Ignition Delay Time Measurements Research Articles

Studies of high pressure 1,3-butadiene flame speeds and high temperature kinetics using hydrogen and oxygen sensitization

The high pressure flame speeds and high temperature kinetics of 1,3-butadiene mixtures are studied by using spherical flames at fuel rich and lean conditions at 1–18 atm with H2 and O2 additions. H2 and O2 are added in the mixture to perturb concentrations of H/O radicals and the flame speed sensitivity to 1,3-butadiene + O/H reactions. The presently measured 1,3-butadiene/air flame speeds agree with previous studies at ambient pressure, but are lower at 5 atm. Comparison between the new experimental data and the prediction by the recently developed 1,3-butadiene model (Zhou et al., 2018) shows over-prediction of the flame speeds, especially at high pressure and fuel lean conditions. With H2 and O2 sensitization, the discrepancy between the model prediction and experiments becomes even larger. Sensitivity analysis shows that the flame speed is very sensitive to 1,3-butadiene + O/H reactions, especially to the branching and termination reaction ratio of 1,3-C4H6 + O = C2H3 + CH2CHO (R1) and 1,3-C4H6 + O = CH2O + C3H4-a (R2). The sensitivity of the flame speed to the branching ratio of these two reactions increases with the increase of H2 and O2 enrichment. Due to the lack of accurate theoretical calculations of these two reaction rates, the new flame speed data and previously measured ignition delay time were used to assess the uncertainty of the branching ratio and reaction rates of R1 and R2. It shows that the optimized branching ratio significantly improves the flame speed and ignition delay time predictions, especially at high pressure and fuel lean conditions. The present study reveals that H2 and O2 sensitization in flames provides an important way to identify the uncertainties of fuel + O/H reactions and to improve the model predictability for flames.

Shock tube study of normal heptane first-stage ignition near 3.5 atm

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

Shock tube studies of ethanol preignition

Understanding premature ignition or preignition is of great importance as this phenomenon influences the design and operation of internal combustion engines. Preignition leading to super-knock restricts the efficiency of downsized boosted engines. To gain a fundamental understanding of preignition and how it affects an otherwise homogeneous ignition process, a shock tube may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In a previous work by Javed et al. (2017), ignition delay time measurements of n-heptane showed significantly expedited reactivity compared to well-validated chemical kinetic models in the intermediate-temperature regime. In the current work, ethanol is chosen as a representative fuel that, unlike n-heptane, does not exhibit negative temperature coefficient (NTC) behaviour. Reactive mixtures containing 2.9% and 5% of ethanol at equivalence ratios of 0.5 and 1 were used for the measurement of ignition delay times behind reflected shock waves at 2 and 4 bar. Effect of bath gas was studied with mixtures containing either Ar or N2. In addition to conventional side-wall pressure and OH* measurements, a high-speed imaging setup was utilized to visualize the shock tube cross-section through a transparent quartz end-wall. The results suggest that preignition events are more likely to happen in mixtures containing higher ethanol concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging shows that low-temperature ignition process is usually initiated from an individual hot spot that grows gradually, while high-temperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously.

High-Pressure Oxy-Syngas Ignition Delay Times With CO2 Dilution: Shock Tube Measurements and Comparison of the Performance of Kinetic Mechanisms

In this study, syngas combustion was investigated behind reflected shock waves in CO2 bath gas to measure ignition delay times (IDT) and to probe the effects of CO2 dilution. New syngas data were taken between pressures of 34.58–45.50 atm and temperatures of 1113–1275 K. This study provides experimental data for syngas combustion in CO2 diluted environments: ignition studies in a shock tube (59 data points in 10 datasets). In total, these mixtures covered a range of temperatures T, pressures P, equivalence ratios φ, H2/CO ratio θ, and CO2 diluent concentrations. Multiple syngas combustion mechanisms exist in the literature for modeling IDTs and their performance can be assessed against data collected here. In total, twelve mechanisms were tested and presented in this work. All mechanisms need improvements at higher pressures for accurately predicting the measured IDTs. At lower pressures, some of the models agreed relatively well with the data. Some mechanisms predicted IDTs which were two orders of magnitudes different from the measurements. This suggests that there is behavior that has not been fully understood on the kinetic models and is inaccurate in predicting CO2 diluted environments for syngas combustion. To the best of our knowledge, current data are the first syngas IDTs measurements close to 50 atm under highly CO2 diluted (85% per vol.) conditions.

An experimental and chemical kinetic modeling study of 1,3-butadiene combustion: Ignition delay time and laminar flame speed measurements

Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation.A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.

The unimportance of the reaction H2 + N2O ⇆ H2O + N2: A shock-tube study using H2O time histories and ignition delay times

The hydrogen-nitrous oxide (H2–N2O) system is of paramount importance in safety considerations for nuclear waste management and semiconductor manufacturing. For less-dilute H2–N2O mixtures, a reaction of key importance in determining ignition delay times is the direct reaction between H2 and N2O to give H2O and N2, although this reaction has received only one direct investigation in the work of Kosarev et al. (2007). To examine the importance of the title reaction, new H2O time histories were obtained using a laser absorption technique at 1.39 µm in a mixture of 0.222% N2O/1.778% H2/Ar between 1414 and 1811 K near 1.2 atm. Additionally, the ignition delay time measurements of Kosarev et al. (2007) were repeated using endwall emission and pressure diagnostics. The new datasets show excellent agreement with the predictions of a recent mechanism when the title reaction is reduced by a factor of 30 relative to the work of Kosarev et al. Furthermore, the accurate mechanism predictions of H2–N2O ignition delay time data available in the literature (other than those of Kosarev et al.) were not degraded when the proposed value of k1 = 7.0×1013exp(−16,356/T) was used (k1 in cm3 mole−1 s−1, T in K). This expression for k1 should be viewed as an upper limit on the rate for the title reaction and should make the title reaction negligibly important in future NOx modeling efforts; the true value of k1 is likely even lower than this expression. A factor of 2 uncertainty is assigned to this expression for k1. Finally, it is recommended that the data of Kosarev et al. (2007) be neglected in future mechanism validations as these data appear to be inaccurate, in part due to non-ideal pressure rise effects and choice of emission diagnostic.

Ignition delay time measurements and modeling for gasoline at very high pressures

Ignition delay times (IDT) for high-octane-number gasolines and gasoline surrogates were measured at very high pressures behind reflected shock waves. Fuels tested include gasoline, gasoline with oxygenates, and two surrogate fuels, one dominated by iso-octane and one by toluene. RON/MON for the fuels varied from 101/94 to 106.5/91.5. Measurements were conducted in synthetic air at pressures from 30 to 250 atm, for temperatures from 700 to 1100 K, and equivalence ratios near 0.85. Results were compared with a recent gasoline mechanism of Mehl et al. (2017). IDT measurements of the iso-octane-dominated surrogate were very well reproduced by the model over the entire pressure and temperature range. IDT measurements for the toluene-dominated surrogate were also reproduced by the model to a lesser extent. By contrast, IDT measurements for the neat gasoline and gasoline with oxygenates, show excellent agreement with the trends of the Mehl et al. model only below 900 K. Above 900 K, the model returned IDT values for the two gasolines that were approximately 1.6× the measured values. Finally, we observed that IDT measurements for the toluene-dominated surrogate fuel and the two gasolines, near 70 atm and below 900 K, appeared to be shortened, possibly by non-homogeneous ignition or non-ideal gas processes. This dataset provides a critically needed set of IDT targets to test and refine boosted gasoline models at high pressures.

Towards a kinetic understanding of the NOx sensitization effect on unsaturation hydrocarbons: A case study of ethylene/nitrogen dioxide mixtures

Abstract Ignition delay time (IDT) measurements for ethylene (C2H4)/nitrogen dioxide (NO2)/oxygen/argon mixtures with various blending ratios of NO2/C2H4 = 0/100, 15/85 and 50/50 were performed in a shock tube. The sensitization effect of NO2 on C2H4 ignition was investigated at pressures ranging from 1.2 to 10 atm, in the temperature range 920–1780 K, and at equivalence ratios from 0.5 to 2.0. Similar to our recent studies of NO2 effects on CH4 and C2H6 ignition (Deng et al., 2016; Deng et al., 2017), the effect of NO2 addition on measured IDTs of the unsaturated hydrocarbon C2H4 also exhibits both temperature- and pressure- dependences. NO2 addition generally promotes the reactivity of ethylene and the promotion effect is more pronounced for the mixtures with high NO2 content at higher pressures (p > 4.0 atm), at lower temperatures (T 1250 K). A detailed kinetic mechanism of C2H4/NO2 was proposed by refining critical reactions through incorporating ab-initio calculations from this study and recently published rate constant data. This mechanism is capable of reproducing the new data reported in this work and literature ones as well and has thus been used to perform sensitivity and flux analyses to kinetically interpret the interaction of NO2 with C2H4.

Ignition delay times of methane and hydrogen highly diluted in carbon dioxide at high pressures up to 300 atm

The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306 nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578 K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27–286 atm and H2/O2/CO2 mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms.

Experimental and chemical kinetic modeling investigation of methyl butanoate as a component of biodiesel surrogate

Biodiesel is a potential alternative to fossil diesel. In combustion simulations, in order to circumvent the difficulty in integrating reaction schemes for biodiesels, which are typically of a large size and not well understood, a surrogate approach to simplify the representation of its long chain methyl ester components is adopted. In this work, a compact reaction scheme for methyl butanoate, which is a potentially important candidate for biodiesel surrogates, is derived from a detailed reference mechanism (Dooley et al., 2008). An existing well-validated model for n-dodecane (Narayanaswamy et al., 2014) oxidation, which is a suitable base to model biodiesel surrogates, is augmented with the oxidation pathways of methyl butanoate. The resulting combined mechanism is comprehensively assessed for methyl butanoate kinetic description. Several rate constants pertaining to methyl butanoate kinetics are updated in the resulting chemical mechanism based on recent rate recommendations from the literature in a consistent manner. The revised kinetic model has been validated comprehensively against a wide range of experimental data and found to be satisfactory. In addition, auto-ignition delay times of methyl butanoate have been measured in a rapid compression machine (RCM). The ignition delay time measurements cover a wide range of experimental conditions: temperatures of 850–1100 K and pressures of 10–40 bar. The impact of varying equivalence ratios on ignition delay times has also been investigated for ϕ = 0.5–1.5 and ignition delay times are reported for the rich mixtures for the first time as a part of this work. No two-stage ignition or negative temperature coefficient (NTC) behavior has been observed for methyl butanoate in the experimental investigation. The effect of addition of low-temperature chemistry pathways to the methyl butanoate chemical kinetic mechanism has also been explored.

Experimental and theoretical investigations of methyl formate oxidation including hot β-scission

Abstract Recently the possibility of hot β-scission pathways gained attention. These reactions give a shortcut during the important fuel consumption phase in combustion processes leading from H-atom abstraction directly to the β-scission products without fuel radical thermalization. Methyl formate (MF) was shown to be prone to hot β-scission due to a low β-scission barrier height. Furthermore, MF as smallest methyl ester can be considered as biodiesel surrogate and it is an important intermediate product during combustion of various ethers. In this work a predominantly ab-initio derived detailed kinetic model of MF combustion is developed including hot β-scission pathways and compared to a sophisticated literature model based on classical estimation methods. For this, new stoichiometric MF in air ignition delay time measurements in a shock tube and a rapid compression machine over a wide temperature range (790 K–1250 K) and pressures of 10, 20 and 40 bar served as validation targets. The experimental ignition delay times (IDT) show Arrhenius type behavior in both facilities at all conditions. The newly developed quantum-based model catches the pressure dependency and low-temperature reactivity well although overpredicting the IDT at higher temperatures. It was found that hot β-scission is the major depletion pathway of formate group-centered MF radicals. This, however, does not change the overall reactivity of MF combustion due to the low stability of the alkyl peroxide (RO2) at the formate group. For species with competing thermal β-scission and RO2 formation, however, hot β-scission may have a significant impact.

Ignition delay times of decalin over low-to-intermediate temperature ranges: Rapid compression machine measurement and modeling study

Autoignition characteristics of decalin, a bicyclic alkane, were investigated in a heated rapid compression machine (RCM) over a wide range of conditions. Ignition delay time (IDT) was measured at compressed pressures of 10, 15 and 20 bar, for equivalence ratios of 0.5, 1.0, 1.5 and 2.0, and at temperatures in the range 631−930 K. Negative temperature coefficient (NTC) behavior of decalin ignition delay time was observed within the temperatures of 750−860 K, in which the ignition delay time increases with rising temperature. The dependence of ignition delay time on compressed pressure, equivalence ratio, and oxygen concentration was systemically studied. A reasonable modification was made to a literature mechanism. The simulation results using the tuned mechanism are found to well capture the dependence of the measured ignition delay time on temperautre, pressure, equivalence ratio, and oxygen concentration over the entire temperature range. Correlation formulas of the simulated and measured ignition delay times were proposed to quantitatively reveal the ignition delay time dependence and to evaluate the mechanism performance. A reaction pathway analysis was carried out at low temperature (700 K), NTC temperature (850 K), and high temperature (1000 K), respectively, to identify the dominant reaction pathways consuming decalin and intermediate species. A sensitivity analysis was also performed at different temperatures and equivalence ratios to find out the important reactions that promote and inhibit decalin autoignition.

Autoignition of varied cetane number fuels at low temperatures

The reactivity of six kerosene based control fuels, specifically formulated for cetane number variation, are investigated by measuring ignition delay time in a heated rapid compression machine. Cetane numbers vary from 30 to 55 (increment of 5) while holding other properties relatively constant by adjusting chemical group composition. Main cetane variation was controlled through the distribution of normal alkanes and isoalkanes, which was fine-tuned using additives. Other fuel properties such as density, viscosity, H/C ratio, etc. were balanced using cyclic compounds and aromatics. Fuels were tested in the RCM at compressed pressures of Pc = 10 and 20 bar, equivalence ratios of ϕ = 0.25, 0.5 and 1.0, in the low to intermediate temperature range (620 K ≤ Tc ≤ 730 K). Relations between cetane number and ignition delay time have been evaluated at multiple test conditions, and further analysis on multistage ignition has been conducted. Ignition delay times of fuels with higher cetane numbers are shorter at these temperatures for most conditions. First stage ignition delay time measurements have been observed to be relatively insensitive to Pc, ϕ, and fuel type, while deviations in overall ignition delay times are mainly attributed to second stage ignition delay time, impacted by variations in the first stage temperature. Control fuels of this type offer an opportunity to be used in practical experiments to determine the impact of cetane number on combustion dynamics.

Shock-tube study of the ignition and product formation of fuel-rich CH4/air and CH4/additive/air mixtures at high pressure

Higher-value chemicals can be produced from methane with small exergy losses by partial oxidation if the chemical conversion proceeds in an internal combustion engine (ICE) as a polygeneration process (Gossler and Deutschmann, 2015). Kinetics models are not sufficiently validated for the very fuel-rich and high-pressure conditions relevant for this process. Therefore, ignition delay times of fuel-rich methane/(additive)/air mixtures were measured in a shock tube at about 30 bar and temperatures between 600 and 1650 K. n-heptane and diethylether were used as additives to increase the reactivity of the fuel so that the polygeneration process can be realized in an ICE at HCCI conditions at lower compression temperatures. At ϕ = 2, measured ignition delay times agree well with simulations using different mechanisms from literature. Synthesis gas (CO, H2) is the main product at these conditions (Sen et al., 2016). For the production of higher hydrocarbons, the equivalence ratio must be increased. Very fuel-rich mixtures (ϕ = 10) were used because the temperature increase during the reaction of these mixtures is quite low (<450 K), so that post-ignition temperatures stay below the lower limit of soot formation. Only for mixtures with n-heptane as additive, good agreement of measured and simulated ignition delay times is found. The other mixtures show strong deviations with all mechanisms. As a further parameter to improve and validate the mechanisms at ϕ = 10, product distributions after ignition were determined by sampling in the cooling phase with a fast-opening valve and GC/MS analysis. Besides H2 and H2O, CO and higher hydrocarbons like C2H2, C2H4, C2H6, and C6H6 were detected as main products. About half of the carbon of the consumed methane is converted to CO, the other half to higher hydrocarbons. The product distributions are well predicted by simulations.

Shock tube study of the rate constants for H + O2 + M → HO2 + M (M = Ar, H2O, CO2, N2) at elevated pressures

The rate constants for the reaction H + O2 + M→HO2 + M were investigated at elevated pressures from 12 to 33 atm using ignition delay time (IDT) measurements behind reflected shock waves in H2/O2/M mixtures with different collision partners M = Ar, H2O, N2 and CO2. The temperature and pressure ranges where the rate constants of the reactions H + O2 + M→HO2 + M and H + O2- > OH + O dominate the IDT sensitivity were selected as optimum test conditions using the detailed H2/O2 mechanism of Hong et al. (2011). The current study thus provides a quantitative and relatively direct method for determining the rate constants for H + O2 + M→HO2 + M using simple IDT measurements. The rate constants found are consistent with earlier studies, but with reduced uncertainties.

Concerning shock-tube ignition delay times: An experimental investigation of impurities in the H2/O2 system and beyond

Shock-tube ignition delay time data have recently been called into question on the basis of possible hydrocarbon impurities that may serve to artificially accelerate ignition delay times. To assess potential sources of impurities, systematic tests were performed in highly dilute H2/O2/Ar and CH4/O2/Ar mixtures using H2O laser absorption at 1.388 µm and OH* emission at 307 nm near 1 atm and between 1257 and 2108 K. Factors investigated included common sources of impurity, namely shock-tube cleanliness, Ar purity, diaphragm fragments, leftover cleaning substances, turbopumping duration, and mixing tank cleanliness, but only mixing tank cleanliness was found to have any quantifiable effect on measured ignition delay times. Rate measurements of H + O2⇆OH + O using H2O time-histories were found to be unperturbed by impurity effects and were in excellent agreement with the Hong et al. measurement. Mixing tank impurities were found to perturb ignition delay times of a stoichiometric H2/O2 mixture in 98% Ar but had no effect on a stoichiometric CH4/O2 mixture in 99% Ar. Even with clean mixing tank conditions, H2/O2 ignition delay times in 98% and 99% Ar displayed a 25–30% discrepancy with predicted values but showed remarkable agreement with two sets of historical data. This discrepancy was modeled by including trace hydrocarbons (within certified purity levels) in simulations. For less-dilute (≤ 94% Ar) H2/O2 mixtures, literature data seem to agree rather well and were insensitive to trace hydrocarbons according to simulations. Furthermore, experimental and modeling evidence strongly suggests that almost every hydrocarbon C1 and higher is insensitive to impurities with respect to ignition delay times. It is thus concluded that the only data for which an impurity effect seems to exist is for highly dilute (98–99% Ar) H2/O2 data, while H2/O2 data below 94% dilution and almost every hydrocarbon C1 and greater are insensitive to common impurities.

An Investigation of Combustion Properties of Butanol and Its Potential for Power Generation

Over the last years, global concerns about energy security and climate change have resulted in many efforts focusing on the potential utilization of nonpetroleum-based, i.e., bioderived, fuels. In this context, n-butanol has recently received high attention because it can be produced sustainably. A comprehensive knowledge about its combustion properties is inevitable to ensure an efficient and smart use of n-butanol if selected as a future energy carrier. In the present work, two major combustion characteristics, here laminar flame speeds applying the cone-angle method and ignition delay times applying the shock tube technique, have been studied, experimentally, and by modeling exploiting detailed chemical kinetic reaction models, at ambient and elevated pressures. The in-house reaction model was constructed applying the reaction model generation (RMG)-method. A linear transformation method recently developed, linTM, was exploited to generate a reduced reaction model needed for an efficient, comprehensive parametric study of the combustion behavior of n-butanol-hydrocarbon mixtures. All experimental data were found to agree with the model predictions of the in-house reaction model, for all temperatures, pressures, and fuel-air ratios. On the other hand, calculations using reaction models from the open literature mostly overpredict the measured ignition delay times by about a factor of two. The results are compared to those of ethanol, with ignition delay times very similar and laminar flame speeds of n-butanol slightly lower, at atmospheric pressure.

A shock tube study of ignition delay times in diluted methane, ethylene, propene and their blends at elevated pressures

Ignition delay times (IDT) of small hydrocarbons at elevated pressures provide a valuable constraint for the refinement of the core small-hydrocarbon sub-mechanisms used in all combustion kinetics. Current knowledge of these core mechanisms is based largely on low-pressure data, with only limited high-pressure data available. To remedy this, the present study focuses on ignition delay times in methane, ethylene, propene and their blends at elevated pressures. IDT measurements were performed in 4% O2, balance Ar mixtures, over the temperature range of 950–1800 K, at pressures of 14–60 atm and equivalence ratios of 1 and 2. IDT was determined from recorded sidewall pressure, OH∗ emission measurements and fuel time-histories measured using laser absorption at 3.39 μm. These measurements extend the test conditions of earlier studies, with the advantage that they have all been performed at similar conditions and with the same facility and should provide a uniform set of kinetics targets for the evaluation of core small-hydrocarbon mechanisms. This dataset also allowed the temperature variation of the pressure and equivalence ratio scaling for methane and ethylene IDT to be investigated.

Shock-Tube Experiments and Chemical Kinetic Modeling Study of CH4 Sensitized by CH3NHCH3

Ignition delay times of stoichiometric CH4/CH3NHCH3/O2/Ar mixtures are measured with a shock tube in the temperature range of 1100–2000 K. Different pressures (4, 8, and 18 atm) and CH3NHCH3 blending ratios (0, 0.05, 0.1, 0.2, 0.5, and 1) are explored. CH3NHCH3 promotes CH4 ignition. Correlations for the measured ignition delay times are inferred through multiple linear regression. The measured ignition delay times are compared to the predictions of a recently developed CH3NHCH3 kinetic model with C0–C2 reaction subsets, and a new model for CH4/CH3NHCH3/O2/Ar mixtures is assembled and validated against the present measurements. To elucidate the effect of CH3NHCH3 on CH4 ignition, kinetic analyses including sensitivity and rate of production are performed. Perturbation of the radical pool (H, CH3, OH, and HO2) leading to the observed effects is discussed.

Shock tube measurements and kinetic modeling study on autoignition characteristics of cyclohexanone

Cyclohexanone was identified as an attractive second-generation biofuel that can be used as an alternative fuel or blending component, thereby reducing greenhouse gas and particulate emissions from internal combustion engines. This paper investigates auto-ignition characteristics of cyclohexanone/O2/Ar mixtures behind reflected shock waves. Ignition delay time measurements cover the temperature range of 1255−1646 K, with pressures near 2.5, 5, and 10 atm, argon/oxygen dilution ratios of 23.9, 48.9 and 98.9, and equivalence ratios of 0.5, 1.0, and 2.0. Five kinds of mixtures were prepared to determine the effects of pressure, equivalence ratio and dilution ratio on the reactivity over the entire temperature range. An ignition delay correlation, τ=6.87×10−6P−0.52Φ0.70D0.66exp(45.59/RT)(μs), has been developed for cyclohexanone, including dependencies on pressure, equivalence ratio and dilution ratio. Experimental data have been compared to the kinetic modeling predictions of Serinyel et al. [Proc. Combust. Inst. 35 (2015) 507−514]. In general, the predictions overestimate the experimental data by 30–50%. With several reaction rate constants of selected small radical reactions updated, the predictions by modified mechanism present satisfactory agreement with the measured ignition delay data. Also, simulations based on the original model and modified model were compared to species concentrations measured in a jet-stirred reactor at 10 atm from the literature. Reaction pathway and sensitivity analyses were performed to identify the fuel consumption paths and key reactions during the ignition process of cyclohexanone. To our knowledge, the auto-ignition measurement of gas-phase cyclohexanone presented here are the first measurement for this compound.