Ignition Delay Research Articles

Machine Learning Approaches for Predicting Ignition Delay in Combustion Processes: A Comprehensive Review

Machine Learning Approaches for Predicting Ignition Delay in Combustion Processes: A Comprehensive Review

An improved particle swarm algorithm-based method for kinetic modeling study of ammonia/air laminar flame speed

In recent years, ammonia has garnered increasing attention as a promising carbon-free fuel. Laminar flame speed is a critical property of ammonia fuel that has been extensively studied by researchers using chemical kinetics mechanisms. However, some deviations still remain in the numerical predictions. To further improve the prediction accuracy of the laminar flame speed for ammonia/air flame, an ammonia kinetic mechanism is developed in this work. Reactions from Gri-Mech 3.0, Li et al. mechanism, and Han et al. mechanism that have significant impact on laminar flame speed were first assembled to the Okafor et al. mechanism, and H/OH sub-mechanisms were merged to establish a kinetic mechanism containing 43 species and 142 reactions. A particle swarm algorithm with improved inertia weights is proposed and the fitness function for outputting laminar flame speed prediction error is customized based on Cantera codes, then forming the algorithm framework used to optimize the kinetic mechanism. The pre-exponential factor A, the temperature exponent n and the activation energy Ea of the four reactions in the merged mechanism that have high sensitivity to the laminar flame speed are selected as the independent variables for optimization, and the reactions rate constants corresponding to the high prediction accuracy of the laminar flame speed is finally obtained. The numerical prediction indicates a reduction in mean absolute percentage error of laminar flame speed from 22.563 % to 10.649 % using the optimization mechanism. The results of the sensitivity and reaction pathways analyses demonstrated that more H-related reactions were considered in the optimized mechanism, and the relative ROP of H-related reactions were adjusted at different equivalence ratios, resulting in higher or lower predictions of laminar flame speed compared with the unoptimized Okafor et al. mechanism. Ignition delay time as well as the species distribution of NH3, NO, and N2O was studied. The ignition delay time predictions using the optimized mechanism are in better agreement with the experiment data at P = 11 atm and 30 atm. The optimization mechanism demonstrated accurate predictions for NH3 and NO while exhibiting an overestimation for N2O, but the N2O prediction error is smaller compared to the Okafor et al. mechanism.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Performance of a single cylinder diesel engine fuelled with emulsified residual oleins and standard diesel fuel

Emulsified residual oleins and standard diesel fuel were evaluated. The engine tests for the fuels analyzed were performed in a Petter single cylinder direct injection diesel engine under steady state conditions at fixed torque values (20 Nm and 34 Nm) and the engine speeds (n) between 1300-1700 rpm. Power output, specific fuel consumption, ignition delay and exhaust composition were evaluated. The emulsified residual oleins (ERO) shown lower power output joint to a higher specific fuel consumption related to the lower heating value of residual oleins compared to diesel fuel. A shorter ignition delay was observed for the ERO. Decreases in NOx emission were obtained for ERO compared to diesel fuel. The use of ERO in order to be a partial or full alternative to the use of diesel fuel for energy production was achieved.

Engine performance of a single cylinder direct injection diesel engine fuelled with blends of Jatropha Curcas oil and standard diesel fuel

Blends of Jatropha Curcas oil and standard diesel fuel were evaluated (without pre-heating). The engine tests for the blends were performed in a Petter single cylinder direct injection diesel engine under steady state conditions at high loads. Engine speeds between 1300-1700 rpm were selected for the engine tests. Torque, power output, specific fuel consumption, in cylinder pressure, ignition delay, rate of heat released and exhaust composition were evaluated. The tested blends between 10-20% of oil shown lower effective torque and power output joint to a higher specific fuel consumption related to the lower heating value of Jatropha oil compared to diesel fuel. Lower pressure peaks and rates or pressure rises were observed when Jatropha blends are used. A decrease in the rate of heat released and shorter ignition delay were observed for the blends. Decreases in HCand CO emissions were observed for blends compared to diesel fuel. Both alternatives assessed shown that the differences observed compared to diesel fuel, can be partially restored with engines regulation. The use of Jatropha oil in order to be a partial or full alternative to the use of diesel fuel for energy production was achieved.

Experimental analysis and numerical simulation of ignition delay time of diesel fuel using a shock tube

The present work consisted in developing a computational routine for prediction and characterization ignition delay time, pressure, temperature generated and energy released in the combustion process in a shock tube. Mathematical modeling considered the high-pressure region of the shock tube known as the conducted section. The Lazarus code compiler was used to generate the unstructured triangular meshes and to implement the computational routines to simulate the propagation of shock waves inside the shock tube. The mathematical modeling used the geometric parameters of the shock tube of the combustion laboratory of the Federal University of Minas Gerais and was based only on the displacement of the shock wave after the diaphragm rupture. The main objective of the work was the development of computational routines to characterize the shock wave parameters of the ignition delay times of conventional diesel to validate the experimental tests carried out in the shock tube conducted by Santana et al. [3] and compare it with the experimental tests and numerical simulations carried out by other authors available in the literature. A linear regression of the experimental tests conducted by Santana et al. [3] with conventional diesel was performed to obtain the Arrhenius equation for numerical simulation of the ignition delay time of diesel under the following conditions: temperatures from 880 to 1300 K, pressures 24 bar and equivalence ratio 1. The results show good prediction between experimental tests and numerical simulations. Were found delay times ranging from 425 to 1890 μs. Considering all temperature range, the difference between the experimental and simulated test was approximately 15%, this difference also can be explained by the measurement in the shock tube, that the ignition delay time was calculated by the time difference between the passage of the shock wave by the pressure sensor and the start of the ignition detected by the luminosity detection sensor. This work is relevant because it was developed computational routine in open software and the results achieved in the simulation were considered satisfactory, since they are in the same order of magnitude as the results found in the experimental tests carried out by Santana et al. [3] and other works.

Modeling and chemical kinetic analysis of methanol and reformed gas (H2/CO2) blending with ammonia under lean-burn condition

Hydrogen can be easily produced by methanol steam reforming. This work aims to investigate the effects of methanol addition and its reformed gas of H2/CO2 on ammonia combustion under lean-burn operation. A newly-developed reaction mechanism for NH3, H2, CH3OH, CO and their blends was established based on the highly-precise validations of laminar burning velocity (LBV), nitrogen monoxide (NO) and ignition delay time (IDT). The premixed flame analysis showed that methanol blends and reforming with hydrogen addition can greatly enhance the combustion rate, resulted from the chemical effect. The equivalence ratio (ER) can be extended to 0.45 for a methanol blending ratio of 60 % and reforming ratio of 90 % (M60R90) to achieve the similar level of LBV at pure ammonia at an ER of 1.0 under ambient condition. The methanol blends and reforming lead to the increase of NO, while the lean-burn operation can help to reduce the NO emission.

Engineering High-Performance Hypergolic Propellant by Synergistic Contribution of Metal-Organic Framework Shell and Aluminum Core.

Hypergolicity is a highly desired characteristic for hybrid rocket engine-based fuels because it eliminates the need for a separate ignition system. Introducing hypergolic additives into conventional fuels through physical mixing is a feasible approach, but achieving highly reliable hypergolic ignition and energy release remains a major challenge. Here, the construction of core-shell Al@metal organic framework (MOF) heterostructures is reported as high-performance solid hypergolic propellants. Upon contact with the liquid oxidizer the uniformly distributed hypergolic MOF (Ag-MOF) shell can induce the ignition of hypergolic-inert fuel Al, resulting in Al combustion. Such a synthetic strategy is demonstrated to be favorable in hotspot generation and heat transfer relative to a simple physical mixture of Al/Ag-MOF, thus producing shorter ignition delay times and more efficient combustion. Thermal reactivity study indicated that the functionalization of the Ag-MOF shell changes the energy release process of the inner Al, which is accompanied by a thermite reaction. The synergistic effect of implantation of hypergolic MOF and high energy Al contributes to high specific impulses of 230-270 s over a wide range of oxidizer-to-fuel ratios.

Optimizing combustion and emissions in natural gas/diesel dual-fuel engine with pilot injection strategy

For the clean and efficient operation of agricultural power, a non-road common-rail engine was modified to achieve natural gas/diesel dual-fuel (NDDF) combustion mode. Under low load conditions, NDDF combustion deteriorates and fuel economy decreases due to lower pressure and temperature inside the cylinder. Therefore, effects of injection pressure, pilot injection timing (SOIpilot) and pilot injection quantity (Qpilot) on in-cylinder combustion, emission characteristics and fuel economy of NDDF engine with pilot injection strategy were investigated at low load. Results show that with the increment of injection pressure, the diesel fuel beam penetration distance increased, prompting the diesel more quickly and fully mixed with fresh charge. The maximum in-cylinder pressure increased, the combustion center (CA50) moved forward, the ignition delay period was shortened, and soot, HC, C2H4 and C3H6 emissions were reduced, while BTE was improved. When SOIpilot moved forward, the pilot injected diesel and natural gas were more fully mixed, which promoted the generation of activation radicals and combustion intensity of main injected diesel. Pmax first increased and then decreased, the peak value of first heat release rate (HRRP1) gradually decreased, the peak value of second heat release rate (HRRP2) continuously increased, the combustion duration decreased, and the soot, CO and HC emissions decreased. At SOIpilot = −30 °CA ATDC, unregulated emissions reached the lowest level, while BTE was 35.55 %. With the increment of Qpilot, both Pmax and HRRP1 increased, CA05 and CA50 moved forward, combustion duration was extended, COV decreased significantly, and all emissions except NOx decreased, especially aldehyde emissions.

Ignition behavior of a mixture of brown coal and biomass during the movement of fine particles in a hot air flow

The combustion of brown coal, biomass and their mixtures was studied experimentally and analytically. The experimental research was conducted using a laboratory setup that reproduced the conditions of combustion of pulverized solid fuel in a boiler furnace. Thermogravimetric analysis was employed as part of the analytical study to determine the key characteristics of combustion of solid fuels, such as temperatures at which the ignition and burnout occur, as well as the combustion index. Scanning electron microscopy was utilized to analyze the surfaces of coal and wood particles to detect pores, cracks and channels. To determine the tendency of the solid fuel mixture to slag heat exchangers in boiler furnaces, the ash residue characteristics (iron oxide deposits, active alkali, calcium sulfate compounds) were obtained. Biomass is more reactive than brown coal. Its ignition delay times at 500 °C and 750 °C are 57 % and 25 % lower, respectively, than those for coal. The higher the biomass content in the solid fuel mixture, the lower the ignition delay time. The heating source temperature at which the coal/biomass fuel mixture ignites corresponds to the temperature at which biomass ignites. With a larger proportion of biomass, the temperature during the fuel mixture combustion goes down, while the combustion index increases. On the basis of the findings of the conducted research, optimal proportions of solid fuel components were found (80 % brown coal, 20 % biomass). Also, fuel preparation and feeding systems for pulverized fuel combustion and fluidized bed combustion of optimal fuel mixtures in a coal-fired boiler furnace were proposed.

Optical investigation of the influence of high-reactivity iso-octane turbulent jet ignition on the combustion characteristics of ammonia/air mixtures

Ammonia has attracted considerable attention as a zero-carbon fuel; however, challenges such as ignition difficulty and slow flame propagation persist. These challenges can be addressed by employing a highly reactive fuel in the pre-chamber to initiate the ignition of ammonia. This work aims to experimentally investigate the influence of pre-chamber fuel composition and concentration on ammonia combustion characteristics across various equivalence ratios. The results are as follows: elevating iso-octane equivalence ratios in the pre-chamber enhances stoichiometric ammonia combustion characteristics, reducing ignition delay and burn duration. However, excessive values result in significant delays and a decrease in flame speed. At an iso-octane equivalence ratio of 0.8, the main chamber exhibited the shortest combustion duration. By utilizing the optimal pre-chamber total equivalence ratios of 2.0 and 2.3, the ammonia lean combustion limit was achieved at an excess air ratio of 1.89 under experimental conditions (0.8 MPa, 440 K). As main chamber equivalence ratios decreased, ignition delay shortened. The pre-chamber total equivalence ratios exert a more pronounced impact on the main chamber ignition delay. In contrast, the main chamber equivalence ratios substantially influence the middle and later stages of combustion.

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.

Experimental study on microwave ignition of ADN-based liquid propellant droplets doped with alumina nanoparticles

The objective of this study was to investigate the effects of alumina (Al2O3) nanoparticles on the evaporation, ignition, and combustion of ammonium dinitramide (ADN)-based liquid propellants under microwave irradiation. The alumina nanoparticles were uniformly dispersed in the ADN-based propellant at mass ratios of 25, 50, and 100 ppm, resulting in the formation of nano-fuel. Experimental methods were employed to examine the influence of alumina nanoparticles on the micro-explosion intensity, ignition delay time, combustion duration, critical ignition power, and emission spectra of the nano-fuel droplets. The experimental findings revealed a significant increase in the intensity and frequency of micro-explosion events with the inclusion of alumina nanoparticles. Compared to pure propellant, the nano-fuel with a concentration of 25 ppm exhibited a reduction of 33.5% in ignition delay time and a decrease of 9% in critical ignition power. Furthermore, the emission spectra characteristics of the pure propellant and nano-fuel were analyzed, indicating that alumina nanoparticles notably enhanced the thermal decomposition of ADN and the exothermic reaction of methanol combustion. This study provides an effective approach to improve the evaporation and combustion performance of ADN-based liquid propellants, offering potential possibilities for future applications.

A theoretical and experimental study of 2-ethylfuran + OH reaction

2-Ethylfuran (EF2), having similar thermophysical properties as gasoline, can be produced from biomass and has received increasing attention in recent years. The reaction of EF2 with OH radical plays a fundamental role in combustion chemistry of furan fuels. This work provides a comprehensive investigation on the reaction kinetics of the title reaction. The performance of several density functional theory methods is evaluated against the DLPNOCCSD(T)/CBS(T-Q) benchmark method. The results show that M08-HX/jun-cc-pVTZ is the best one for both abstraction and addition reactions with the lowest average unsigned deviations of 0.61 and 0.63 kcal mol−1, respectively. Multi-structural variational transition state theory combined with small-curvature tunneling approximation (MS-CVT/SCT) is employed to calculate the high-pressure limit (HPL) rate constants over a wide temperature range of 500−2000 K. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) method is used to obtain the pressure-dependent rate constants for addition reactions at several selected pressures from 0.01 to 100 atm. We have measured the rate constants of EF2 + OH reaction behind reflected shock wave at temperatures of 985−1342 K and pressure around 1 bar. The measured rate constants exhibit a positive temperature dependence and may be described by a three-parameter Arrhenius expression: k=1.28×10−10×T6.89exp(5318T) (cm3 mol−1 s–1). The calculated results are in good agreement with measurements and the total rate constants exhibit a non-monotonic temperature dependence due to the competition between the abstraction and addition channels. At the HPL, the addition reaction is dominant, and the branching ratio is larger than abstraction reaction at 500–1800 K and is 0.92 at 500 K. Finally, to evaluate the performance of our calculations on the ignition delay time of EF2, three available kinetic models (Somers et al., Xu et al. and Li et al.) of EF2 are updated and the results show that the predictions of Li's model are improved.

Effects of ignition voltage and electrode structure on electric ignition and combustion characteristics of Ammonium Dinitramide (ADN)-based liquid propellants in electric ignition mode in inert gas environment

Hydrazine is toxic and carcinogenic, which greatly increases the difficulty of application and no longer meets the needs of green aerospace. As a green propellant, the Ammonium Dinitramide (ADN)-based liquid propellant has the advantages of higher specific impulse, being non-toxic, pollution-free, and easy storage. However, an ADN-based space engine in orbit has exposed the problems of high-temperature deactivation of catalysts and cold-start failure. An active ignition technology—electric ignition technology was explored in this paper to break through the technical bottleneck of catalyst deactivation and the inability to a cold start. An experimental system of a constant-volume combustor for the ADN-based liquid propellant based on the electric ignition method was established. The electric ignition and combustion characteristics of the ADN-based liquid propellant in a volume combustor with an electric ignition method were studied. The influencing mechanisms of the ignition voltage and the electrode structure on the electric ignition characteristics of the ADN-based liquid propellant were investigated. An elevation of the ignition voltage could facilitate the ignition process of the ADN-based liquid propellant, curtail electric energy input and heating effect, while exerting an adverse impact on the combustion process of the propellant. An increase in the ignition voltage enhanced the ignition process of the propellant while simultaneously suppressing its combustion process when utilizing mesh electrodes. Compared to the strip electrodes, the mesh electrodes increased the contact area between the electrodes and the propellant, increased the electric energy input power in the electric ignition process, and reduced the ignition delay time. The mesh electrodes could promote the combustion process of the propellant to a certain extent.

Experimental and modeling study on ignition kinetics of ethyl methyl carbonate

Ethyl methyl carbonate (EMC) is an essential electrolyte of lithium-ion batteries (LIBs), and the spontaneous ignition of electrolyte is considered to be an important inducement for the thermal runaway (TR) combustion of LIBs. To better understand its ignition chemistry and combustion kinetics, the ignition delay times (IDTs) of ethyl methyl carbonate were measured by using a Rapid Compression Machine (RCM) over a wide range of temperatures (810–980 K), pressures (20 and 40 bar), equivalence ratios (φ = 0.5, 1.0, and 2.0), and a fuel concentration of 2 %, yielding results that are in substantial variance with those calculated with detailed kinetics in the literature over the entire range of conditions investigated. An improved chemical kinetic model was developed to simulate the ignition and combustion characteristics of EMC under intermediate-temperature region by employing the updated kinetic parameters. Results show that the overall performance of the present kinetics model is substantially improved with respected to the prediction of ignition delay times, intermediate species profiles, as well as laminar flame speeds across different temperature, pressure, and equivalence ratio conditions. Furthermore, sensitivity and pathway analyses show that the ignition chemistry of EMC is mainly controlled by retro-ene reaction and H abstraction reactions.

Development of a Compact Skeletal Mechanism for Thermally Cracked Hydrocarbon Fuels

ABSTRACT The construction of a compact kinetic mechanism for thermally cracked hydrocarbon fuels holds great importance for developing advanced engine with regenerative cooling. In the present study, a skeletal oxidation mechanism consisting of 104 species and 373 reactions, which can be divided into two parts, i.e. a core mechanism for pyrolysis gas, and a skeletal sub-mechanism for pyrolysis liquid, was developed by a decoupling method. The combustion process of pyrolysis gas and its pure constituents was depicted by incorporating detailed reaction pathways of H2/CO/C1-C4 molecules, and variations of hydrocarbon classes, as well as carbon number distributions were also considered by introducing eight skeletal sub-mechanisms for heavy hydrocarbons which covered chain alkanes, cycloalkanes, and monocyclic aromatics in pyrolysis liquid, thus the newly developed mechanism was suitable for thermally cracked hydrocarbon fuels over a wide range of pyrolysis temperatures. The skeletal mechanism was extensively validated by experimental data of laminar burning velocities and ignition delay times, confirming its applicability for reproducing the flame characteristics of pyrolysis gases, pyrolysis liquids, and thermally cracked fuels. Sensitivity analyses showed that laminar burning velocities of thermally cracked fuels were only affected by reactions involving small radicals when the pyrolysis temperature was lower than 600°C, while reactions involving benzene rings started to dominate the flame propagation of thermally cracked fuel at 650°C under fuel-lean, stoichiometric, and fuel-rich conditions. Reactions of small radicals and heavy hydrocarbons both affected the ignition process of thermally cracked fuels.

Experimental investigation of fuel injection timing effects on a CRDI diesel engine running on hydrogen-enriched waste plastic oil

In response to fossil fuel concerns and the imperative of effective plastic waste management, this study investigates the impact of hydrogen-plastic oil (WPO) blends on a CRDI diesel engine. Six combinations (WPO10, WPO20, WPO30, WPO10 + H2, WPO20 + H2, WPO30 + H2) are examined across varying loads and fuel injection timings. Notably, WPO10 + H₂ (H2 energy share i.e., 8 lpm) exhibits a 15.1 % increase in brake thermal efficiency, an 11.2 % fuel consumption reduction, improved peak cylinder pressure (66 bar), and maximum heat release rate (40.16 J/°CA) compared to diesel. It also shows reduced emissions of CO, HC, CO2, and smoke opacity—34.8 %, 15.1 %, 10.7 %, and 28.5 %, respectively, relative to diesel. Elevated nitrogen oxide (NOx) emissions are observed across all blends. This study underscores the environmental benefits of blending waste plastic oil with diesel, promoting recycling, reducing reliance on traditional diesel, lowering greenhouse gas emissions, and supporting sustainability. Advanced fuel injection timing at higher loads slightly mitigates NOx emissions, while increasing the blend proportion leads to prolonged ignition delays, reduced thermal efficiency, and heightened emissions.

Low-temperature oxidation of n-butanol in a jet-stirred reactor: Detailed species measurements and modeling studies

n-Butanol is a new generation of liquid biofuel. A detailed understanding of the low-temperature combustion chemistry of n-butanol is instructive for its practical application in advanced low-temperature engines. The high-temperature combustion chemistry of n-butanol has been extensively studied, but its low-temperature combustion chemistry remains underexplored, especially the detailed species distribution. In this work, ozone addition was used to enhance the low-temperature oxidation reaction activity to achieve the oxidation of n-butanol in a jet-stirred reactor (JSR) at atmospheric pressure and in the temperature range of 400–800 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for detailed measurements of species distribution. The identification of some intermediate species contributed to elucidating and verifying the low-temperature oxidation reaction pathways of n-butanol, such as butanal, C4 hydroxyalkyl hydroperoxides, C4 hydroxyl cyclic ether, and C4 keto-hydroperoxide. Furthermore, the formation of several elusive intermediates was also identified (e.g., C2H4O2, C3H6O2, C4H6On (n = 1∼3), and C4H8O2), highlighting the necessity of further kinetic studies on the low-temperature oxidation of n-butanol. Based on detailed experimental measurements, the n-butanol model was further developed to improve the model predictions by updating the rate constants for the H-atom abstraction reactions of n-butanol by the ȮH radical, adding the detailed butanal sub-mechanism, etc. Especially, the premature formation of CO2 in the present experiment was satisfactorily explained and predicted by the addition of a generally accepted reaction class (i.e., RȮ2 + RȮ2 = RȮ + RȮ + O2 of butanal). Overall, this updated model satisfactorily predicts both the ignition delay time reported in the literature and the species distribution obtained in this work.

Exergy destruction behavior of chemical reactions during the auto-ignition of methane doped with hydrogen

Chemical reaction is the major source of combustion irreversibility in premixed conditions, and revealing the details of exergy destruction can provide a more essential perspective on exploring high-efficiency combustion strategies. The present study focuses on the homogeneous combustion process of CH4/H2/air mixtures, aiming to reveal the exergy destruction behavior of chemical reactions from the viewpoint of different combustion stages and oxidation paths. Results indicate that exergy destruction is mainly accumulated in the ignition delay stage, and the exergy destruction caused by different elementary reactions depends on the temperature at which the reaction takes place and the resulting change of free energy. By dividing the chemical reactions into four groups, the main reaction channels that contribute to the change in exergy destruction with combustion boundaries have been identified. Global reaction flux analysis reveals the effect mechanism of different oxidation paths on exergy destruction. Furthermore, the staged combustion concept with endothermic reactions reacting firstly at lower temperatures followed by dilute combustion in the high-temperature heat release stage is discussed from the point of the exergy destruction evolution, which offers the potential to simultaneously reduce exergy destruction and suppress NOx formation.