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The pyrolysis of propane plays an important role in determining the combustion properties of natural gas mixtures and offers insight into the cracking patterns of larger fuels. This work investigates propane pyrolysis behind reflected shock waves with a multiwavelength laser‐absorption speciation technique. Nine laser wavelengths, sensitive to key pyrolysis species, were used to measure absorbance time histories during the decomposition of 2% propane in argon between 1022 and 1467 K, 3.7‐4.3 atm. Absorbance models were developed at each diagnostic wavelength to interrogate common initial conditions, and time histories of all major species are reported at 1250, 1290, 1330, 1370, and 1410 K. Nearly complete carbon recovery observed at lower temperatures enabled the inference of hydrogen formation from atomic conservation, while decaying carbon recovery at high temperatures suggests the formation of allene and 1‐butene. The results show systematically faster pyrolysis than predicted by kinetic modeling and motivate further study into the kinetics of propane pyrolysis. [ABSTRACT FROM AUTHOR]

Homogeneous and inhomogeneous ignition modes of n -heptane were studied using high-speed imaging in a high-pressure shock tube (HPST). n -Heptane, a fuel with strong negative temperature coefficient (NTC) behavior, was mixed with 4%-21% oxygen in argon or nitrogen and ignited over a wide temperature range (700–1250 K) and at elevated pressures (> 10 atm). Ultraviolet (UV) images of OH* emission were captured through a sapphire shock-tube end wall using a high-speed camera and a UV intensifier. The current study demonstrates the capability to study auto-ignition modes using high-speed imaging in a high-pressure shock tube. Both homogeneous and inhomogeneous auto-ignition events were observed with the latter generally confined to intermediate temperatures and reactive n -heptane mixtures. We also observed that conventional sidewall diagnostic signals are, in many cases, sufficient to identify inhomogeneous ignitions that are not accurately modeled under the assumption of spatially uniform chemistry. [ABSTRACT FROM AUTHOR]

Abstract The need for more efficient power cycles has attracted interest in super-critical CO 2 (sCO 2) cycles. However, the effects of high CO 2 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 CO 2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O 2 /CO 2 mixtures and hydrogen/O 2 /CO 2 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., CH 4 /O 2 /CO 2 mixtures at 27–286 atm and H 2 /O 2 /CO 2 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. [ABSTRACT FROM AUTHOR]

Abstract The rate constants for the reaction H + O 2 + M→HO 2 + M were investigated at elevated pressures from 12 to 33 atm using ignition delay time (IDT) measurements behind reflected shock waves in H 2 /O 2 /M mixtures with different collision partners M = Ar, H 2 O, N 2 and CO 2. The temperature and pressure ranges where the rate constants of the reactions H + O 2 + M→HO 2 + M and H + O 2 - > OH + O dominate the IDT sensitivity were selected as optimum test conditions using the detailed H 2 /O 2 mechanism of Hong et al. (2011). The current study thus provides a quantitative and relatively direct method for determining the rate constants for H + O 2 + M→HO 2 + M using simple IDT measurements. The rate constants found are consistent with earlier studies, but with reduced uncertainties. [ABSTRACT FROM AUTHOR]

Abstract A novel, multi-species, combined laser absorption/gas chromatography (GC) speciation diagnostic was used to quantify intermediate species present in the low-temperature oxidation of 1.0% n-heptane in 20.8% O 2 /Ar and 20.8% O 2 /0.21% CO 2 /Ar (equivalence ratio 0.53) at 760 K, 4.9 atm. Laser absorption techniques were used to measure initial fuel and time-resolved temperature, CO 2 , H 2 O, and C 2 H 4. Sampled-gas GC analysis was used in conjunction with a variable-test-time shock tube facility to obtain quasi-time-resolved measurements of n-heptane, C 2 H 4 , CO, H 2 , C 3 H 6 , and CO 2 in the same experiments. Measurements obtained using both techniques are compared to each other, and to initial results predicted by a detailed kinetic model using the experimentally measured pressure trace to constrain the model. Discrepancies between measured and predicted ignition delay times indicate the overestimation of three primary RO 2 isomerization reaction rates. The three reaction rates were modified to improve agreement of modeled ignition delay times with the measurements. Final results produced using the modified mechanism are compared to the experimental results; the comparison shows close agreement between the two experimental measurement techniques, and measured species yields confirm the low-temperature reaction pathways that govern n-heptane decomposition and C 2 H 4 , CO, H 2 , and C 3 H 6 production. [ABSTRACT FROM AUTHOR]

The first implementation of a combined laser absorption diagnostic/gas chromatography (GC) sampling system for the measurement of combustion-relevant species in a conventional shock tube configuration is reported, with ethylene pyrolysis as an example application. A heated, endwall sampling system is used to extract a post-shock sample for GC analysis. Analysis of the gas sample yields a measurement of the ultimate mole fraction values of multiple species (currently ethylene, acetylene, hydrogen, and methane) at the end of the reflected shock test time. A 10.532-µm laser absorption diagnostic is simultaneously used to measure time-resolved ethylene. A method to accurately model sampled speciation results using published kinetic models is discussed. A method for extending laser measurements into the expansion fan region for direct comparison with sampled GC results has also been developed. The combined optical and sampled-gas measurement techniques were used to study ethylene pyrolysis (1.0% mole fraction ethylene/argon) at approximately 5 atm, over a range of temperatures (1200–2000 K). The ethylene mole fraction measurements obtained using both techniques show close agreement. [ABSTRACT FROM AUTHOR]

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% O 2 , 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. [ABSTRACT FROM AUTHOR]

In this study we report a novel UV laser absorption diagnostic for sensitive and time-resolved measurement of methyl radicals (CH 3 ) in shock tube kinetics studies, utilizing a frequency-quadrupled, high-repetition-rate (78 MHz) ps-pulsed Ti-sapphire laser targeted at the strong rovibronic transition of CH 3 at 216.62 nm. This diagnostic employs a cavity-enhanced absorption spectroscopy (CEAS) strategy that amplifies the absorbance signal by a gain factor of about 50, which enables sub-ppm detection limit of CH 3 to be achieved under typical shock conditions of 1200 K < T < 2500 K and P ∼ 1 atm, in a shock tube of ∼15 cm inner diameter. The use of a pulsed laser also enables an on-axis CEAS configuration, which yields a time resolution of 2–4 µs. For example, CH 3 measurements were performed in shock tube decomposition experiments of dilute ethane and methane mixtures, demonstrating the substantial improvement in detection sensitivity of the current diagnostic compared to conventional single-pass absorption measurements. This new diagnostic promises to be a powerful tool for future shock tube chemical kinetics studies, and can be potentially extended to the detection of other UV-absorbing species. [ABSTRACT FROM AUTHOR]

The overall rate constants for the H-abstraction reactions of a series of aldehydes, namely isobutyraldehyde (i-C 3 H 7 CHO), n -valeraldehyde ( n -C 4 H 9 CHO), isovaleraldehyde (i-C 4 H 9 CHO), trans -2-pentenal (C 2 H 5 CH=CHCHO), trimethylacetaldehyde ((CH 3 ) 3 CCHO) and benzaldehyde (C 6 H 5 CHO), by hydroxyl radicals (OH), were studied behind reflected shock waves at temperatures of 976–1346 K and pressures around 0.6 atm. The OH radicals, produced pyrolytically from tert-butyl hydroperoxide (TBHP), were quantitatively measured by UV absorption using a narrow-linewidth laser near 306.69 nm, targeting the well-characterized R 1 (5) rovibronic transition of OH in the A-X (0, 0) band. The high sensitivity of this OH diagnostic allowed for very dilute reactant concentrations, which simplified the reaction systems to pseudo-first-order kinetics. For each aldehyde + OH, its overall rate constant was inferred by fitting simulated OH profiles to the measured OH time histories when a detailed reaction mechanism for this aldehyde was available, or directly from exponential fits to the measured OH time-histories (after correcting for a small effect of different TBHP/aldehyde ratios). Both approaches yielded similar results. The resulting reaction rate constants, measured for the first time at temperatures above 1000 K, can be expressed in Arrhenius equations in units of cm 3 mol −1 s −1 as follows: k iC 3 H 7 CHO+OH + = 7.70 × 10 13 exp(–1700 K/T) ± 7%, valid over 976 – 1346 K; k nC 4 H 9 CHO+OH + = 1.03 × 10 14 exp(–1730 K/T) ± 7%, valid over 986 – 1313 K; k iC 4 H 9 CHO+OH + = 1.02 × 10 14 exp(–1750 K/T) ± 10%, valid over 976 – 1322 K; k (CH 3 ) 3 CCHO+OH + = 7.20 × 10 13 exp(–1670 K/T) ± 8%, valid over 979 – 1243 K; k C 2 H 5 CHCHCHO+OH + = 6.93 × 10 13 exp(–1340 K/T) ± 8%, valid over 993 – 1285 K; k C 6 H 5 CHO+OH + = 3.10 × 10 13 exp(–1380 K/T) ± 8%, valid over 983 – 1333 K. [ABSTRACT FROM AUTHOR]

High-temperature methyl acetate (MA) pyrolysis and oxidation were studied behind reflected shock waves using laser-absorption species time-history measurements of CO, CO 2 , OH and H 2 O. The shock tube experiments with very dilute fuel mixtures covered a temperature range of 1401–1605 K for MA pyrolysis (0.2% MA/Ar) and 1423–1674 K for MA oxidation (0.4% O 2 , ϕ = 1), and pressures around 1.5 atm. The dominant sensitivity of CO 2 concentration to MA unimolecular decomposition reactions enabled accurate determination of the rate coefficient and dissociation branching ratio by monitoring the CO 2 time-history during MA pyrolysis. A recent kinetic mechanism developed by Yang et al. [13] originally for interpreting flow reactor and low-pressure flame data was adopted to simulate and compare with the current shock tube data. The measured CO and CO 2 time-histories during MA pyrolysis were both well-predicted by the modified Yang et al. [13] mechanism. A relatively complete description of MA oxidation behavior was given by measuring CO, CO 2 , OH and H 2 O time-histories at the same temperature and pressure (1480 K, 1.5 atm). A unique two-stage CO 2 formation during MA oxidation was clearly observed in the measured CO 2 time-histories over the entire temperature range, with the rapid pre-ignition CO 2 formation analyzed to be associated with the initial MA dissociation. Despite the overprediction of MA ignition delay times by 18–40% between 1623 and 1423 K, the kinetic model successfully captures the plateau levels and the peak values of all the measured species profiles, as well as simulates the characteristic two-stage formation of CO 2 observed experimentally. [ABSTRACT FROM AUTHOR]

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%O 2 /Ar mixtures behind reflected shocks using an aerosol shock tube: τ ign [ ms ] = 2.24 × 10 - 6 [ ms ] ( P [ atm ] ) - 0.41 ( ϕ ) 0.30 ( C n ) - 0.61 exp 37.1 [ kcal/mol ] R ̂ u [ 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%O 2 /Ar mixtures. [ABSTRACT FROM AUTHOR]

The overall rate constants for the reactions of hydroxyl radicals (OH) with a series of aldehydes, formaldehyde (CH 2 O), acetaldehyde (CH 3 CHO), propionaldehyde (C 2 H 5 CHO) and n-butyraldehyde (n-C 3 H 7 CHO), were studied behind reflected shock waves at temperatures of 950–1400 K and pressures of 1–2 atm. OH radicals were produced by rapid thermal decomposition of tert-butyl hydroperoxide (TBHP), and OH time-histories were monitored by narrow-linewidth UV laser absorption of the well-characterized R 1 (5) line in the OH A-X (0, 0) band near 306.69 nm. The overall rate constants were inferred by fitting simulated OH profiles to the measured OH time histories using detailed mechanisms of Veloo et al. (2013), USC Mech-II (2007) and GRI Mech 3.0. The measured high-temperature aldehydes + OH rate constants can be expressed in modified Arrhenius equations, in units of cm 3 mol −1 s −1 and K, as k CH 2 O = 1.02 × 10 7 T 1.92 exp ( 779 / T ) ± 13 % k CH 3 CHO = 4.32 × 10 6 T 2.02 exp ( 716 K / T ) ± 22 % k C 2 H 5 CHO = 5.94 × 10 6 T 1.98 exp ( 823 K / T ) ± 27 % k nC 3 H 7 CHO = 5.36 × 10 6 T 2.06 exp ( 658 K / T ) ± 25 % No pressure dependence was observed in these measurements. The measured rate constant for the formaldehyde + OH reaction is consistent with previous experimental work from Vasudevan et al. (2005) within ±25%. For C 2 –C 4 aldehydes + OH, this study provides the first direct rate constant measurement at high temperatures. More general rate constant expressions covering a much wider temperature range (200–1400 K) were also determined by combining current measurements with existing low temperature data in the literature. These wide-range expressions were seen to be in excellent agreement with most existing experimental data. [ABSTRACT FROM AUTHOR]

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, 1047–1520 K, and equivalence ratios, 0.25–2.2, in two pressure and mixture regimes: for fuel/air mixtures at 2.07–8.27 atm, and for fuel/4%oxygen(O 2 )/argon(Ar) mixtures at 15.9–44.0 atm. 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. [ABSTRACT FROM AUTHOR]

Abstract: Ignition delay times behind reflected shock waves are strongly sensitive to variations in temperature and pressure, yet most current models of reaction kinetics do not properly account for the variations that are often present in shock tube experiments. Particularly at low reaction temperatures with relatively long ignition delay times, substantial increases in pressure and temperature can occur behind the reflected shock even before the main ignition event, and these changes in thermodynamic conditions of the ignition process have proved difficult to interpret and model. To obviate such pressure increases, we applied a new driven-gas loading method that constrains the volume of reactive gases, thereby producing near-constant-pressure test conditions for reflected shock measurements. Using both conventional operation and this new constrained-reaction-volume (CRV) method, we have collected ignition delay times for 1-butanol/O2/N2 mixtures over temperatures between 716 and 1121K and nominal pressures of 20 and 40atm for equivalence ratios of 0.5, 1.0, and 2.0. The equivalence ratio dependence of 1-butanol ignition delay time was found to be negative when the oxygen concentration was fixed, but positive when the fuel concentration was held constant. Ignition delay times with strong pre-ignition pressure increases in conventional-filling experiments were found to be significantly shorter than those where these pressure increases were mitigated using the CRV strategy. The near-constant-pressure ignition delay times provide a new database for low-temperature 1-butanol mechanism development independent of non-idealities caused by either shock attenuation or pre-ignition perturbations. Comparisons of these near-constant-pressure measurements with predictions using several reaction mechanisms available in the literature were performed. To our knowledge this work is first of its kind that systematically provides accurate near-constant-enthalpy and -pressure target data for chemical kinetic modeling of undiluted fuel/air mixtures at engine relevant conditions. [Copyright &y& Elsevier]

Abstract: Laser absorption diagnostic methods were developed for the quantitative measurement of formaldehyde (CH2O) and acetaldehyde (CH3CHO) at high temperatures in shock tube kinetic studies. Investigation of the high-temperature CH2O spectrum has shown that the optimal wavelength for CH2O detection using commercially available lasers is near 2896cm−1. By exploiting the structural difference between the absorption spectra of CH2O and that of broadband interfering species, a two-color (2895.92cm−1 and 2895.60cm−1) interference-free detection scheme for CH2O sensing in a combustion environment was developed. A third color (32601.10cm−1) has also been added to develop a UV/IR detection scheme for combined CH3CHO/CH2O measurements. To implement these schemes, aldehyde cross-sections at all three colors were measured behind reflected shock waves over a wide span of temperatures (600–1800K) and pressures (0.8–3.6atm), with an uncertainty of ±5%, and the diagnostic schemes were validated using two controlled experiments with well-established chemistry. Applications of these diagnostics were also demonstrated in shock tube pyrolysis experiments of 1,3,5-trioxane, CH2O and CH3CHO. The unimolecular decomposition rate of 1,3,5-trioxane was determined over 869–1037K at an average pressure of 2.1atm: kI =3.58×1012 exp (−18,590K/T)s−1, with an overall uncertainty of less than 20%. [Copyright &y& Elsevier]

Abstract: Fuel decomposition time-history measurements during morpholine pyrolysis and ignition delay time measurements during morpholine oxidation were carried out behind reflected shock waves in a high-pressure shock tube. For the pyrolysis studies, morpholine concentrations of 5000ppm in argon were employed, with experiments covering the temperature range 1086–1404K and pressure range 20–23.6atm. For the ignition delay time measurements, experiments covered the temperature range 866–1197K with pressures near 15 and 25atm, oxygen mole fractions of 4% and 21%, and equivalence ratios of 0.5, 1, and 2. A morpholine reaction set developed to simulate low-pressure flames was updated based on the current shock tube data, recently published rate constants for relevant reactions, and newly calculated thermochemistry. The simulations from the updated set were in good agreement with the current shock tube experiments. [Copyright &y& Elsevier]

Abstract: High-temperature acetone and 2-butanone pyrolysis studies were conducted behind reflected shock waves using five species time-history measurements (ketone, CO, CH3, CH4 and C2H4). Experimental conditions covered temperatures of 1100–1600K at 1.6atm, for mixtures of 0.25–1.5% ketone in argon. During acetone pyrolysis, the CO concentration time-history was found to be strongly sensitive to the acetone dissociation rate constant k 1 (CH3COCH3 →CH3 +CH3CO), and this could be directly determined from the CO time-histories, yielding k 1(1.6atm)=2.46×1014 exp(−69.3 [kcal/mol]/RT) s−1 with an uncertainty of ±25%. This rate constant is in good agreement with previous shock tube studies from Sato and Hidaka (2000) [3] and Saxena et al. (2009) [4] (within 30%) at temperatures above 1450K, but is at least three times faster than the evaluation from Sato and Hidaka at temperatures below 1250K. Using this revised k 1 value with the recent mechanism of Pichon et al. (2009) [5], the simulated profiles during acetone pyrolysis show excellent agreement with all five species time-history measurements. Similarly, the overall 2-butanone decomposition rate constant k tot was inferred from measured 2-butanone time-histories, yielding k tot(1.5atm)=6.08×1013 exp(−63.1 [kcal/mol]/RT) s−1 with an uncertainty of ±35%. This rate constant is approximately 30% faster than that proposed by Serinyel et al. (2010) [11] at 1119K, and approximately 100% faster at 1412K. Using the measured 2-butanone and CO time-histories and an O-atom balance analysis, a missing removal pathway for methyl ketene was identified. The rate constant for the decomposition of methyl ketene was assumed to be the same as the value for the ketene decomposition reaction. Using the revised k tot value and adding the methyl ketene decomposition reaction to the Serinyel et al. mechanism, the simulated profiles during 2-butanone pyrolysis show good agreement with the measurements for all five species. [Copyright &y& Elsevier]

Abstract: HO2 and CH3 are major intermediate species presented during the oxidation of natural gas at intermediate temperatures and high pressures. Previous theoretical calculations have identified several product channels for HO2 and CH3 reactions, with CH3 +HO2 →CH3O+OH and CH3 +HO2 →CH4 +O2 being the dominant reaction pathways. Both reaction pathways play an important role in the kinetics of CH4 oxidation as CH3 +HO2 →CH3O+OH is a chain-branching reaction whereas CH3 +HO2 →CH4 +O2 a chain termination reaction. H2O2/CH4/Ar mixtures were shock-heated to a temperature between 1054 and 1249K near 3.5atm to initiate the reaction. OH radicals yielded from H2O2 thermal decomposition react with H2O2 and CH4 respectively to produce HO2 and CH3 in the reacting system. Using laser absorption spectroscopy, time-histories of H2O, OH and HO2 were measured behind reflected shock waves. The rate constant of reaction CH3 +HO2 →CH3O+OH was determined to be 6.8×1012 cm3 mol−1 s−1 with an uncertainty factor of 1.4. The rate constant of the competing CH3 +HO2 →CH4 +O2 reaction was determined to be 4.4×1012 cm3 mol−1 s−1, with an uncertainty factor of 2.1. In addition, the rate constants of two other major reactions of the reacting system, H2O2 (+M)→2OH (+M) and OH+CH4 →CH3O+OH, were found to have excellent agreement with values recommended in literature. [Copyright &y& Elsevier]

Abstract: OH concentration time-histories during n-heptane and methylcyclohexane (MCH) oxidation were measured behind reflected shock waves in a heated, high-pressure shock tube. Experimental conditions covered temperatures of 1121 to 1332 K, pressures near 15 atm, and initial fuel concentrations of 750 and 1000 ppm (by volume), and an equivalence ratio of 0.5 with O2 as the oxidizer and argon as the bath gas. OH concentrations were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH system at 306.47 nm. These current measurements together with our recent results for n-dodecane oxidation [S.S. Vasu, D.F. Davidson, Z. Hong, V. Vasudevan, R.K. Hanson, Proc. Combust. Inst. 32 (2009), doi:10.1016/j.proci.2008.05.006] provide critically needed validation targets for jet fuel surrogate kinetic mechanisms and further improve understanding of high-pressure, high-temperature oxidation chemistry. Detailed comparisons of these OH time-histories with the predictions of various kinetic mechanisms were made. Sensitivity and pathway analyses for these reference fuel components were performed, leading to reaction rate recommendations with improved model performance. Current results are the first quantitative measurements of OH time-histories during high-pressure oxidation of these fuels, and hence are a critical step toward development of accurate reaction models for jet fuel surrogates. [Copyright &y& Elsevier]

Abstract: Ignition delay times were measured for gas-phase jet fuel (Jet-A and JP-8) in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: temperatures of 715–1229 K, pressures of 17–51 atm, equivalence ratios of 0.5 and 1, and oxygen concentrations of 10 and 21% in synthetic air. Ignition delay times were measured using sidewall pressure and OH* emission at 306 nm. Longer ignition delay times at low temperatures (715–850 K) were accessed by utilizing driver-gas tailoring methods. Also presented is a review of previous ignition delay time measurements of kerosene-based fuels and recent work on surrogate fuel and kinetic mechanism development. To our knowledge, we report the first gas-phase shock tube ignition delay time data for JP-8, and our measurements for Jet-A are for a broader range of conditions than previously available. Our results have very low scatter and are in excellent agreement with the limited previous shock tube data for Jet-A. Although JP-8 and Jet-A have slightly different compositions, their ignition delay times are very similar. A simple dependence was found for ignition delay times from 874 to 1220 K for the pressure range studied for both fuels. Ignition delay time variations with equivalence ratio and oxygen concentration were also investigated. The new experimental results were compared with predictions of several kinetic mechanisms, using different jet fuel surrogate mixtures. [Copyright &y& Elsevier]

Abstract: The two-channel thermal decomposition of toluene, C6H5CH3 →C6H5CH2 +H (1) and C6H5CH3 →C6H5 +CH3 (2), was investigated in shock tube experiments over the temperature range of 1400–1780K at a pressure of 1.5 (±0.1) bar. Rate coefficients for reactions (1) and (2) were determined by monitoring benzyl radical (C6H5CH2) absorption at 266nm during the decomposition of toluene diluted in argon and modeling the temporal behavior of the benzyl concentration with a kinetic model. The first-order rate coefficients determined at a pressure of 1.5bar are expressed by k 1(T)=2.09×1015 exp(−87510 [cal/mol]/RT) [s−1] and k 2(T)=2.66×1016 exp(−97880 [cal/mol]/RT) [s−1]. The resulting branching ratio, k 1/(k 1 + k 2), ranges from 0.8 at 1350K to 0.6 at 1800K. [Copyright &y& Elsevier]

Abstract: The two-channel thermal decomposition of formaldehyde [CH2O], (1a) CH2O+Ar→HCO+H+Ar, and (1b) CH2O+Ar→H2 +CO+Ar, was studied in shock tube experiments in the 2258–2687K temperature range, at an average total pressure of 1.6atm. OH radicals, generated on shock heating trioxane–O2–Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C3H6O3] was used as the CH2O precursor in the current experiments. H-atoms formed upon CH2O and HCO decomposition rapidly react with O2 to produce OH via H+O2 →O+OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k 1a and k 1b, applicable in this temperature range, are: Uncertainty limits for k 1a and k 1b were estimated to be ∼±25%. The reaction between CH2O and O2, CH2O+O2 →HO2 +HCO, was also investigated in shock tube experiments. The rapid thermal decomposition of HCO and HO2 generate H-atoms that react with O2 to produce OH. Rate coefficients were, as in the CH2O decomposition experiments, inferred by matching measured and modeled OH time-histories behind the reflected shock, under conditions where interference from secondary chemistry is minimal. A two-parameter, least-squares fit of the current data, valid over the 1480–2367K temperature range, yields the following rate expression: The uncertainty in k 2 was estimated to be ∼±35%. [Copyright &y& Elsevier]

Abstract: The reaction of toluene with molecular oxygen to yield benzyl and hydroperoxyl radicals has been studied using ultraviolet laser absorption of benzyl radicals at 266 nm in shock-heated gases. Test gas mixtures of toluene with excess oxygen diluted in helium and argon were heated in reflected shock waves to temperatures ranging from 1117 to 1366 K at total pressures around 1.7 bar. The growth in benzyl absorbance was monitored at 266 nm, allowing determination of the rate coefficient for the C6H5CH3 + O2 → C6H5CH2 + HO2, reaction (1). The high signal-to-noise ratio provided by laser absorption provides rate coefficient determinations with an estimated uncertainty of . Fitting both these high-temperature shock tube results and the rate recommendation of Ingham et al. [Proc. Combust. Inst. 25 (1994) 767–774] at 773 K, the rate coefficient for reaction (1) can be described with a three-parameter Arrhenius expression by . In addition, the measured benzyl time-histories can be used as experimental targets for the development and validation of detailed mechanisms for toluene oxidation. [Copyright &y& Elsevier]

Abstract: The absorption of ultraviolet narrow-line laser radiation by methyl radicals in the electronic system has been studied at high temperatures behind shock waves. Methyl radicals at high temperatures were generated by the shock heating of methyl precursors: azomethane, methyl iodide, and ethane. The spectral shape and intensity of the broadband absorption feature from 211.5 to 220nm at high temperature (1565K) has been measured. The absorption coefficient of at 216.62nm, the wavelength of peak absorption at high temperatures in the band, has been determined from 1200 to 2500K. Additionally, the absorption coefficients of several interfering UV-absorbing combustion species (, and ) have been determined at 216.62nm. [Copyright &y& Elsevier]

Abstract: The decomposition rates of ethane and propane in the falloff regime at high temperatures were studied in a shock tube using UV narrow-line laser absorption of CH3 at 216.6nm. Experimental conditions ranged from 1343 to 2034K and 0.13 to 8.4atm with mixtures varying in concentration from 100 to 402ppm of ethane or propane diluted in argon. Decomposition rate coefficients were determined by monitoring the formation rate of CH3 immediately behind shock waves and modeling the CH3 formation with a detailed kinetic model. Calculations were performed using RRKM/master equation analysis with a restricted (hindered) Gorin model for the transition state and fit to the current high-temperature dissociation data as well as previous low-temperature recombination measurements. The decomposition rate coefficient for ethane decomposition from 700 to 1924K can be described, using the Troe pressure broadening formulation, by k ∞,1 (T)=1.88×1050 T −9.72 exp(−54,020K/T)s−1, k 0,1 (T)=3.72×1065 T −13.14 exp(−51,120K/T)cm3 mol−1 s−1, and F cent,1 (T)=0.61 exp(−T/100K)+0.39 exp(−T/1900K)+ exp(−6000K/T). The decomposition rate coefficient for propane decomposition from 600 to 1653K can be described by k ∞,2 (T)=1.29×1037 T −5.84 exp(−49,010K/T)s−1, k 0,2 (T)=5.64×1074 T −15.74 exp(−49,680K/T)cm3 mol−1 s−1, and F cent,2 (T)=0.69 exp(−T/50K)+0.31 exp(−T/3000K)+exp(−9000K/T). [Copyright &y& Elsevier]

High-temperature methane and propane laminar flame speed measurements were conducted behind reflected shock waves in a shock tube. A high-power Nd:YAG laser was used to spark-ignite the shock-heated gas mixtures and initiate laminar flame propagation. High-speed, OH* endwall imaging was used to record the propagation of the spherically expanding flames in time, and a non-linear stretch correlation was applied and used to determine the unburned, unstretched laminar flame speed. "Low-temperature" (<600 K) flame speed results are presented for stoichiometric methane/air and propane/air mixtures at initial unburned gas conditions of 489–573 K and 391–556 K, respectively, and 1 atm. The low-temperature measurements show close agreement with available literature data and kinetic modeling results, thereby validating the shock-tube laminar flame speed measurement approach. "High-temperature" (>750 K) flame speed results are presented for a propane/21% O 2 -47% N 2 -32% He mixture (ϕ = 0.8) at initial unburned gas conditions of 764–832 K, 1 atm. The high-temperature measurements fall between kinetic model predictions, but the kinetic model results show significant disagreement, highlighting the need for high-temperature flame speed validation data of this kind. We believe that these results represent the first laminar flame speed measurements conducted in a shock tube, and that the high-temperature results are the highest-temperature, 1-atm flame speed measurements available in the literature. [ABSTRACT FROM AUTHOR]

Abstract 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%O 2 /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%O 2 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. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fuel decomposition for two conventional fuels and six alternative fuels were measured. [•] Ethylene time-histories were measured for those eight fuels. [•] Current study can help refine surrogate compounds and detailed kinetic mechanisms. [Copyright &y& Elsevier]

Abstract: Laser-based OH, CO, CH3, and H2O species histories during 3-pentanone oxidation were measured behind reflected shock waves over the temperature range of 1277–1678K at pressures around 1.6atm, and for equivalence ratios of 0.5, 1.0 and 1.5, complementing recent work also conducted in this laboratory (Lam et al., 2012). These data are used to develop an improved detailed kinetic mechanism for 3-pentanone pyrolysis and oxidation. A 3-pentanone submechanism consisting of 13 species and 61 reactions is superimposed upon the H2/CO/C1–C4 base mechanism of JetSurF 2.0 (Wang et al., 2010), with several reactions updated in order to obtain better agreement with shock tube oxidation data. The improved mechanism shows satisfactory agreement with the wealth of shock tube data presented here and previously published, although there remain some conditions where the model can benefit from further improvements. Major rate parameter uncertainties in the model are also discussed and will be used in a companion work addressing model optimization and experimental design of shock tube studies using data obtained with the Stanford shock tube. [Copyright &y& Elsevier]

Highlights: [•] We measured the ignition delay times for three branched alkanes: 2,4-dimethylpentane, 2,5-dimethylhexane, and iso-octane. [•] The measured ignition delay times for branched alkanes are higher than their normal alkane isomers. [•] The measured ignition delay times increase with the carbon branching ratio. [ABSTRACT FROM AUTHOR]

Abstract: CH4 and C2H4 concentration time-histories were measured behind reflected shock waves during the pyrolysis of two n-alkanes: n-butane and n-heptane. Experiments were conducted at temperatures of 1200–1600K and at pressures near 1.5atm, with fuel concentrations of 1% in Ar. A mid-IR scanned-wavelength laser absorption diagnostic with a difference frequency generation (DFG) laser near 3.43μm was used to measure CH4 concentration time-histories. C2H4 was measured using a fixed-wavelength absorption scheme at 10.532μm with a CO2 laser. The mechanism of Wang et al. with a constant volume gasdynamic model was used to calculate temperature and pressure profiles and the mole fraction time-histories of CH4 and C2H4. The measured CH4 and C2H4 time-histories in n-butane pyrolysis were compared to simulations based on the comprehensive n-alkane mechanism by Wang et al. and the detailed n-butane mechanism by Marinov et al. Based on these comparisons, the n-butane decomposition rates measured by Oehlschlaeger et al. were incorporated into the Wang et al. mechanism and two additional butane abstraction reaction rate constant adjustments were also made. The measured CH4 and C2H4 time-histories during n-heptane pyrolysis were also compared to simulations based on the mechanisms by Wang et al. and Curran et al. The overall n-heptane decomposition rate measured by Davidson et al. was incorporated into the Wang et al. mechanism, and the two methyl abstraction reactions from n-heptane were adjusted, and the H-abstraction reaction rate from ethylene was updated. Using these modified mechanisms the agreement between simulation and experimental time-histories of CH4 and C2H4 were both significantly improved for n-butane and n-heptane pyrolysis. [Copyright &y& Elsevier]

Abstract: The kinetics of 2-butanol and i-butanol pyrolysis were investigated by measuring multi-species time histories using shock tube/laser absorption methods. Species time histories of OH, H2O, C2H4, CO, and CH4 were measured behind reflected shock waves using UV and IR laser absorption during the high-temperature decomposition of 1% butanol/argon mixtures. Initial reflected shock temperatures and pressures for these experiments covered 1270–1640K and 1.3–1.9atm. These are the first multi-species time history measurements in shock tubes for 2-butanol and i-butanol. Production pathways and reaction rate sensitivities for the measured species are analyzed using the recent Sarathy et al. [36] detailed mechanism. It is observed that radical branching in 2-butanol is a highly complicated process with multiple reactions indistinguishably affecting the measured species profiles. However, i-butanol exhibits relatively simple radical branching, and evidence is presented that demonstrates radical branching must be adjusted to favor the iC4H8OH-β channel. Recommendations for i-butanol+H reactions are suggested and significant improvements between measurements and simulations using the Sarathy et al. [36] mechanism are achieved with these revised values. [Copyright &y& Elsevier]

Abstract: The kinetics of 1-butanol pyrolysis were investigated by measuring multi-species time histories using shock tube/laser absorption methods. Species time histories of OH, H2O, C2H4, CO, and CH4 were measured behind reflected shock waves using UV and IR laser absorption during the high-temperature decomposition of 1% 1-butanol/argon mixtures. Initial reflected shock temperatures and pressures for these experiments covered 1250–1650K and 1.3–1.9atm. Measured OH and H2O time histories are in good agreement with previous experimental studies; measured C2H4, CO, and CH4 time histories are the first reported for this fuel in shock tube experiments. Production pathways and sensitivities for the measured species are analyzed using the recent Sarathy et al. (2012) [37] detailed mechanism. Simulations using this mechanism underpredict H2O, OH, and C2H4 mole fractions, overpredict CH4 mole fractions, and significantly underpredict CO mole fractions at early times. As discussed in past papers and confirmed in this study, the branching ratios of H abstraction rates from 1-butanol, which are not precisely known, can significantly affect H2O time history simulations. These simulations show that H2O is produced primarily through H-atom abstraction from 1-butanol by OH, and therefore H2O time histories are extremely sensitive to 1-butanol decomposition channels that contribute to the OH radical pool. Simulations also show that more C2H4 would be produced by faster decomposition of 1-butanol through several channels that also affect H2O production. Finally, simulations show that CO time histories are strongly sensitive to 1-butanol decomposition into nC3H7 and CH2OH, especially at early times. Evidence is presented that indicates this decomposition pathway is too slow in the simulations by a factor of three to five at conditions of the current study. [Copyright &y& Elsevier]

Abstract: Ignition delay times were measured behind reflected shock waves for cyclohexane, methylcyclohexane, and n-butylcyclohexane at 1.5 and 3atm, equivalence ratios near 1 and 0.5, and temperatures between 1280 and 1480K. The observed ignition delay times can be summarized as follows: methylcyclohexane> n-butylcyclohexane≈cyclohexane. Several reasons are suggested to explain the ordering of the ignition delay times for these three naphthenes. We believe that this work provides the first set of ignition delay time data for n-butylcyclohexane. In addition, H2O and OH time-histories were recorded during the oxidation of cyclohexane, methylcyclohexane, n-butylcyclohexane, iso-octane and n-heptane under similar test conditions. OH time-histories near time zero are distinctive for each type of fuel studied, and these early-time OH profiles provides critical insight into the influence of molecular structures on ignition behavior, particularly in the case of the cycloalkanes. Comparisons of measured time-histories with simulations from recent cycloalkane oxidation mechanisms are also presented. [Copyright &y& Elsevier]

A high-pressure shock tube was used to study ignition delay times (IDT) of CH 4 /O 2 /Ar and natural gas/O 2 /Ar mixtures behind reflected shock waves. Reaction progress was monitored using sidewall pressure and direct laser absorption diagnostics of CH 4 near 3.175 µm and ethylene near 10.532 µm. Stoichiometric, fuel-rich and fuel-lean mixtures of CH 4 /O 2, highly dilute in argon, were studied over a temperature range from 1450 to 1850 K and pressures between 10 and 55 atm. Of note are the experiments conducted with fuel-rich mixtures, as there is a lack of literature data in this regime. In the current study, the methane absorption diagnostic provided a unique tool enabling both speciation of methane and a clear definition of ignition delay time. In addition to methane oxidation, we have measured ignition delay times of commercial natural gas blends over a temperature range of 1408–1541 K, at pressures near 12 atm, and at an equivalence ratio of 1. To understand the effects of minor constituents (such as ethane and propane) in commercial natural gas blends, ethylene concentration during pyrolysis experiments was monitored using a two-wavelength scheme (10.532 µm and 10.674 µm) using a CO 2 gas laser. The deficiency of existing kinetic models towards predicting the high-temperature kinetics of natural gas blends was highlighted through our measurements. Therefore, this study also provides data critical for refining these models. Extensive sensitivity analysis emphasizes the importance of the reaction CH 3 +C 2 H 6 →CH 4 +C 2 H 5 during natural gas pyrolysis, and the accuracy of the chemical kinetic models is significantly improved by using a revised reaction rate constant (Shao et al. 2019) for this reaction. These measurements extend the test conditions of earlier studies of methane and commercial natural gas. [ABSTRACT FROM AUTHOR]

The development of renewable, alternative jet fuels presents an exigent challenge to the aviation community. In this work, a streamlined methodology for building computationally efficient kinetic models of real fuels from shock tube experiments is developed and applied to a low cetane-number, broad-boiling alternative jet fuel (termed C-4). A multi-wavelength laser absorption spectroscopy technique was used to determine species time-histories during the high-temperature pyrolysis of C-4, and a batch gradient descent optimization routine built a hybrid-chemistry (HyChem) kinetic model from the measured data. The model was evaluated using combustor-relevant, high-pressure ignition delay time measurements with satisfactory agreement. The present model enables predictive simulations of C-4 in practical environments, while the underlying methodology described here can be readily extended to build kinetic models for a broad range of real fuels of interest. [ABSTRACT FROM AUTHOR]

Abstract The reaction rate of H + O 2 +M = HO 2 +M in the low-pressure limit was determined in the temperature range of 1450– 2000 K, with Argon, Nitrogen, and Carbon Dioxide as the third-body collision partners, by measuring the OH time-history after the induction time during lean oxidation of Hydrogen. Test conditions were optimized to suppress the sensitivity of OH to interfering reactions after the induction time, using dilute and extremely lean mixtures at these temperatures. The strong Q 1 (5) transition in the A-X(0,0) band of OH was used as the probing wavelength for the diagnostic. Calibration measurements were performed prior to the experiments to estimate the effects of pressure shift and collision broadening on the absorption coefficient of OH for each of the bath gas species, which enabled a reduction in the uncertainty in the absorption cross-section of OH. Aided by the calibrated and improved OH diagnostic, the reaction rate constants were determined at high temperatures, with low scatter, and tight uncertainty bounds. The measured rate constants also agree with the extrapolation of previous investigations at lower temperatures. Combined rate constant expressions based on the lower temperature measurements of Shao et al. (2018) and the current high-temperature measurements are proposed, which are valid over the temperature range of 1000–2000 K: k 2 , A r = (2.66 * 10 19) * T − 1.36 c m 6 mo l − 2 s − 1 (± 9 %) k 2 , N 2 = (2.25 * 10 21) * T − 1.95 c m 6 mo l − 2 s − 1 (± 23 %) k 2 , C O 2 = (2.23 * 10 18) * T − 0.79 c m 6 mo l − 2 s − 1 (± 28 %) [ABSTRACT FROM AUTHOR]

Abstract Ignition delay times (IDT) of n-heptane, iso- octane, n-dodecane, and iso- octane/n-dodecane blends, in stoichiometric mixtures with air, were measured behind reflected shock waves in a heated, high-pressure shock tube. Measurements were taken at temperatures of 665–1250 K, pressures of 28–70 atm, and equivalence ratios near unity. Pressure time-history recorded from sidewall piezo-electric transducers, fuel-concentration time-history obtained from fixed-wavelength laser absorption at 3.39 µm, and OH∗ (306 nm) emission time-history recorded by a Si detector, were used to determine IDT. The staged ignition phenomenon in the low temperature regime was also examined with attention on the 1st stage fuel decomposition fraction. IDT measurements were also made using the constrained-reactive-volume strategy, which has the capability to eliminate non-ideal effects such as remote ignition, and were compared with measurements using a conventional filling technique. The current measurements provide a wide range (28–70 atm) of ignition delay times for key surrogate fuels under practical engine conditions, and hence provide validation targets for refinement of chemical kinetic models. [ABSTRACT FROM AUTHOR]

Highlights • Multi-wavelength speciation methods for high-temperature pyrolysis in shock tubes. • Review of fixed-wavelength, high-temperature laser absorption diagnostics for jet fuel fragments. • Database of high temperature absorption cross-section measurements. Abstract A framework for the study of high-temperature hydrocarbon pyrolysis is presented. The proposed framework details a multi-wavelength speciation method using convex optimization, a review of complementary fixed-wavelength laser diagnostics, and a database of high-temperature absorption cross-sections to enable use of the framework for 11 different species. The proposed speciation method involves a vectorized Beer-Lambert system solved under solution constraints using convex optimization. In support of the proposed method, a review of pertinent laser diagnostics and a database of high-temperature absorption cross-sections are also provided. The database includes measured absorption cross-sections of methane (CH 4), ethylene (C 2 H 4), propene (C 3 H 6), isobutene (i-C 4 H 8), 1-butene (1-C 4 H 8), benzene (C 6 H 6), toluene (C 7 H 8), and four jet fuels (samples of Jet A, JP5, JP8, and Gevo ATJ) at the wavelengths of 3.1758 µm, 3.17595 µm, 3.283 µm, 3.392 µm, 3.410 µm, 10.532 µm, 10.675 µm, 10.89 µm, 10.958 µm, 10.962 µm, and 11.345 µm within the range of 300–1600 K and 0.25–4 atm. The database is comprised of over 1000 high–temperature absorption cross-section measurements from new shock tube experiments, new Fourier transform infrared spectrometer (FTIR) experiments, and an aggregated dataset from the literature. The convex speciation method and cross-section database provided will facilitate laser absorption studies of hydrocarbon pyrolysis used in the development of chemical kinetic mechanisms. [ABSTRACT FROM AUTHOR]

We report the development of a novel laser absorption diagnostic for accurate, time-resolved and in situ measurement of various hydrocarbon fuels in combustion systems. This diagnostic method utilized a wavelength-tunable interband cascade laser operated near 3.41 µm, providing improved performance in several aspects over the conventional 3.39-μm He–Ne gas laser diagnostic. First, it enabled a simplified and more compact experimental setup that significantly reduced the measurement complexity. Second, it improved the long-term stability over the 3.39-μm diagnostic by at least a factor of 2, leading to substantially reduced measurement uncertainties. Lastly, the new diagnostic also avoided a cluster of CH 4 transitions that coincide with the He–Ne wavelength, and hence minimized CH 4 interference in other hydrocarbon measurements. Absorption cross-sections of a variety of hydrocarbons at both 3.39 and 3.41 µm were measured in a high-purity shock tube over 531–1659 K, 0.34–3.1 atm, and reported here as functions of temperature. Example applications of this new diagnostic in shock tube pyrolysis studies of methylcyclohexane, n-heptane and iso-octane are also presented. These studies have yielded an improved value of the overall decomposition rate constant of methylcyclohexane as k d  = 3.3 × 10 15 exp(−38000 K/T) s − 1  + 28%/−34%, which is valid over 1260–1400 K and near 1.5 atm. [ABSTRACT FROM AUTHOR]

The unimolecular decomposition kinetics of methyl propanoate (MP), including the direct C−O/C−C bond fissions and molecular reaction channels, were studied by using high-level ab initio calculations and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) theory. Four homolytic bond-fission and ten hydrogen transfer reactions of the MP unimolecular decomposition were identified. The phenomenological rate constants were determined using the RRKM/ME theory over a temperature range of 1000−2000 K and a pressure range of 0.01 atm to the high-pressure limit. At 1 atm, the branching ratios show that the dissociation reactions MP ↔ •CH 2 C( O)OCH 3 + CH 3 , MP ↔ CH 3 OC•( O) + C 2 H 5 and MP ↔ CH 3 CH 2 C( O)O• + CH 3 dominate MP pyrolysis over the temperature range of 1000−1500 K. Our calculated rate constants were adopted in a detailed kinetic model to reproduce the laser-absorption measured CO and CO 2 concentration time-histories during the pyrolysis of 0.2% MP/Ar in a shock tube from 1292−1551 K and at 1.6 atm. The updated mechanism accurately predicted the early-time CO and CO 2 formation over the entire temperature range. In particular, our mechanism well reproduced the CO 2 time-histories from the early-time formation to the final plateau level. [ABSTRACT FROM AUTHOR]

In view of a desired transition from fossil fuels to sustainably produced biofuels that should contribute to a net reduction of CO 2 emissions, promising fuel candidates have been identified including 2-butanone (methyl ethyl ketone, MEK) that is qualified for use in spark-ignition (SI) engines. To support a potential, rapid integration of such biofuels into the existing infrastructure, fundamental studies of their combustion and emission behavior are highly important. In the case of 2-butanone specifically, only very few fundamental combustion experiments have been performed to date, with a notable shortage of detailed speciation data. For predictive model development, accurate and reliable species measurements are needed to describe the oxidation and combustion of 2-butanone and to elucidate the kinetic mechanism. The present study relies on three different experiments: a laminar flow reactor coupled with molecular-beam mass spectrometry (MBMS, Bielefeld), a rapid compression machine (RCM, Aachen), and a shock tube using advanced laser absorption techniques (Stanford). This combination ensured coverage of a wide regime in temperature, pressure, and mixture composition while providing numerous species profiles. The species measurements in the flow reactor were performed at stoichiometric (Φ = 1.0) and fuel-rich (Φ = 2.0) equivalence ratios at temperatures between about 800–1100 K with an argon dilution of 95%. Ignition delay times in the RCM were measured in air for equivalence ratios of 0.5, 2.0 in the temperature range of 840–945 K to reflect application-relevant conditions. Shock tube measurements were performed at stoichiometric conditions at 1303–1509 K in argon. To provide insight into the oxidation mechanism of 2-butanone, the newly measured experimental data was used to develop and validate a detailed chemical kinetic model. To this end, the reactions for the low-temperature regime, especially concerning early fuel consumption, radical formation, and subsequent low-temperature oxidation, were examined in detail, and used to extend and adapt an existing reaction mechanism (Burke et al. , 2016 [23] ) to more accurately predict the new low-temperature conditions studied. The resulting model that incorporates the latest theoretical kinetic calculations available in the literature was compared to the measurements presented here as well as validated using literature data. To the authors' knowledge, the present model represents the most robustly validated mechanism available for the prediction of 2-butanone combustion targets to date. [ABSTRACT FROM AUTHOR]

Experimental data obtained in this study (Part II) complement the speciation data presented in Part I, but also offer a basis for extensive facility cross-comparisons for both experimental ignition delay time (IDT) and laminar flame speed (LFS) observables. To improve our understanding of the ignition characteristics of propene, a series of IDT experiments were performed in six different shock tubes and two rapid compression machines (RCMs) under conditions not previously studied. This work is the first of its kind to directly compare ignition in several different shock tubes over a wide range of conditions. For common nominal reaction conditions among these facilities, cross-comparison of shock tube IDTs suggests 20–30% reproducibility (2σ) for the IDT observable. The combination of shock tube and RCM data greatly expands the data available for validation of propene oxidation models to higher pressures (2–40 atm) and lower temperatures (750–1750 K). Propene flames were studied at pressures from 1 to 20 atm and unburned gas temperatures of 295–398 K for a range of equivalence ratios and dilutions in different facilities. The present propene–air LFS results at 1 atm were also compared to LFS measurements from the literature. With respect to initial reaction conditions, the present experimental LFS cross-comparison is not as comprehensive as the IDT comparison; however, it still suggests reproducibility limits for the LFS observable. For the LFS results, there was agreement between certain data sets and for certain equivalence ratios (mostly in the lean region), but the remaining discrepancies highlight the need to reduce uncertainties in laminar flame speed experiments amongst different groups and different methods. Moreover, this is the first study to investigate the burning rate characteristics of propene at elevated pressures (>5 atm). IDT and LFS measurements are compared to predictions of the chemical kinetic mechanism presented in Part I and good agreement is observed. [ABSTRACT FROM AUTHOR]

Abstract: Methanol and methyl formate pyrolysis were studied by measuring CH3OH and CO concentration time-histories behind reflected shock waves. In the study of methanol pyrolysis, experimental conditions covered temperatures of 1266–1707K, pressures of 1.1–2.5atm, and initial fuel concentrations of 1% and 0.2% with argon as the bath gas. Detailed comparisons of CH3OH and CO concentration profiles with the predictions of the detailed kinetic mechanism of Li et al. (2007) [8] were made. Such comparisons combined with sensitivity analysis identified the need to include an additional methanol decomposition channel, CH3OH↔CH2(S)+H2O, into the mechanism. Pathway and sensitivity analyses for methanol decomposition were performed, leading to rate constant recommendations both for CH3OH unimolecular decomposition and H-abstraction reactions with improved model performance. In the study of methyl formate pyrolysis, methanol concentration time-histories were measured at temperatures over the range of 1261–1524K, pressures near 1.5atm, and initial fuel concentrations of 1% with argon as the bath gas. Our current work, and CO time-histories from previous work, indicates that the Dooley et al. (2010) [3] model is able to accurately simulate most species concentrations in shock tube experiments at early times. However, model improvement is still needed to match the CH3OH and CO time-histories at later times. Incorporation of the modified rate constants in the methanol sub-mechanism leads to good predictions of the full methanol time-histories at all temperatures. The kinetic implications of some aspects of the CO time-histories and suggestions for further improving the predictive capabilities of these mechanisms are discussed. The current results are the first quantitative measurements of CH3OH time-histories in shock tube experiments, and hence are a critical step toward understanding of the chemical kinetics of oxygenates. [Copyright &y& Elsevier]

Abstract: We report a constrained-reaction-volume strategy for conducting kinetics experiments behind reflected shock waves, achieved in the present work by staged filling in a shock tube. Using hydrogen–oxygen ignition experiments as an example, we demonstrate that this strategy eliminates the possibility of non-localized (remote) ignition in shock tubes. Furthermore, we show that this same strategy can also effectively eliminate or minimize pressure changes due to combustion heat release, thereby enabling quantitative modeling of the kinetics throughout the combustion event using a simple assumption of specified pressure and enthalpy. We measure temperature and OH radical time-histories during ethylene–oxygen combustion behind reflected shock waves in a constrained reaction volume and verify that the results can be accurately modeled using a detailed mechanism and a specified pressure and enthalpy constraint. [Copyright &y& Elsevier]

Abstract: High temperature dimethyl ether (DME) pyrolysis was studied behind reflected shock waves by measuring time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%, and 2% DME in argon respectively. Experiments were conducted at temperatures of 1300–1600K and pressures near 1.5atm. A direct absorption strategy with a fixed wavelength (2193.359cm−1) using a quantum cascade laser (QCL) was used to measure CO concentration time histories. A mid-IR scanned-wavelength laser absorption diagnostic with a difference frequency generation (DFG) laser near 3.43μm was used to measure CH4 concentration time histories. C2H4 was measured using a two-wavelength absorption scheme at 10.532μm and 10.675μm with a CO2 laser. The mechanism of Curran et al. with a constant volume gasdynamic model was used to calculate temperature and pressure profiles and to infer the mole fractions of CO, CH4 and C2H4. The concentration time-histories of CO, CH4 and C2H4 were all found to be strongly sensitive to the DME decomposition rate k 1 (CH3OCH3 (+M)→CH3 +CH3O (+M)), which was recently measured by Cook et al. at conditions similar to the current work. This measured k 1 value was incorporated into two major DME decomposition mechanisms of Curran et al. and Zhao et al. The modified Curran et al. mechanism was found to predict the time histories of CH4 and C2H4 significantly better than the modified Zhao et al. mechanism. [Copyright &y& Elsevier]

Abstract: High-temperature 3-pentanone pyrolysis and oxidation studies were performed behind reflected shock waves using laser-based species time-history measurements (3-pentanone, CH3, CO, C2H4, OH and H2O) and ignition delay time measurements. The overall 3-pentanone decomposition rate coefficient was inferred from the measured 3-pentanone and CH3 time-histories during pyrolysis at temperatures of 1070–1530K and a pressure of 1.6atm., and yielded a mathematical expression for k tot =4.383×1049 T−10 exp(−44,780/T)s−1 with an uncertainty of ±35% over 1070–1330K. The measured species time-histories and ignition delay times were also compared to simulations from a detailed kinetic mechanism of Serinyel et al. (2010) [14]. The measured k tot was approximately 3.5 times faster than the value used by Serinyel et al. Additionally, the absence of a methyl ketene decomposition reaction was identified as the cause of a deficiency in the O-atom balance of the measured 3-pentanone and CO time-histories. Using the revised overall 3-pentanone decomposition rate coefficient and an additional methyl ketene decomposition pathway, the modified mechanism was able to successfully simulate all six species time-histories, and showed a significant improvement in the predictions of ignition delay times. Finally, a comparison of ignition delay times and OH species time-histories during 3-pentanone, 2-pentanone and acetone oxidation found that 3-pentanone was the most reactive of the three ketones. [Copyright &y& Elsevier]

Abstract: Methyl butanoate pyrolysis and decomposition pathways were studied in detail by measuring concentration time-histories of CO, CO2, CH3, and C2H4 using shock tube/laser absorption methods. Experiments were conducted behind reflected shock waves at temperatures of 1200–1800K and pressures near 1.5atm using mixtures of 0.1%, 0.5%, and 1% methyl butanoate in Argon. A novel laser diagnostic was developed to measure CO in the ν 1 fundamental vibrational band near 4.56μm using a new generation of quantum-cascade lasers. Wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) was used to measure CO2 near 2752nm. Methyl radical was measured using UV laser absorption near 216nm, and ethylene was monitored using IR gas laser absorption near 10.53μm. An accurate methyl butanoate model is critical in the development of mechanisms for larger methyl esters, and the measured time-histories provide kinetic targets and strong constraints for the refinement of the methyl butanoate reaction mechanism. Measured CO mole fractions reach plateau values that are the same as the initial fuel mole fraction at temperatures higher than 1500K over the maximum measurement time of 2ms or less. Two recent kinetic mechanisms are compared with the measured data and the possible reasons for this 1:1 ratio between MB and CO are discussed. Based on these discussions, it is expected that similar CO/fuel and CO2/fuel ratios for biodiesel molecules, particularly saturated components of biodiesel, should occur. [Copyright &y& Elsevier]

Abstract: Ignition delay times of the four isomers of butanol were measured behind reflected shock waves over a range of experimental conditions: 1050–1600K, 1.5–43atm, and equivalence ratios of 1.0 and 0.5 in mixtures containing 4% O2 diluted in argon. Additional data were also collected at 1.0–1.5atm in order to replicate conditions used by previous researchers. Good agreement is seen with past work for 1-butanol ignition delay times, though our measured data for the other isomers were shorter than those found in some previous studies, especially at high temperatures. At most conditions, the ignition delay time increases for each isomer in the following order: 1-butanol, 2-butanol and i-butanol nearly equal, and t-butanol. In addition, t-butanol has a higher activation energy than the other three isomers. In a separate series of high-pressure experiments, ignition delay times of 1-butanol in stoichiometric air were measured at temperatures as low as 800K. At temperatures below 1000K, pre-ignition pressure rises as well as significant rollover of ignition delay times were observed. Modeling of all collected data using several different chemical kinetic mechanisms shows partial agreement with the experimental data depending on the mechanism, isomer, and conditions. Only the mechanism developed by Vranckx et al. partially explains the rollover and pre-ignition observed in stoichiometric experiments in air. [Copyright &y& Elsevier]