Laminar Flame Speed Research Articles

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.

Deflagration characteristics of N2 diluted propanal/air mixtures at elevated pressures

Hydrocarbon fuels, in addition to being direct sources of energy, also serve as crucial raw materials for numerous industrial processes. In this study, the oxidation of propanal to propionic acid under elevated pressures has garnered our attention due to the substantial heat release during the mixing of propanal and oxidants, posing considerable hazards to production. Fortunately, the introduction of N2 effectively mitigates these risks by reducing the laminar burning speed (SL) and peak deflagration pressure (Pm), and even preventing the ignition of mixtures. On this basis, the deflagration characteristics of propanal/N2/air mixtures were investigated in a 20 L spherical vessel at 348 K under various equivalent ratios (φ, 0.5 - 2.0), dilution rates (D, 0 - 0.5) and initial pressures (P0, 0.1 - 1.0 MPa). The combustion characteristics of propanal/N2/air were also analyzed through the 1-D laminar flame speed model, and the sensitivity analysis was performed to elucidate the effect of φ, D and P0 on the combustion of propanal. Results reveal that the addition of N2 stabilizes the flame propagation, and the Pm of propanal/N2/air mixtures decreases with the increasing D, with a sudden drop observed at a certain D level. However, the reduction of SL does not undergo an abrupt decrease across the entire dilution rate range. Furthermore, Pm exhibits a linear relationship with P0 at lower D, but this linear trend disappears once D approaches the critical suppression concentration. Interestingly, the critical suppression concentration of N2 increases with P0 at fuel-rich conditions, yet remains unaffected by P0 at fuel-lean conditions. Near the critical suppression concentration, a limiting burning speed (Slim) emerges, below which the flame is incapable of spontaneous propagation. Notably, Slim is not a fixed value, but varies according to the composition and initial state of the mixture. For a given gas component, an increase in P0 leads to a decline in Slim, while for gases with varying components, Slim is observed to be lower under fuel-rich conditions compared to fuel-lean scenarios. Also, both the addition of N2 and the augmentation of P0 significantly reduce the SL. When SL is comparable to the gas rise speed due to natural convection, the flame propagation becomes vulnerable to buoyancy instability. Meanwhile, the flame propagation may also be influenced by thermal diffusion instability and hydrodynamic instability. Under the combined effect of these instabilities, the flame is prone to be unstable. More importantly, when the flame contacting the upper wall of the is in a critical state where it can or cannot propagate downward, the irregularity of the disturbed flame may introduces uncertainty into the subsequent propagation, resulting in markedly different Pm even among mixtures with identical compositions.

Effects of compositional uncertainties in cracked NH3/biosyngas fuel blends on the combustion characteristics and performance of a combined-cycle gas turbine: A numerical thermokinetic study

Blending of partially cracked ammonia with biosyngas is an attractive strategy for improving NH3 combustion. In practice, products of biomass gasification and those of thermo-catalytic cracking of NH3 are subject to some compositional uncertainties. Despite their practical importance, so far, the effects of such uncertainties on combustion systems remained largely unexplored. Hence, this paper quantifies the effects of small compositional uncertainties of reactants upon combustion of partially cracked NH3/syngas/air mixtures. An uncertainty quantification method, based on polynomial chaos expansion and a data-driven model, is utilised to investigate the effects of uncertainty in fuel composition on the laminar flame speed (SL) and adiabatic flame temperature (Tad) at different inlet pressures (Pi). The analysis is then extended to the power output of a combined-cycle gas turbine fuelled by the reactants. It is found that 1.5% fuel compositional uncertainty can cause 12–21% of SL uncertainty depending on the inlet pressure. Furthermore, the effect of compositional uncertainty on Tad increases at higher ratios of H2 to NH3. Sensitivity analysis reveals that the uncertainty of CO contribution to SL uncertainty is higher than that of NH3, while the trend is reversed for the Tad uncertainty. In addition, the power output from the combined-cycle gas turbine system varies between 4 and 6% with 1.5% of fuel compositional uncertainty. This become more noticeable at elevated Pi [5–10 atm], particularly when the fuel mixture contains high H2 which is the main contributor to Tad variability.

Initiation and Carbene Induced Radical Chain Reactions in CH2F2 Pyrolysis.

High temperature dissociations of organic molecules typically involve a competition between radical and molecular processes. In this work, we use a modeling, experiment, theory (MET) framework to characterize the high temperature thermal dissociation of CH2F2, a flammable hydrofluorocarbon (HFC) that finds widespread use as a refrigerant. Initiation in CH2F2 proceeds via a molecular elimination channel; CH2F2→CHF+HF. Here we show that the subsequent self-reactions of the singlet carbene, CHF, are fast multichannel processes and a facile source of radicals that initiate rapid chain propagation reactions. These have a marked influence on the decomposition kinetics of CH2F2. The inclusion of these reactions brings the simulations into better agreement with the present and literature experiments. Additionally, flame simulations indicate that inclusion of the CHF+CHF multichannel reaction leads to a noticeable enhancement in predictions of laminar flame speeds, a key parameter that is used to determine the flammability of a refrigerant.

Effects of Pressure and Characteristic Scales on the Structural and Statistical Features of Methane/Air Turbulent Premixed Flames

In this study, the highly nonlinear and multi-scale flame-turbulence interactions prevalent in turbulent premixed flames are examined by using direct numerical simulation (DNS) datasets to understand the effects of increase in pressure and changes in the characteristic scale ratios at high pressure. Such flames are characterized by length-scale ratio (ratio of integral length scale and laminar thermal flame thickness) and velocity-scale ratio (ratio of turbulence intensity and laminar flame speed). A canonical test configuration corresponding to an initially laminar methane/air lean premixed flame interacting with decaying isotropic turbulence is considered. We consider five cases with the initial Karlovitz number of 18, 37, 126, and 260 to examine the effects of an increase in pressure from 1 to 10 atm with fixed turbulence characteristics and at a fixed Karlovitz number, and the changes to characteristic scale ratios at the pressure of 10 atm. The increase in pressure for fixed turbulence characteristics leads to enhanced flame broadening and wrinkling due to an increase in the range of energetic scales of motion. This further manifests into affecting the spatial and state-space variation of thermo-chemical quantities, single point statistics, and the relationship of heat-release rate to the flame curvature and tangential strain rate. Although these results can be inferred in terms of an increase in Karlovitz number, the effect of an increase in pressure at a fixed Karlovitz number shows differences in the spatial and state-space variations of thermo-chemical quantities and the relationship of the heat release rate with the curvature and tangential strain rate. This is due to a higher turbulent kinetic energy associated with the wide range of scales of motion at atmospheric pressure. In particular, the magnitude of the correlation of the heat release rate with the curvature and the tangential strain rate tend to decrease and increase, respectively, with an increase in pressure. Furthermore, the statistics of the flame-turbulence interactions at high pressure also show sensitivity to the changes in the characteristic length- and velocity-scale ratios. The results from this study highlight the need to accurately account for the effects of pressure and characteristic scales for improved modeling of such flames.

Shock tube and comprehensive kinetic modeling study of ammonia/diethyl ether (DEE) mixtures

Ammonia (NH3) is a promising alternative clean fuel due to its carbon-free and high hydrogen content, along with the well-established infrastructure for storage and distribution. To overcome the issue of the low reactivity of NH3, a feasible strategy is co-burning ammonia with highly reactive fuels. Diethyl ether (DEE) is considered a promising alternative and biomass-oxygenated clean diesel substitute. Therefore, the NH3/DEE blend is also a promising carbon neutral alternative fuel. In this work, we measured the ignition delay times (IDTs) of NH3/DEE mixtures with DEE fractions (XDEE) of 0.05, 0.10, 0.30, and 1.00 at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 1.75 and 10 bar, and temperature ranges from 1102 K to 1673 K in a shock tube. The DEE-NH3 model was proposed in this work, which included the DEE sub-model, NH3 sub-model, and some new cross-reactions between N-containing species and C-containing species. The DEE sub-model from Shrestha et al. (Fuel Communications, 2022) was modified in this work, NH3 sub-model was from our previous work (Reaction Chemistry & Engineering, 2023). The DEE-NH3 model was extensively validated by the IDTs, laminar flame speeds (LFSs), and species profiles (SPs) of NH3/DEE mixtures as well as pure DEE and NH3. The comparison of the prediction performance between the DEE-NH3 model and the Shrestha model was conducted for the ignition, flame propagation, and NH3 consumption. The effects of the cross-reactions on the NH3/DEE ignition and combustion were studied in detail.

Flow, combustion and species fields of premixed CH4/H2/CO syngas oxy-flames on a swirl burner: Effects of syngas composition

Addressing burning characteristics of syngas under oxy-combustion conditions in substantially CO2-diluted combustion environment for gas turbine combustion applications has rarely been reported. A detailed modeling study was performed over a broad range of syngas compositions at fixed inlet flow velocity of 6 m/s, fixed equivalence ratio of 0.42, and fixed oxygen fraction (OF) of 60% (by vol.). For validating the numerical model, the calculated results were compared against experimental results recorded from the same physical combustion setup. The effects of syngas composition on flow field, laminar flame speed (LFS), flame thickness, adiabatic flame temperature (AFT), combustor power density (PD), thermal power (TP), and species distributions were investigated. Adiabatic flame temperature increased from 2312 to 2388 K, LFS increased from 0.53 to 1.32 m/s, PD reduced from 2854 to 2672 kW/m3, and TP decreased from 4.0 to 3.7 kW when the hydrogen fraction (HF) was changed from 0 to 80%.When the HF was reduced from 90% to 10%, with excluding CH4, the AFT increased from 2447 to 2560 K, the LFS decreased from 2.01 to 0.46 m/s, the TP increased from 3.64 to 4.1 kW, and the PD increased from 2620 to 2923 kw/m3.

Combustion Analysis of Active Pre-Chamber Design for Ultra-Lean Engine Operation

<div>In this article, the effects of mixture dilution using EGR or excessive air on adiabatic flame temperature, laminar flame speed, and minimum ignition energy are studied to illustrate the fundamental benefits of lean combustion. An ignition system developing a new active pre-chamber (APC) design was assessed, aimed at improving the indicated thermal efficiency (ITE) of a 1.5 L four-cylinder gasoline direct injection (GDI) engine. The engine combustion process was simulated with the SAGE detailed chemistry model within the CONVERGE CFD tool, assuming the primary reference fuel (PRF) to be a volumetric mixture of 93% iso-octane and 7% n-heptane. The effects of design parameters, such as APC volume, nozzle diameter, and nozzle orientations, on ITE were studied. It was found that the ignition jet velocity from the pre-chamber to the main chamber had a significant impact on the boundary heat losses and combustion phasing. The simulation showed that, under 16.46 compression ratio (CR) and 8.93 bar indicated mean effective pressure (IMEP) condition, it is possible to achieve the peak ITE of 49.85% with λ = 2.23.</div>

Flame morphology and laminar flame assessments affected by flames interaction using multi-ignition sources of NH3/H2-air flames

A detailed assessment of flame–flame interaction and laminar flame evolution using multi-ignition sources is experimentally studied. To understand the flame interaction, the centrally ignited flame is measured and calculated for comparison with multi-ignition sources hydrogen–ammonia/flame. The location of the external ignition source, the delay time, the hydrogen blending, and the mixture equivalence ratio at an initial pressure of 0.1 MPa affect the propagation and morphology of the flame. It can be observed that the advancement of the pressure wave of the external flame causes deformation to the central flame front; This deformation occurs even before the interaction of the flames. The deformation can be decomposed into horizontal deformation, which decelerates the flame front as a result of the drag or accelerates due to the thrust of the flow field on the flame front. At the same time, vertical deformation is influenced by drag and thrust-lift forces. Therefore, the equivalent flame decelerates with time. This effect gives a nonsymmetric shape for expanding flame, and the shape changes from spherical to ellipsoidal, then a triaxial quasi-ellipsoid flame (scalene). The equivalent flame speed and laminar burning velocity are maximized near stoichiometry for all delay times and locations of the ignition source. As the delay time of the stoichiometric hydrogen ammonia/air increases, the equivalent laminar flame speed and laminar burning velocity monotonously decrease, as well as the time and location of the interaction. The equivalent flame speed and laminar burning velocity for ignition sources 1 and 2 decreases with delay time, and this becomes evident on the rich side. While employing a third ignition source increases with delay time since the drag force get eliminated from the horizontal axis. Furthermore, the hydrogen blending effect enhances and highlights these tendencies.

Comparison and optimization of strategies for ammonia-diesel dual-fuel engine based on reactivity assisted jet ignition and reactivity turbulent jet disturbance under high load conditions

Ammonia (NH3), as a zero carbon, easy to synthesize, and easily obtainable fuel, is one of the choices for future internal combustion engine fuels. However, the slow laminar flames speed and high ignition temperature of NH3 seriously hinder the development of NH3 engines. Efficiently igniting NH3 and ensuring its stable combustion is currently a challenging research topic. Utilizing diesel to optimize the NH3 combustion process is one effective solution. However, the increase of unburned ammonia (UNH3) and unstable combustion, under high load conditions have always been technical challenges that constrain the development of ammonia-diesel dual-fuel (ADDF) engines. Previous studies have found that pre-chambers (PC) have a significant effect on improving ammonia combustion characteristics. Therefore, based on the PC system, this study proposes two combustion modes: Reactivity Assisted Jet Ignition (RAJI) and Reactivity Turbulent Jet Disturbance (RTJD), which greatly optimize the combustion characteristics of ADDF engines under high load conditions. The research results indicate that the RAJI mode effectively improves the thermal environment inside the cylinder. Setting the start of diesel injection of pre-chamber (SODI-PC) at −14 °CA after top dead center (ATDC) can increase the cylinder temperature by 25.05% before the start of diesel injection of main chamber (SODI-MC). The improvement of thermal environment reduces the average generation of N2O to 13.56% of the traditional mode. However, if SODI-PC is postponed to −8 °CA ATDC, the effect of creating a thermal environment is no longer significant. Research on the RTJD mode has found that adopting the RTJD mode can significantly optimize emissions. Compared with traditional modes, N2O, UNH3, and Soot decreased by 28.81%, 27.79%, and 35.08%, respectively. Additionally, SODI-PC is delayed until after 4 °CA ATDC, CO and UHC will be further reduced to 50.00% and 12.24% of the traditional mode, respectively. In addition, the RTJD mode overcomes the deficiency of the slow laminar flame speed of NH3, shortening the combustion duration by 23.46%, and effectively improving the gross indicated thermal efficiency (ITEg) by an average increase of 1.08%, with the highest increase reaching 47.57%. This paper provides ideas for optimizing the structure and strategy of ADDF engines, as well as theoretical support and empirical reference for achieving clean and efficient combustion of engines.

Comparison of Tabulated and Complex Chemistry Approaches for Ammonia–Diesel Dual-Fuel Combustion Simulation

<div>Using ammonia as a carbon-free fuel is a promising way to reduce greenhouse gas emissions in the maritime sector. Due to the challenging fuel properties, like high autoignition temperature, high latent heat of vaporization, and low laminar flame speeds, a dual-fuel combustion process is the most promising way to use ammonia as a fuel in medium-speed engines.</div> <div>Currently, many experimental investigations regarding premixed and diffusive combustion are carried out. A numerical approach has been employed to simulate the complex dual-fuel combustion process to better understand the influences on the diffusive combustion of ammonia ignited by a diesel pilot. The simulation results are validated based on optical investigations conducted in a rapid compression–expansion machine (RCEM). The present work compares a tabulated chemistry simulation approach to complex chemistry-based simulations. The investigations evaluate the accuracy of both modeling approaches and point out the limitations and weaknesses of the tabulated chemistry approach. When using two fuels, the tabulated chemistry approach cannot reproduce misfiring events due to inherent model limitations. By adjusting the model parameters of the tabulated chemistry model, it is possible to reproduce experimental results accurately for a specific case. However, using the adjusted parameters for simulations with changed injection timing or interaction angle between the sprays shows that no predictive calculations are possible. The parameter set is only valid for a single operation point.</div> <div>Further simulations show that the complex chemistry approach can capture the complex interaction between both directly injected fuels for different operation points. It correctly predicts the ignition as well as heat release. Therefore, the approach allows predictive combustion simulations. Furthermore, it reproduces the occurrence of misfiring in cases of unsuitable interaction of both sprays and injection timing.</div>

Assessing Hydrogen–Ammonia Ratios to Achieve Rapid Kernel Inception in Spark-Ignition Engines

Abstract In the quest for decarbonizing internal combustion engines, ammonia (NH3) is recognized as a viable alternative fuel due to its zero-carbon emission profile, positioning it as a potential substitute for conventional petroleum fuels. However, the suboptimal combustion characteristics of ammonia pose challenges for its direct application in engines. The introduction of hydrogen (H2) as a combustion enhancer shows promise in improving ammonia viability for engine use. While previous studies have confirmed the benefits of hydrogen addition to ammonia for enhanced engine performance, comprehensive analysis of the precise ammonia-to-hydrogen ratio for optimal efficacy remains scarce. This research aims to bridge this gap by evaluating hydrogen–ammonia mixtures for achieving methane-equivalent laminar flame speeds under typical engine conditions, with a focus on the kernel inception process primarily driven by laminar flames. The findings indicate that a minimum of 20% hydrogen mixed with ammonia is necessary to facilitate rapid spark inception, although it does not reach the laminar flame speed of methane. Additionally, employing a high compression ratio and operating near stoichiometry could lower the required hydrogen–ammonia ratio. Considering the challenges in generating ample hydrogen with NH3 dissociators and the need for operational conditions like full-load and low-speed to lessen hydrogen demand, ammonia–hydrogen fuel blends are deemed most suitable for stationary engine applications in the near term.

Study on suppression mechanism of NH4H2PO4 inhibiting gas flame of red pine pyrolysis

Ammonium phosphate fire extinguishing agents, due to their excellent cost-effectiveness and suitability for forest fires, have become one of the most commonly used fire suppressants for forest fire control. Among them, NH4H2PO4 fire extinguishing agent is the most widely applied. In this study, NH4H2PO4 (>90%) as the main component of dry powder fire extinguishers was selected as the suppressant to investigate the reaction mechanism of NH4H2PO4 interacting with red pine wood pyrolysis gas flames. Initially, we constructed a Bunsen burner experimental system suitable for NH4H2PO4/red pine wood pyrolysis gas/air. Although experimental methods yield the most intuitive empirical conclusions, measuring only the laminar flame speed of red pine wood pyrolysis gas/NH4H2PO4/air mixture does not adequately analyze the inhibitory mechanism of NH4H2PO4 on red pine wood pyrolysis gas flames. Additionally, due to limitations in experimental equipment and methods, it is not feasible to extend equivalence ratios and NNH4H2PO4 doses to large or small working conditions. Therefore, in this study, we utilized Chemkin software to construct theoretical models of flames under different conditions and performed calculations. We comprehensively analyzed the kinetic regulatory mechanisms of NH4H2PO4 interacting with red pine wood pyrolysis gas flames from aspects such as chemical inhibition, sensitivity of elementary reactions, elementary reaction yields, and chemical reaction pathways.

Modeling Ammonia-Hydrogen-Air Combustion and Emission Characteristics of a Generic Swirl Burner

Abstract The combustion process of both pure NH3 and a NH3/H2 fuel blends is here analyzed using two kinetics processors, i.e., Chemkin-Pro-and CANTERA: detailed kinetic mechanisms have been tested and compared in terms of laminar flame speed and ignition delay time (IDT) with the aim to identifying the most suitable ones for the evaluation of NOx emissions. The generic swirl burner being used in Cardiff University's Gas Turbine Research Center has been considered as validation test case. In addition, this paper presents an experimental campaign followed by a computational fluid dynamics (CFD) approach for the assessment of NOx emission using axisymmetric Reynolds-Averaged Navier–Stokes (RANS) simulations, leading to a significant reduction of the computational time. Different pressures and mass flow rates are evaluated to understand correlations of NOx formation for pollutants reduction purpose. A direct comparison between experimental and numerical results is carried out in terms of flow field, flame shape, and NOx emissions. Results show that the increase in pressure from 1.1 bar to 2 bar results in reduction of NOx emissions from 2515 ppmv to 885 ppmv, also indicating guidelines for using a simplified RANS analysis, which leads to improved computational efficiency, allowing wide sensitivity and optimization analysis to support the design development of an industrial combustion system.

Development of a reduced mechanism for methane combustion in OpenFOAM: A computational approach for efficient and accurate simulations

The primary aim of this study is to develop a more efficient reaction mechanism for accurately simulating methane combustion, with a specific focus on ignition delay, laminar flame speed, and 2-D simulated flames, while also reducing computational time. Ten reduced reaction mechanisms for methane combustion were evaluated, with only one, "SK30," meeting the required accuracy standards. However, SK30 proved to be computationally intensive when simulating a 2-D premixed flame at a microscale. To address this challenge, a two-step reduction process was implemented. Firstly, an automated algorithm utilized direct relation graphs and sensitivity analysis with ignition delays as a reference to streamline the mechanism while maintaining accuracy. Subsequently, the second step involved identifying key reactions that had a more significant impact on flame speed than ignition delay through sensitivity analysis. Any missing reactions were then added judiciously, prioritizing the retrieval of the missing but important reactions to overall accuracy while minimizing computational cost. This process resulted in a novel mechanism comprising 25 species and 132 reactions for methane-air combustion. The validity of this mechanism was confirmed through comparison with a benchmark model, demonstrating satisfactory agreement in 1-D flame speed and 2-D premixed flame modeling. Most notably, the new mechanism substantially reduced processing time, achieving a 50 % speedup compared to SK30.

The trade-off characteristic between battery thermal runaway and combustion

Lithium iron phosphate (LFP) lithium-ion batteries are widely believed to be more thermally safe than nickel-rich layered LiNixCoyMnzO2 (NCM) batteries because LFP cathodes are more stable. However, LFP batteries have been reported to cause serious combustion accidents. The relationship between internal thermal runaway and external combustion in LFP and NCM batteries remain unclear. Herein, we found that there is a trade-off between thermal runaway within the battery and external combustion. Cathode oxidizability is linearly correlated with the intensity of thermochemical reactions within battery components. The nickel-rich LiNi0.9Co0.05Mn0.05O2|Graphite (NCM955|Gr) battery exhibits the most intense exothermic reactions and energy release due to the strong oxidation of its cathode, leading to the highest thermal runaway temperature of 862.4 °C within the battery. The LiFePO4|Graphite (LFP|Gr) battery with the weak cathode oxidizability shows the lowest temperature of 297.4 °C within the battery, but with the highest laminar flame speed of 55 cm s−1 outside the battery. The reason for this is that the LFP cathode rarely oxidizes the electrolyte, instead, the electrolyte reacts with the anode primarily, thereby generating substantial reductive gases and accumulating the explosion risks. This work provides the theoretical basis and data support for future battery selection and design.

Toward zero carbon emissions: High thermal efficiency low speed two-stroke marine engine using pure ammonia fuel

As the future development trend and the crucial link to achieving the carbon peaking and carbon neutrality goals in China, the achievement of “zero carbon” emissions in low-speed two-stroke marine engines is currently in the preliminary stages of exploration. Ammonia fuel is one of the main zero-carbon energy sources of interest for marine engines. The primary challenges associated with ammonia application include its high self-ignition temperature, low laminar flame speed, and lengthy ignition delay time. Although stable combustion can be achieved with the assistance of a small amount of highly active carbon-based fuel, carbon emissions are still inevitable. To obtain “zero carbon” emissions, the high-temperature cylinder gas recirculation strategy (HTCGR) was innovatively proposed to realize the possible application of pure ammonia. However, the heat transfer loss by using pure ammonia fuel is enhanced by 289% compared to the diesel mode. Therefore, the main objective of this article is to investigate ways to decrease heat transfer loss and further improve the thermal efficiency of the two-stroke engine from the perspective of energy distribution, and the effect of the circulating high-temperature gas, swirl ratio, and fuel distribution on heat transfer loss was systematically numerically studied in this manuscript. As a result, the heat transfer losses can be decreased up to 38.80% by lowering the CGR rate and adjusting the fuel distribution, and the ITE can be improved from 48.85% in the diesel model to 51.19% in the ammonia mode. However, to avoid incomplete combustion in the engine, a minimum CGR rate of 13.3% is basically required for the compression ignition of pure ammonia fuel.

Laminar flame speed measurement and combustion mechanism optimization for ethylene–air mixtures

AbstractEthylene plays a crucial role as an intermediate component in the cracking and combustion processes of large molecular alkane and olefins. In this article, the laminar flame speed of ethylene–air mixtures was measured using the heat flux method. The mechanism of ethylene was simplified by utilizing the error propagation directed relationship graph (DRGEP) and sensitivity analysis (SA), and the Arrhenius pre‐exponential factors for 20 selected reactions in the skeletal mechanism were optimized using the particle swarm optimization (PSO) algorithm. Finally, an ethylene optimization mechanism including 39 species and 85 reactions was obtained. The prediction results for flame speed, ignition delay time, and species concentration were compared with experimental data and other mechanisms, covering a wide range of temperatures (298–1725 K), pressures (1–22.8 atm), and equivalence ratios (0.5–2.0). The findings demonstrate that the optimization mechanism not only improves the prediction results of laminar flame speed in the rich combustion zone and low oxygen environment but also enhances the prediction accuracy of the ignition delay time at high pressure and in the lean combustion zone, as well as the prediction accuracy of C2H4 and H2O radicals. In conclusion, the optimized mechanism exhibits higher accuracy and broader applicability.

Chemical kinetic model reduction based on species‐targeted local sensitivity analysis

AbstractReduction of large combustion mechanisms is usually conducted based on the detection and elimination of redundant species and reactions. Reaction elimination methods are mostly based on sensitivity analysis, which can provide insight into the kinetic system, while species elimination methods are more efficient. In this work, the species‐targeted local sensitivity analysis (STLSA) method is proposed to evaluate the importance of species and eliminate non‐crucial species and their related reactions to simplify kinetic models. This paper comprehensively evaluates the effectiveness of STLSA across various combustion scenarios, including high and low‐temperature ignition and laminar flame speed, using diverse mechanisms like USC Mech II, JetSurf 1.0, POLIMI_TOT_1412, NUIGMech1.1 and so on. Comparisons with graph‐based methods, such as DRG and DRGEP, highlight STLSA's superior efficiency and accuracy. Moreover, STLSA is compared to species‐targeted global sensitivity analysis (STGSA), demonstrating significant computation cost savings and comparable model reduction capabilities. The study concludes that STLSA is a robust and versatile tool for mechanism reduction, offering substantial improvements in computational efficiency while maintaining high accuracy in predicting key combustion properties.

Development of a surrogate and its comprehensive compact chemical kinetic mechanism for the combustion of Alcohol-To-Jet fuel

This study develops a compact high-fidelity chemical kinetic mechanism for a proposed surrogate of Alcohol-To-Jet fuel, capable of modelling ignition delay time, laminar flame speed, and species concentration. A combination of 85% iso-dodecane and 15% iso-cetane by wt% that is a close approximation to the real fuel composition was selected as the ATJ surrogate to investigate the simulation of the three given combustion properties. A relatively good agreement has been observed for ignition delay, laminar flame speed, and mole fraction of some species, between the simulation by the Chemkin-Pro software package and the available experimental data in the literature. The authors suggest that this proposed surrogate with its compact validated mechanism can be used by researchers to study the combustion behaviour of ATJ fuel including the investigation on the complex geometries of combustion system such as Equivalent Reactor Network Analysis which require a compact accurate chemical kinetic mechanism.