Ignition Delay Time Research Articles

EFFECT OF MAGNESIUM ON IGNITION AND COMBUSTION OF BORON-LOADED GELLED JET A-1 FUEL

Boron (B) is a desirable fuel for explosives and propellants due to its high-energy density. However, the combustion of B particles is difficult because its native oxide layers serve as inhibitors and cover the particles. This oxide layer makes the combustion process more difficult. However, using B as an additive in gelled fuel has significant potential use in ramjets. The present study used magnesium (Mg) with B-loaded gel-based fuel as a burning promotor. Different combustion parameters were evaluated for four different fuel compositions. The present investigation employed a droplet combustion setup and high-speed imaging technique to examine the impact of Mg on B particles in gel fuel. The influences of Mg addition on the droplet combustion flame, B ignition delay time, boron oxide (BO<sub>2</sub>) intensity, and post-combustion particles were studied. The addition of Mg significantly influenced B in reducing the ignition delay time, where the reduction was almost 70.58% at higher Mg loadings (GBMg30). The combustion of B-based samples resulted in the emission of BO<sub>2</sub>, which was characterized by a greenish appearance. The findings were corroborated by spectroscopic analysis. The analysis of the burning events was conducted utilizing a combination of color and high-speed camera techniques in conjunction with a 546 nm BO<sub>2</sub> filter. Post-combustion particles were analyzed with the help of scanning electron microscopy images and thermogravimetric analysis.

Numerical study of the effect of NH3 addition on CH4/air combustion characteristics under gas turbine operating conditions

In this study, the effects of different NH3 additions on the laminar premix combustion characteristics of CH4/air flame were investigated, including laminar combustion velocity, ignition delay time, four important free radicals and NO emission.

A comprehensive experimental and kinetic modeling study of methyl tert-butyl ether combustion

A comprehensive understanding of the combustion chemistry of methyl tert‑butyl ether (MTBE) is of key importance in its application as an additive in gasoline fuels. Ignition delay times (IDTs) of MTBE/air mixtures have been measured in both a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) at equivalence ratios of 0.5, 1.0, and 2.0 in air, at pressures of 10 and 30 bar over the temperature range 600 – 1350 K. Species profiles for MTBE oxidation were obtained in a jet-stirred reactor (JSR) at 1 bar, at equivalence ratios of 0.5, 1.0, and 2.0 in the temperature range 700 – 1100 K.A detailed reaction mechanism, comprising 813 species and 4319 reactions, has been developed and predicts well all of the experimental data obtained in this work and also texisitng literature data. Pressure- and temperature-dependent rate constants for the MTBE unimolecular elimination reaction producing isobutene and methanol were calculated using high-level ab-initio calculations. A sensitivity analysis reveals that this elimination reaction is important, and significantly inhibits fuel reactivity at temperatures above 1300 K. At intermediate temperatures (850 – 1300 K), the reaction MTBE + ȮH = TĊ4H8OCH3 + H2O plays a crucial role in promoting the reactivity of MTBE oxidation, whereas the reaction MTBE + ȮH = TC4H9OĊH2 + H2O is the most inhibiting reaction. At low temperatures (600 – 850 K), the isomerization reaction of TC4OCȮ2–1 ⇌ TĊ4OCO2H-2 significantly promotes the reactivity. Conversely, the reaction 2TC4OCȮ2–1 ↔ 2TC4OCȮ-1 + O2 inhibits reactivity the most. The NTC behavior in MTBE oxidation can be explained by the competition between the reactions involving the formation and consumption of cyclic ethers from TĊ4H8OCH3 radicals and the reactions associated with the formation and consumption of carbonyl hydroperoxide species.

The role of chemistry in the retardant effect of dimethyl methylphosphonate in flame–wall interaction

Organophosphorous compounds can act as chemical inhibitors for fire suppression and have recently received significant attention in the combustion community due to their potential to induce flame extinction through a radical recombination process. To optimize their use, a comprehensive understanding of the extinction dynamics is essential. In this study, mixtures of methane with dimethyl methylphosphonate (DMMP) are investigated in laminar conditions, with a specific focus on flame–wall interaction. The ultimate objective is obtaining a deeper insight into the flame quenching process, which is synergistically enhanced by heat transfer to the wall and the chemical retardant effect of DMMP. Detailed chemistry information is implemented by setting up an ad-hoc skeletal kinetic mechanism. This is developed and validated for flame configurations with increasing complexity, including freely propagating premixed flames, head-on-quenching flames, and side-wall quenching flames. The results indicate that the skeletal mechanism is able to reproduce available experimental data for flame speed and ignition delay time, as well as the main flame features and quenching characteristics in flame–wall interactions for an increasing level of DMMP. Simulations of a side-wall quenching burner, incorporating a secondary fuel injection through a porous insert at the wall, are carried out. Chemical analyses provide detailed insight into the role of flame retardant during the flame quenching process. The results show that the HOPO/PO2 catalytic cycle is of major importance for the suppressant effect, further supported by the formation of CH3PO2 as an intermediate, increasing the formation of HOPO itself. Moreover, the CO mass fraction is observed to increase also because of the radical scavenging effect, inhibiting the conversion to CO2 via CO ＋ OH → CO2 ＋ H. Overall, this study advances the understanding of the chemical features of flame quenching in the presence of flame retardants containing implications for fire safety applications.

Low-temperature ignition and oxidation mechanisms of tetrahydropyran

Cyclic ethers are relevant as next-generation biofuels and are also combustion intermediates that follow directly from unimolecular decomposition of hydroperoxyalkyl radicals. Accordingly, cyclic ether reactions are crucial to understanding low-temperature oxidation for advanced compression-ignition applications in combustion where peroxy radials are central to degenerate chain-branching pathways. Reaction mechanisms relevant to low-temperature ignition of cyclic ethers contain intrinsic complexities due to competing reactions of carbon-centered radicals formed in the initiation step undergoing either ring-opening or reactions with O2. To gain insight into mechanisms describing tetrahydropyran combustion, ignition delay time and speciation measurements were conducted. The present work integrates measurements below 1000 K of ignition delay times and species profiles in rapid compression machine experiments from 5 – 20 bar, spanning several equivalence ratios, with jet-stirred reactor experiments at 1 bar and stoichiometric conditions. The experiments are complemented with the development of the first chemical kinetics mechanism for tetrahydropyran that includes peroxy radical reactions, including O2-addition to tetrahydropyranyl (Ṙ), HOȮ-elimination from tetrahydropyranylperoxy (ROȮ) and hydroperoxytetrahydropyranyl (Q˙OOH), bi-cyclic ether formation, β-scission of Q˙OOH, and ketohydroperoxide formation.Negative-temperature coefficient (NTC) behavior is exhibited in the experiments and is reflected in the model predictions, which were within experimental uncertainty for several conditions. Disparities between the model predictions and experiments were analyzed via sensitivity analysis to identify contributing factors from elementary reactions. The analyses examine the role of ring-opening products of tetrahydropyranyl and hydroperoxytetrahydropyranyl isomers to identify reaction mechanisms that may contribute to model uncertainties. The detection of 66 species in the JSR experiments indicates that tetrahydropyran undergoes complex reaction networks, which includes interconnected reactions of aldehyde radicals and alkyl radicals. Primary radicals pentanal-5-yl and butanal-4-yl are derived from ring-opening reactions of tetrahydropyran-1-yl and undergo subsequent decarbonylation to form alkyl radicals (1-butyl and 1-propyl) that undergo reaction with O2 and may contribute to chain-branching, in addition to pathways involving tetrahydropyran-derived ketohydroperoxides.

The influence of clustering in homogeneous isotropic turbulence on the ignition behavior of iron particles

We conduct carrier-phase direct numerical simulations (CP-DNS) to investigate the ignition of iron particles in homogeneous isotropic turbulence, and characterize the connection between particle clustering and particle ignition. A pseudo-spectral reacting multiphase flow solver using a low Mach number approximation is employed. It features an established point-particle sub-model for reacting iron particles and describes the mass, heat and momentum transfer across the particle boundary layers in a two-way coupled regime. Within this setup, we perform a series of simulations covering a broad range of Reynolds and Stokes numbers. The results confirm the well-known observation from incompressible dispersed multiphase flows that particle clustering is most pronounced for particle clouds with Stokes numbers of order one. Here, strong particle clustering additionally facilitates the earlier ignition of heterogeneously burning iron particles. This is because groups of neighboring particles locally deposit more heat per volume than a single isolated particle, thus more strongly increasing the local fluid temperature and shifting particle reactions from kinetically-limited to diffusion-limited particle conversion. A Voronoi tessellation analysis shows that the particles packed together in clusters tend to ignite first, even for small Stokes numbers, at which clustering is less prominent. An increase in Reynolds number reduces the ignition delay times, especially for high Stokes numbers, since particles with higher inertia are more sensitive to the larger scales of the turbulent motion. The present findings are of practical importance for the design and flame stabilization mechanisms in future iron combustion burners.

Auto-ignition characteristics of oxygenated aromatic compounds: Benzyl alcohol, benzaldehyde, and phenol

The growing attention towards biomass and biomass-derived fuels for transportation, energy production, forest fire prevention, and safety applications stimulated significant research activities focused on understanding the chemical kinetics of oxygenated aromatic hydrocarbons, which represent important reference components for biofuels. This motivates the current investigation of the ignition behavior of benzyl alcohol, benzaldehyde, and phenol, which exhibit intertwined reaction pathways. Novel ignition delay time data were measured in a shock tube and a rapid compression machine covering pressures from 10 to 40 bar and temperatures from 860–1250 K. In addition, a kinetic model was developed and used to analyze critical reaction pathways in comparison to models from literature. It is shown that the phenyl-phenoxy-phenyl peroxy subsystem plays a key role not only in the oxidation of benzaldehyde, but also in the oxidation of benzene and phenol. Comparison of the current model with a recent version of the CRECK mechanism reveals that differences are present with respect to the dominant reaction pathways that have a strong influence on the formation of Ö radicals. Further investigations are required to resolve this discrepancy reconciling theory, modelling, and the unfortunately limited number of systematic experimental data to which this work significantly contributes.

Study on synergistic hydrogen generation from aluminum-based composites in different forms of water.

The environmental suitability of hydrogen storage materials is significantly influenced by the way aluminum reacts synchronously with water, ice, and water steam. The straightforward ball milling process was used to synthesize Al-based composite materials with carbon nanotubes (CNTs) or graphene oxide (GO). The reactivity of the composites in various types of water was investigated. The Al/Bi/CNT and Al/Bi/GO composites may react in liquid water, low-temperature ice, and high-temperature steam. The hydrolysis promotion of Al-based composites by CNTs is superior to that of GO, whether in liquid water at 20 °C or ice at -20 °C. The maximum hydrogen generation rate of Al/Bi/CNT composites can reach 34.6 mL g-1 s-1 at 20 °C. The hydrogen generation volume of Al/Bi/CNT can reach 700 mL g-1 in 15 min on ice at -20 °C. Moreover, the ignition temperature and ignition delay time of Al/Bi/CNT are shorter than those of Al/Bi/GO in high-temperature steam. The hydrogen generation volume from Al/Bi/CNT at 200 °C can reach 853 mL g-1. These may originate from the unique one-dimensional nanostructure of CNTs, which provides more surface area or reaction sites during the hydrolysis of the composite.

Study on the effect of graphite-based materials on the reactivity of activated aluminum composites in different forms of water

For further energy enhancement of water-reactive metal fuels, it is crucial to find aluminum composites that can react synchronously and quickly with multiple types of water (ice, steam, and water). In this paper, two graphite-based aluminum composites, aluminum/bismuth/graphite (Al/Bi/GI) and aluminum/bismuth/graphene nanosheets (Al/Bi/GE), were prepared. The morphology and crystal structure of the composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The reactivity of Al/Bi/GI and Al/Bi/GE with water from 20 °C to 80 °C and ice at 0 °C and −15 °C was characterized using hydrogen generation properties. Their ignition and combustion properties in high-temperature water steam from 300 °C to 900 °C were also investigated. The results show that Al/Bi/GI and Al/Bi/GE composites prepared by ball milling have blocky and multilayered lamellar structures, respectively. That ball milling does not result in the formation of new alloys. The thickness of graphite material affects the reactivity of activated aluminum composites. The hydrogen generation rates of Al/Bi/GE in water and ice are much higher than that of Al/Bi/GI. The maximum hydrogen generation rates of Al/Bi/GE were 28.8 and 46.6 times higher than those of Al/Bi/GI in water at 20 °C and ice at −15 °C, respectively. In high-temperature water steam, the ignition temperature and ignition delay time of Al/Bi/GE are lower than those of Al/Bi/GI. Al/Bi/GE achieves a rapid reaction that can be synchronized with three forms of water and is expected to be applied to water-reactive metal fuel propellants.

Application of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry in the combustion simulation of ethylene-fueled scramjet combustor.

The high-fidelity reduced mechanism is one of the key elements in the combustion simulation of scramjet combustors to reveal their combustion and flow phenomena. In the present work, the hierarchically constructed NUIGMech1.2 (2857 species and 11 814 reactions) is applied to the combustion simulation of an ethylene-fueled scramjet combustor using the method of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry (TDAC). The integrated skeletal reduction strategy successively consists of species elimination using the revised directed relation graph with error propagation method of fixed species scheme and improved sensitivity analysis method, and reactions elimination based on computational singular perturbation importance index. A preferred ethylene skeletal mechanism (26 species and 117 reactions) is obtained through the integrated skeletal reduction strategy under target working conditions of temperature range of 900-1800 K, pressure range of 1-4 atm, and equivalence ratio range of 0.25-5.0. The compact skeletal mechanism is comprehensively validated against the experimental results of ignition delay times, laminar flame speeds, and key species concentration profiles. Meanwhile, it shows consistent results with the detailed mechanism on the adiabatic flame temperature profiles and "S"-curves. When applying this skeletal mechanism to combustion simulations of ethylene-fueled scramjet combustor with double parallel cavities, the path flux analysis method and in situ adaptive tabulation algorithm of TDAC is further utilized to speed up the chemical reaction solution process at run-time. Under the scramjet and ramjet modes, the corresponding simulation results in terms of flame luminosity images, schlieren images, and static pressure distributions, coincide well with those of experimental measurements. The combustion and flow characteristics of the two modes are investigated and analyzed comparatively based on above results and combustion performance parameters. Present work contributes to the application of fuel kinetic mechanisms in scramjet combustor combustion simulation.

Mechanism development for larger alkanes by auto-generation and rate rule optimization: A case study of the pentane isomers

The core chemistry and thermodynamic data of large alkanes in NUIGMech1.3 were recently updated. In the present work, the set of rate rules for large alkanes is optimized against experimental data to improve the predictive capability of the mechanism. As an initial step in developing a consistent set of rate rules for any larger alkane, we optimized the mechanism of the pentane isomers, whose mechanisms were generated based on 185 rate rules in 24 reaction classes using the MAMOX++ code. The final combined mechanism contains 1427 species and 6676 reactions. For the efficient optimization of such a large mechanism, the Optima++ code was extended to rate rules and was linked with the Zero-RK simulation code. As reference data, first-stage and total ignition delay times measured in shock tubes and rapid compression machines, and species concentrations measured in jet-stirred reactors were collected in wide ranges of conditions. The prior uncertainties of the Arrhenius equations of the 185 rate rules were determined based on a review of alkane fuel rate constant studies. The PCA-SUE method was used for the selection of the influential rate rules. This method identified 94 important rate rules, whose Arrhenius parameters were subsequently optimized within their prior uncertainty ranges using Optima++ against a representative subset of the data collection with a moderate computational effort. The optimization notably improved the accuracy of the mechanism, which now performs significantly better than the Bugler et al. mechanism (PROCI, 2017). The present study demonstrates the effectiveness of the proposed methodology, thereby paving the way to the optimization of a complete set of rate rules that can be used for the generation of a reliable combustion mechanism for any larger alkane, and with some extensions even for unsaturated fuels or oxygenated fuels such as biodiesels.

Predicting the autoignition behaviour of tailorable advanced biofuel blends using automatically generated mechanisms

Emerging processes such as biomass alcoholysis have the potential to provide tailorable, advanced biofuels to replace conventional fossil fuels. Knowledge of the engine-relevant behaviour for such fuels is evolving but is currently limited. Simulation tools may assist in the exploration of this behaviour but are reliant on the availability of robust, detailed kinetic mechanisms to produce accurate predictions. Automatic mechanism generation (AMG) techniques may be applied to facilitate the production of such mechanisms, as long as the utilised databases contain high-quality kinetic and thermochemical information of relevance to the functional groups of interest. To model the combustion characteristics of complex fuel blends, Reaction Mechanism Generator (RMG) is applied in this work to produce detailed ethyl (ethyl levulinate, diethyl ether, ethanol) and butyl (n‑butyl levulinate, di-n‑butyl ether, n-butanol) kinetic mechanisms. The predictive capabilities of these mechanisms are evaluated against a combination of literature data for individual components and new experimental measurements of ethyl and butyl blends. Stoichiometric blend ignition delay times are measured in a rapid compression machine at a compressed pressure of 20 bar and compressed temperatures of 645–960 K. The investigated blends are formulated to achieve a desired research octane number (blends ELV1 and BLV1) or to match physical property limits of diesel fuels (blends ELV2 and BLV2). Ethyl blend measurements show clear examples of complex low temperature oxidation behaviour, but this is not observable in the butyl cases, as the high boiling point of butyl levulinate necessitates the use of significant dilution in ignition delay experiments. The generated models show a high degree of accuracy when compared to measurements at thermodynamic conditions of relevance to engine technologies. The blending behaviour shown by the experimental measurements is also well predicted. This work highlights the importance of database additions/modifications based on low uncertainty, high quality, literature data when using AMG methods.

Carrier-phase DNS study of particle size distribution effects on iron particle ignition in a turbulent mixing layer

The ignition and combustion of iron particles in a turbulent mixing layer is studied by means of three-dimensional carrier-phase direct numerical simulations (CP-DNS). A particular focus is set on particle size distribution (PSD) effects on the ignition behaviour by comparing CP-DNS results from using a realistic experimental PSD to DNS data based on a monodisperse (MD) particle cloud with the same equivalence ratio. The CP-DNS solves the Eulerian transport equations of the reacting gas phase and resolves all turbulent scales, while the particle boundary layers are modelled in the Lagrangian point-particle framework. A previously validated sub-model for the oxidation of iron to Wüstite (FeO) that accounts for both diffusion- and kinetically-limited combustion is employed. The mixing layer is initialised with an upper stream of air carrying cold iron particles and an opposed lower stream of hot air. Simulation results show distinct differences in the ignition behaviour between the MD and PSD cases. The ignition of the PSD case is delayed compared to the MD case and does not show any significant particle clustering prior to ignition. Further investigations indicate that the particle size has a crucial effect on the mixing process and ignition time. Small particles start their oxidation process early and already consume some of the available oxygen, while not crucially affecting the gas temperature due to their limited iron mass contribution. Conversely, a slower entrainment into the lower stream combined with higher thermal inertia and the prior oxygen depletion by the small particles leads to a delayed oxidation of the larger particles. As a net result, the PSD case shows a wide spread of individual particle ignition delay times and overall delayed bulk ignition compared to the MD case, where the majority of the particles ignites over a shorter period of time.

Theoretical insight into key reactions in DME/NH3 co-firing: A detailed kinetic study and implications for rational combustion modelling

Dimethyl ether (DME) is promising as an additive for co-firing with ammonia (NH3) to solve the problem of its low reactivity combustion. However, the combustion modeling of blended fuels is challenging, and previous efforts primarily relied on the adjustment of rate constants for key reactions to bridge the gap between experiments and simulations. In the present work, ab initio kinetic studies were performed for the key reactions of OH/HO2/NH2 with DME in blended combustion. High-level theoretical calculations were carried out for all saddle points and a benchmark was implemented for accurate evaluation of the barrier heights. The direct dynamics calculations used variational transition state theory with interpolated single-point energies method. The kinetic studies took into account the small-curvature tunneling, low-pressure limit or pre-equilibrium model and the multistructural torsional anharmonicity effect. The calculated rate constants are in very good agreement with the available experimental kinetic data. The mechanism of Xiao et al. (Combust. Flame 245 (2022) 112372) was updated based on this kinetic study and the simulations of the ignition delay times with modified rate constants better agree with the measurements than with the original mechanism. Especially, the present model avoids decreasing the cross-reaction rate constants to improve the simulation results. This work provides detailed theoretical insights and updated models with good predictive performance based on ab initio kinetics, which have more rigorous theoretical significance.

High-pressure ignition behaviors of methane/ethane/propane-n-heptane mixtures representing natural gas-diesel dual fuel

Considering that natural gas is the key transition fuel towards the carbon-neutral future, the objective of the present study is to gain insight into evolution features of natural gas-diesel dual fuel during ignition process. Firstly, new experimental data of ignition delay times for the stoichiometric methane/ethane/propane-n-heptane mixtures, which is of significance for validating and optimizing chemical kinetic models of the dual fuel at engine-relevant conditions, were acquired through a shock-tube facility at pressure of 40 atmopshere within temperature range of 1200-1600 K, and quantitative influences of components of the fuel mixtures on ignition were determined. Then importance of species including typical radicals and alkenes during ignition processes were identified. Besides, stage characteristics of the fuel mixtures during ignition processes were analyzed. The result shows that the ignition of real natural gas which contains some ethane and propane is greatly different from that of methane. It can be seen that the C2 substances are significant to control ignition of the mixtures. For methane-n-heptane and methane/ethane-n-heptane mixtures, the whole ignition process can be divided into decomposition and oxidation stages. While for the fuel mixtures containing propane and n-heptane, it seems to be more reasonable to divide the whole ignition process into three-stages, i.e., decomposition, mixed and oxidation stages.

Spray and combustion characterization under an ultra-high-density condition – Multi-fuel comparison

This study reveals for the first time the ignition and combustion characteristics of different fuel properties under ultra-high-density ambient condition. A new pre-burn type optically accessible constant volume combustion chamber was established, allowing for operation at pressures exceeding 300 bar. This chamber was utilized to generate a spray combustion environment at 150 bar and 1000 K, corresponding to a bulk-density of 50.82 kg/m3. Diesel was used as the reference fuel, and three types of primary reference fuel (PRF), i.e., PRF0 (n-heptane), PRF50, and PRF100 (isooctane), were used as the main test fuels. OH* chemiluminescence was utilized to measure the stable flame lift-off length, while the broadband chemiluminescence technique was used to measure the ignition delay time and to capture time-resolved flame development. The results show that the ignition delay of PFR100 is significantly larger than that of other fuels due to its lowest cetane number. Flame visualization results indicate that diesel spray combustion aligns well with the conceptual model proposed by Dec [SAE transactions, 1997: 1319–1348], whereas noticeable wrinkles are observed in the flames of PRF50. The flame lift-off length during the quasi-steady period shows to be independent of fuel properties, but the value has notably reduced to less than 9 mm. Comparison of the multi-fuel luminosity images reveals that diesel experiences the most pronounced combustion recession. Spatially integrated natural luminosity analysis indicates that diesel has the longest combustion duration, chiefly due to a larger rate of injection, whereas the difference in combustion duration between different PRFs is negligible. The heat release rate error range for PRF0 is the largest, suggesting stronger combustion variations. The heat release rate of diesel combustion is the most robust. The peak value of AHRR of PRF50 and PRF100 is marginally higher than that of diesel, which can be attributed to their higher low heating value.

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed prevaporized n-propanol/air and /isopropanol/air flames

The idea of utilizing lean, premixed and prevaporized (LPP) combustion could be a step towards sustainable aviation in the future, with the absolute absence of soot particles and very low nitrogen oxide emissions. However, premixing with compressed hot air presents a challenge for both conventional jet fuel and synthetically produced sustainable aviation fuel (SAF) due to self-ignition and flashbacks. Our group is thus seeking LPP-compatible advanced sustainable aviation fuels, which are evaluated based on their flashback propensity. Preliminary studies indicate that short-chain alcohols, with their higher ignition delay times and lower flame velocities, may be appropriate for LPP combustion concepts. In this study, the isomers of propanol, n-propanol, and isopropanol, are the first representatives of the short-chain alcohols to be investigated experimentally and numerically with respect to their flame stability limits. The experimental determination of stability limits for n- and isopropanol/air premixed flames was conducted on a generic, non-swirled atmospheric burner for an equivalence ratio range of 0.82 to 1.11. A fully automated measurement procedure was used to generate 119 measurement points with Reynolds numbers in the range from 1430 to 2750. The influence of the positional isomerism and of the preheat temperature (422 K, 446 K and 466 K) of the unburned mixture are investigated. A clear influence of the isomeric fuel structure is found. A theoretical analysis shows that it can be related to the different laminar flame speed and laminar flame thickness of the isomers. Moreover, a detailed analysis shows that the critical velocity gradient concept to describe boundary layer flashback is well usable with a Péclet number model.

Evaluation of the corrections methodologies for improving the ignition delay time calculation in hydrogen/air mixtures using a design of experiments approach

Evaluation of the corrections methodologies for improving the ignition delay time calculation in hydrogen/air mixtures using a design of experiments approach

Probing O2-dependence of cyclopentyl reactions via isomer-resolved speciation

Modeling chemical kinetics relevant to low-temperature combustion requires complete description of reactions involving critical species such as hydroperoxyalkyl radicals, Q˙OOH, which undergo competing unimolecular reactions and bimolecular reactions with O2. The balance of flux across the two pathways affects rates of chain-branching and depends on temperature, pressure, and oxygen concentration. Accordingly, the influence of [O2] on product formation from alkyl + O2 reactions and the subsequent fate of Q˙OOH and related products is central to the development of an accurate chemical kinetics mechanism. However, chemical reactions consuming Q˙OOH-mediated species are often simplified to such a degree that mechanism truncation error (uncertainty derived from incomplete reaction networks) becomes significant.For the specific purpose of determining the extent to which expanded sub-mechanisms ameliorate inaccuracies in model predictions resulting from mechanism truncation error, the present work integrates speciation measurements from jet-stirred reactor experiments on cyclopentane oxidation with modeling using an expanded detailed chemical kinetics mechanism. The experiments utilize vacuum ultraviolet-absorption spectroscopy and mass spectrometry for isomer-resolved speciation of intermediates produced from cyclopentane oxidation at 835 Torr from 500 – 1000 K. [O2]-dependent experiments were also conducted from 0.12 – 3.66 · 1018 molecules cm–3 at 825 K to examine the influence on species profiles. The expanded mechanism is produced by merging detailed sub-mechanisms for Q˙OOH-mediated species (cyclopentene and 1,2-epoxycyclopentane) produced using Reaction Mechanism Generator (RMG) with an existing detailed mechanism for cyclopentane.Model predictions using the expanded mechanism yielded improvements in species mole fractions for both temperature- and O2-dependence. Ignition delay time simulations were also conducted and show appreciable sensitivity in the negative-temperature coefficient region due to incorporation of the detailed sub-mechanisms due to influence on O˙H radical populations. Sensitivity analysis revealed reactions that merit theoretical rate calculations for additional improvements and include species such as H2O2 and species related to R˙ + O2 reactions of cyclopentene and 1,2-epoxycyclopentane.

Transition to detonation in hydrogen-air mixtures due to shock focusing in a 3-wall 90-deg corner

This work presents an experimental investigation on the transition to detonation due to shock-wave focusing in a 3-wall 90-deg corner in hydrogen-air mixtures (17.5 – 60% H2) initially at 298 K and 1 bara. The reflector was placed at one side of 2 – m long, 0.14 m internal diameter tube. The other side of the tube was equipped with an ignition source and a flame acceleration section. The shock wave approached the reflector with a velocity in the range of 580 – 970 m/s. The reflector was placed symmetrically to the tube axis. The research proved that transition to detonation due to shock focusing highly depends on the shock wave strength and can be triggered in a stoichiometric mixture at shock speed VS = 605 m/s which corresponds to M = 1.485 or 59.9% of the speed of sound in combustion products. The ignition delay time recorded for transition to detonation was at the order of few microseconds. The detonability range observed with the 3-wall 90-deg reflector proved to be 15 - 70 % H2 in air compared to 18 - 59% H2 observed in work performed for a 90-deg wedge reflector configuration at the same initial conditions. Additionally, the scaling factor between critical shock velocity between both types of reflectors is almost constant and equal to 0.84 throughout the investigated mixtures. The high focusing ability of the 3-wall corner has also proved the minimum transition pressure recorded in the corner tip at the level of 8.95 MPa in a stoichiometric mixture. This value corresponds to a nearly 17-fold increase in pressure compared to normal shock post-reflection conditions. Considering the shock focusing as the major trigger for detonation the presented results may explain the differences between the detonability limits observed in obstacle-laden ducts and large-scale experimental setups.