Detailed Chemical Kinetic Model Research Articles

Revealing the flame inhibition effect of phytic acid (PA) by PIV measurements and detailed chemical kinetic modeling of counterflow CH4/PA/air flames

As industrialization continues to deepen, polymers have become ubiquitous in daily life. However, their flammable characteristics pose significant safety risks. Therefore, supporting the global goal of sustainable development necessitates the development of high-performance, environmentally friendly flame retardants. Biomass phytic acid (PA) has emerged as a promising option because of its high phosphorus content and excellent biocompatibility. However, its combustion behaviors and chemical kinetic mechanisms remain unclear. In this study, quantum chemical calculation methods were used to construct a detailed PA chemical reaction kinetic model. A series of experiments were conducted to validate the model and evaluate PA's flame suppression effect. Utilizing a counterflow flame burner and particle image velocimetry (PIV), the inhibitory effect of various PA concentrations was examined based on the laminar flame speed of CH4/PA/Air mixture. The results revealed that a merely 0.2 % addition of PA could reduce the laminar flame speed by 38.9 %, demonstrating its significant flame suppression effect. Building on the foundational GRI-Mech 3.0 and integrating PA's pyrolysis and reaction mechanism, this study developed for the first time a detailed chemical model of CH4/PA/Air combustion. This model integrated PA's thermal decomposition module and thermodynamic data via ab-initio quantum chemical calculations, thereby accurately predicting global kinetic indicators such as laminar flame speed. Results suggested that the key reactions, such as PO2+H + M→HOPO + M, HOPO2+H→PO2+H2O, and HOPO + OH→PO2+H2O primarily influenced the laminar flame speed. Moreover, the CFD simulation elucidated the complex interaction between PA and flame structures. A detailed analysis of the spatial distribution of key parameters such as temperature, combustion radicals, and effective inhibition radicals unveiled the PA's flame suppression mechanism in support of the practical application of this eco-friendly flame retardant.

Experimental and kinetic study of methanol/ethanol assisted oxidative degradation of ammonia in supercritical water

Ammonia is a refractory intermediate during supercritical water oxidation (SCWO) of nitrogen-containing wastes, and alcohols are usually introduced as auxiliary fuels to accelerate its degradation. The kinetics of methanol/ethanol assisted oxidative degradation of ammonia in supercritical water have been investigated in a flow reactor, covering alcohol-to-ammonia ratios of 0.2–2.0. During SCWO of ammonia-methanol mixtures, ammonia was mostly converted to N2, with N2O as the main by-product. Increasing methanol concentration facilitated effectively the degradation of ammonia. For the cases with ethanol assistance, high concentrations of HCN were detected at the investigated temperature. Increase in ethanol concentration enhanced ammonia conversion, but had a non-monotonic influence on N2 production. A detailed chemical kinetic model was developed by integrating the nitrogen chemistry with methanol/ethanol mechanisms. The model performance was further validated by comparing to the present and literature data. The model reproduced the ammonia conversions at different conditions fairly well, but overpredicted the N2 yield and underpredicted the NO2 and HCN yields slightly. Based on the model, reaction mechanism was inferred and important elementary steps were identified through kinetic analyses. Finally, the influence of process parameters on ammonia degradation rate was numerically explored. This study is expected to facilitate the application of SCWO technology for N-containing wastes treatment.

A detailed chemical kinetic model for methanol and ammonia co-oxidation under hydrothermal flames: Reaction mechanism and flame characteristics

A detailed chemical kinetic model for ammonia and methanol co-oxidation under hydrothermal flames was established with pressure and thermodynamic corrections. Simulation model was validated by comparing with experimental temperature rise, and methanol and ammonia removal efficiencies. Species evolutions, reaction paths, and reaction sensitivities for pure ammonia combustion, and ammonia and methanol co-oxidation under hydrothermal flames were analyzed. Ammonia concentration begins to decrease only when methanol is consumed in a certain amount in the ammonia and methanol co-oxidation under hydrothermal flames, indicating the addition of methanol promotes the degradation of ammonia. Moreover, reaction heat is more conductive to ignition and the conversion of ammonia to nitrogen than active free radicals provided from methanol. The ignition delay time presents a minimum as a function of ammonia/methanol concentration ratio, with the minimum values of ignition delay time present at approximately ωNH3/ωCH3OH = 1 for different methanol concentrations. Higher preheating temperatures favor more NOX but less N2O formation, while higher ammonia concentrations favor both NOX and N2O formations. The presence of ammonia increases the laminar flame speed and permits a lower extinction temperature, indicating the mixture of methanol and ammonia can improve the stability of hydrothermal flames.

Species selection for automatic chemical kinetic mechanism generation

AbstractMany important chemical kinetic systems require detailed chemical kinetic models to resolve. These detailed kinetic models can involve thousands of species and hundreds of thousands of chemical reactions, making them difficult to construct by hand. Modern automatic mechanism generation algorithms can mostly be divided into two classes: rule and rate based. Rule‐based generators choose species based on user defined constraints on species and reaction classes. Rate‐based generators generate a much larger set of potentially important species and reactions and then choose which ones to add based on running simulations of species and reactions deemed important and calculating the flux to potentially important species. In principle, the latter is preferable, as it requires the user to make far fewer assumptions about what is important in the system. However, while the effectiveness of the rate‐based approach has been demonstrated in a wide variety of systems, it has also been demonstrated to have difficulty picking up important low‐flux chemistries. Here we present a discussion of the challenges associated with rate‐based mechanism generation and new algorithms that are able to efficiently mitigate these challenges improving species selection during mechanism generation in a set of case studies.

Comparative Study of n-C5 Bond Dissociation Energies and H-Atom Abstraction via H for Alkane, Alcohol, Methyl Ether, Nitroso, Nitro, Nitrite, and Nitrate.

Energetic materials have a wide range of applications, for many of which new materials are constantly developed. For understanding how energetic materials are chemically converted, it is essential to systematically understand how relevant functional groups affect the stability and properties of such materials. Here, the impact of nitroso, nitro, nitrite, and nitrate functional groups on a pentyl moiety is studied theoretically, focusing on bond dissociation energies and H atom abstraction via Ḣ. The nitroso group is found to imply the strongest effects, while the nitro group appears to exert a stabilizing effect on the pentyl moiety. Importantly, the nitrogenated functional groups generally stabilize the pentyl moiety, in contrast to alcohol and methyl ether functional groups. Therefore, it is concluded that using analogies to the chemistries of the latter two functional groups for chemical kinetic modeling of nitrogenated functional groups is not adequate. Based on the presented bond dissociation energies and rate coefficients, it is to be expected that unimolecular bond fissions at the nitrogenated functional groups will dominate over a wide temperature range. Building on the provided data, future detailed chemical kinetic modeling efforts for nitroso, nitro, nitrite, and nitrate compounds can be aided or initiated.

The pronounced NTC behavior of 1,2,4-trimethylbenzene at high-pressure oxidation

This study presents experimental and modeling results of the 1,2,4-trimethylbenzene (T124MBZ) oxidation in a jet-stirred reactor at equivalence ratios of 0.4 and 2.0, temperature range of 450–1000 K, and pressure of 12.0 atm. Compared with the oxidation at atmospheric pressure, a pronounced negative temperature coefficient (NTC) behavior of T124MBZ was observed between 622 K and 773 K under fuel-lean condition. A detailed chemical kinetic model involving 865 species and 5092 reactions was developed. In addition, the model was validated against experimental results of laminar burning velocities and ignition delay times. The present model reasonably reproduces these experimental data. The dominant consumption channels for T124MBZ are H-abstraction reactions on methyl groups by OH radicals. The H-abstraction reactions on the 1- and 2- methyl sites by OH radicals are the most promoting in pre- and mid- NTC phases, and H2O2(+M) = 2OH(+M) is the most promoting in the post-NTC phase, while the reaction on the 4-methyl site is the most inhibiting across pre-, mid-, and post-NTC phases. This difference arises from the fact that the dominant consumption pathway for 2,4-dimethylbenzyl and 2,5-dimethylbenzyl involves H-transfer, while the 4-methyl site cannot undergo an H-transfer reaction due to the absence of adjacent methyl groups. The competition between the two consumption channels of ·QOOH radicals (decomposition and second O2 addition), resulting in the reduction of OH radicals as temperature increases, leads to diminished OH production. This causes the emergence of pronounced NTC behavior at high pressure.

An experimental and modeling study of propane oxidation kinetics in low temperature supercritical water

Propane oxidation in supercritical water was investigated at iso-thermal iso-baric conditions using a batch reactor facility. Mixtures were comprised of 0.014 % propane by volume with an equivalence ratio of 0.8 and a total density of 222 mg/mL or 610 mg/mL. Reaction times ranged from 8 to 30 min for a temperature of 375ºC at 220 or 400 bar, or 400ºC at 220 bar. Major reaction products were CO and CO2 and minor products were propene, acetone, ethene, ethanol, methane, methanol and hydrogen. New detailed chemical kinetic models were developed by combining and refining existing models using genetic optimization. Model predictions exhibited excellent agreement with experimental observations, and indicated that rates of H-abstraction and OH addition reactions involving alkanes and alkenes are affected by the supercritical water environment. Model accuracy was highly sensitive to the rates of CH3O2H = CH3O + OH and CH3 + H2O2 = CH4 + HO2.

Development of Detailed and Reduced Chemical Kinetic Models for Ammonia Gas Turbines

Abstract The power generation sector has been recently moving towards decarbonization and there is an increased interest in replacing conventional fossil fuels with fuels that produce reduced/zero carbon emissions. One such fuel is ammonia (NH3). However, ammonia is hard to ignite, has a low flame speed, and produces a significantly large amount of nitrogen oxide (NOx) emissions. Hence, using 100% ammonia as fuel in gas turbines requires significant modifications and the development of novel combustors. Blending hydrogen with ammonia, however, helps in having better control over the combustion properties. Before utilizing hydrogen-blended ammonia in an actual gas turbine combustor, thorough simulation studies are required to evaluate its performance, possible hazards, and emissions. The literature lacks well-validated chemical kinetic models for the combustion of hydrogen-blended ammonia for undiluted mixtures at gas turbine-relevant conditions (~20 bar). Hence, in this work, we develop a detailed chemical kinetic model for hydrogen blended ammonia combustion and validate it with a wide range of experimental data for both dilute and undiluted mixtures relevant to gas turbine operating conditions. The detailed chemical kinetic mechanism was reduced to a smaller version (32 species mechanism) without significant loss in accuracy using the direct relation graph with error propagation (DRGEP) and full species sensitivity analysis. The resultant mechanism can predict a wide range of experimental results with the least cumulative error and will be a valuable tool in CFD simulations that will enable the development of gas turbines for zero-carbon power generation.

Experimental and kinetic modeling study of 1,3,5-trimethylbenzene oxidation at elevated pressure

This work presents the experimental and modeling results of 1,3,5-trimethylbenzene (T135MB) oxidation in a jet-stirred reactor at equivalence ratios (Φ) of 0.4 and 2.0, temperature range of 660–1020 K and pressure of 12.0 atm. 5 aromatic compounds, 5 oxygenated species and 6 light hydrocarbons were identified and quantified using GC and GC-MS. Compared with previous oxidation studies, 1-ethyl-3,5-dimethylbenzene, 3,5-dimethylbenzaldehyde, 3,5-dimethylphenol, ethylene and acetylene were newly detected. A detailed chemical kinetic model involving 911 species and 5370 reactions was developed and validated against experimental results of T135MB oxidation speciation at jet-stirred reactor and shock tube, laminar burning velocities and ignition delay times. Rate-of-production analysis shows that the dominant consumption channel of T135MB is H-abstraction by OH radicals on methyl sites, and 3,5-dimethylbenzaldehyde is a key intermediate in the oxidation of T135MB. Compared with atmospheric pressure, sharp consumption of T135MB was observed (the fuel conversion rate changes from 25 % at 838 K to 95 % at 839 K) under fuel-lean condition at 12 atm, which results from the rapid growth of OH radicals controlled by H2O2(+M) = 2OH(+M). Sensitivity analysis reveals that H-abstraction reactions of T135MB by OH and H on methyl sites are the most inhibiting reactions at Φ = 0.4 and 2.0, respectively, when T135MB conversion rate is 25 %. However, these two reactions become promoting at 95 % T135MB conversion. The H-abstraction reaction of T135MB with O2 on methyl sites is the most promoting at 25 % T135MB conversion, shifting to an inhibiting reaction at 95 % conversion.Novelty and significance statement: The novelty of this research is that high-pressure and low-temperature oxidation experiment of T135MB covering both fuel-lean and fuel-rich conditions was first reported in this work with abundant intermediates information. Rapid consumption of T135MB under fuel-lean condition was observed, and new intermediates such as 3,5-dimethylbenzaldehyde were detected which plays an important role in the fuel conversion. Additionally, a comprehensive kinetic model for T135MB was developed, which well predicts speciation, ignition delay times and laminar flame speeds under wide range of conditions. It is significant because the high pressure data is very important for the model validation since it is more close to the real engine-operating conditions. Moreover, the model provides insight on the high pressure chemical kinetics of the aviation surrogate fuel consumption and deepens the understanding of low-temperature oxidation of alkyl aromatics.

Optimization models for photosynthetic bioenergy generation in building façades

Façade design optimization has a significant impact on promoting resilient solar harvesting methods in urban areas. This study introduces a new framework toward the optimal inclusion of microalgae photobioreactors in different façade geometries by developing a detailed chemical kinetics model aided by optimization techniques. Although algal bioreactive façades in recent literature are restricted to flat façades, the present study reveals that bioenergy generation can be enhanced by up to 29.6 % through optimized folded façades. Hooke-Jeeves algorithm demonstrates remarkable computational efficiency in optimizing such façades, achieving the optimal outcome with only 88 function evaluations. However, the Hooke-Jeeves algorithm has limitations when applied to free-form façades by displaying noticeable instability and reporting an optimal value approximately 40 % lower than Genetic Algorithms. Hooke-Jeeves algorithm has the possibility to conclude misleading outcomes as optimal points. This is mitigated by applying multiple initial points and verifying the outcomes with graphical optimization. In terms of energy performance, free-form façades have higher irradiation of up to 210 W/m2 compared to 180 W/m2 for flat façades. Optimal distribution of renewable modules on free-form façades increases energy generation by 14.3 %. South-facing façades produce the highest bioenergy at 2.4 W/m2, while west and east orientations generate the lowest at 0.8 W/m2.

An experimental and chemical kinetic modeling study of octane isomer oxidation. Part 1: 2,3,4-trimethyl pentane

2,3,4-trimethyl pentane (234-TMP) is an isomer of octane with the same number of methyl branching groups as 2,2,4-trimethyl pentane (iso-octane). However, there are very few studies of this fuel available in the literature. In this work, a detailed chemical kinetic model is developed to describe the oxidation of 234-TMP using NUIGMech1.3 as the core mechanism. The rate constants for some important reaction classes are updated following a review of literature rate constants. Additionally, the impact of each rate constant on simulated ignition delay times for 234-TMP compared to 2,2,3-trimethyl pentane and 2,2,4-trimethyl pentane (iso-octane) is discussed. The thermodynamic data of the alkanes (RH), alkyl (Ṙ), alkyl peroxy (RȮ2), hydroperoxy-alkyl Q˙OOH, and peroxy hydroperoxyalkyl (Ȯ2QOOH) radicals are newly estimated based on recently updated group values in the literature. Moreover, this study presents the first set of data available for the oxidation of 234-TMP at higher pressures (15 and 30 atm), in the temperature range 600–1600 K, and at fuel/‘air’ equivalence ratios (φ) of 0.5, 1.0 and 2.0. The chemical kinetic model shows general good agreement with the experimental measurements. In addition, flux and sensitivity analyses are conducted to identify the important pathways and reactions controlling fuel oxidation at different temperatures. Furthermore, the reactivity of 234-TMP is compared to that of iso-octane, indicating that 234-TMP is slower to react as it has more tertiary carbon sites compared to iso--octane.

Synergistic inhibition of methane/air explosions by NaHCO3 particles with a bimodal size distribution

To reveal the inhibitory effect of stoichiometric methane/air explosions, NaHCO3 with a bimodal distribution were selected as inhibitors. A 5 L vertical transparent duct was used. All the results revealed that Blend82, whose mass fraction contained 80% OP500–600 and 20% OP200–220 NaHCO3, exhibited the optimal inhibitory effect. For the bimodal distribution of NaHCO3, assessing the inhibitory effect solely on the basis of a single particle size parameter is unreliable. Synergistic effects between particles of different sizes should be considered when the interaction index IS < 1. Multiple size classes of bimodal NaHCO3 were proposed for use in response surface methodology to predict the inhibitory effect of methane/air flames. Furthermore, we employed a detailed chemical kinetic model to quantify the endothermic and chemical effects of NaHCO3 on the laminar burning velocity of methane/air flames and established a good relationship between the explosion parameters and laminar flame properties.

Heterogeneous CPU-GPU parallelization for modeling supersonic reacting flows with detailed chemical kinetics

Accurate simulations of complex combustion phenomena associated with supersonic reacting flows require the use of detailed chemical kinetic mechanisms. However, detailed chemical mechanisms may consist of a large number of species and reactions resulting in extremely high computation cost. In order to accelerate the simulations of supersonic reacting flows with detailed chemical mechanisms, a heterogeneous CPU-GPU parallel algorithm and its portable software implementation are presented. The parallel algorithm was broken down into two parts: the CPU handled the computation of fluid dynamics while the GPU evaluated the chemical source terms and gas physical properties. The use of overlapping computations of chemical source terms on GPU and calculations of viscous flux on CPU is also presented. A study of performance tests was conducted. The performance results show that evaluating chemical source terms and gas physical properties on 2 GPUs are about 157.8× and 78.5× faster than running on the 16-core CPU when using the most complex mechanism on a grid of 3.3 million cells, respectively, resulting in an excellent speedup of the whole iteration up to 47. The significant performance improvement provided by the parallel algorithm can provide a significant perspective for designing heterogeneous CPU-GPU algorithms for applications in simulating supersonic reacting flows with detailed chemical kinetics. Program summaryProgram Title: OpenHurricaneCPC Library link to program files:https://doi.org/10.17632/m9pphg9cjj.1Licensing provisions: GPLv3Programming language: C++, CUDANature of problem: The computation cost in evaluating chemical source terms and theirs diagonal Jacobian matrices, and in calculating gas physical properties dominates the simulations of supersonic reacting flows with detailed chemical kinetic models. And the parallelization of these computations can yield great benefits from GPU acceleration.Solution method: The parallel implementation of evaluating the chemical source terms and theirs diagonal Jacobian matrices, and calculating gas physical properties were conducted on GPU to speed up the computation.

Subgraph Isomorphic Decision Tree to Predict Radical Thermochemistry with Bounded Uncertainty Estimation.

Detailed chemical kinetic models offer valuable mechanistic insights into industrial applications. Automatic generation of reliable kinetic models requires fast and accurate radical thermochemistry estimation. Kineticists often prefer hydrogen bond increment (HBI) corrections from a closed-shell molecule to the corresponding radical for their interpretability, physical meaning, and facilitation of error cancellation as a relative quantity. Tree estimators, used due to limited data, currently rely on expert knowledge and manual construction, posing challenges in maintenance and improvement. In this work, we extend the subgraph isomorphic decision tree (SIDT) algorithm originally developed for rate estimation to estimate HBI corrections. We introduce a physics-aware splitting criterion, explore a bounded weighted uncertainty estimation method, and evaluate aleatoric uncertainty-based and model variance reduction-based prepruning methods. Moreover, we compile a data set of thermochemical parameters for 2210 radicals involving C, O, N, and H based on quantum chemical calculations from recently published works. We leverage the collected data set to train the SIDT model. Compared to existing empirical tree estimators, the SIDT model (1) offers an automatic approach to generating and extending the tree estimator for thermochemistry, (2) has better accuracy and R2, (3) provides significantly more realistic uncertainty estimates, and (4) has a tree structure much more advantageous in descent speed. Overall, the SIDT estimator marks a great leap in kinetic modeling, offering more precise, reliable, and scalable predictions for radical thermochemistry.

Data-driven prediction of laminar burning velocity for ternary ammonia/hydrogen/methane/air premixed flames

Zero-carbon fuels such as hydrogen and ammonia play a pivotal role in the energy transition by offering cleaner alternatives to natural gas (methane), especially in industrial combustion systems. Binary and ternary blends of these fuels offer a transitionary, low-carbon solution in the near future. Laminar burning velocity (LBV), as a fundamental combustion property, is significantly different for ammonia, hydrogen, and methane. Although the LBV of binary blends of these fuels is well-studied, ternary blends have not been extensively studied. In this study, the primary objective is to employ a simple ensemble learning method to predict the LBV of ternary ammonia/hydrogen/methane/air mixtures. The training dataset consists of experimental data sourced from a large number of publications (3,846 data points), as well as synthetic data generated by 1D freely propagating premixed flame simulations in Cantera using a detailed chemical kinetic model. Three machine learning algorithms, namely artificial neural networks, gaussian process regression, and extreme gradient boosting trees are trained and optimised. Then, a simple ensemble averaging method is used to reduce overfitting and improve robustness. The ensemble model achieves coefficient of determination (R2) of 0.991 on the test set with an inference time that is approximately 8,000 times faster than the 1D simulation run time. The ensemble model is capable of predicting LBVs of ammonia/hydrogen/methane/air mixtures for T=[295–756K], P=[1–10bar], ϕ=[0.5–1.8] across all possible blending ratios.

Effect of hydrogen fraction and initial pressure on the inhibition of methane/hydrogen/air explosions by NaHCO3

To explore the effects of the hydrogen fraction and initial pressure on the inhibition of methane/hydrogen/air explosions, the inhibition of stoichiometric methane/hydrogen/air explosions (hydrogen fraction ranging from 0 to 0.8) by NaHCO3 at various initial pressures (0.8 atm, 1.0 atm, 1.4 atm) was investigated in a 36 L spherical vessel. The experimental findings indicate that the effectiveness of NaHCO3 in suppressing methane/hydrogen/air explosions is significantly influenced by the hydrogen fraction and initial pressure. For a given hydrogen fraction, higher initial pressures weaken the inhibitory effect of a given NaHCO3 concentration on explosions. The drop ratios in the maximum explosion pressure for initial pressures of 0.8 atm, 1.0 atm, and 1.4 atm are 41.2 %, 19.7 %, and 15 %, respectively, at a hydrogen fraction of 0.8 and a NaHCO3 concentration of 400 g/m3. Additionally, the inhibitory impact of a given NaHCO3 concentration diminishes as the hydrogen fraction increases at the given initial pressure. An increasing hydrogen fraction decreases the drop ratio of the maximum explosion pressure while simultaneously increasing the maximum pressure rise rate and deflagration index. A thermodynamic equilibrium model for the endothermic decomposition of NaHCO3 particles was built to explore the impact of the hydrogen fraction and initial pressure on the decomposition characteristics of NaHCO3 particles of different sizes. The laminar burning velocity was derived using a theoretical model through the experimental pressure history and was calculated through detailed chemical kinetic modeling. The effect of the hydrogen fraction on the suppression mechanisms of NaHCO3 was evaluated by using the normalized laminar burning velocity. The dominant inhibition mechanism of NaHCO3 on the laminar burning velocity changes from physical inhibition to chemical inhibition with increasing hydrogen fraction.

An experimental, theoretical, and kinetic modeling study of post-flame oxidation of ammonia

The post-flame oxidation rate of ammonia was investigated in a novel atmospheric pressure flow reactor at temperatures of 1280 ± 16 K and as a function of residence time and mixture composition (1-10% O2, dry and moist). The experimental results, as well as selected data from literature, were analyzed using an updated detailed chemical kinetic model. The medium temperature, very lean conditions enhance the importance of reactions of the nitroxyl (HNO) intermediate. High-level theory was used to calculate the rate constant for HNO + NH2, indicating that this step is significantly faster than values used in literature. Furthermore, a trajectory based approach was used to determine collision efficiencies for selected bath gases for HNO + M. The experimental results show that the NH3 oxidation rate increases with temperature and O2 concentration, while the presence of water vapor slightly inhibits reaction. Formation of NO and N2O was strongly promoted at higher levels of O2. Modeling results agreed well with the measurements, except at the lowest level of O2. The predicted oxidation rate of NH3 was shown to result from a delicate balance between chain branching and terminating steps involving NH2, H2NO, and HNO. Recent theoretical work on reactions of these species by Klippenstein and coworkers and Stagni et al. was instrumental in improving modeling predictions. After initiation, NO reached a pseudo-steady-state level, where the pathways to NO were largely balanced by the NH2 + NO reaction. Nitric oxide was partly oxidized to NO2, with the NH2 + NO2 reaction responsible for most of the N2O formation. Novelty and significance statement:This study provides the first detailed kinetic analysis of the lean post-flame oxidation of ammonia, based on time-resolved flow reactor data in a novel reactor. In addition to the post-flame oxidation rate of ammonia, data for formation of NO and N2O were compared with modeling predictions. The medium temperature, very lean conditions enhance the importance of reactions of the HNO and H2NO intermediates. Inclusion in the model of results from recent high-level theoretical work, including present calculations for HNO + NH2 and HNO + M, was crucial for capturing the observed behavior. It is argued that the post-initiation steady-state NH3 oxidation rates constitute important data for model validation, along with ignition delays and laminar flame speeds.

Optical diagnostics and chemical kinetic analysis on partially premixed combustion characteristics fueled with methanol and various cetane improvers

The need for decarbonization has inspired the utilization of methanol in transportation industry. Via both experimental and modeling frameworks, this study, for the first time, reveals the ignition, heat release and flame development characteristics of methanol mixed with different cetane improvers, i.e., 2-ethylhexyl nitrate (EHN) and di-tertiary‑butyl peroxide (DTBP), under practical, advanced engine conditions. Particularly, a detailed chemical kinetic model is newly developed for methanol/EHN/DTBP mixtures, incorporating the most updated sub-chemistries for relevant fuel components and critical intermediates, to assist in further analysis on the effects of EHN and DTBP on ignition and emission formation under engine thermodynamic environments derived from recorded in-cylinder pressure histories. Results show that stable combustion of methanol/EHN mixtures can be realized under low direct injection (DI) pressure, high in-cylinder thermodynamic condition, featuring the lowest coefficient of variation of indicated mean effective pressure (COVIMEP) of 1.46 %. Throughout the combustion process, the yellow and white flame dominates. At the same DI timing, methanol/EHN mixtures present shorter ignition delay, faster heat release rate (HRR) and more advanced combustion phasing compared with methanol/DTBP mixtures. At similar combustion phasing, the peak flame/OH* natural luminosity intensity of methanol/EHN mixtures is higher. KL factor, which is utilized to evaluate soot concentration, also shows similar trend, indicating higher sooting tendency of methanol/EHN mixtures. Flux analysis further highlights higher production of soot precursor (i.e., C2H2) with EHN than DTBP, primarily via the pathway of C2H4→ C2H3→C2H2. The active atmosphere created by the decomposition of EHN or DTBP has significant effects on decreasing methanol ignition delay in the early stage of their exothermic process, while the active atmosphere of EHN remains impactful whereas that of DTBP diminishes at later stages. The thermal atmosphere shows greater effects on methanol ignition at later stages of the exothermic process of EHN or DTBP decomposition than the early stages.

Pyrolysis and kinetic modeling study of tetramethylethylenediamine: A potential green propellant

Tetramethylethylenediamine (TMEDA) is a promising green propellant that has been studied as a substitute for hydrazine. An in-depth study of the TMEDA thermal decomposition mechanism is important for future applications in engines. In this work, pyrolysis experiments of TMEDA were carried out in an atmospheric jet-stirred reactor (JSR) from 610 to 1000 K. Abundant intermediate species were analyzed and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). The results show that the pyrolysis of TMEDA mainly produces hydrocarbons, amines, imines, and nitriles, among which HCN and C2H5N are the main nitrogen-containing intermediates. Based on the experimental results, a detailed chemical kinetic model for describing the thermal decomposition of TMEDA was updated to improve predictions. The updated model can satisfactorily predict the consumption of TMEDA and the generation of most intermediates. Furthermore, detailed experimental measurements combined with kinetic analysis are used to clarify the decomposition reaction pathways of TMEDA. The rate of production (ROP) analysis indicates that unimolecular C-C bond dissociation is one of the leading consumption pathways of TMEDA, forming the (CH3)2NCH2 radical. In addition, the H-atom abstraction reactions involving CH3 radicals formed by the C-N bond dissociation of (CH3)2NCH2 radicals further promote the consumption of TMEDA. Sensitivity analysis highlights that the unimolecular C-C bond dissociation reaction of TMEDA and the reactions involving CH3 radicals mainly control the pyrolysis behavior of TMEDA. The detailed experimental results and kinetic analysis in this work are expected to contribute valuable references to further research on the combustion mechanism of TMEDA.

A Simplified Chemical Reactor Network Approach for Aeroengine Combustion Chamber Modeling and Preliminary Design

In a time when low emission solutions and technologies are of utmost importance regarding the sustainability of the aviation sector, this publication introduces a reduced-order physics-based model for combustion chambers of aeroengines, which is capable of reliably producing accurate pollutant emission and combustion efficiency estimations. The burner is subdivided into three volumes, with each represented by a single perfectly stirred reactor, thereby resulting in a simplified three-element serial chemical reactor network configuration, reducing complexity, and promoting the generality and ease of use of the model, without requiring the proprietary engine information needed by other such models. A tuning method is proposed to circumvent the limitations of its simplified configuration and the lack of detailed geometric data for combustors in literature. In contrast to most similar frameworks, this also provides the model with the ability to simultaneously predict the combustion efficiency and all pollutant emissions of interest (NOx, CO and unburnt hydrocarbons) more effectively by means of implementing a detailed chemical kinetics model. Validation against three correlation methods and actual aeroengine configurations demonstrates accurate performance and emission trend predictions. Integrated within two distinct combustion chamber low-emission preliminary design processes, the proposed model evaluates each new design, thereby displaying the ability to be employed in terms of optimizing a combustor’s overall performance given its sensitivity to geometric changes. Overall, the proposed model proves its worth as a reliable and valuable tool for use towards a greener future in aviation.