely fitted pseudo-first-order kinetics scheme.

The impact of lake shape and size on lake breezes and air-lake exchanges on Titan

Titan, the largest moon of Saturn, has many lakes on its surface, formed mainly of liquid methane. Like water lakes on Earth, these methane lakes on Titan likely profoundly affect the local climate. Previous studies (Rafkin and Soto, 2020; Chatain et al., 2022) showed that Titan's lakes create lake breeze circulations with characteristic dimensions similar to the ones observed on Earth. However, such studies used a model in two dimensions; this work investigates the consequences of the addition of a third dimension to the model. Our results show that 2D simulations tend to overestimate the extension of the lake breeze over the land, and underestimate the strength of the subsidence over the lake, due to divergence/convergence geometrical effects in the mass conservation equations. In addition, 3D simulations including a large scale background wind show the formation of a pocket of accelerated wind behind the lake, which did not form in 2D simulations. An investigation of the effect of shoreline concavity on the resulting air circulation shows the formation of wind currents over peninsulas. Simulations with several lakes can either result in the formation of several individual lake breeze cells (during the day), or the emergence of a large merged cell with internal wind currents between lakes (during the night). Simulations of several real-shaped lakes located at a latitude of 74°N on Titan at the autumn equinox show that larger lakes trigger stronger winds, and that some sections of lakes might accumulate enough methane vapor to form a thin fog. Additionally, we adapted the Turbulent Kinetic Energy closure scheme of the model to better represent the extremely low turbulence at the surface of Titan, of 2 10−4 m2.s−2 above the land, and inferior to 3 10−5 m2.s−2 above the lake. The addition of a third dimension, along with adjustments in the parametrizations of turbulence and subsurface land temperature, results in a reduction in the magnitude of the average lake evaporate rate, namely to ∼6 cm/Earth year.

3D study of a chemically reacting fixed-bed under gasification conditions for non-porous char: Laminar flow

This paper reports a computational investigation of the combustion of char particles in a fixed-bed using a three-dimensional model. Numerical simulations were carried out using the commercial software AnsysⓇ Fluent, implementing a six semi-global kinetic scheme, where carbon monoxide combustion, forward and backward water-gas-shift reaction (WGS), Boudouard reaction, and two other heterogeneous reactions take place inside the reactor composed of 85 spherical particles. The diameter of the particles is 5.6 mm. Laminar flow regime (Rein = 50, 75 and 100), inflow gas temperature in the range of 900–1500 K, and mass fraction of oxygen at the inlet (YO2,in) of 0.05, 0.11, and 0.233 are the parameters of interest to study the heterogeneous combustion of carbon char particles inside the unit. The Boudouard reaction and oxidation of carbon monoxide are dominant among heterogeneous and homogeneous chemical reactions, respectively, being the inflow gas temperature a critical parameter to assess, since the kinetic scheme is dramatically activated at higher temperatures. A linear dependency has been found for species concentration and temperature profile in most of the simulations using YO2,in=0.05. Further studies are necessary to extend this research to intrinsic chemical reactions (tracking of porosity and particle radius), and the hybridization of the bed zone by incorporating inert solid particles into the system.

Multi-step kinetic mechanism coupled with CFD modeling of slow pyrolysis of biomass at different heating rates

This study employed numerical modeling to investigate the reactivity and kinetics involved in the slow pyrolysis of lignocellulosic biomass. The model integrates a biomass multi-step kinetics scheme with heat, momentum, and mass transfer, and it was based on the transport of species and flow in a porous medium approach. To develop the multi-step kinetics model, two samples of biomass, avocado stone (AS) with a high hemicellulose content and α-cellulose (CEL), were subjected to TGA experiments in an inert atmosphere. Furthermore, several experimental data in the literature on the evolution of slow pyrolysis products and data obtained by TGA experiments were considered to refine and validate the proposed kinetic mechanism. The temperature range, from 25 to 700 °C, was explored using different heating rates (10, 20, and 40 °C/min). Experimental results showed that CO and CO2 are the predominant gases during primary devolatilization, whereas H2 and CH4 result from secondary reactions at temperatures above 400 °C. The proposed mechanism involves computational fluid dynamic (CFD) simulations of laboratory-scale biomass pyrolysis, comparing temperature and species concentrations with experimental data. The predicted results for individual non-condensable gas mass yields showed an average relative error of below 6.90 % and 11.59 % for CEL and AS, respectively. In the case of biochar, the error was 6.41 % and 9.74 % for AS and CEL, respectively. The developed kinetic model can be applied to simulate the slow pyrolytic degradation of biomass based on its chemical composition.

Adaptation of the Kinetical Scheme to Ethylene Combustion Conditions at Temperatures Above 1200 K

In the paper authors present original methods for analyzing the kinetic scheme and reaction rateconstants for calculation of the ignition delay time and the laminar velocity of the combustion wave forC2H4–O2–Ar and C2H4-air mixtures. The kinetic scheme under consideration will further be applied inproblems of plasma-assisted combustion in a supersonic flow using a discharge. After made changes to thereaction system, a good agreement between the calculation results and experimental data was obtained.

Simulation of filamentation dynamics of microwave discharge in nitrogen

The present study deals with numerical simulations of microwave (MW) discharges in nitrogen based on extended fluid-dynamic model. The set of governing equations for non-equilibrium gas-discharge plasma includes conservation equations for species number densities, electron energy density, Poisson equation for the electric field coupled to the multi-temperature Navier–Stokes fluid-dynamic equations taking into account thermal nonequilibrium; the power transmitted from MW radiation to electrons is determined from the Helmholtz equation. The kinetic scheme includes 61 reactions involving neutral molecules and atoms in the ground and electronically excited states, ions and electrons. The set of equations is solved for a two-dimensional problem under conditions of experiments at a pressure of 40 and 50 Torr and different electromagnetic wave frequencies and pulse duration. The dynamics of discharge formation and transition from the diffuse to the filament form is studied. The results are compared with experimental data, and a good agreement is shown for the time larger than 10 µs. The possible reasons for discrepancies at a shorter time are discussed and the effect of small oxygen impurities on the quantitative characteristics of the discharge are evaluated. The presence of a small oxygen impurity and seed electrons in the region of discharge formation yields a better agreement between numerical and experimental data.

Adaptation of the Kinetic Scheme to Ethylene Combustion Conditions at Temperatures Above 1200 K

Adaptation of the Kinetic Scheme to Ethylene Combustion Conditions at Temperatures Above 1200 K

Comparative Analysis of Jet Fuel Surrogates Based on a Comprehensive Kinetic Scheme

Comparative Analysis of Jet Fuel Surrogates Based on a Comprehensive Kinetic Scheme

An immersed boundary method‐discrete unified gas kinetic scheme simulation of particle‐laden turbulent channel flow on a nonuniform orthogonal mesh

AbstractParticle‐resolved simulations of turbulent particle‐laden flows provide a powerful research tool to explore detailed flow physics at all scales. However, efficient particle‐resolved simulations for wall‐bounded turbulent particle‐laden flows remain a challenging task. In this article, we develop a simulation approach for a turbulent channel flow laden with finite‐size particles on a nonuniform mesh by combining the discrete unified gas kinetic scheme (DUGKS) and the immersed boundary method (IBM). The standard discrete delta function was modified according to reproducible kernel particle method to take into account mesh non‐uniformity and correctly conserve force moments. Simulation results based on uniform and nonuniform meshes are compared to validate and examine the accuracy of the nonuniform mesh DUGKS‐IBM. Finally, the computational performance of the nonuniform mesh DUGKS‐IBM is discussed.

Analysis of the possibility of solid-phase ignition of coal fuel

The article presents the results of theoretical and experimental studies of the ignition process of coal fuel particles in a pelletized state during high-temperature radiation-convective heating in an oxidizing environment. A new, different from the known, “two-temperature” mathematical model of the ignition process of a cylindrical coal pellet under intense heating conditions is presented. During the modeling, a detailed kinetic scheme of the reaction of gaseous products of thermal decomposition of coal with atmospheric oxygen was considered. The mathematical model was tested through a comparative analysis of the theoretical and experimental values of the ignition delay times of coal pellet fuel with experimental data. A wide range of heating conditions that could potentially lead to solid-phase ignition were analyzed. The research results showed that even under conditions of high oxygen content in the original coal, solid-phase ignition of coal pellets does not occur during intense heating. Based on the results of numerical modeling, it was established that the ignition process occurs in the gas phase. The oxygen released during the thermal decomposition of coal is not enough for stable gas-phase or heterogeneous ignition of pyrolysis products in the intrapore space of the fuel pellet. Based on the results of the numerical modeling, it was established that the oxygen released during the thermal decomposition of coal is significantly deficient in the intrapore space. The O2 concentration is not enough for a significant (in terms of heat flow) thermochemical intrapore reaction with gaseous and solid products of thermal decomposition.

Instability of mixing layers: Momentum and thermal transport in the continuum breakdown regime.

We examine the momentum and thermal transport in the continuum breakdown regime of a mixing layer flow, which exhibits Kelvin-Helmholtz instability under ideal continuum conditions. The Grad 13 moment model is used as it provides an adequate description of the flow physics (second-order accurate in Knudsen number) in the transition regime. Analytical solutions are developed under breakdown conditions for two-dimensional, compressible, parallel shear flows. It is shown that the deviation of viscous stress and heat flux from the Navier-Stokes-Fourier system follows two different scaling regimes depending upon the Mach number. At low Mach numbers, the departure of all stress and heat-flux components depends only upon the Knudsen number. At high Mach number, the scaling of shear stress and transverse heat flux depends on the product of the Knudsen and Mach numbers. The normal stresses depend individually on the Knudsen and Mach number. The scaling results are verified against numerical simulations of compressible mixing layers performed using the unified gas kinetic scheme for various degrees of rarefaction.

Well-balanced kinetic schemes for two-phase flows

At equilibrium state of a two-phase fluid system, the chemical potential is constant and the velocity vanishes. However, such equilibrium state usually cannot be captured by numerical methods due to discretization errors. Consequently, inconsistent thermodynamic interfacial properties due to non-constant chemical potential, and spurious velocities due to discrete force imbalance, are frequently encountered in numerical simulations. Therefore, numerical schemes with well-balanced properties, which can capture the equilibrium state at discrete level, are preferred for simulating two-phase flows. In this paper, we report the progress of developing well-balanced mesoscopic numerical methods based on kinetic theory, focusing on the lattice Boltzmann equation (LBE) method and the discrete unified gas-kinetic scheme (DUGKS). First, the discrete balance property of a free-energy LBE for single-component two-phase flows is analyzed to identify the structure of force imbalance. Then, a well-balanced LBE (WB-LBE) model which has the same algorithm structure as the standard one is reviewed. The WB-LBE is theoretically shown to be able to achieve the discrete equilibrium state, and the well-balance properties are confirmed numerically. Some extensions to other two-phase LBE models are discussed with enhanced numerical stability. The well-balanced DUGKS, which can use non-uniform meshes and exhibits better numerical stability for large density ratio systems is also reported.

Kinetic study of the vibrational relaxation of CH3Zn by collisions with He

Methyl zinc CH3Zn generated in the photolysis of dimethyl zinc (CH3)2Zn at 223 nm was detected with laser-induced fluorescence (LIF) via the A∼2E-X∼2A1 transition. The time-resolved LIF intensity of the level X∼2A1,00 grew with time, indicating that the nascent CH3Zn was highly vibrationally excited. The He-pressure dependence of the growth rates of the LIF intensity was analyzed by the kinetic scheme with a rate-determining step, giving the rate coefficient for vibrational relaxation to the X∼2A1,00 level to be [8.2±0.9(2σ)]×10-13 cm3 molecule−1 s−1. The probability of vibrational relaxation of CH3Zn per collision with He nicely correlates with those of non- or less-polar diatomic molecules.

Streamer-to-filament transition in pulsed nanosecond atmospheric pressure discharge: 2D numerical modeling

The streamer-to-filament transition in air at atmospheric pressure, in a nanosecond pin-to-pin discharge, is studied by a 2D model. The main aim is to implement a kinetic scheme providing a sharp electron density increase to the 2D PASSKEy code, to validate the results on the available experimental data, and to investigate the mechanisms responsible for the transition. Results show that after the formation of a conductive channel across the discharge gap, two discharge modes appear during a few nanoseconds: a glow phase, with a relatively homogeneous distribution with the electron density of cm−3, and a filamentary phase, with the electron density of cm−3. Two filaments appear at the cathode and anode respectively, and propagate towards the middle of the gap with a velocity of about cm s−1, forming a narrow channel. Simultaneously, the gas temperature increases from 350 to 2800 K. The diameter of the channel at the middle gap decreases from 210 to 90 µm. Dissociation and ionization of the electronically excited states of molecules N2(A, B, a, C), and ionization of ground and electronically excited states of O and N atoms are the most important processes for the transition. Numerical results also reveal the influence of the memory effect (pre-heating, pre-ionization, and pre-dissociation) from previous pulses on the streamer-to-filament transition.

Kinetic Analysis of Dry Reforming of Methane on Traditional and Membrane Catalysts

The article presents an analysis of the results of a kinetic study of dry reforming of methane (UCM) in reactors with traditional (TC) and membrane catalysts (MC). The kinetic experiment in reactors with MC and TC was performed in the temperature range of 820–900°C and the ratio CH4 : CO2 = 1 : 1. In the experiment, an intensification of the process of the methane cracking reaction was established, the rate constant of which increases by an order of magnitude. Such a difference in the results of the DRM on the studied catalysts are due to the intensification of mass transfer on the MC, which is based on the phenomenon of thermal slip. A mathematical description corresponding to the kinetic scheme of DRM process is proposed, and the rates constants of direct and reverse reactions in both reactors are found. In the DRM process, water gas is formed on the TC, and synthesis gas is formed on the MC. At TC, the DRM process is accompanied by the accumulation of carbon deposits (CD), and at MC this accumulation is absent. The DRM process on both catalysts is characterized by three main reactions (methane cracking, gasification of the CD with carbon dioxide and/or water vapor and the reverse water gas shift), which were assumed to be reversible under experimental conditions. It turned out that on TC the gasification of the CD occurs in the reverse reaction of methane cracking, and on the MC—in the reactions of gasification by water vapor (mainly) and carbon dioxide. The process on the MC is characterized by irreversible reactions of methane cracking, gasification of the CD with water vapor and carbon dioxide. The reverse water gas shift reaction on the MC remains reversible, and its rate constants of the direct and inverse reactions turned out to be an order of magnitude lower than similar constants on the TC.

The Hodgkin-Huxley-Katz Prize Lecture: A Markov model with permeation-dependent gating that accounts for resurgent current of voltage-gated Na channels.

Many neurons that fire high-frequency action potentials express specialized voltage-gated Na channel complexes that not only conduct transient current upon depolarization, but also pass resurgent current upon repolarization. The resurgent current is associated with recovery of transient current, even at moderately negative potentials where fast inactivation is usually absorbing. The combined results of many experimental studies have led to the hypothesis that resurgent current flows upon repolarization when an endogenous blocking protein that occludes open channels at depolarized potentials is expelled by inwardly permeating Na ions. Additional observations have suggested that the position of the voltage sensor of domain IV regulates the affinity of the channel for the putative blocker. To test the effectiveness of a kinetic scheme incorporating these features, here we develop and justify a Markov model with states grounded in known Na channel conformations. Simulations were designed to investigate whether including a permeation-dependent unblocking rate constant and two open-blocked states, superimposed on conformations and voltage-sensitive movements present in all voltage-gated Na channels, is sufficient to account for the unusual gating of channels with a resurgent component. Optimizing rate constant parameters against a wide range of experimental data from cerebellar Purkinje cells demonstrates that a kinetic scheme for Na channels incorporating the novel aspects of a permeation-dependent unblock,as well as distinct high- and low-affinity blocked states, reproduces all the attributes of experimentally recorded Na currents in a physiologically plausible manner.

Stationary concentration of 1O2 over 1O2 quantum yield: Next level of photoactivity analysis

The investigation of the activity of photosensitizers is a multivariate challenge for their further application in photocatalysis. Porphyrins are attractive photoactive molecules for chemical and biochemical applications, while their observed photocatalytic efficiency is a function of a variety of characteristics. In the present work the influence of porphyrin substituents on their photoactivity and photostability along with the solvent effect were studied and discussed in details. Series of experiments with 9,10-diphenylanthracene (DPA) as a selective 1O2 trap allowed to develop the approach for direct measurement of [1O2] and quantitatively evaluate the influence of various factors onto it. The processes of 1O2 relaxation, as well as its chemical and physical quenching were separated and analyzed individually. A kinetic scheme for the photosensitization process was treated with steady-state approximation to develop a mathematical apparatus, where a non-linear dependence between photosensitizer and singlet oxygen concentration was found. The general character of the derived non-linear dependence was proved experimentally for each studied porphyrin. Physical and chemical quenching rate constants of the studied porphyrins were calculated. DFT calculations allowed to find an empirical correlation between HOMO energies and observed rate constants. A plausible reaction mechanism between TPP and 1O2 was examined with Fast-NEB-TS method.

Fast functional mapping of ligand-gated ion channels

Ligand-gated ion channels are formed by three to five subunits that control the opening of the pore in a cooperative fashion. We developed a microfluidic chip-based technique for studying ion currents and fluorescence signals in either excised membrane patches or whole cells to measure activation and deactivation kinetics of the channels as well as ligand binding and unbinding when using confocal patch-clamp fluorometry. We show how this approach produces in a few seconds either unidirectional concentration-activation relationships at or near equilibrium and, moreover, respective time courses of activation and deactivation for a large number of freely designed steps of the ligand concentration. The short measuring period strongly minimizes the contribution of disturbing superimposing effects such as run-down phenomena and desensitization effects. To validate gating mechanisms, complex kinetic schemes are quantified without the requirement to have data at equilibrium. The new method has potential for functionally analyzing any ligand-gated ion channel and, beyond, also for other receptors.