Detailed Gas-phase Research Articles

Thermodynamics second-law analysis of hydrocarbon diffusion flames: Effects of soot and temperature

In this paper, a computation of the ethylene co-flow diffusion flame is carried out using the Co-flame code with a detailed gas-phase mechanism and soot model. A numerical analysis based on the thermodynamics second-law was conducted for developing a high-efficiency combustion condition of hydrocarbon diffusion flames. The entropy generations and exergy loss ratios due to thermal radiation, heat conduction and convection, and chemical reactions are numerically calculated, in which entropy generations caused by both gasses and soot particle are considered. The effects of soot (controlled by changing stoichiometric mixture fraction Zst from 0.1 to 0.62) and adiabatic flame temperature Tad (2300 K, 2500 K, 2700 K) on entropy generation and exergy loss ratio in flame are investigated. The results suggest that, entropy generation due to soot radiation and soot surface growth and oxidation accounts for about 15% and 5% in total radiation entropy generation and total chemical entropy generation, respectively. Entropy generation due to thermal radiation decreases when soot reduces, and entropy generation due to heat conduction and convection, mass diffusion and chemical reactions increases when Zst increases. When Tad is 2300 K, the change of exergy loss ratio is slight with a reducing of soot and increasing of Zst. When Tad is 2500 K and 2700 K, the exergy loss ratio has tendency of increasing with a reducing of soot and increasing of Zst. The exergy loss ratio increases with increasing of Tad.

Modelling the biomass updraft gasification process using the combination of a pyrolysis kinetic model and a thermodynamic equilibrium model

Conversion of biomass into gas suitable for further exploitation is one of the valuable renewable energy pathways due to the wide distribution and availability of raw materials. Biomass gasification is a thermochemical process of partial combustion in a reduced oxygen environment that aims to produce hydrogen-enriched syngas. Updraft gasifier design, with its advantages of high efficiency, produces syngas with higher hydrogen yield compared to other gasifier designs. The main drawback of the updraft gasifier is high yield of tars in the outflow gas decreasing its lower heating value. Recently, significant research efforts have focused on the optimization of the updraft gasifiers operating conditions, especially by developing numerical models as a complementary approach to experiments. The simplest modelling approach for predicting biomass gasification behaviour is the thermodynamic equilibrium model. When describing the behaviour of an updraft gasifier, special focus needs to be given to the pyrolysis stage, since in this type of reactor pyrolysis products directly outflow from the gasifier. In this work, a pilot-scale biomass gasifier was modelled using a combination of a pyrolysis kinetic model with a thermodynamic equilibrium model. To describe the pyrolysis behaviour, the CRECK-S-BIO and two secondary gas-phase mechanisms with distinct levels of complexity were used. The gasification and oxidation of char were modelled using a thermodynamic equilibrium model through the minimization of Gibbs free energy approach. The predicted results of dry clean syngas were compared to the experimental data considering eleven different operating conditions. The model combination that used the detailed secondary gas-phase mechanism achieved generally lower average prediction errors. Although some discrepancies were observed in the predictions, these preliminary results show that the model approach considered in this study represents a good basis for future development of the model.

Improving Hydrothermal Carbonization (htc) Processes by Hydrochar Gasification

Hydrochar is produced by means of hydrothermal carbonization at relatively low temperatures (180-260°C) in sub-critical water. While, in many respects similar to biochar, its physical and chemical properties differ significantly (Basu, 2018). Of particular interest for this work are the higher energy density and lower ash content, that make hydrochar a possible feedstock for gasification plants with energy generation purposes. A gasification step is used instead of direct combustion to convert solid fuels in energy to achieve cleaner combustion and higher efficiency (Wang and Stiegel, 2016). In this work a 400 kg/h hydrochar gasification plant was modelled to identify optimal conditions and energetic yield of hydrochar obtained from municipal sewage. The fixed bed, updraft, gasification reactor was modeled in detail using a multi-scale, multiphase methodology already widely tested on biomass (Corbetta et al., 2015; Ranzi et al., 2014). The gas solid kinetic model was coupled with a detailed gas-phase kinetic scheme with over 200 species, including reaction intermediates, and 2000 reactions for reliable product yield prediction. Using Visual Basic Application as an interface, the predictions from the detailed simulation package were delivered to a commercial simulation package to model the energy generation section of the plant. Aspen HYSYS V10 was used for this purpose for the simplicity of integration and its widespread use in similar industrial plants. The gasification was carried out with air, air enriched in oxygen to 28%, air enriched in oxygen to 35% and pure oxygen with different amounts of steam to control the temperature in the chamber and at different values of equivalence ratio. The gasification performance was evaluated in terms of lower heating value of the generated fuel gas while the H2S formation was accounted for only in a superficial manner using rules of thumb derived from previous experimental experience.

The Simulation of Different Combustion Stages of Micron-sized Aluminum Particles

In this study, a quasi-steady combustion model of an aluminum particle is established, which is more accurate to simulate the physical combustion process. Detailed gas-phase reaction mechanism and surface reaction mechanism are considered. Moreover, the particle temperature is not constant in this work, which is calculated in different combustion stages. The judgement standard of each combustion stage is from observational data in the experiment and the simulation results of combustion durations of each stage, and distribution of ambient temperature and gas-phase species profiles are in good agreement with experimental results. The calculation results show that in the first stage, burning rate of the particle is the fastest, and in the second stage, particle temperature can drop to more than 100 K below the boiling point for the large particles, which is slightly below the boiling point for small ones. As the combustion stage changes, Da number is going to keep going down, which will lead to the transition of combustion method from diffusion-limited control to kinetic-limited control for an aluminum particle.

Development, simulation, and laboratory test of novel gas-solid-liquid separator for UASB/EGSB reactor of wastewater treatment

For structure simplification and material conservation, three novel candidates of the gas-liquid-solid (GLS) separator were developed for the up-flow reactors, typically UASB and EGSB, used for wastewater treatment. These proposed GLS separation prototypes were designed on a completely different separation mechanism that the European prototype adopted. This mechanism was observed and analyzed both in laboratory experiments and in computational fluid dynamic (CFD)-based 3D simulation. The three new GLS separators could reduce more than 19% in the height of dimensions and 20% in material consumption compared with the classical GLS separator. Nineteen experimental runs with the laboratory-scale fabricated separators, including both purchased granular sludge and locally obtained floccular sludge. suggested that the GLS separation prototypes were as excellent in accomplishing the supposed separation task as the presently widely used traditional counterparts. For further in-depth and detailed internal flow fields and gas phase behavior investigation, 3D CFD simulations were employed. The simulation revealed clearly and accurately the flow fields and retention properties of solid particles and gas bubbles among the layers of the GLS separator’s baffles and was in good agreement with the laboratory experiment. Analysis of Variance (ANOVA) of the simulation results was conducted for stability evaluation and dynamic characteristics analysis, which may be useful in the GLS separator’s routine operation.

Analysing Mouse Skin Cell Behaviour under a Non-Thermal kHz Plasma Jet

Plasma jets are extensively used in biomedical applications, particularly for exploring cell viability behaviour. However, many experimental parameters influence the results, including jet characteristics, secondary liquid chemistry and protocols used, slowing research progress. A specific interest of the presented research was skin cell behaviour under a non-thermal kHz plasma jet—a so-called cold plasma jet—as a topical skin treatment. Our research was focused on in vitro mouse skin cell direct plasma treatment with argon as an operating gas. The research was complemented with detailed gas-phase diagnostics and liquid-phase chemical analysis of the plasma and plasma-treated medium, respectively. The obtained results showed that direct plasma jet treatment was very destructive, leading to low cell viability. Even with short treatment times (from 35 s to 60 s), apoptosis was observed for most L929 murine fibroblasts under approximately the same conditions. This behaviour was attributed to plasma species generated from direct treatment and the types of cell lines used. Importantly, the research exposed important points that should be taken under consideration for all further research in this field: the urgent need to upgrade and standardise existing plasma treatment protocols of cell lines; to monitor gas and liquid chemistries and to standardise plasma discharge parameters.

Parametric study of methane catalytic combustion in a micro-channel reactor: Effects of porous washcoat properties

A washcoat resolved numerical model of catalytic micro combustor fueled by methane is developed. Detailed gas phase mechanism and alumina dioxide supported Pt-based surface mechanism are adopted. This model proves to capture flow, temperature and species fields precisely including impermeable flow and products accumulation in porous washcoat layer. The novelty and superiority of this research compared to traditional modeling method in which instantaneous diffusion onto catalytic wall is assumed lies in that: it is possible to investigate effects of porous washcoat properties with this full-scale numerical model. Computational results show that homogeneous reactions (HR) are impaired and heterogeneous reactions (HTR) are strengthened as channel confinement increases. Da number analysis reveals that the micro combustor works under a mixed controlled mode in which catalytic reaction rate and internal mass diffusion rate are comparable. When washcoat layer thickness increases from 15 μm to 75 μm, overall catalyst usage roughly decreases from 0.34 to 0.05. It is concluded washcoat thickness can’t be too thin in order to maintain HTR intensity and too thick since catalyst usage is extremely low. Porous washcoat properties impact combustion characteristics in a similar mechanism: modifying effective mass diffusion coefficient. In general, increasing porosity, average pore diameter and decreasing tortuosity will enhance HTR and impair HR. In real application, these parameters can be utilized to modify combustion temperature, emission performance and stability operation map.

Systematic evaluation and kinetic modeling of low heating rate sulfur release in various atmospheres

In the present work, a systematic experimental and numerical study of sulfur release from coal in varying atmospheres at low heating rates (LHR) is presented. To this aim, two bituminous coals were investigated, Colombian hard coal (K1) with typical sulfur content, and American high-sulfur coal (U2), with elevated sulfur content. Mass loss and release of target volatile species - H2S, COS, SO2- were tracked using a TG-MS. The samples were heated at 10 Kmin−1under different atmospheres: argon, CO2, synthetic air (21 vol% O2/79 vol% Ar) and oxy-fuel (21 vol% O2/79 vol% CO2). The role of the different atmospheres in the sulfur release was elucidated as well as the fate of the volatile species in the gas-phase. The advantage of investigating the release at LHR is that heat and mass transfer effects can be neglected, as the experimental conditions allow the process to remain in the kinetic regime. The successive increase in atmosphere complexity allowed to individuate the chemical paths leading to the formation of SOXand its precursors in each of the conversion steps: devolatilization, char conversion as well as the coupling to gas-phase reactions. The experiments were further analyzed with a kinetic model for the solid-phase of coal conversion, coupled with a detailed gas-phase kinetic mechanism. The solid-phase kinetic model was modifiedaccounting for the particularities of the fuels, for the effects of oxy-fuel atmosphere. A small number of kinetic parameters was adjusted for improved predictions of the release rate and the yields of sulfur species.

Detailed gas-phase kinetics and reduced reaction mechanism for methane pyrolysis involved in CVD/CVI processes

A detailed gas-phase kinetic model consisting of 37 species and 318 reactions is developed for methane pyrolysis involved in the chemical vapour deposition/infiltration (CVD/CVI) processes. The model was used to perform the simulations of CH4 pyrolysis in a hot-wall CVD reactor over a wide range of operating conditions and validated against the experimental data. The overall trend of the concentration profiles of notable species is in good agreement with experimental results. The sensitivity analysis and reaction path flux diagram revealed that CH4 pyrolysis directly proceeds without any induction period. C2 species are the primary intermediates for the formation of acetylene, ethylene, and benzene. Besides, the accuracy of the present model is better than the models available in the literature. The detailed kinetic model was then systematically reduced to a small mechanism comprising 13 species and 29 reactions. The predictions of reduced mechanism also agreed well with the detailed mechanism at low inlet partial pressure (<10 kPa). Although the error in the mole fraction of light species was minimal, the relatively large error for C6H6 species was observed at high inlet partial pressure of CH4.

Macroscopic Modeling of Liquid Water in Channel of Jari Cell Based on Detailed Two-Phase CFD

In this work, we modeled liquid water behavior in channels based on detailed two-phase CFD for application on Full-Cell scale multiphysics simulation. In our work, in order to execute efficient simulations in full-cell scale, all relevant transport phenomena in PEFC (mass, chemical species, and heat) are considered macroscopic, where the phenomena are coupled with electrochemical reactions. In order to apply this method for conditions in which liquid water is important, two-phase fluid models should be constructed properly.In order to model two-phase flow in channels, we evaluated the relationship between liquid water volume fraction, pressure drop, and liquid water velocity in macroscopic control volume with detailed CFD using OpenFOAM. The calculation target is the JARI cell channel and mesh size is 50 micrometer order. We used the interIsoFoam for the solver, which uses the VOF method for two-phase flow and geometric surface reconstruction method for surface capturing. The relationship is expressed by the table data that shows the relationship between the liquid phase volume fraction and liquid phase velocity or pressure loss multiplier. The table data shows a good representation of the below. Dependency of pressure drop on the water volume fractionDependency of drainage droplet size on the gas flow rateDehydration speed In order to evaluate the obtained models, we calculated PEFC simulations with the models and validated the calculations.In our simulator, in order to estimate the performance of PEFC cells taking all relevant transport phenomena (mass, chemical species, and heat) is solved on the coarse-grained mesh, coupled with electrochemical reactions. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA is also considered. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from the gas phase to reaction sites in catalyst layers, etc. Electrochemical reactions are modeled by the Butler-Volmer equation, in the manner of lumped parameter models.The calculation using the derived two-phase model well reproduce the below phenomena. Various Overpotentials difference between drive conditionsDrainage speed difference depending on gas flow ratesLiquid water difference between under the rib and the channelOxygen depletion depending on stoichiometry and liquid water distribution

Description and evaluation of a detailed gas-phase chemistry scheme in the TM5-MP global chemistry transport model (r112)

Abstract. This work documents and evaluates the tropospheric gas-phase chemical mechanism MOGUNTIA in the three-dimensional chemistry transport model TM5-MP. Compared to the modified CB05 (mCB05) chemical mechanism previously used in the model, MOGUNTIA includes a detailed representation of the light hydrocarbons (C1–C4) and isoprene, along with a simplified chemistry representation of terpenes and aromatics. Another feature implemented in TM5-MP for this work is the use of the Rosenbrock solver in the chemistry code, which can replace the classical Euler backward integration method of the model. Global budgets of ozone (O3), carbon monoxide (CO), hydroxyl radicals (OH), nitrogen oxides (NOx), and volatile organic compounds (VOCs) are analyzed, and their mixing ratios are compared with a series of surface, aircraft, and satellite observations for the year 2006. Both mechanisms appear to be able to satisfactorily represent observed mixing ratios of important trace gases, with the MOGUNTIA chemistry configuration yielding lower biases than mCB05 compared to measurements in most of the cases. However, the two chemical mechanisms fail to reproduce the observed mixing ratios of light VOCs, indicating insufficient primary emission source strengths, oxidation that is too fast, and/or a low bias in the secondary contribution to C2–C3 organics via VOC atmospheric oxidation. Relative computational memory and time requirements of the different model configurations are also compared and discussed. Overall, the MOGUNTIA scheme simulates a large suite of oxygenated VOCs that are observed in the atmosphere at significant levels. This significantly expands the possible applications of TM5-MP.

Effects of C2H2 and C2H4 radiation on soot formation in ethylene/air diffusion flames

The involvement of hydrocarbons such as C2H4 and its combustion intermediate species C2H2 in thermal radiation has not been accounted in the numerical simulations of literature studies, which may in turn cause errors in estimating the soot formation processes. Numerical calculations were conducted using detailed gas-phase chemistry and thermal and transport properties in laminar coflow ethylene/air diffusion flames. The SNBCK model parameters for C2H2 and C2H4 were generated based on HITRAN database. The results show that the position of soot formation is affected by the radiation absorption of C2H4 at low temperatures and the radiation emission of C2H2 at high temperatures. The maximum C2H2/C2H4 radiation effect is 9.46% for air condition case and 9.87% for oxygen-enriched case. The height corresponding to the maximum soot volumetric fraction increases for the air condition while it decreases for the oxygen-enriched condition when the radiation effect is considered. The calculations reproduced well the experimental data of soot volumetric fraction in the literature and the numerical results were improved by 10.4% when considering the C2H2/C2H4 radiation. The results indicate that the radiation heat transfer of C2H2 and C2H4 needs to be taken into account in the numerical modeling of the ethylene/air diffusion flames.

Understanding the dynamic growth environment of silicon dioxide in atmospheric pressure plasma using multiphysics modeling

Atmospheric pressure plasma (APP) is an emerging technology for surface modification and depositing plasma coating without a vacuum chamber in ambient atmosphere to improve corrosion protection and paint adhesion for aerospace and energy applications. Current APP-based platforms lack the ability to control the surface chemistry of the coating for diverse operating environments. Here, we describe a predictive multiphysics model of protective layer deposition using microwave-assisted APP system. We link this model to the detailed gas-phase and surface-plasma chemistry to predict and optimize the growth environment of the oxide film on the substrate. The proposed modeling approach systematically identifies, couples, and characterizes multiple chemical and physical phenomena under a modular framework. This multiphysics model quantitatively correlates the rate of growth of oxide film to the progression of surface chemical reactions during the deposition process, which is very challenging to determine using in situ experiments. The modeling approach also incorporates a modified organosilicon evaporation model to calculate evaporation rates of a precursor, tetraethyl orthosilicate (TEOS). We have assessed the presented model based on the results obtained from experiments. The dependencies of the growth rate of the film on the processing parameters were investigated to understand the factors behind efficient APP deposition. The results show that growth rates strongly depend on gas temperatures over the surface, flow patterns, and torch-substrate distance, and depend less on the angle of attack. Finally, for large area applications, the movement of the torch and the impact on coating uniformity have been modeled using time-dependent multiphysics simulations.

Formation and sink of glyoxal and methylglyoxal in a polluted subtropical environment: observation-based photochemical analysis and impact evaluation

Abstract. The dicarbonyls glyoxal (Gly) and methylglyoxal (Mgly) have been recognized as important precursors of secondary organic aerosols (SOAs) through the atmospheric heterogeneous process. In this study, field measurement was conducted at a receptor site in the Pearl River Delta (PRD) region in southern China, and an observation-based photochemical box model was subsequently applied to investigate the production and evolution of Gly and Mgly as well as their contributions to SOA formation. The model was coupled with a detailed gas-phase oxidation mechanism of volatile organic compounds (VOCs) (i.e., Master Chemical Mechanism, MCM, v3.2), heterogeneous processes of Gly and Mgly (i.e., reversible partitioning in aqueous phase, irreversible volume reactions and irreversible surface uptake processes), and the gas–particle partitioning of oxidation products. The results suggested that without considering the heterogeneous processes of Gly and Mgly on aerosol surfaces, the model would overpredict the mixing ratios of Gly and Mgly by factors of 3.3 and 3.5 compared to the observed levels. The agreement between observation and simulation improved significantly when the irreversible uptake and the reversible partitioning were incorporated into the model, which in total both contributed ∼ 62 % to the destruction of Gly and Mgly during daytime. Further analysis of the photochemical budget of Gly and Mgly showed that the oxidation of aromatics by the OH radical was the major pathway producing Gly and Mgly, followed by degradation of alkynes and alkenes. Furthermore, based on the improved model mechanism, the contributions of VOC oxidation to SOA formed from gas–particle partitioning (SOAgp) and from heterogeneous processes of Gly and Mgly (SOAhet) were also quantified. It was found that o-xylene was the most significant contributor to SOAgp formation (∼ 29 %), while m,p-xylene and toluene made dominant contributions to SOAhet formation. Overall, the heterogeneous processes of Gly and Mgly can explain ∼ 21 % of SOA mass in the PRD region. The results of this study demonstrated the important roles of heterogeneous processes of Gly and Mgly in SOA formation and highlighted the need for a better understanding of the evolution of intermediate oxidation products.

Effects of hydrogen enrichment and water vapour dilution on soot formation in laminar ethylene counterflow flames

Effects of simultaneous, together with separate addition of hydrogen to the fuel and water vapour to the oxidizer on soot formation in laminar, counterflow ethylene diffusion flames at atmospheric pressure were studied numerically. A polycyclic aromatic hydrocarbon (PAH)-based sectional soot model coupled with detailed gas-phase chemistry, was used to investigate the underlying chemical pathways of soot formation. In agreement to available studies, the results showed that, in addition to the dilution effects, the fuel-side enrichment of hydrogen and/or oxidizer side dilution of water vapour suppress the soot formation through chemical effects. The reduction in soot formation through chemical effects of H2 and H2O addition is mainly attributed to the reduced rates of soot surface growth due to suppression of the H-abstraction reaction in the hydrogen-abstraction-C2H2-addition (HACA) sequence as a result of an increased molecular hydrogen concentration. The simulations indicate that the chemical effects tend to increase PAH concentration within the soot formation region, which results in increased soot nucleation rates and number density. However, the contribution of soot nucleation in the overall soot formation process becomes secondary, while the soot surface growth predominantly governs the chemically inhibiting effects of H2 and H2O addition. Compared to separate addition, simultaneous addition of hydrogen to the fuel and water vapour to the oxidizer proves to be more effective in soot suppression for the same dilution level, since there exist weak synergistic effects between H2 and H2O. The numerical analysis further demonstrates that the separate, as well as simultaneous addition of H2 and H2O, tend to decrease the number density of larger-sized particles, causing the reduction in average soot particle size.

Uncertainty Analysis of the Chemical Reactor Network Approach for Predicting the Pollutant Emissions in a Double-Swirl Combustor

Abstract The purpose of this study is to predict the pollutant emissions generated within an aero-engine combustor model using the computational fluid dynamics-chemical reactor network (CFD-CRN) approach by modeling combustion in highly swirled flows. The selected test case is a laboratory double swirled combustor that came with an extensive experimental database from previous works for CH4/air diffusion flames at atmospheric pressure. The CFD-CRN modeling approach is initiated by solving Reynolds-averaged Navier–Stokes (RANS) equations for a 3D computational domain. The numerically achieved time-averaged values of the velocity components are in good agreement with the experimental data for two different thermal power. The CRN is obtained by dividing the flow field into ideal chemical reactors using various filters on the CFD results. The temperature, axial velocity, CH4, and O2 mass fractions distributions are selected as the splitting criteria for constructing the CRN. An uncertainty analysis is carried out to investigate the effects of different splitting approaches for the temperature criteria since it significantly affected the pollutant emissions in the gas turbine combustor. The simulations of the pollutant emissions are performed via the detailed gas-phase chemical kinetic mechanism of GRI-Mech 3.0. The nonlinear distribution of the temperature intervals result in lower uncertainty and provide reliable results even with a small number of ideal reactors. Also, it is observed that the CRN can be used in different operating conditions and provide suitable results if it is constructed with exceptional consideration. Moreover, a parametric study is performed by varying the equivalence ratio and air inlet temperature to investigate the trends of the NO and CO emissions.

Numerical Study of Hydrogen Addition Fuel on Soot Formation in Axisymmetric Laminar Methane/Air Diffusion Flames

This article employs the CoFlame Code to investigate the effects of hydrogen addition to fuel on soot formation characteristics in laminar coflow methane/air diffusion flames at atmospheric pressure. Numerical calculations were carried out using a detailed C1-C2 gas phase reaction mechanism and a soot model consisting of two pyrene molecules colliding into a dimer as soot nucleation, hydrogen abstraction acetylene addition (HACA) and pyrene condensation as surface growth, and soot oxidation by O2, O and OH radicals. Calculations were conducted for five levels of hydrogen addition on volume basis. To quantify the chemical effect of hydrogen, additional calculations are performed for addition of inert pseudo-hydrogen (FH2). The addition of H2 or FH2 does not have a strong influence on flame temperature. The results confirm that hydrogen addition can inhibit soot formation in the methane/air diffusion flame by reducing both the nucleation and surface growth steps of soot formation process. The effect of FH2 addition on soot formation suppression is more remarkable than H2, indicating that the chemical effect of hydrogen added to methane prompts soot formation. The dilution effect of hydrogen addition on soot formation suppression is stronger than its chemical effect on soot formation enhancement the present findings are consistent with those of previous numerical studies.

Mapping Metallicity Variations across Nearby Galaxy Disks

Abstract The distribution of metals within a galaxy traces the baryon cycle and the buildup of galactic disks, but the detailed gas phase metallicity distribution remains poorly sampled. We have determined the gas phase oxygen abundances for 7138 H ii regions across the disks of eight nearby galaxies using Very Large Telescope/Multi Unit Spectroscopic Explorer (MUSE) optical integral field spectroscopy as part of the PHANGS–MUSE survey. After removing the first-order radial gradients present in each galaxy, we look at the statistics of the metallicity offset (ΔO/H) and explore azimuthal variations. Across each galaxy, we find low (σ = 0.03–0.05 dex) scatter at any given radius, indicative of efficient mixing. We compare physical parameters for those H ii regions that are 1σ outliers toward both enhanced and reduced abundances. Regions with enhanced abundances have high ionization parameter, higher Hα luminosity, lower Hα velocity dispersion, younger star clusters, and associated molecular gas clouds showing higher molecular gas densities. This indicates recent star formation has locally enriched the material. Regions with reduced abundances show increased Hα velocity dispersions, suggestive of mixing introducing more pristine material. We observe subtle azimuthal variations in half of the sample, but cannot always cleanly associate this with the spiral pattern. Regions with enhanced and reduced abundances are found distributed throughout the disk, and in half of our galaxies we can identify subsections of spiral arms with clearly associated metallicity gradients. This suggests spiral arms play a role in organizing and mixing the interstellar medium.

On the Use of Fractional-Order Quadrature-Based Moment Closures for Predicting Soot Formation in Laminar Flames

ABSTRACT The accurate numerical prediction of soot formation in practical combustion devices remains a challenge. Several new quadrature-based moment closures based on fractional-order moments of soot particle volume are proposed for the prediction of soot formation in laminar diffusion flames at atmospheric and elevated pressures. Both univariate Quadrature Method of Moments (QMOM) models based on a classical particle volume formulation and bivariate Conditional Quadrature Method of Moments (CQMOM) models based on a new particle volume/primary particle number formulation are proposed. The soot models include detailed gas-phase chemistry along with nucleation, surface growth, oxidation, and coalescence/coagulation soot chemistry source terms. Initial comparisons to predictions of a sectional method for space homogeneous simulations illustrate well the improved predictions of soot number density and volume fraction are provided by the fractional-order moment closures compared to integer-order moment approaches. Furthermore, additional comparisons of soot prediction of methane/ethanol laminar diffusion flames at elevated pressures indicate that the proposed bivariate CQMOM, with a specified soot inception size, offer significantly improved results when compared to the other variants and available experimental data.

Multi-scale modeling of gas-phase reactions in metal-organic chemical vapor deposition growth of WSe2

We present a multi-scale computational approach to model the gas-phase chemical kinetics for Metal-Organic Chemical Vapor Deposition (MOCVD) of WSe2 using W(CO)6 and H2Se as gas-phase precursors. This framework combines Quantum Mechanical (QM) methods based on Density Functional Theory (DFT), ReaxFF-based reactive molecular dynamics, and Computational Fluid Dynamics (CFD) to efficiently model the gas-phase physiochemical processes leading to WSe2 growth in a cold-wall horizontal MOCVD chamber. A detailed gas-phase chemical kinetic reaction model is developed to describe all major chemical reaction pathways from the precursors W(CO)6 and H2Se to the most thermodynamically stable molecules, with quantified kinetic rate constants. First QM calculations are performed to suggest key reaction types and to provide the necessary training set to determine ReaxFF reactive force-field parameters for the W/H/C/O/Se system. Using the developed force field, ReaxFF simulations are performed to identify all major chemical reaction pathways and determine their associated activation energies. Other kinetic parameters, together with the thermal and transport properties of all species involved, are estimated using well-established theories or correlations. This chemical kinetic model with thermal and transport parameters is then integrated into a reacting flow solver for full-scale CFD simulations of the MOCVD chamber under realistic operating conditions, to demonstrate its capabilities in predicting the major processes in the gas phase and qualitatively estimating thin film growth behavior. The predicted gas-phase concentrations of tungsten chalcogenides at the growth substrate correlate well with experimental measurements of average film thickness across the substrate, which suggests that crystal growth may result from surface deposition reactions of these species. This computational framework for the gas-phase chemical kinetics in MOCVD (prior to surface deposition and subsequent crystal growth) can thus test experimental MOCVD conditions, generate insights into more effective growth protocols, shed light on the significance of reactor geometry, and improve the reproducibility of MOCVD results across different experimental growth chambers. The computational framework is also transferable to other CVD chemistries.