Stoichiometric Regime Research Articles

Numerical Study of Turbulent Swirling Diffusion Flame Under Lean and Rich Conditions Using Turbulence Realizable k-epsilon Model

ABSTRACT This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio.

Multi-active Site Dynamics on a Molecular Cr/Co/Se Cluster Catalyst.

This study uncovers the interconnected reactivity of the three catalytically active sites of an atomically precise nanocluster Cr3(py)3Co6Se8L6 (1(py)3, L = Ph2PNTol-, Ph = phenyl, Tol = 4-tolyl). Catalytic and stoichiometric studies into tosyl azide activation and carbodiimide formation enabled the isolation and crystallographic characterization of key catalytically competent metal-imido intermediates, including the tris(imido) cluster 1(NTs)3, the catalytic resting state 1(NTs)3(CNtBu)3, and the site-differentiated mono(imido) cluster 1(NTs)(CNtBu)2. In the stoichiometric regime, nitrene transfer proceeds via a stepwise mechanism, with the three active sites engaging sequentially to produce carbodiimide. Moreover, the chemical state of neighboring active sites was found to regulate the affinity for substrates of an individual Cr-imido edge site, as revealed by comparative structural analysis and CNtBu binding studies.

Impact of the precursor gas ratio on dispersion engineering of broadband silicon nitride microresonator frequency combs

Microresonator frequency combs, or microcombs, have gained wide appeal for their rich nonlinear physics and wide range of applications. Stoichiometric silicon nitride films grown via low-pressure chemical vapor deposition (LPCVD), in particular, are widely used in chip-integrated Kerr microcombs. Critical to such devices is the ability to control the microresonator dispersion, which has contributions from both material refractive index dispersion and geometric confinement. Here, we show that modifications to the ratio of the gaseous precursors in LPCVD growth have a significant impact on material dispersion and hence the overall microresonator dispersion. In contrast to the many efforts focused on comparisons between Si-rich films and stoichiometric (Si3N4) films, here, we focus on films whose precursor gas ratios should nominally place them in the stoichiometric regime. We further show that microresonator geometric dispersion can be tuned to compensate for changes in the material dispersion.

Decoupling between plant growth and functional traits of the free-floating fern Salvinia natans under shifted water nutrient stoichiometric regimes

Eutrophication is a prominent component of global environmental change in aquatic ecosystems and usually triggers the aggressive growth of free-floating aquatic plants. An overload of nutrients, including nitrogen (N) and phosphorus (P), causes eutrophication. However, the effects of water nutrient stoichiometry on aquatic plants under eutrophic conditions have been understudied. The present study subjected Salvinia natans (L.) All., a free-floating fern, to shifted water nutrient stoichiometric regimes, and indicators for plant growth, including biomass accumulation and relative growth rate, and resource use functional traits, including specific leaf area, leaf area ratio, specific length of root-like leaves, specific area of root-like leaves, chlorophyll content and maximal fluorescence, were evaluated. We found that water nutrient stoichiometry affected plant growth traits rather than functional traits. In particular, P eutrophication profoundly facilitated the growth of S. natans. Therefore, P rather than N appeared to determine the growth of S. natans. However, the functional traits were unresponsive to the shift in the water nutrient stoichiometric regime. A decoupling between plant growth and functional traits in response to water nutrient stoichiometric change existed for S. natans generally. Since S. natans infests rice paddy agriculture in southern China, the regulation of P addition, for instance, curtailing the use of P-enriched pesticides and fertilizers in agriculture, may benefit the control of this nuisance species.

Hydroarylation of olefins catalysed by a dimeric ytterbium(II) alkyl

Although the nucleophilic alkylation of aromatics has recently been achieved with a variety of potent main group reagents, all of this reactivity is limited to a stoichiometric regime. We now report that the ytterbium(II) hydride, [BDIDippYbH]2 (BDIDipp = CH[C(CH3)NDipp]2, Dipp = 2,6-diisopropylphenyl), reacts with ethene and propene to provide the ytterbium(II) n-alkyls, [BDIDippYbR]2 (R = Et or Pr), both of which alkylate benzene at room temperature. Density functional theory (DFT) calculations indicate that this latter process operates through the nucleophilic (SN2) displacement of hydride, while the resultant regeneration of [BDIDippYbH]2 facilitates further reaction with ethene or propene and enables the direct catalytic (anti-Markovnikov) hydroarylation of both alkenes with a benzene C-H bond.

The strengths and limitations of using biolayer interferometry to monitor equilibrium titrations of biomolecules.

Every method used to quantify biomolecular interactions has its own strengths and limitations. To quantify protein-DNA binding affinities, nitrocellulose filter binding assays with 32 P-labeled DNA quantify Kd values from 10-12 to 10-8 M but have several technical limitations. Here, we considered the suitability of biolayer interferometry (BLI), which monitors association and dissociation of a soluble macromolecule to an immobilized species; the ratio koff /kon determines Kd . However, for lactose repressor protein (LacI) and an engineered repressor protein ("LLhF") binding immobilized DNA, complicated kinetic curves precluded this analysis. Thus, we determined whether the amplitude of the BLI signal at equilibrium related linearly to the fraction of protein bound to DNA. A key question was the effective concentration of immobilized DNA. Equilibrium titration experiments with DNA concentrations below Kd (equilibrium binding regime) must be analyzed differently than those with DNA near or above Kd (stoichiometric binding regime). For ForteBio streptavidin tips, the most frequent effective DNA concentration was ~2 × 10-9 M. Although variation occurred among different lots of sensor tips, binding events with Kd ≥ 10-8 M should reliably be in the equilibrium binding regime. We also observed effects from multi-valent interactions: Tetrameric LacI bound two immobilized DNAs whereas dimeric LLhF did not. We next used BLI to quantify the amount of inducer sugars required to allosterically diminish protein-DNA binding and to assess the affinity of fructose-1-kinase for the DNA-LLhF complex. Overall, when experimental design corresponded with appropriate data interpretation, BLI was convenient and reliable for monitoring equilibrium titrations and thereby quantifying a variety of binding interactions.

Trait convergence and niche differentiation of two exotic invasive free-floating plant species in China under shifted water nutrient stoichiometric regimes.

The effects of eutrophication on the growth and phenotypic performance of macrophytes have been widely studied. Experimental evidence suggests that an increase in the water nutrient level would promote the performance of several invasive free-floating macrophytes. However, few studies have focused on how a shift in water nutrient (nitrogen and phosphorus) stoichiometric regimes may influence the performance of invasive free-floating macrophytes. In the present study, two exotic invasive plant species, free-floating Eichhornia crassipes and Pistia stratiotes, were subjected to different water nutrient stoichiometric regimes, and their phenotypic performance was studied. We found that the two species converged in several resource use traits and diverged in lateral root length. This implied that their similarities in fitness-correlated traits and their underwater niche differentiation probably contribute to their stable coexistence in the field. Additionally, the eutrophic conditions in the different N:P regimes scarcely altered the performance of both species compared to their performance in the oligotrophic condition. Based on previous studies, we predicted that moderate eutrophication with slight overloading of nitrogen and phosphorus would not improve the performance of several invasive free-floating plants and thus would scarcely alter the invasive status of these species. However, moderate eutrophication may cause other problems, such as the growth of phytoplankton and algae and increased pollution in the water.

Polymer electrolyte membrane fuel cell operating in stoichiometric regime

In this work, we report the existence of a stoichiometric regime where the performances of operating polymer electrolyte membrane (PEM) fuel cells are entirely governed by the mass transport in the cathode channels. An analytical model of the fuel cell stoichiometric regime is derived and evidenced experimentally: from the cell spectral signature at low frequency based on electrochemical impedance spectroscopy, and from the current density distribution measured using a segmented current collector. The existence of such regime provides a simple way to characterize, model and predict PEM fuel cell performances.

Impacts of Dilution on Hydrogen Combustion Characteristics and NOx Emissions

Combustion characteristics at small scales have been studied continuously due to the potential applications in portable power devices. It is known that heat release impacts at small scales result in different flame behavior as compared to conventional scales. The impacts of geometry, stoichiometry, flow rates, wall temperatures, etc., are widely studied in the literature. However, dilution impacts still need to be further studied due to its important role on controlling the flame behavior and subsequent pollutants emissions at these scales. In this work, premixed hydrogen/air combustion is simulated at an axis-symmetric microchannel (with diameter D = 0.8 mm and length L = 10 mm), where detailed chemical kinetics are implemented in simulations (32 species and 173 reactions). The heat transfer on the wall is considered by imposing a hyperbolic temperature profile on the wall, where the wall temperature increases from 300 K at the inlet to 1300 K at the outlet. With this setup, a range of equivalence ratios including a typical fuel-lean regime (ϕ = 0.7), stoichiometric regime (ϕ = 1.0), and two cases at an ultra-rich regime (ϕ = 2.0 and ϕ = 3.0) are investigated. For each equivalence ratio, excess dilution (using N2) is introduced to the mixture, and its impact is compared with other cases. With that, the impacts of dilution variations on the combustion characteristics of premixed hydrogen/air are investigated for different equivalence ratios. More specifically, several incidents such as flame dynamics, flame stabilization, extinctions, and NOx emissions are studied for the aforementioned operating conditions.

Implicit LES study of spark parameters impact on ignition in a temporally evolving mixing layer between H2/N2 mixture and air

The paper presents numerical studies on forced (spark) ignition in a temporally evolving mixing layer between a fuel (mixture of hydrogen and nitrogen) and air. The research was performed in the framework of low Mach number approximation using implicit large eddy simulation (ILES) approach with detailed chemistry for hydrogen combustion (9 species, 19 reactions) for which the filtered source terms were computed directly using filtered quantities. The computations were performed applying a high-order numerical code on a numerical mesh with cell sizes close to the Kolmogorov scale, thus ensuring that the ILES results were credible. The spark was modelled by an energy deposition model and we considered various sizes and energies of the sparks. We analyzed the impact of these parameters on a success of ignition and flame development process. In case of successful ignition events we observed that depending on the spark locations and its characteristic the maximum temperature during the spark deposition (0.5 ms) varied in the range 2000 K4000 K and stabilized around 1800K shortly after the spark has been switched-off. It was found that the location of the spark in the flow region characterized by low turbulence intensity and proximity of the stoichiometric regimes almost always resulted in successful ignition, independently of the spark parameters (the energies 1mJ-4.5 mJ and sizes 1.2mm-3.3 mm). On the other hand, in the regions of high turbulence the maximum energy of the spark had to be large to ignite the flow. From this point of view it was observed that in the cases in which the total amount of energy supplied to the sparks was defined a priori, the sparks which were smaller but more intense were more effective than the sparks larger in size but weaker. We found that development of the flame and its growth after switching-off the spark cannot be directly related to the maximum temperature levels during the ignition. It was shown that in this respect the locations of the sparks relative to vortical structures play much more important role.

Theoretical, numerical, and experimental investigation of pressure rise due to deflagration in confined spaces

Estimating pressure rise due to deflagration in a fully or partially confined space is of practical importance in safety design of a petrochemical plant. Herein, we have developed a new theoretical model to predict the pressure rise due to deflagration in both fully and partially confined spaces. First, the theoretical model was compared and validated against experimental data from the closed-space experiments with hydrogen, methane, propane, and ethane. The theory predicted accurate pressure rises near the stoichiometric regime for all fuel types; outside the stoichiometric regime, especially, for rich mixtures of hydrocarbons with air, the theory over-predicted pressure rise since it does not account for soot formation and the associated energy losses by radiation. Experimental investigation of propane and hydrogen deflagration was conducted in a partially confined space and the theory-based predictions agreed with the data up to 5%. Parametric numerical study was performed to investigate the effect of the initial pressure and temperature of gaseous fuels on pressure rise.

Characteristics of gas phase-controlled flame spread over liquid fuels

Comparative flame spread tests were conducted on the basis of a typical hydrocarbon fuel mixture (No. 3 jet fuel, RP-3) and a typical pure alcohol fuel (n-butanol), with the initial fuel temperature above the flashpoint of each fuel. Flame spread appearances, velocity of flame spread, and controlling mechanism of flame spread were discussed and achieved. Based on the experimental results, a flame spread model as well as a scaling analysis of velocity with non-dimensional temperature was established. Results show that the gas phase-controlled flame spread is separated into super-flash lean regime and super-flash stoichiometric regime at 64.9°C for RP-3 and 46.1°C for n-butanol. A blue precursor flame periodically appears in the super-flash lean regime with the velocity significantly larger than the yellow main flame speed, whereas both flames possess the same flame speed in the super-flash stoichiometric regime. Moreover, the value of the stoichiometric temperature of n-butanol is predicted as approximately 46.7°C based on the liquid evaporation theory, which agrees well with the experimental value of 46.1°C. Scaling analysis of gas phase-controlled flame spread indicates that the variation in velocity of flame spread Vf is directly proportional to 1T01/3(Tb-T0)2.

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (tBuPCP)IrH(Cl) (1; tBuPCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be moderately active in NHMe2·BH3 (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (tBuPCP)IrHCl(η1-BH3·NHMe2) (3) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3·NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of 1) the h...

Filtration combustion of carbon in the presence of endothermic oxidizers

This paper presents a model for the steady-state filtration combustion of carbon mixtures with a solid incombustible material in opposed gas flow containing an endothermic oxidizer such as steam and/or carbon dioxide. A computation scheme for calculating the characteristics of the process (combustion temperature and composition of the products) is proposed for the case where the product composition is determined by the thermodynamic equilibrium in the hightemperature zone. For stoichiometric regimes, finite analytical expressions are obtained that relate the combustion temperature and the composition of the products with the oxidizer gas composition and the proportion of carbon in the carbon/solid inert material mixture. Predictions of the model are in qualitative agreement with published experimental data.

Model for filtration combustion of carbon: Approximation of a thermodynamically equilibrium composition of combustion products

A model for stationary filtration combustion of mixtures of carbon with a solid incombustible material in a counterflow a gas containing oxygen and a chemically inert gas is proposed. If the rates of chemical reactions in the combustion zone are sufficiently high, the ratio of carbon monoxide and dioxide in combustion products is assumed to be thermodynamically equilibrium at the combustion temperature. This assumption provides a closure for the balance equations for carbon, oxygen, and energy in the combustion front and gives explicit analytical expressions for the combustion temperature and composition of the products as functions of the composition of the carbon/inert mixture and of the gaseous oxidizer in stoichiometric regimes. Combustion modes are numerically calculated for a wide variation of the compositions. The upper limit of the reagent supply rate until which equilibrium relations can be used is found. A comparison of model predictions with experimental results shows their reasonable qualitative agreement.

Anti-flooding of polymer electrolyte membrane fuel cell with in-plate adverse-flow flow-field

The stoichiometric ratios and related regimes, which can promote anti-flooding of polymer electrolyte membrane fuel cell (PEMFC) with in-plate adverse-flow flow-field (IPAF), were investigated. Two flow combinations, which are the simple and complex adverse-flow between plates (ABP) that can be realized by IPAF, were employed. Constant stoichiometric ratios examination indicates that the complex ABP could improve anti-flooding of PEMFC better in the medium (greater than 200 mA/cm2 and less than 1 000 mA/cm2) and high (greater than 1 000 mA/cm2) current densities than the simple ABP. More stoichiometric ratios were introduced to find the cathode critical stoichiometry. Under the condition of cathode critical stoichiometry, the maximal local relative humidity of both electrodes of complex ABP is equal to 100% and below while the anti-flooding of the cathode of simple ABP is not satisfactory in the medium and high current densities. Further study shows that the mechanism of fuel cell, which is the interdependence between the electrodes effect, can make significant contribution to anti-flooding.

Buoyancy-driven convection may switch between reactive states in three-dimensional chemical waves

Traveling waves in an extended reactor, whose width cannot be neglected, represent a three-dimensional (3D) reaction-diffusion-convection system. We investigate the effects of buoyancy-driven convection in such a setting. The 3D waves traveled through horizontal layers of the iodate-arsenous acid (IAA) reaction solution containing excess of arsenous acid. The depth of the reaction solution was the examined parameter. An increase in the intensity of buoyancy-driven flow caused an increase of the traveling wave velocities. Convection distorted the front of the chemical waves. For layers deeper than h>13 mm, heat release became smaller than heat production causing the emergence of Rayleigh-Bénard convection cells. At the interface, a dependency of wave shape on solution depth was observed. For h<7 mm, the waves adopted a stable V-like shape, while for h>13 mm a parabolic shape dominated. For 7<h<13 mm, both shapes were realized with the same probability. Finally, an intermittent switch between stoichiometric regimes is observed as an unexpected effect of the buoyancy-driven convection. The switch is expressed by iodine enrichment in the product. Hence, the experiments demonstrate that the buoyancy-driven convective flow can cause long-lived, but nevertheless transient, changes in the chemical composition by inducing a local transition between different regimes of the IAA reaction.

BiotecVisions 2012, February

BiotecVisions 2012, February

Ge–Si–O phase separation and Ge nanocrystal growth inGe:SiOx/SiO2 multilayers—a new dc magnetron approach

Ge:SiOx/SiO2 multilayers are fabricated using a new reactive dc magnetron sputtering approach. Theinfluence of the multilayer stoichiometry on the ternary Ge–Si–O phase separation and thesubsequent size-controlled Ge nanocrystal formation is explored by means ofx-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Ramanspectroscopy. The ternary system Ge–Si–O reveals complete Ge–O phase separation at400 °C which does not differ significantly to the binary Ge–O system. Ge nanocrystals of < 5 nm size are generated after subsequent annealing below700 °C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by asurrounding unsaturated silica matrix. A stoichiometric regime was found where almost noGeO2 is present after annealing. Thus, the Ge nanocrystals become completely embedded ina stoichiometric silica matrix favouring the use for photovoltaic applications.

Al-doped SrTiO 3: Part I, anomalous oxygen nonstoichiometry

Oxygen nonstoichiometry ( δ) was measured on the system of SrTi 1− x Al x O (3− x/2)− δ ( x = 0.018) as a function of oxygen activity in the range of 10 − 18 ≤ Po 2/atm ≤ 1 at elevated temperatures of 800° to 1000 °C by a coulometric titration technique with a resolution of ∼ 10 ppm. It has been found that in the stoichiometric regime ( δ ≈ 0) where oxygen vacancy concentration is essentially fixed by the majority dopant concentration (2[V O ] ≈ [Al Ti′]), the isothermal variation of nonstoichiometry deviates anomalously (apparently with three inflections) from the usual, sinehyperbolic trend with a single inflection. This is interpreted as being due to a valence change of variable–valent, background impurities or the like in the minority, and their amount (∼ 80 ppm) estimated from the deviation. Subsequently, the related defect chemical parameters, including the equilibrium constants for the ionization of the unknown variable–valent entities and the hole trapping by the majority dopants Al Ti ′, are evaluated.