Detailed Kinetics Research Articles

On the Effectiveness of Aerosol Extinguishing Agents for Battery Vent Gases and Hydrogen

Abstract It is well known that Lithium-ion batteries in thermal runaway can emit gases with high hydrogen concentrations which, in case of delayed ignition, can cause a gas explosion. Aerosol suppression agents are sometimes used to suppress fires and/or prevent ignition of released gases in such situations. The agent has been studied for suppression of fires (diffusion flames), but less is known about the ability of such aerosols to inert highly reactive gas mixtures. In this paper, a commercially available aerosol suppression agent is mixed with a highly reactive gas mixture representing the gas composition from an NCA battery in thermal runaway as well as with pure hydrogen, and the effect on burning velocity is assessed based on OH-PLIF measurements on a Bunsen-burner type setup. The experiments were complemented by 1D flame simulations using a detailed chemical kinetics scheme including relevant combustible gases and the suppression agent. The results show that, although the agent is highly effective in gaseous form, evaporation of aerosols in the pre-flame-zone is prevented by the lower radiative fraction and results in higher burning velocity of hydrogen-rich mixtures. The local cooling induced by the evaporation of the aerosol in the flame leads to an increased flame area and thereby total burning velocity. This effect is further exaggerated by the preferential diffusion of hydrogen. Therefore, modification of the system is needed before applying for this purpose.

Development of a Genetic Algorithm Tool for the Optimization of the Methanol Oxidation

AbstractThis work presents an automatic optimization tool using the genetic algorithm (GA) for the chemical kinetic model of methanol (CH3OH) oxidation. A total of 54 parameters of 40 important reactions of the reaction model have been optimized. Ignition delay times measured in shock tubes, concentration profiles measured in plug flow reactors, and laminar flame speeds were used for the model validation. Compared to the results of the initial model, the optimized model exhibits a significantly improved predictive capability for the experimental targets. The GA tool developed in this study has been proven effective for optimizing detailed chemical kinetics models.

Comparative study of crystallization kinetics and phase segregation of triple cation and methylammonium lead iodide perovskites on moisture probing using synchrotron X-ray based radiation.

3D mixed perovskites have achieved substantial success in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here we present detailed crystallization kinetics of mixed perovskites FA0.83MA0.17Pb(I0.83Br0.17)3, where FA is formamidinium and MA is methylammonium, with the addition of Cs+ to form a triple cation perovskite (3-CAT), in a comparison with the perovskite building block MAPbI3 (MAPI) via static grazing-incidence wide-angle X-ray scattering (GIWAXS) and micro-diffraction measurements. Spin-coated films produced α-perovskite peaks with no PbI2 or δ-intermediate phases, which was a promising result for the 3-CAT perovskite from micro-diffraction measurements. However, the 3-CAT did not remain stable on probing with varied relative humidity (RH) conditions as segregation back to the δ-intermediate and PbI2 phase after 10 s of exposure to an RH value of 11% was found to occur from the GIWAXS results. When RH levels were elevated to over 100%, segregation peaks of PbI2 and δ-intermediate (2H, 4H and 6H) became conspicuous as the α-phase intensity diminished, unlike for MAPI that remains relatively stable. The possible cause of this is hydrophilic bonds that form between the 3-CAT crystals and the small annealing window of the best composition perovskite (5% Cs+) film.

EVOLUTION OF THE SYNTHESIS PRODUCT COMPOSITION OF Ti-B-Al SYSTEM IN COMBUSTION MODE

A three-dimensional combustion synthesis model of the composite material in a titanium (Ti)-boron (B)-aluminum (Al) system, which takes into account the melting of all components, detailed kinetics of the process, evolution of the porosity, and dependence of the thermophysical properties on the structures and composition of the material, is proposed. It was found that the main phases were TiB<sub>2</sub> and TiAl in the final composite. The fraction of the other phases was less than 1%; preheating of the reaction mixture from 20°C to 300°C led to a slight increase in the intermetallic phases (about 1%) and a decrease in the boride phases. The results showed that the synthesis of the composite material with a composition of (1-a) · (Ti + Al) + a · (Ti + 2B) under the experimental conditions, in which a = 0-0.5, proceeded in the wave mode without twisting of the reaction front. The theoretical results were consistent with the experimental data.

Three-dimensional vorticity effects on extinction behaviour of laminar flamelets

A recent rotational flamelet model (Sirignano [Three-dimensional, rotational flamelet closure model with two-way coupling, J. Fluid. Mech. 945 (2022), p. A21; Inward swirling flamelet model, Combust. Theory Model. 26 (2022),pp. 1014–1040; Stretched vortex layer flamelet, Combust. Flame. 244 (2022), p. 112276]) is developed and tested with an improved framework of detailed chemistry and transport. The rotational flamelet model incorporates the effects of shear strain and vorticity on local flame behaviour and is three-dimensional by nature. A similarity solution reduces the three-dimensional governing equations to ODEs involving a transformation to a non-Newtonian reference frame. A 9-species chemical kinetics model is used for H 2 - O 2 combustion with non-reacting N 2 . In all non-premixed flame cases, the oxidiser is pure O 2 while the fuel ( H 2 ) is diluted with N 2 . Multiple flamelet cases including non-premixed, premixed, and partially-premixed flames are performed. Across all cases, vorticity extends flammability limits by up to 30% in terms of the ambient extinction strain rate and modifies both local flame structure and mixture composition. For N 2 -diluted H 2 - O 2 non-premixed flames, where the location of minimum density coincides with the location of peak temperature, the centrifugal force induced by vorticity reduces the mass flow rate through the flame, effectively lowering the local strain rate. This increases residence time, thus extending flammability limits and reducing burning rates. This analysis is done also for premixed and partially-premixed flames. For pure H 2 - O 2 non-premixed flames, where minimum density lies between the flame zone and the fuel inlet boundary, centrifugal forces do not significantly modify flame behaviour. Stable and unstable branches of S-curves for non-premixed and partially-premixed flames and stable branches for premixed flames show extended flammability limits due to vorticity. The capabilities of the rotational flamelet model reveal that vital physics are currently missing from two-dimensional, irrotational, constant-density, flamelet models. Improvements of detailed chemical kinetics, transport formulation, and thermo-physical properties bring the new flamelet model to par in these areas with existing models, while adding new features in terms of physical emulation.

On the Modeling of Small-Angle Neutron Scattering Data to Analyze the Early Stage of Phase Separation in Fe-Cr-based Alloys

AbstractSmall-angle neutron scattering (SANS) is a valuable method for the analysis of phase decomposition in Fe-Cr alloys; however, quantification of the decomposition requires careful modeling of the scattering data considering factors such as interface character and short-range order. Here, we quantify the phase decomposition in a high-performance super duplex stainless steel in situ during accelerated aging in the early stage of decomposition by modifying a previously suggested quantitative SANS data modeling method. The proposed revised method can accurately model the SANS data and paves the way for revisiting the detailed phase decomposition kinetics in situ during aging in various Fe-Cr-based alloys.

Kinetic study of the growth of PAHs from biphenyl with the assistance of phenylacetylene

Biphenyl is a crucial precursor to polycyclic aromatic hydrocarbons (PAHs), and phenylacetylene is an abundant product in aromatic hydrocarbons combustion. By exploring the reaction kinetics of phenylacetylene with biphenyl radicals, we further explore the novel hydrogen-abstraction phenylacetylene-addition (HAPaA) mechanism which is recently proposed to account for alternative mass growth pathways of PAHs. A combination of M06–2X/6–311+G(d,p) and PWPB95-D3/def2-QZVPP calculations were performed to construct the potential energy surfaces, and the rate coefficients were determined via solution of transition state theory based master equations. We demonstrate the capability of biphenyl species to grow with the assistance of phenylacetylene by unraveling the ring growth process of biphenyl radicals, establishing the main evolution routes of key intermediates, and quantifying the competition relationship between various channels. Energetic analysis and kinetic calculations demonstrate that the initial orientations of the reacting moieties do have a remarkable impact on the detailed kinetics of the entrance channels. However, the two adducts formed from initial additions achieve a rapid equilibrium because of the largest rate constants of the interconversion reactions between them, which counteracts the orientation effect on the overall kinetics. Further reaction pathways and corresponding products are related to the aryl radical position. Specifically, the aromatics of 4-phenylphenanthrene or phenanthrene, formed through cyclization reactions followed by hydrogen or phenyl eliminations, are preferred for the “armchair” type 2-biphenyl radical. In contrast, products featuring a triple bond generated through CH β-scission reactions are favored for the “free” type 3-biphenyl and 4-biphenyl radicals. The present research can serve as a good basis for further experimental and modeling studies of PAHs and soot formation.

Investigation on Swelling of Agar-Based Antibacterial Hydrogels for Hard-to-Heal Wound Dressings.

Despite a wide range of available wound treatments, hard-to-heal wounds still pose a challenge. Hydrogels are often used as dressings for these wounds, because they sustain moisture in the wound environment, supporting the natural healing process. However, it is still not fully understood how physicochemical properties of hydrogel matrix affect the drug release process. Thus, detailed swelling kinetics examination coupled with modeling is needed together with studies on drug release. In this regard, several hydrogels based on plant-derived agar and modified with amikacin sulfate were investigated. The main properties of hydrogels were examined focusing on detailed swelling kinetics. Drug release was studied as microbiological activity against E. coli and S. Epidermidis strains. The obtained hydrogels were characterized by high swelling, reaching values in range from 465-1300 %, fitting the second order kinetics mode and exhibiting the quasi-Fickian diffusion properties. Furthermore, there was no correlation found between swelling properties and antibacterial activity against tested strains. The results confirmed that presented hydrogel materials have desirable properties for application as dressings for hard-to-heal wounds. The suggested compositions are a promising base for modification with other active substances (e. g., regenerative, anti-inflammatory) and studying the broader correlation between swelling and drug release.

Unraveling the Anion Storage Properties of Manganese-Doped SrTiO3.

Strontium titanate (STO), a cubic perovskite material, has gained recent attention as a supercapacitor active material with its pseudocapacitive energy storage attributed to anion intercalation. However, very few in-depth studies have been conducted to understand the anion storage properties of STO and its metal-doped derivative compounds. In this study, we explored the anion-insertion storage mechanism of Mn-doped strontium titanate (Mn-STO) compared to pristine STO. The polycrystalline Mn-STO, synthesized via solid-state reaction, showed 3-fold times higher electrochemical surface area and exhibited enhanced anion storage compared to pristine STO. Detailed anion kinetics and diffusion studies reveal that the anion storage in Mn-STO is dominated by the bulk diffusion-controlled pseudocapacitive process than in STO. Further, the supercapacitor fabricated with Mn-STO in a 3 M KOH aqueous electrolyte with 0.1 M MnSO4 additives demonstrated excellent cycling stability, retaining 100% capacitance after 10,000 cycles, highlighting the potential of Mn-STO as an electrode material for supercapacitor applications.

Butadiene hydrogenation on N-doped carbon-hosted non-noble metal nanostructures

Diverse N-doped carbon-supported non-noble metal nanostructures (Ni, Co, Fe, Cu) are designed, and explored in selective butadiene hydrogenation. Focusing on particle size and composition, optimal catalytic performance is observed with Ni catalysts, where smaller metallic Ni particles of ca. 6.2 nm exhibit superior activity and larger ones (14–47 nm) display much higher total butene selectivity. An integral approach combining detailed kinetics, chemisorption, and dual-beam Fourier transform infrared spectroscopic study is performed to rationalize the Ni particle size effect. The findings reveal that smaller Ni particles offer improved activation of butadiene and hydrogen due to advantageous adsorption dynamics. Spectroscopic examinations further suggest different adsorption configurations existing on Ni particles, with larger particles displaying strong π-adsorption, which impedes hydrogen replacement. Additionally, the stability of the catalysts is scrutinized under various reaction conditions, revealing that deactivation occurs more rapidly at lower temperatures, primarily due to mild coke deposition.

Transport properties of high Mach number hypersonic air plasmas

Abstract The collisional and transport properties of hypersonic plasmas have been simulated by an air plasma chemistry model, which includes collisions with electrons, ions and neutral species, elastic scattering, vibrational relaxation and a transport model. Species and thermal fluxes at the surface of the vehicle have been investigated as a function of post-shock gas temperature and species sticking coefficient for a fixed altitude of 50 km. At temperatures below about 5000 K, the leading species are vibrationally excited nitrogen molecules, while at higher temperatures the atomic oxygen and nitrogen dominate. Large fluxes of oxygen atoms have been observed in the simulations, on the order of 1020 cm − 2 s − 1, which may lead to high rates of surface oxidation. Another subject of this study is temperature equilibration. For electrons, equilibration takes tens of microseconds, while the vibrational temperature equilibration time vary widely; from a few microseconds to hundreds of microseconds, depending on the gas temperature. Finally, the adiabatic parameter γ was calculated for post-shock gas temperatures 5000 K and 10 000 K and total gas density 1.2 × 10 17 c m − 3 . For post-shock gas temperature 5000 K, only a slight reduction of γ from the ideal gas value of γ = 1.4 is observed, while for 10 000 K it drops to γ ≅ 1.26, which has a considerable impact on plasma properties. It was further established that the vibrational kinetics plays a leading role. While a single excited vibrational level of the N2 molecule captures the salient properties of the plasma, a detailed vibration kinetics is required for high-fidelity simulations.

Gradient dynamics approach to reactive thin-film hydrodynamics

Wetting and dewetting dynamics of simple and complex liquids is described by kinetic equations in gradient dynamics form that incorporates the various coupled dissipative processes in a fully thermodynamically consistent manner. After briefly reviewing this, we also review how chemical reactions can be captured by a related gradient dynamics description, assuming detailed balanced mass action type kinetics. Then, we bring both aspects together and discuss mesoscopic reactive thin-film hydrodynamics illustrated by two examples, namely, models for reactive wetting and reactive surfactants. These models can describe the approach to equilibrium but may also be employed to study out-of-equilibrium chemo-mechanical dynamics. In the latter case, one breaks the gradient dynamics form by chemostatting to obtain active systems. In this way, for reactive wetting we recover running drops that are driven by chemically sustained wettability gradients and for drops covered by autocatalytic reactive surfactants we find complex forms of self-propulsion and self-excited oscillations.

Kinetic investigation on palladium‐catalyzed carbonylation of allyl alcohol

AbstractPalladium‐catalyzed carbonylation of allyl alcohol to 3‐butenoic acid has been investigated. A significant effect of halide promoters, p‐tolylsulfonic acid (TsOH), water, solvents, and PPh3 concentration activity and selectivity has been studied. Detailed kinetics of this reaction was investigated in a temperature range of 363–383 K. The influence of parameters such as stirring speed, allyl alcohol, catalyst, benzyltriethylammonium chloride (BTEAC), TsOH concentrations, and CO partial pressures on the activity and selectivity has been studied. An empirical rate equation was suggested and found to be fairly consistent with observed rate data. In addition, the activation energy and kinetic parameters were evaluated.

Consolidated Description of Explosion Limits and Ignition Delays: Hydrogen and C1-C7 N-Alkanes

ABSTRACT This study computationally investigates the auto-ignition delay times and explosion limits of H2/O2 and C1-C7 n-alkanes/O2 mixtures based on detailed chemical kinetics. Comprehensive ignition responses are consolidated through three-dimensional response surfaces of the ignition delay time as function of the system temperature and pressure, yielding the essential kinetic characteristics governing ignition. The intersection curves of the ignition surface with the cross-section perpendicular to the three coordinate axes are the characteristic response curves of the ignition delay time and explosion limit. Therefore, the ignition delay time and explosion limit are the changing characteristics of the three-dimensional ignition surface observed from different perspectives. Sensitivity analysis demonstrates that while the significant variations of the reaction pathways among different fuels are the primary reason of the wide spreading of the explosion limits curves for the various fuels in the low-temperature regime, their chain branching reactions tend to be similar in the intermediate temperature regime such that the explosion limit curves of different fuels are closely coalesced. Furthermore, in the high-temperature regime, the chain branching reactions of alkanes are primarily governed by small radicals, such as CH3, while it is still dominated by the H+O2=OH+O for H2.

Fossil Diesel, Soybean Biodiesel and Hydrotreated Vegetable Oil: A Numerical Analysis of Emissions Using Detailed Chemical Kinetics at Diesel Engine Like Conditions

Nowadays, emissions from internal combustion engines are a relevant topic of investigation, taking into account the continuous reduction of emission limits imposed by environmental regulatory agencies around the world, obviously as the result of earnest studies that have pointed out the impact on the human health of high levels of contaminants released into the environment. Over recent years, the use of biofuels has contributed to attenuating this environmental issue; however, new problems have been raised, such as NOx emissions tend to increase as the biofuel percentage in the fuel used in engines increases. In this research, the emissions of a compression ignition internal combustion engine modeled as a variable volume reactor with homogeneous combustion were numerically investigated. To analyze the combustion process, a detailed kinetics model tailored specifically for this purpose was used. The kinetics model comprised 30,975 chemical reactions involving 691 chemical species. Mixtures of fuel surrogates were then created to represent the fuel used in the Brazilian fuel marketplace, involving (i) fossil diesel—“diesel A”, (ii) soybean diesel—“biodiesel”, and (iii) hydrotreated vegetable oil— “HVO”. Surrogate species were then selected for each of the aforementioned fuels, and blends of those surrogates were then proposed as mixture M1 (diesel A:biodiesel:HVO—90:10:0), mixture M2 (diesel A:biodiesel:HVO—85:15:0), and mixture M3 (diesel A:biodiesel:HVO—80:15:5). The species allowed in the kinetics model included all the fuel surrogates used in this research as well as the target emission species of this study: total hydrocarbons, non-methane hydrocarbons, carbon monoxide, methane, nitrogen oxides, carbon dioxide, soot, and soot precursors. When compared to experimental trends of emissions available in the literature, it was observed that, for all the proposed fuel surrogates blends, the numerical approach performed in this research was able to capture qualitative trends for engine power and the target emissions in the whole ranges of engine speeds and engine loads, despite the CO and NOx emissions at specific engine speeds and loads.

Tailoring Light-Harvesting in Zn-Porphyrin and Carbon Fullerene based Donor-Acceptor Complex through Ethynyl-Extended Donor π-Conjugation.

Organic photovoltaic efficiency though currently limited for practical applications, can be improved by means of various molecular-level modifications. Herein the role of extended donor -conjugation through ethynyl-bridged meso-phenyl/pyridyl on the photoinduced charge-transfer kinetics is studied in noncovalently bound Zn-Porphyrin and carbon-fullerene based donor-acceptor complex using time-dependent optimally tuned range-separated hybrid combined with the kinetic rate theory in polar solvent. Noncovalent dispersive interaction is identified to primarily govern the complex stability. Ethynyl-extended -conjugation results in red-shifted donor-localized Q-band with substantially increased dipole oscillator strength and smaller exciton binding energy, suggesting greater light-harvesting efficiency. However, the low-lying charge-transfer state below to the Q-band is relatively less affected by the ethynyl-extended -conjugation, yielding reduced driving forces for the charge-transfer. Detailed kinetics analysis reveals similar order of charge-transfer rate constants (~1012 s-1) for all donor-acceptor composites studied. Importantly, enhanced light-absorption, smaller exciton binding energy and similar charge-transfer rates together with reduced charge-recombination make these complexes suitable for efficient photoinduced charge-separation. These findings will be helpful to molecularly design the advanced organic donor-acceptor blends for energy efficient photovoltaic applications.

Numerical investigation of lean methane flame response to NRP discharges actuation

This study investigates the response of a laminar methane-air flame to Nanosecond Repetitively Pulsed (NRP) discharges in a canonical wall-stabilized burner using a combined experimental and numerical approach. The flow and flame behaviors were modeled using Direct Numerical Simulation (DNS) with an Analytically Reduced Chemistry for a precise chemical description. A phenomenological model incorporating detailed plasma kinetics and experimental observations was developed to simulate plasma effects. Zero-dimensional plasma reactor simulations were used to build up a reduced-order model describing discharge energy distribution in the specific conditions studied. Experimental measurements of electrical profiles identified two discharge regimes: a low-energy Corona discharge and a higher-energy Glow discharge, characterized by distinct spatial energy distributions. Experimental flame response analysis revealed three major phases: marginal response up to 100 pulses, a downstream shift of the flame tip, and stabilization after 400 pulses. Numerical simulations indicated that the Corona regime is crucial for explaining initial flame responses, while the Glow regime influences later stages. Adjustments in the Vibrational–Translational (VT) energy relaxation time and energy deposition ratios between fresh and burnt gases were necessary to match experimental observations. Additionally, an accurate modeling of the transient and steady-state flame responses requires integrating both the specificity of the Corona and the Glow discharge regimes. Future work should focus on measuring or theoretically calculating N2(v) relaxation times in CH4-H2O-CO2 mixtures and analyzing the spatial energy distribution of discharges interacting with flames to enhance plasma-combustion coupled models.Novelty and significanceIn this work, a phenomenological plasma-assisted combustion model has been developed, to investigate a laminar premixed stagnation plate burner, focusing on VT energy relaxation time and spatio-temporal energy distribution modeling. For the first time, not only O2, N2 and O but also the fuel and combustion intermediates and products have been considered in the VT relaxation model. It revealed their strong influence on the overall flame response in a case where the discharge crosses a flame front, highlighting the strong beneficial effect of energy deposited in vibrational form. The study questions and investigates the energy distribution from fresh to burnt gases, challenging the conventional uniform energy distribution assumption. The experimental identification of two specific plasma regimes was necessary to predict the transient flame response. Additionally, energy deposited downstream of the flame, in fresh gases, was found to more efficient than in hot gases to enhance combustion.

HomeRNA self-blood collection enables high-frequency temporal profiling of presymptomatic host immune kinetics to respiratory viral infection: a prospective cohort study.

Early host immunity to acute respiratory infections (ARIs) is heterogenous, dynamic, and critical to an individual's infection outcome. Due to limitations in sampling frequency/timepoints, kinetics of early immune dynamics in natural human infections remain poorly understood. In this nationwide prospective cohort study, we leveraged a Tasso-SST based self-blood collection and stabilization tool (homeRNA) to profile detailed kinetics of the presymptomatic to convalescence host immunity to contemporaneous respiratory pathogens. We enrolled non-symptomatic adults with recent exposure to ARIs who subsequently tested negative (exposed-uninfected) or positive for respiratory pathogens. Participants self-collected blood and nasal swabs daily for seven consecutive days followed by weekly blood collection for up to seven additional weeks. Symptom burden was assessed during each collection. Nasal swabs were tested for SARS-CoV-2 and common respiratory pathogens. 92 longitudinal blood samples spanning the presymptomatic to convalescence phase of eight SARS-CoV-2-infected participants and 40 interval-matched samples from four exposed-uninfected participants were subjected to high-frequency longitudinal profiling of 785 immune genes. Generalized additive mixed models (GAMM) were used to identify temporally dynamic genes from the longitudinal samples and linear mixed models (LMM) were used to identify baseline differences between exposed-infected (n = 8), exposed-uninfected (n = 4), and uninfected (n = 13) participant groups. Between June 2021 - April 2022, 68 participants across 26 U.S. states completed the study and self-collected a total of 691 and 466 longitudinal blood and nasal swab samples along with 688 symptom surveys. SARS-CoV-2 was detected in 17 out of 22 individuals with study-confirmed respiratory infection, of which five were still presymptomatic or pre-shedding, enabling us to profile detailed expression kinetics of the earliest blood transcriptional response to contemporaneous variants of concern. 51% of the genes assessed were found to be temporally dynamic during COVID-19 infection. During the pre-shedding phase, a robust but transient response consisting of genes involved in cell migration, stress response, and T cell activation were observed. This is followed by a rapid induction of many interferon-stimulated genes (ISGs), concurrent to onset of viral shedding and increase in nasal viral load and symptom burden. Finally, elevated baseline expression of antimicrobial peptides were observed in exposed-uninfected individuals. We demonstrated that unsupervised self-collection and stabilization of capillary blood can be applied to natural infection studies to characterize detailed early host immune kinetics at a temporal resolution comparable to that of human challenge studies. The remote (decentralized) study framework enables conduct of large-scale population-wide longitudinal mechanistic studies. This study was funded by R35GM128648 to ABT for in-lab developments of homeRNA and data analysis, a Packard Fellowship for Science and Engineering from the David and Lucile Packard Foundation to ABT, and R01AI153087 to AW.

Ammonia pyrolysis oxidation excited by nanosecond pulsed discharge: Global/fluid models hybrid solution

The kinetic characteristics of plasma-assisted oxidative pyrolysis of ammonia are studied by using the global/fluid models hybrid solution method. Firstly, the stable products of plasma-assisted oxidative pyrolysis of ammonia are measured. The results show that the consumption of NH3/O2 and the production of N2/H2 change linearly with the increase of voltage, which indicates the decoupling of non-equilibrium molecular excitation and oxidative pyrolysis of ammonia at low temperatures. Secondly, the detailed reaction kinetics mechanism of ammonia oxidative pyrolysis stimulated by a nanosecond pulse voltage at low pressure and room temperature is established. Based on the reaction path analysis, the simplified mechanism is obtained. The detailed and simplified mechanism simulation results are compared with experimental data to verify the accuracy of the simplified mechanism. Finally, based on the simplified mechanism, the fluid model of ammonia oxidative pyrolysis stimulated by the nanosecond pulse plasma is established to study the pre-sheath/sheath behavior and the resultant consumption and formation of key species. The results show that the generation, development, and propagation of the pre-sheath have a great influence on the formation and consumption of species. The consumption of NH3 by the cathode pre-sheath is greater than that by the anode pre-sheath, but the opposite is true for OH and O(1S). However, within the sheath, almost all reactions do not occur. Further, by changing the parameters of nanosecond pulse power supply voltage, it is found that the electron number density, electron current density, and applied peak voltages are not the direct reasons for the structural changes of the sheath and pre-sheath. Furthermore, the discharge interval has little effect on the sheath structure and gas mixture breakdown. The research results of this paper not only help to understand the kinetic promotion of non-equilibrium excitation in the process of oxidative pyrolysis but also help to explore the influence of transport and chemical reaction kinetics on the oxidative pyrolysis of ammonia.

Defying the D2-law in fuel droplet combustion under gravity

We show both experimentally and theoretically for the first time that the widely used D2-law for describing the shrinking kinetics of a vaporizing droplet is not the true law for single burning droplets under the influence of gravity. Experimentally, the instantaneous diameter D for such a droplet is identified to obey a new kinetic law: Dn decreases linearly with time t, with the exponent n=2.53±0.30−2.69±0.14 for a variety of common liquid fuels such as alkanes and alcohols of low and high boiling hydrocarbons. Theoretically, we develop a phenomenological theory to show n=8/3≈2.67 well capturing the experimental values. The burning rate constant in this new D8/3-law no longer behaves like the thermal diffusivity α of the gas phase but turns into α3ℓ−7/3/g due to the additional inherent fluid-property-determined buoyancy length ℓ under the influence of the gravitational acceleration g. This non-square power law is purely transport determined. It is a consequence of simultaneous momentum, heat, and mass transfer resulted from buoyant convection setup by the blazing flame around the droplet, insensitive to combustion chemistry and detailed reaction kinetics. This study provides not only renewed insights into the multifaceted droplet combustion phenomenon but also possibly new paradigms for a better understanding or characterization of actual fuel droplet combustion processes in normal gravity environments.