Reactant Density Research Articles

Effects of micrometer-scale cavities on the shock-to-detonation transition in a heterogeneous LX-17 energetic material

Cavities and other fracture structures within energetic materials may have significant impact on their performance. The mechanism on how hot spots induced by cavity collapse affect the detonation initiation process is still not fully understood. In this work, two-dimensional simulations are conducted for heterogeneous LX-17 energetic material containing array-distributed cavities to investigate the detonation initiation process induced by the impaction of the incident shock wave (ISW), and the impacts of cavity size and volume fraction on the shock-to-detonation transition (SDT) are also evaluated. First, we fix the cavity radius to be 40 μm and the cavity volume fraction to be 12.57%, and compare the detonation initiation processes for neat and heterogeneous LX-17 energetic materials. The results indicate that cavities within LX-17 can accelerate the detonation initiation, i.e., shortening the initiation distance and time. Then, the flow characteristics and incident shock wave evolutions during the cavity collapse process are analyzed. The results show that the interaction between the cavity and the incident shock wave results in the local hot spots and causes LX-17 reactant to auto-ignite, so as to accelerate the shock-to-detonation transition. Finally, the influence of the cavity size and volume fraction on the detonation initiation process is assessed. It is found that as the cavity volume fraction increases, the detonation initiation distance and time increase and even become larger than the results predicted of the neat case, i.e., the acceleration effect of cavities on the detonation initiation weakens and the cavities even inhibits the shock-to-detonation transition. When the cavity volume fraction is fixed, it is found cases of small-size cavity predict longer initiation distance and time than cases of large-size cavity. The analysis indicates that increasing cavity volume fraction corresponds to smaller density of LX-17 reactant, and the hot spot duration time is shorter for cases of small-size cavity than cases of large-size cavity. Therefore, the detonation initiation distance and time increase as the cavity volume fraction increases and the cavity size decreases.

Transient behavior of liquid water distribution in a lung-inspired PEM fuel cell

Water management is a crucial aspect of hydrogen fuel cell operation because inadequate management can lead to significant losses in mass transport, limitations in oxygen diffusion, and membrane durability issues. This study investigates the impact of various operational conditions on the initial formation and evolution of liquid water content and distribution, as well as water evacuation, within a lung-inspired PEM fuel cell with a 50 cm2 active area. A series of experiments were conducted to assess the effects of cell pressure, relative humidity of the reactant (anode and cathode), temperature, and cell current density. Neutron imaging was utilized as it has been shown to be an effective technique for quantitative analysis of water distributions. The results indicate that water initially appears in the sponges located in the central region of the cathode, but due to considerable back-diffusion, water predominantly accumulates in the area of the anode channels in contact with the cathode sponges. The amount of water in the cell increases faster when the relative humidity of the cathode and anode is higher, and high pressure and low temperature also favours water accumulation. After the experiments, the design of the cathode sponge facilitates faster water evacuation compared to the anode side. Overall, this study offers valuable insights into the behaviour of water in a hydrogen fuel cell and the factors influencing its distribution.

Hydrolysis of MgH2 enhanced by acetic acid for stable hydrogen generation

Magnesium hydride (MgH2) is known for its high hydrogen yield, reaching up to 1703 mL/g through hydrolysis. However, the formation of a passive Mg(OH)2 layer impedes the reaction. To address this, acetic acid was used to enhance the hydrolysis. The experimental system achieved a nearly 98% hydrogen yield by injecting acetic acid, and a possible mechanism involving the neutralization reaction with the passivation layer was proposed. By optimizing the molar ratios of CH3COOH to MgH2 and H2O to MgH2, the hydrogen storage density of the hydrolysis reactants reached 1.43 wt%. Using these optimized parameters, a hydrogen generation setup was designed, achieving an average rate of 8.4 L/min. Controlled addition of acetic acid enabled continuous and regulated hydrogen production. This study offers a novel strategy for designing hydrogen supply systems.

Flow field configuration design for a large-scale hydrogen polymer electrolyte membrane fuel cell

To maximize the power performance of a 108 cm2 hydrogen polymer electrolyte membrane fuel cell (PEMFC), a newly modified parallel flow field configuration (MPFFC) is presented. The new MPFFC has double inlets and outlets as well as a partially boarded flow section downstream of the channel. The various structural parameters for the new MPFFC with respect to the arrangement of inlet and outlet positions, the number of branching channels, and the width of the boarded flow section are optimized by a 3D multiphase physical PEMFC model. The local physical parameter distributions within the PEMFC are investigated and evaluated by a homogeneity index. Moreover, an entropy weighting method is employed to estimate the overall PEMFC performance of various MPFFCs. Results demonstrate that the new MPFFC has superior power performance, uniformity of parameter distribution and decreased pressure loss along the channel compared to a conventional parallel (CP) FFC. Specifically, compared with a CPFFC, the new optimal MPFFC increases the maximum power density, uniformity of reactant and current density distribution by 4.4%, 8.5% and 31.8%, respectively, while reducing pressure drop by 73.6%. Overall, the optimal MPFFC improves the comprehensive evaluation index for overall PEMFC performance from 0.0095 to 0.1671 compared with a CPFFC.

Efficient olefin epoxidation with high turnover rates over ordered mesoporous silica nanoparticles with uniform manganese sites

Olefin epoxidation has been studied over a series of mesoporous manganese silicate nanoparticles and tBuOOH. Partial substitution of Si by Mn in silica framework leads to highly dispersed, distorted Mn(III) sites, confirmed by various physicochemical analyses. High conversion and stable styrene oxide, stilbene oxide and epoxy cyclohexane yield have been achieved with MS75 with 1:1.5 ratio of olefin: oxidant. However, at faster reaction rates lower styrene oxide yield is observed at lower Mn-containing catalyst (MS100). High TON, TOF over manganese sites shows accessible monomeric Mn sites. Attributed to high reactant density at redox-active Mn silicate sites due to facile diffusion across shallow pore depth channels of nanoparticulate om-MSx and surface hydrophobic characteristics. The catalysts were found to be stable under recycling and filtration experiments. The excellent stability against leaching of active manganese species indicates stable sites and good textural properties in om-MSx, essential for extended activity.

Investigation of spatial distribution of EEPFs and neutral species in nitrogen inductively coupled plasmas by 2D hybrid simulation

Neutral species in nitrogen plasmas play a crucial role in many applications related to semiconductor fabrication. In this research, a two-dimensional fluid/electron Monte Carlo hybrid model is employed to simulate nitrogen inductively coupled plasmas, and the spatial distributions of electron energy probability distributions (EEPFs), as well as their influence on the neutral species, are discussed under various pressures. It is found that the EEPF in the bulk region is relatively uniform, and it exhibits a bi-Maxwellian distribution at 3 mTorr. As pressure increases, the high energy tail declines due to the more frequent collisions. Moreover, a hole appears at around 3 eV in the EEPF above the substrate, and it becomes less obvious toward the skin layer below the dielectric window. Moreover, the maxima of metastable species densities, i.e., N2(A3Σu+), N(2D), and N(2P), are located at the center of the chamber at low pressure, and they gradually move to the skin layer under the coils as pressure increases. The behaviors of neutral species can be understood by examining the reactant densities of the main generation and loss mechanisms, as well as the corresponding rate coefficients which are calculated according to EEPFs. In addition, since the ground state N(4S) is mainly produced by the quenching of metastable atoms and neutralization of ions at the walls, the maximum of the N(4S) density appears below the dielectric window and above the substrate at 3 mTorr, and the peak under the dielectric window becomes more obvious at higher pressure due to the stronger locality.

Numerical Investigation of Tapered Flow Field Configurations for Enhanced Polymer Electrolyte Membrane Fuel Cell Performance

A tapered flow field configuration (FFC) is proposed to improve the oxygen transport, water removal and performance of polymer electrolyte membrane fuel cells. By performing a three-dimensional multiphase fuel cell model, the influence of the tapered FFC on the internal physicochemical process and overall cell performance are numerically investigated in this study. The tapered FFC without and with consideration of the electric contact resistance (ECR) between bipolar plates and gas diffusion layers are comparatively evaluated. Compared with the conventional FFC, for the tapered FFC without considering the ECR, an increased ratio of the side length for the inlet to that for the outlet (LI/O) enhances oxygen transport, water removal and cell performance. However, for tapered FFCs considering the ECR, an increased ratio of LI/O initially increases and then weakens oxygen transport, water removal and cell performance. Nevertheless, regardless of the tapered FFC with or without considering the ECR, a decreased ratio of LI/O degrades the overall cell performance. In comparison with all tapered FFC designs, the optimal tapered FFC design with an LI/O of 1.2 exhibits a more uniform reactant and current density distributions, which reduces the coefficient of variation of the current density and oxygen molar concentration by approximately 21.4% and 8.5%, thereby improving overall cell performance.

Hydrodynamically enhanced electrochemical mass transfer on the surface of an electrically conductive droplet

A fully coupled model is proposed to investigate the influence of flow on electrochemical mass transfer at the interface between the electrolyte and an electrically conductive droplet. The electric current flows through the droplet, and consequently the droplet acts as both anode and cathode. Computations of flow, concentration of reactant, and electric current density fields were carried out. Various droplet sizes (0.5, 2, 4 mm) under different flow regimes considering Reynolds number (Re = 0.2, 2, 20, 40 and 80) were investigated. An iterative numerical method is proposed to determine the concentration of reactant and electric current density at droplet-electrolyte interface considering the reaction kinetics (Butler-Volmer) formula and the diffusion-advection of the reactant through the hydrodynamic boundary layer around the droplet. With the increase of Reynolds number, the amount of electric current density which flows through the droplet increases. It is found that the mass transfer at droplet-electrolyte interface is controlled by reaction kinetics for the small droplet (0.5 mm). However, the diffusion of the reactant governs the efficiency of mass transfer with the increase of the droplet size (2 and 4 mm). With the increase of Reynolds number, the anodic area on the surface of droplet is enlarged.

Rational strain engineering of single-atom ruthenium on nanoporous MoS2 for highly efficient hydrogen evolution

Maximizing the catalytic activity of single-atom catalysts is vital for the application of single-atom catalysts in industrial water-alkali electrolyzers, yet the modulation of the catalytic properties of single-atom catalysts remains challenging. Here, we construct strain-tunable sulphur vacancies around single-atom Ru sites for accelerating the alkaline hydrogen evolution reaction of single-atom Ru sites based on a nanoporous MoS2-based Ru single-atom catalyst. By altering the strain of this system, the synergistic effect between sulphur vacancies and Ru sites is amplified, thus changing the catalytic behavior of active sites, namely, the increased reactant density in strained sulphur vacancies and the accelerated hydrogen evolution reaction process on Ru sites. The resulting catalyst delivers an overpotential of 30 mV at a current density of 10 mA cm−2, a Tafel slope of 31 mV dec−1, and a long catalytic lifetime. This work provides an effective strategy to improve the activities of single-atom modified transition metal dichalcogenides catalysts by precise strain engineering.

Metabolic activity of an anammox population affected by major environmental factors

Anammox is known as a cost-effective approach to nitrogen removal from ammonium-rich wastewater. However, the eco-physiological characteristics of anammox population have not well been understood so far, which has obstructed the practical application of anammox process. In the present research work, an enrichment culture of anammox population was collected from an expanded granular sludge bed (EGSB) reactor, and the anammox activity was evaluated by various pH, temperature and reactant density, progressively. The results showed that the optimal ecological amplitude of pH and temperature for the anammox population was ranged from 7.0-7.5 and 30°C-35°C, respectively. To make anammox process more efficient, the density of NH4+-N and NO2−-N should be around 10.0 and 4.0 mmol·L−1, respectively. It was found that anammox population was more sensitive to NO2−-N density than NH4+-N density, and then NO2−-N density was proposed as a key indicator to practicing anammox process in engineering. With the findings, the present research will be significant in practicing annamox process.

Prediction of mean radical concentrations in lean hydrogen-air turbulent flames at different Karlovitz numbers adopting a newly extended flamelet-based presumed PDF

A recent analysis (Lipatnikov et al., 2020) of complex-chemistry direct numerical simulation (DNS) data obtained from lean hydrogen-air flames associated with corrugated-flame (case A), thin-reaction-zone (case B), and broken-reaction-zone (case C) regimes of turbulent burning has shown that the flamelet concept (i) can predict mean concentrations of various species in those flames if the probability density function (PDF) for the fuel-based combustion progress variable c is extracted from the DNS data, but (ii) poorly performs for the mean rate W¯c of product creation. These results suggest applying the concept to evaluation of mean species concentration (but not the mean rate) in combination with another closure relation for W¯c whose predictive capabilities are better. This proposal is developed in the present paper whose focus is placed on studying a new flamelet-based presumed PDF P(c) for predictions of mean concentration of radicals in engineering computational fluid dynamics (CFD) applications. Analysis of the DNS data shows that (i) the flamelet PDF performs well at intermediate values of c in cases A and B, but should be truncated at small and large c, (ii) modeling P(c) in the radical recombination zone (i.e., at large c) is of importance for predicting mean concentrations of H,O, and OH. Accordingly, the flamelet PDF is truncated and combined with a uniform P(c) at large c. Moreover, the mean rate W¯c extracted from the DNS data is used to calibrate the PDF (the rate is considered to be given by another model). Assessment of the approach against the DNS data shows that it well predicts mean density, temperature, and concentrations of reactants, product, and the aforementioned radicals in cases A and B. In case C, the approach performs worse for OandOH at large c¯ and moderately underestimates the mean concentration of H in the entire flame brush.

Mathematical Analysis and Simulation of Ahrens Model in Pulse Combustor

Ahrens model accepted large data of Kilicarslan but with some real differences. In Kilicarslan model, released heat in combustion having more I2R that is released under Ahrens. As per Ahrens if any specific object divides reaction and cool zone then heat release / cycle is very less compare with Kilicarslan. But with one condition of one object that is, it should be pressure independent otherwise density of reactants remains constant (or not enough increased); as a result, more heat released with less satisfied output. Mathematical model of Ahrens brief about object zones and its separation. This paper also brief about dynamic work function of tailpipe with adjustment of frequency which will proceed for system stability and its accuracy.

A Diffusion Based Embedding of the Stochastic Simulation Algorithm in Continuous Space

A variety of simulation methodologies have been used for modeling reaction-diffusion dynamics -- including approaches based on Differential Equations (DE), the Stochastic Simulation Algorithm (SSA), Brownian Dynamics (BD), Green's Function Reaction Dynamics (GFRD), and variations thereon -- each offering trade-offs with respect to the ranges of phenomena they can model, their computational tractability, and the difficulty of fitting them to experimental measurements. Here, we develop a multiscale approach combining efficient SSA-like sampling suitable for well-mixed systems with aspects of the slower but space-aware GFRD model, assuming as with GFRD that reactions occur in a spatially heterogeneous environment that must be explicitly modeled. Our method extends the SSA approach in two major ways. First, we sample bimolecular association reactions following diffusive motion with a time-dependent reaction propensity. Second, reaction locations are sampled from within overlapping diffusion spheres describing the spatial probability densities of individual reactants. We show the approach to provide efficient simulation of spatially heterogeneous biochemistry in comparison to alternative methods via application to a Michaelis-Menten model.

Scalar characterisations of three-dimensional shock-flame interactions: similarity and inhomogeneity

ABSTRACTThree-dimensional shock-flame interactions are simulated by the ninth-order WENO scheme to study the characterisations of scalar under Richtmyer-Meshkov instability (RMI) with reaction. Initially, a planar incident shock with Ma = 1.7 accelerates the single-mode diffusive flame interface with initial wavelength 4 mm and amplitude 0.4 mm from the reactant C2H4+3O2+4N2 medium with temperature 293 K and pressure 13.3 kPa; then the transmitting shock branch during incident shock-flame interaction is reflected at the end boundary to form a reflected shock which disturbs the flame interface again. The reflected shock-flame interaction will form transmitting and reflecting shock branches where the reflecting branch reflects at the end boundary and accelerates the flame interface again. During successive interactions between flame interface and shocks, a large range of flow is generated. Physical processes in the flame interface are examined related to the evolution of reactant density and internal energy density, including convection, molecule diffusion, reaction, pressure-dilatation, and heat conduction. Similarity in scalars and velocity is examined in both spatial and spectral spaces and inhomogeneity related to similarity is provided.

(Invited) High-Temperature Mixed Conducting Membranes and Reactors for Direct CO2 Capture and Conversion

Direct CO2 capture and conversion have been an active research area in recent years due to its importance in addressing the current global warming/climate change and future depletion of fossil fuels and creating a carbon neutral ecosystem. The benchmark technology for CO2 capture is the post-combustion amine absorption, which is energy intensive and cost prohibitive to large-scale commercial implementation. In comparison, the CO2 conversion technologies are still at their infancy with many technical challenges in efficiency and selectivity to be overcome, but primarily being explored in low-temperature solution-based and high-temperature solid-oxide-based electrochemical cells with solar or electrical energy as the input. Here we present a new type of high-temperature membrane reactors operated on mixed conducting chemistry for direct CO2 separation and instant conversion. In the presentation, we will first discuss the chemistries of three basic types of mixed ionic/electronic conductors designed for CO2 separation and conversion. We will then present materials and synthesis of the membranes, followed by testing the performance of membrane reactors with mock-up flue gas as the feeding gas and methane/ethane as the sweeping gases. The performances of oxidative conversions of methane and ethane are characterized by conversion rate of the reactants, selectivity and yield of the products and flux density of both reactant and product species. Finally, the fundamental separation and conversion mechanisms will be discussed in detail.

Effect of Cathode Catalyst Layer Structure on the Performance of PEFC

One of the routes to promote PEFC commercialization is to reduce its manufacture cost. Platinum represents more than 50% of PEFC cost. Therefore, two targets can be considered; reduction of platinum loading (g/cm2) at cathode, and increase specific catalytic activity towards reaction (mA/cm2 pt). On the other hand, the effectiveness factor has an inherent relation with current density and mass transfer of reactants (oxygen and proton), which are existing simultaneously. For investigating cathode behavior, two dimensionless moduli, representing diffusion limitations due to oxygen and proton mass transfer resistances, have been proposed 1. To estimate mass transport resistances, effectiveness factor and intrinsic reaction rate constant, experiments were carried out by varying cathode catalyst layer (CCL) structure on a membrane electrode assembly (MEA). The MEA, fabricated by decal transfer method, consisted of a Nafion membrane (NR-212, 50 μm) with Pt/C weight ratio of 0.3, 0.4 and 0.5 in 10 μm CCL. The ionomer (I)/C ratio was 1. The CCL was prepared by two methods. The first, Pt particles were supported on carbon (Ketjen black) relevant to the required Pt/C ratio (Standard catalyst preparation). The second, lower Pt/C ratios were manipulated by adding carbon black (CB) to a prepared catalyst mixture contained 50% Pt/C where Pt loading was 0.3 mg/cm2 (CB dilution method). The IR corrected voltage (electromotive force) was calculated by measuring the polarization curves and membrane resistance using an electrochemical measurement system. The impedance spectra were measured in a range of frequency from 10 mHz to 100 kHz. The analysis of the results using our dimensionless moduli approach may support having deeper insight that could accelerate the development of PEFC. Reference: M. Kawase et al., AIChE J., 63, 249–256 (2017) http://doi.wiley.com/10.1002/aic.15545. Figure 1

Active diffusion limited reactions.

We investigate the one- and two-dimensional diffusion limited reactions A + A → 0 and A + B → 0 with A active Janus particles and B passive particles in thermal equilibrium. We show that by increasing the self-propulsion time of the A particles, the reactant densities decay faster, at least for time transients of potential interest for chemical applications, e.g., to develop smart drug delivery protocols. Asymptotic and transient density decays obey power laws with exponents that depend on the actual annihilation reaction and its dimensionality.

Microwave assisted biodiesel production using a novel Brønsted acid catalyst based on nanomagnetic biocomposite

In the present study, a novel magnetic sulfonated solid acid catalyst was synthesized using cassava peel, a biomass waste, by incorporating Fe3O4 nanoparticles. The cassava peel biochar was prepared by slow pyrolysis which was further sulfonated with concentrated H2SO4. The magnetic Fe3O4 nanoparticles synthesized by co-precipitation method was incorporated in the sulfonated biochar by ultrasonication to obtain a magnetized sulfonated biochar catalyst. Fe3O4 nanoparticles give excellent stability as the acid sites on the shell structure are easily accessible to reactant sites and also aids in the separation of the catalyst after the transesterification process. The characterization results confirmed that magnetized sulfonated biochar catalyst contains three functional acidic sites: weak (OH) and strong (COOH and SO3H) acidic groups. The catalytic activity and reusability of the magnetic biocatalyst were investigated by microwave assisted biodiesel production from Millettia pinnata seed oil. The biodiesel yield of 98.7% was obtained under the optimum parametric conditions. The enhanced stability and activity of the magnetic catalyst are due to the strong ionic interactions between particles, high sulfonic acid group density and higher accessibility of reactants towards acidic sites. The reusability of catalyst was studied by using it for five cycles obtaining a yield of more than 75%. The fuel properties of the synthesized biodiesel were found to meet international biodiesel standards.

Charge Density in Enzyme Active Site as a Descriptor of Electrostatic Preorganization

Large protein macromolecules in enzymatic catalysis have been shown to exert a specific electric field that reduces the reorganization energy upon barrier crossing and thus reduces the reaction free energy barrier. In this work we suggest that the charge density in the active site of an enzyme investigated using formalisms embodied by the quantum theory of atoms in molecules (QTAIM) provides a sensitive and quantum mechanically rigorous probe of electrostatic preorganization. We focus on the active site of ketosteroid isomerase, a well-studied enzyme for which electrostatic preorganization has been modeled theoretically and studied experimentally. We study the charge density in the active site and the reaction mechanism in the presence of small external electric fields of various directions and magnitudes. We show that the geometry of the full charge density is a sensitive reporter on the external field experienced by the active site. Changes are observed in the relative positions of critical points and amount of charge at critical points as a function of the field. At the same time, a subset of these features correlates linearly with the barrier of the first reaction step in catalysis. Small changes in the barrier, within 1-2 kcal/mol, are reflected in the charge density, suggesting the existence of a field - reactant state charge density - reaction barrier correlation. Hence, QTAIM can be used for the analysis of electric field in enzyme active sites, and further investigations and exploitations of the found correlations may prove useful in enzyme design where preorganization is optimized.

Influence of Tight Confinement on Selective Oxidative Dehydrogenation of Ethane on MoVTeNb Mixed Oxides

M1 phase MoVTeNb mixed oxides exhibit unique catalytic properties that lead to high C2H4 yields in oxidative conversion of C2H6 at moderate temperatures. The role of the heptagonal channel micropores of the M1 phase in regulating reactivity and selectivity is assessed here using reactant size-dependent kinetic probes and density functional theory (DFT) treatments for C2H6 and cyclohexane (C6H12) activations inside and outside the micropores. The sizes of C2H6 and the micropores suggest a tight guest–host fit, but C6H12 cannot access intrapore sites. Measured C2H6 to C6H12 activation rate ratios on MoVTeNbO are much higher than those measured on nonmicroporous vanadium oxides (VOx/SiO2) and estimated by DFT on external surfaces, suggesting that most C2H6 activations on MoVTeNbO occur inside the micropores under typical conditions. C2H6 exhibits higher activation energy than C6H12 on VOx/SiO2, consistent with the corresponding C–H bond strengths; the activation energy difference between C2H6 and C6H12 is lo...