Ignition Delay Time Data Research Articles

Ignition delay time correlations for distillate fuels

Ignition delay times were measured behind reflected shock waves in a shock tube for a wide variety of distillate fuels over a range of temperatures, pressures and mixtures. The fuels studied include: jet fuels (JP-5, JP-8, and Jet A), rocket propellants (RP-2), diesel fuels (F-76 and DF-2) and gasoline. A simple correlation was found to describe the ignition delay times for all these fuel/air experiments for equivalence ratios near unity, temperatures from 1000 to 1400K, and pressures from 6 to 60 atm. A simple correlation was also found for low-fuel-concentration experiments diluted in argon. Previously published ignition delay time data were found to be in good agreement with these correlations. Finally, for several fuels studied, systematic variations were seen in the activation energy for ignition delay time measurements with varying equivalence ratio and oxygen concentration.

Combustion of Synthetic Jet Fuel: Chemical Kinetic Modeling and Uncertainty Analysis

Reaction mechanisms for jet-fuel combustion were built with the aim of providing a better description of the chemistry to reacting flow simulations used to design future aircraft engines. This research effort focused on combustion of Fischer–Tropsch synthetic jet fuel (S-8) in vitiated air at conditions relevant to jet engines, augmentors, and interturbine burners (, , and in air). The complex S-8 fuel mixture was approximated with a two-component surrogate mixture of -decane and iso-octane. A wholly new, elementary-step reaction mechanism for the surrogate consisting of 291 species and 6900 reactions was constructed using automatic mechanism generation software. Statistical analyses were conducted to determine reaction rate-constant sensitivity, model prediction uncertainty, and consistency of the model with published ignition delay time data. As a test application, the S-8 reaction model was used to estimate augmentor static stability using a simple Damköhler number analysis that showed increased stability with temperature from 800 to 1400 K and NO concentration from 0 to 1000 ppm (v/v). The ability to quickly generate accurate mechanisms for simple surrogates allows for new synthetic fuels to be quickly modeled and their behavior predicted for an array of experimental conditions and practical applications.

A quantum chemical and kinetics modeling study on the autoignition mechanism of diethyl ether

A detailed chemical kinetics model has been developed to elucidate the auto-ignition behavior of diethyl ether (DEE) under conditions relevant for internal combustion engines. The present model is composed of a C0–C4 base module from literature and a DEE module. For the low-temperature oxidation mechanism, the reactions of ROO and QOOH radicals were studied previously with a quantum-chemical and transition state theory approach by Sakai et al. (2015). In the present study, the potential energy surfaces for the unimolecular reactions of OOQOOH isomers and 1- and 2-ethoxyethyl radicals were determined with a CBSQB3 composite method. In the presence of an OOH group, the reaction barrier of the hydrogen shift from the β site (terminal carbon atom) decreases as it does in alkane oxidation but there is no effect on the hydrogen shift from the α site (next to the ether oxygen atom). Therefore, the reaction barriers of OOQOOH isomers have the same trend as the corresponding ROO radical and rate constants for the reactions of OOQOOH isomers were determined. The constructed model was validated against the recent data of ignition delay times provided in literature by Werler et al. (2015). The agreement is good over the temperature range 500–1300K and pressure range 1–40bar, although, open questions remain regarding the non-consensus at 900–1150K and 40bar. Reaction-path and sensitivity analyses attribute the importance of the reactivity at the α site to the decrease of the C H bond dissociation energy due to the ether oxygen atom.

The oxidation of 2-butene: A high pressure ignition delay, kinetic modeling study and reactivity comparison with isobutene and 1-butene

Butenes are intermediates ubiquitously formed by decomposition and oxidation of larger hydrocarbons (e.g. alkanes) or alcohols present in conventional or reformulated fuels. In this study, a series of novel ignition delay time (IDT) experiments of trans-2-butene were performed in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM) under conditions of relevance to practical combustors. This is the first IDT data of trans-2-butene taken at engine relevant conditions, and the combination of HPST and RCM results greatly expands the range of data available for the oxidation of trans-2-butene to higher pressures (10–50atm), lower temperatures (670–1350K) and a wide range of equivalence ratios (0.5–2.0). A comprehensive chemical kinetic mechanism has simultaneously been developed to describe the combustion of trans-2-butene. It has been validated using the IDT data measured here in addition to a large variety of literature data: jet-stirred reactor (JSR) speciation data, premixed flame speciation data, flow reactor speciation data and laminar flame speed data. Moreover, the reactivity of trans-2-butene is compared to that of the other two isomers, 1-butene and isobutene, and these comparisons are discussed. Important reactions are highlighted via flux and sensitivity analyses and help explain the differences in reactivity among the butene isomers.

Nitromethane ignition behind reflected shock waves: Experimental and numerical study

Ignition delay times for nitromethane have been measured behind reflected shock waves over wide ranges of temperature (875–1595K); pressure (2.0–35atm); equivalence ratio (0.5, 1.0, and 2.0); and dilution (99, 98, 95, and 90% Ar by volume) using a L9 Taguchi array. Emission from excited-state hydroxyl radials (OH∗) was the primary diagnostic for determining the ignition delay times from the experiments. Results showed that nitromethane’s ignition delay time is very sensitive to most of the experimental parameters that were varied. In addition, the OH∗ profile for nitromethane presents an interesting double feature, with the relative intensity between the two peaks varying greatly depending on the experimental conditions. A detailed chemical kinetics mechanism was assembled from previous work by the authors and from sub-mechanisms from the literature. The latest theoretical work on nitromethane decomposition was used, and the final mechanism satisfactorily reproduces the ignition delay time data from the present study, as well as nitromethane and CH4/NOx ignition delay time data available from the literature.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane.

Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

Large eddy simulation of n-heptane spray combustion in partially premixed combustion regime with linear eddy model

Spray combustion of n-heptane in a constant-volume vessel under engine-relevant conditions was investigated using linear eddy model in the framework of large eddy simulation. In this numerical approach, turbulent mixing was traced by an innovative stochastic approach instead of the conventional gradient diffusion model. Chemical reaction rates were calculated with the consideration of the sub-grid scale spatial fluctuations of reactive scalars. Turbulence-chemistry interactions were represented by the separated treatments of the underlying processes including turbulent stirring, chemical reaction, and molecular diffusion. The model was validated against the experimental data of ignition delay times, chemiluminescence images, and soot images from Sandia National Laboratories. Numerical results showed that the ignition process changed from the temperature-controlled regime to the mixing-controlled regime as the initial ambient temperature increased from 800 K to 1000 K. The premixed flame and the diffusion flame coexisted, while the gross heat release rate was found to be dominated by the premixed flame. The temperature fluctuation was mainly observed around the spray jet due to the cooling effect of the fuel vaporization. The fluctuations were more significantly smoothed out by the high-temperature flame than the low-temperature flame. The mean temperature would be overpredicted if the sub-grid temperature fluctuation was neglected.

An ignition delay time and chemical kinetic modeling study of the pentane isomers

Ignition delay times of n-pentane, iso-pentane, and neo-pentane mixtures were measured in two shock tubes and in a rapid compression machine. The experimental data were used as validation targets for the model described in detail in an accompanying study [14]. The present study presents ignition delay time data for the pentane isomers at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ (additionally, 0.3 in ‘air’ for n-, and iso-pentane) at pressures of 1, 10, and 20 atm in the shock tube, and 10 and 20 atm in the rapid compression machine, as well as data at an equivalence ratio of 1.0 in 99% argon, at pressures near 1 and 10 atm in a shock tube. An infrared laser absorption technique at 3.39 µm was used to verify the composition of the richest mixtures in the shock-tube tests by measuring directly the pentane isomer concentration in the driven section. By using shock tubes and a rapid compression machine, it was possible to investigate temperatures ranging from 643 to 1718 K. A detailed chemical kinetic model was used to simulate the experimental ignition delay times, and these are well-predicted for all of the isomers over all ranges of temperature, pressure, and mixture composition. In-depth analyses, including reaction path and sensitivity analyses, of the oxidation mechanisms of each of the isomers are presented. To the authors’ knowledge, this study covers conditions not yet presented in the literature and will, in conjunction with the aforementioned accompanying study, expand fundamental knowledge of the combustion kinetics of the pentane isomers and of alkanes in general.

Scaling relation for high-temperature biodiesel surrogate ignition delay times

High-temperature Arrhenius ignition delay time correlations are useful for revealing the underlying parameter dependencies of combustion models, for simplifying and optimizing combustion mechanisms for use in engine simulations, for scaling experimental data to new conditions for comparison purposes, and for guiding in experimental design. We have developed a scaling relationship for Fatty Acid Methyl Ester (FAME) ignition time data taken at high temperatures in 4%O2/Ar mixtures behind reflected shocks using an aerosol shock tube:τign[ms]=2.24×10-6[ms](P[atm])-0.41(ϕ)0.30(Cn)-0.61exp37.1[kcal/mol]R̂u[kcal/mol K]T[K]Additionally, we have combined our ignition delay time data for methyl decanoate, methyl palmitate, methyl oleate, and methyl linoleate with other experimental results in the literature in order to derive fuel-specific oxygen-mole-fraction scaling parameters for these surrogates. In this article, we discuss the significance of the parameter values, compare our correlation to others found in the literature for different classes of fuels, and contrast the above expression’s performance with correlations obtained using leading FAME kinetic models in 4%O2/Ar mixtures.

1-, 2- and 3-Pentanol combustion in laminar hydrogen flames – A comparative experimental and modeling study

Oxidation pathways of three different pentanol isomers 1-, 2- and 3-pentanol are investigated using the electron–ionization molecular-beam mass spectrometry (EI-MBMS) technique. New experimental speciation data was obtained from laminar, flat, low-pressure H2/O2/Ar base flames seeded with equal amounts of pre-vaporized 1-, 2- or 3-pentanol. The experimental investigation is supported by kinetic modeling. Here, one single detailed reaction mechanism for the three linear pentanol isomers has been constructed and compared against new and existing experimental data. The model itself was obtained by an open-source reaction model generation software (RMG) and tested against existing ignition delay times, flame speeds and new quantitative experimental results for mole fraction profiles of major and intermediate species.The overall discussion of individual species profiles is guided by the model-based reaction flow analysis focusing on the initial steps of the fuel destruction paths for the three pentanols down to C3-hydrocarbon species. Therefore, the reaction pathways for the initial fuel destruction steps are shown and analyzed using combined experimental and predicted results. Analysis is performed by means of secondary decay products for each of the three pentanols. The kinetic reaction model was successfully tested against available ignition delay time and laminar flame speed data. Comparisons against 23 new quantitative species profiles are presented for each of the pentanol doped flames. In general, a good predictive capability of the detailed model can be noted for all three investigated straight-chain pentanols regarding the mole fraction profiles of major and intermediate species.

Experimental and kinetic modeling study of the shock tube ignition of a large oxygenated fuel: Tri-propylene glycol mono-methyl ether

Tri-propylene glycol monomethyl ether (TPGME) is an important oxygenated fuel additive that can be used to reduce soot in diesel engines. However, a validated chemical kinetic model that incorporates the low- to high-temperature chemistry, needed to simulate ignition in a diesel engine is not available for TPGME. In addition, no fundamental experimental data are available that can be used to validate a TPGME mechanism. In this study, a surrogate chemical kinetic model for TPGME that includes low- and high-temperature chemistry has been developed, and shock tube ignition delay time data has been acquired for its validation at 0.25% TPGME for temperatures in the range of 980–1545K, at pressures of 10 and 20atm, and at equivalence ratios of φ=0.5, 1.0 and 2.0. The predictions from the model have been compared to the experimental measurements with good agreement. Under the experimental conditions investigated in the shock tube, TPGME was found to be consumed by molecular elimination reactions and also H-atom abstraction by Ḣ atoms and by ȮH and HȮ2 radicals. In performing sensitivity analyses it was found that the ignition of TPGME is most sensitive to reactions involving propene. Considering how the sensitivity analyses change with pressure, the most sensitive reactions involved Ḣ atoms at 10atm and HȮ2 radicals at 20atm. With respect to the effect of equivalence ratio, reactions involving Ḣ atoms are relatively more sensitive under fuel-rich conditions while those involving HȮ2 radicals are relatively more sensitive under fuel-lean conditions. Further experimental work is needed to enable validation of the model under low-temperature conditions. TPGME was compared to n-heptane which has similar ignition properties based on Cetane Number. Predictions showed that TPGME has a higher overall reactivity compared to n-heptane. In addition, TPGME is shown to produce significantly less soot precursor species when TPGME predictions are compared to n-heptane.

Experimental and kinetic modeling study of styrene combustion

The aim of this work is to report new experimental data on styrene combustion and develop a kinetic model for styrene combustion. Three laminar premixed flames of styrene with equivalence ratios of 0.75, 1.00 and 1.70 were studied at low pressure (0.0395atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry. The JSR oxidation of styrene/benzene mixtures with three equivalence ratios of 0.50, 1.00, and 1.50 was studied at atmospheric pressure using gas chromatography. The mole fraction profiles of flame species and oxidation species were measured. A kinetic model of styrene combustion with 290 species and 1786 reactions was developed from our recently reported toluene model, and was validated on the new experimental data from this work and previous ignition delay time data. The O-atom attack reactions, H-atom abstraction reactions and other H-atom attack reactions were found to dominate the consumption of styrene. In particular, the O-atom attack reactions on the double bond of styrene play an important role in the JSR oxidation and the lean and stoichiometric flames, while the pyrolytic reactions have increasing contributions as the equivalence ratio increases. The phenyl radical plays a crucial role in the decomposition of styrene since it affords most of the reaction fluxes from styrene. In the aromatics growth process, phenylacetylene, the 1-phenylvinyl radical, the phenyl radical, and styrene serve as major precursors of polycyclic aromatic hydrocarbons (PAHs), leading to the high sooting tendency of styrene compared to toluene and ethylbenzene.

Shock Tube Study of Dimethylamine Oxidation

ABSTRACTDimethylamine (DMA) ignition delay times and OH time histories during the oxidation process were investigated behind reflected shock waves. The ignition delay time measurements cover the temperature range of 1181–1498 K, with pressures near 0.9, 1.5, and 2.8 atm, and equivalence ratios of 0.5, 1, and 2, in 4% oxygen/argon. The ignition delay time data feature low scatter and can be correlated to a single expression with 2 ∼ 0.99: τign = 7.30 × 10−4 −0.68 Φ0.45 exp(18,265/), where τign is in μs, in atm, and in K. OH time histories were measured using laser absorption of the R1(5) line of the A‐X(0,0) transition near 306.7 nm, in stoichiometric mixtures of 500 ppm DMA/O2/argon. The mechanism developed by Li et al. was used initially to simulate the measured DMA ignition delay times and the OH time histories. The Li et al. mechanism was then updated by adding the DMA unimolecular decomposition channel: DMA = CH3NH + CH3, with the reaction rate constant estimated by analogy to dimethyl ether decomposition, previously investigated by Cook et al. The reactions of DMA + OH were also updated based on recent work in our laboratory. The simulation results using the modified Li et al. mechanism are in good agreement with both the ignition delay times and OH time‐history data.

Experimental and modeling study on the high-temperature oxidation of Ammonia and related NOx chemistry

Ammonia oxidation and ignition delay time measurements for pressures above 10atm are scarce. In addition, NH3 is known to adsorb on stainless steel, so measurement results could be in question if wall passivation is not employed for apparatuses utilizing steel. To overcome these measurement difficulties and overall lack of high-pressure data for ammonia, new and methodical ignition delay time measurements have been performed behind reflected shock waves over a wide range of temperatures (1560–2455K), pressures (around 1.4, 11, and 30atm) and equivalence ratios (0.5, 1.0, and 2.0) for mixtures of ammonia highly diluted in Ar (98–99%). The new set of data from the present study was compared to several models from the literature. It was found that a large majority of the models do not predict the ignition delay times with accuracy, and there is a surprisingly wide variation amongst the predictions. One satisfactory model, from Dagaut et al. (2008), was selected and extended to compounds other than NH3 using H2/O2/CO, N2O, NO2, and NNH sub-mechanisms from the literature. The resulting comprehensive mechanism predicts well the ammonia ignition delay time data from the present study along with other NH3, NO2, and N2O data from the authors as well as from the literature with high accuracy. In addition to the new ammonia oxidation data and related model comparisons, the present paper documents a state-of-the-art NOx sub-mechanism that can be used for a wide range of combustion calculations when added to, for example, baseline mechanisms involving hydrogen and hydrocarbon kinetics.

An ignition delay and kinetic modeling study of methane, dimethyl ether, and their mixtures at high pressures

The development of accurate chemical kinetic models capable of predicting the combustion of methane and dimethyl ether in common combustion environments such as compression ignition engines and gas turbines is important as it provides valuable data and understanding of these fuels under conditions that are difficult and expensive to study in the real combustors. In this work, both experimental and chemical kinetic model-predicted ignition delay time data are provided covering a range of conditions relevant to gas turbine environments (T=600–1600K, p=7–41atm, ϕ=0.3, 0.5, 1.0, and 2.0 in ‘air’ mixtures). The detailed chemical kinetic model (Mech_56.54) is capable of accurately predicting this wide range of data, and it is the first mechanism to incorporate high-level rate constant measurements and calculations where available for the reactions of DME. This mechanism is also the first to apply a pressure-dependent treatment to the low-temperature reactions of DME. It has been validated using available literature data including flow reactor, jet-stirred reactor, shock-tube ignition delay times, shock-tube speciation, flame speed, and flame speciation data. New ignition delay time measurements are presented for methane, dimethyl ether, and their mixtures; these data were obtained using three different shock tubes and a rapid compression machine. In addition to the DME/CH4 blends, high-pressure data for pure DME and pure methane were also obtained. Where possible, the new data were compared with existing data from the literature, with good agreement.

Ignition delay times of conventional and alternative fuels behind reflected shock waves

The auto-ignition characteristics of two distillate jet fuels and fifteen alternative fuels (including fuel blends) were investigated using shock-tube/laser-absorption methods. Ignition delay times were measured behind reflected shock waves over a range of temperatures, 1047–1520K, and equivalence ratios, 0.25–2.2, in two pressure and mixture regimes: for fuel/air mixtures at 2.07–8.27atm, and for fuel/4%oxygen(O2)/argon(Ar) mixtures at 15.9–44.0atm. In both pressure ranges, the ignition delay times of the alternative fuels and the blends with conventional fuels were found to be similar to those of conventional fuels but with some small systematic differences manifesting the different fuel types. In particular, for alternative aviation fuels, alcohol-to-jet fuels were found to be generally less reactive than Fischer–Tropsch paraffinic kerosenes or hydro-processed renewable jet fuels. Comparisons were also made of the ignition delay time data with detailed kinetic modeling for selected fuels. These comparisons show that existing multi-component surrogate/mechanism combinations can successfully predict the behavior of these fuels over the conditions studied. For those fuels lacking kinetic models, the current ignition delay time measurements provide useful target data for development and validation of relevant surrogate mixtures and reaction mechanisms.

A second-generation constrained reaction volume shock tube.

We have developed a shock tube that features a sliding gate valve in order to mechanically constrain the reactive test gas mixture to an area close to the shock tube endwall, separating it from a specially formulated non-reactive buffer gas mixture. This second-generation Constrained Reaction Volume (CRV) strategy enables near-constant-pressure shock tube test conditions for reactive experiments behind reflected shocks, thereby enabling improved modeling of the reactive flow field. Here we provide details of the design and operation of the new shock tube. In addition, we detail special buffer gas tailoring procedures, analyze the buffer/test gas interactions that occur on gate valve opening, and outline the size range of fuels that can be studied using the CRV technique in this facility. Finally, we present example low-temperature ignition delay time data to illustrate the CRV shock tube's performance.

Shock tube study of ethylamine pyrolysis and oxidation

Ethylamine (CH3CH2NH2) pyrolysis and oxidation were studied using laser absorption behind reflected shock waves. For ethylamine pyrolysis, NH2 time-histories were measured in 2000ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, and NH2 and OH time-histories were measured in ethylamine/O2/argon mixtures. Measurements covered the temperature range of 1200–1448K, with pressures near 0.85, 1.35 and 2atm, and fuel mixtures with equivalence ratios of 0.75, 1 and 1.25 in 0.2%, 0.8% and 4% O2/argon. Simulations using the recent Li et al. mechanism gave significantly shorter ignition delay times and higher intermediate radical species concentrations than the experimental results. The reaction rate constants for the two major ethylamine decomposition pathways were modified in the Li et al. mechanism to improve the prediction of the time-histories of NH2 and OH in ethylamine pyrolysis. In addition, recommendations from recent studies of ethylamine+OH reactions were implemented. With these modifications, the Modified Li et al. mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay time data.

Global Reduced Model for Conventional and Alternative Jet and Diesel Fuel Autoignition

A global reduced model for the autoignition of conventional petroleum-derived and alternative, hydroprocessed and Fischer–Tropsch, jet and diesel fuels is presented. The model is based on a seven-step model from the literature (Zheng, J.; Miller, D. L.; Cernansky, N. P. A Global Reaction Model for the HCCI Combustion Process, SAE Paper 2004-01-2950, 2004) and includes steps to predict oxidation and autoignition in the three temperature regimes of interest to engine applications, the high-, low-, and negative-temperature-coefficient (NTC) regimes. Here we have included dependence of reaction rate parameters on the derived cetane number (DCN) to predict the influence of fuel variability on autoignition and optimized the reduced model to best fit target shock tube ignition delay time data for jet and diesel fuels. The standard deviation between model predictions for ignition delay and the target shock tube data set is ±21% and the model captures all experimental trends for ignition delay variation with DCN, temperature, pressure, and fuel–air equivalence ratio in all temperature regimes. Comparisons to experimental ignition delay time data not contained in the target data set show model–experiment deviation similar to the comparisons with the target data, with global standard deviation of ±20–30% and differences of at most a factor of 2. The model comparisons with shock tube data suggest that the model is suitable for autoignition prediction for fuels containing large quantities of aliphatic compounds with DCN = 30–80 for 650–1300 K, 8–80 atm, and fuel–air equivalence ratios of 0.25–1.5.

Oxidation of Ethylene—Air Mixtures at Elevated Pressures, Part 2: Chemical Kinetics

A chemical kinetics submechanism for small molecular weight hydrocarbons was modified by adjusting rate constants to produce better agreement with recent ethylene ignition delay time data compared with an earlier version of the mechanism, for temperatures from 1003 to 1401 K, at pressures between 1.1 and 24.9 atm, and for equivalence ratios from 0.3 to 2.0. The improved mechanism captures the pressure and equivalence ratio behavior seen in the data at these intermediate temperatures, such as the smaller-than-expected effect of equivalence ratio at the higher temperatures and an apparent lack of pressure dependence at fuel-lean conditions. By using detailed sensitivity analyses, the important reactions were identified, rectifying the model simulations in predicting the observed experimental behavior of the data in this study. In fact, when the model is used to extend the temperature range above 1400 K and below 1000 K, the same pressure dependence is actually seen for all equivalence ratios, just to a lesser extent at the test temperatures. Hence, the resulting hydrocarbon mechanism is much more robust as a result of this exercise. The initial deficiency and subsequent improvement of the model justify the new ignition delay time data from the companion paper to this study as well as the need for further study on ethylene kinetics.