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Quantum-cascade-laser dual-comb spectroscopy (QCL-DCS) is a promising technology with ultra-fast time resolution capabilities for chemical kinetics, atmospheric gas sensing, and combustion applications. A pair of quantum-cascade frequency combs were used to measure absorbance from methane’s ν 4 band between 1270 and 1315 cmâˆ'1 at high-temperature and -pressure conditions that were generated using a high-pressure shock tube. Results here mark a major improvement over previous QCL-DCS measurements in shock tubes. Improvements came from a unique spectral-filtering strategy to correct for a bimodal power-spectral density of QCL frequency combs and careful optimization of the laser setup and experimental conditions. Our modified QCL-DCS was ultimately used to measure temperature within 2% and methane mole fraction within 5% by fitting HITEMP spectral simulations to spectra recorded at 4 ÎĽs temporal resolution. We measure temperature and species time-histories during methane pyrolysis at conditions between 1212â€"1980 K, and 12â€"17 atm, all at 4 ÎĽs resolution. Good agreement is observed with kinetic models, illustrating the potential of future applications of DCS in kinetics and combustion research. [ABSTRACT FROM AUTHOR]

Validation of high-fidelity models for high-temperature hypersonic flows requires high-accuracy kinetics data for oxygen (O2) reactions, including time-histories and rate parameter measurements. Consequently, shock-tube experiments with ultraviolet (UV) laser absorption were performed to measure quantum-state-specific time-histories and coupled vibration-dissociation (CVDV) rate parameters in shock-heated, nondilute O2 and oxygen–argon (O2–Ar) mixtures. Experiments probed mixtures of 20% O2–Ar, 50% O2–Ar, and 100% O2 for initial post-reflected-shock conditions from 6000 to 14 000 K and 26–210 Torr. Two UV lasers—one continuous-wave laser and one pulsed laser—measured absorbance time-histories from the fifth and sixth vibrational levels of the electronic ground state of O2, respectively. The absorbance time-histories subsequently yielded time-histories for vibrational temperature (Tv) from the absorbance ratio, translational/rotational temperature (Ttr) from energy conservation, total O2 number density ( n O 2 ) from the individual absorbances, and vibrational-state-specific number density ( n v ″ ) from the Boltzmann population fractions. These state-specific temperature and number density time-histories demonstrate the low uncertainty necessary for high-temperature model validation and provide data to higher temperature than previous experiments. Additional analysis of the temperature and number density time-histories allowed inference of rate parameters in the Marrone and Treanor CVDV model, including vibrational relaxation time ( τ O 2 − O 2 ), average vibrational energy loss (ε), vibrational coupling factor (Z), and dissociation rate constants ( k d O 2 − O 2 and k d O 2 − O ). The results for each of these five parameters show reasonable consistency across the range of temperatures, pressures, and mixtures and generally agree with a modified Marrone and Treanor model by Chaudhry et al. ["Implementation of a chemical kinetics model for hypersonic flows in air for high-performance CFD," in Proceedings of AIAA Scitech Forum (2020)]. Finally, the results for τ O 2 − O 2 , k d O 2 − O 2 , and k d O 2 − O exhibit much lower scatter than previous experimental studies. [ABSTRACT FROM AUTHOR]

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]

We report the first shock tube measurements of formaldehyde (CH 2 O) during the first stage ignition of n-heptane, 2-methylhexane and 3,3-dimethylpentane, in highly diluted fuel/oxygen mixtures in the pressure range of 7–10 atm and temperature range of 700–880 K. Combined time histories of all carbonyl (–C = O) species, CO and fuel were also measured simultaneously in an effort to study the impact of fuel structure on the concentration and the rate of evolution of first stage ignition products. Of the three isomers studied in this work, n-heptane was found to be the fastest, while 3,3-dimethylpentane was found to be the slowest. The differences in the time scale of formation, and plateau concentration of the intermediates between the isomers across the entire range of test conditions suggests a strong dependency of the measured time histories to fuel structure. These species therefore act as markers of the Negative Temperature Coefficient (NTC) behavior of fuels and can be used as targets for developing semi-empirical, hybrid chemistry models of complex, multi-component petroleum derived gasoline and jet fuels. The time histories reported in this work should prove very useful in the refinement of detailed kinetic models of n-heptane, and development of rate rules for branched alkane isomers. [ABSTRACT FROM AUTHOR]

Abstract Cavity-enhanced absorption spectroscopy (CEAS) has generated much interest in shocktube kinetics studies because of its recent success in achieving improved sensitivity and high time resolution with robust optical alignment. While recent progress demonstrated experimental schemes including off-axis scanned-wavelength approach and on-axis ps-pulsed laser approach, that both successfully suppressed the laser-cavity coupling noise, this paper develops a theoretical model to predict the CEAS sensor performance that can be used as a design tool applicable to more generalized cases. The method models the optical field in the cavity based on the decentered Gaussian beam model, from which the cavity transmission spectrum and the laser-cavity coupling noise can be numerically calculated. The simulation results predict sensor performance for different cavity configurations and laser characteristics, including various degrees of laser-cavity mode-matching, laser linewidths, scanning rates, and cavity filling conditions. Simulation with example wavelengths in the ultraviolet, near-infrared, and mid-infrared showed increasing mode-matched beam waist size for increasing wavelengths. An off-axis alignment scheme was found to be capable of suppressing the coupling noise by two orders-of-magnitude at a moderate laser linewidth of 1 GHz. Coupling noise level on the order of 1e-5 for scanned-wavelength off-axis alignment case with a narrowband mid-infrared laser was obtained by model calculation and agreed with experimental results within acceptable uncertainty range. The developed method can serve to guide future design and optimization of CEAS system in shocktube studies. [ABSTRACT FROM AUTHOR]

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: Hydrogen peroxide (H2O2) and hydroperoxy (HO2) reactions present in the H2O2 thermal decomposition system are important in combustion kinetics. H2O2 thermal decomposition has been studied behind reflected shock waves using H2O and OH diagnostics in previous studies (Hong et al. (2009) [9] and Hong et al. (2010) [6,8]) to determine the rate constants of two major reactions: H2O2 +M→2OH+M (k 1) and OH+H2O2 →H2O+HO2 (k 2). With the addition of a third diagnostic for HO2 at 227nm, the H2O2 thermal decomposition system can be comprehensively characterized for the first time. Specifically, the rate constants of two remaining major reactions in the system, OH+HO2 →H2O+O2 (k 3) and HO2 +HO2 →H2O2 +O2 (k 4) can be determined with high-fidelity. No strong temperature dependency was found between 1072 and 1283K for the rate constant of OH+HO2 →H2O+O2, which can be expressed by the combination of two Arrhenius forms: k 3 =7.0×1012 exp(550/T)+4.5×1014 exp(−5500/T) [cm3 mol−1 s−1]. The rate constants of reaction HO2 +HO2 →H2O2 +O2 determined agree very well with those reported by Kappel et al. (2002) [5]; the recommendation therefore remains unchanged: k 4 =1.0×1014 exp(−5556/T)+1.9×1011+exp(709/T) [cm3 mol−1 s−1]. All the tests were performed near 1.7atm. [Copyright &y& Elsevier]

The rate constant for the overall reaction OH + 1-butanol→ products was determined in the temperature range 900 to 1200K from measurements of OH concentration time histories in reflectedshock wave experiments of tert-butyl hydroperoxide(TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentrationmeasurement. A detailed kinetic mechanism was constructed that includesupdated rate constants for 1-butanol and TBHP kinetics that influencethe near-first-order OH concentration decay under the present experimentalconditions, and this mechanism was used to facilitate the rate constantdetermination. The current work improves upon previous experimentalstudies of the title rate constant by utilizing a rigorously generatedkinetic model to describe secondary reactions. Additionally, the currentwork extends the temperature range of experimental data in the literaturefor the title reaction under combustion-relevant conditions, presentingthe first measurements from 900 to 1000 K. Over the entire temperaturerange studied, the overall rate constant can be expressed in Arrheniusform as 3.24 × 10–10exp(−2505/T[K]) cm3molecule–1s–1. The influence of secondary reactions on the overallOH decay rate is discussed, and a detailed uncertainty analysis isperformed yielding an overall uncertainty in the measured rate constantof ±20% at 1197 K and ±23% at 925 K. The results are comparedwith previous experimental and theoretical studies on the rate constantfor the title reaction and reasonable agreement is found when theearlier experimental data were reinterpreted. [ABSTRACT FROM AUTHOR]

Abstract: Ethylene concentration time-histories were measured behind reflected shock waves in ethylene/argon and n-heptane/argon mixtures using 10.5μm CO2 laser absorption. Reflected shock conditions covered temperatures between 1350 and 1950K, pressures between 1.3 and 3.3atm, and fuel concentrations of 1% ethylene and 300ppm n-heptane in argon. The ethylene absorption cross-section at this wavelength was determined over similar pressures and temperatures using both absorption measurements in a heated-cell FTIR and laser absorption/shock tube measurements. Measured ethylene concentration time-histories during ethylene pyrolysis were compared to constant volume simulations using the Marinov et al. (1998) mechanism. A sensitivity of the measured profiles to the rate constant of the primary ethylene decomposition reaction, C2H4 →C2H2 +H2 was demonstrated, allowing a determination of the rate constant for this reaction using the ethylene time-histories. Ethylene concentrations, measured during n-heptane pyrolysis, were also compared to simulations using the Sirjean et al. (2009)/JetSurF1.0 mechanism. Incorporation of the new reaction rate constant k 1 provided better agreement between the measured ethylene concentration profiles and the simulations for both ethylene and n-heptane pyrolysis. [ABSTRACT FROM AUTHOR]

Abstract: Laser diagnostic techniques play a large and growing role in combustion research and development. Here we highlight three areas where quantitative sensing based on laser absorption has had strong influence: chemical kinetics, propulsion, and practical energy systems. In the area of chemical kinetics, measurements in shock tubes of high-temperature reaction rate coefficients using species-specific laser absorption techniques have provided new and accurate answers to questions about combustion chemical processes. In the area of propulsion, wide-bandwidth measurements of flow temperatures, species concentrations, and velocity have provided engine designers with the necessary information to improve operation and performance. In the area of practical energy systems, real-time measurements of combustor operating conditions and emissions have enabled needed incremental improvements in large power plants and improved safety of operation. Yet, there is still more to be done, and opportunities for new applications will grow as laser sensors evolve. This review seeks to provide an overview of the current power and future potential of these modern diagnostic tools. [ABSTRACT FROM AUTHOR]

Abstract: The reaction of methyl radicals (CH3) with molecular oxygen (O2) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k 1 = k 1a + k 1b, and individual rate coefficients for the two high-temperature product channels, (1a) producing CH3O+O and (1b) producing CH2O+OH, were determined using ultra-lean mixtures of CH3I and O2 in Ar/He. Narrow-linewidth UV laser absorption at 306.7nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k 1 but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a). Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590–2430K, k 1a =6.08×107 T 1.54 exp(−14005/T)cm3 mol−1 s−1 and k 1b =68.6 T 2.86 exp(−4916/T)cm3 mol−1 s−1. The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k 1a and k 1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range. [Copyright &y& Elsevier]

Abstract. Shock tubes often experience temperature and pressure nonuniformities behind the reflected shock wave that cannot be neglected in chemical kinetics experiments. Because of increased viscous effects, smaller tube diameters, and nonideal shock formation, the reflected-shock nonidealities tend to be greater in higher-pressure shock tubes. Since the increase in test temperature ( $\Delta T_5$ ) is the most significant parameter for chemical kinetics, experiments were performed to characterize $\Delta T_5$ in the Stanford High Pressure Shock Tube using infrared emission from a known amount of CO in argon. From the measured change in vibrationally equilibrated CO emission with time, the corresponding d $T_5/$ d t (or $\Delta T_5$ for a known time interval) of the mixture was inferred assuming an isentropic relationship between post-shock temperature and pressure changes. For a range of representative conditions in argon (24–530 atm, 1275–1900 K), the test temperature 2 cm from the endwall increased 3–8 K after 100 $\mu$ s and 15–40 K after 500 $\mu$ s, depending on the initial conditions. Separate pressure measurements using a shielded piezoelectric transducer confirmed the isentropic assumption. An analytical model of the reflected-shock gas dynamics was also developed, and the calculated $\Delta T_5$ 's agree well with those obtained from experiment. The analytical model was used to estimate the effects of temperature and pressure nonuniformities on typical chemical kinetics measurements. When the kinetics are fast ( $<300\mu$ s), the temperature increase is typically negligible, although some correction is suggested for kinetics experiments lasting longer than 500 $\mu$ s. The temperature increase, however, has a negligible impact on the measured laser absorption profiles of OH (306 nm) and CH $_3$ (216 nm), validating the use of a constant absorption coefficient. Infrared emission experiments are more sensitive to temperature and density changes, so $T_5$ nonuniformities should be taken into account when interpreting ir-emission data. [ABSTRACT FROM AUTHOR]

In this work we present a novel two-color differential absorption diagnostic near 5.6 μ m using an external-cavity quantum-cascade-laser (EC-QCL) for formaldehyde (CH 2 O) detection in high-temperature reacting gases. This diagnostic utilizes the narrow mid-infrared absorption features of CH 2 O in the R branch of the C=O stretching band. Specifically, the online and offline colors were chosen as 1787.05 cm-1 and 1787.85 cm-1, respectively. Compared to the previous Stanford work that targeted the 3.5 μ m band of CH 2 O, this new diagnostic achieved higher sensitivity, better species selectivity and immunity to interference absorption from other combustion gases, especially small hydrocarbons (e.g. methane). Absorption cross sections of CH 2 O at the two colors were measured in shock-heated 1,3,5-trioxane/argon mixtures over a wide range of temperatures and pressures: 870 K ≤ T ≤ 1800 K, 0.7 atm ≤ P ≤ 4.5 atm. These cross section measurements were further expressed as analytical functions of temperature and pressure with 2 σ statistical uncertainties of ± 4.9%. Initial application of the diagnostic was demonstrated in a shock tube study of the thermal decomposition of methanol/argon mixture at 1427 K and 2.77 atm, where the measured CH 2 O time-history was compared with simulations from multiple combustion mechanisms. We expect this new diagnostic to be useful in future combustion studies. [ABSTRACT FROM AUTHOR]

• High-T pyrolysis of ten n- and iso- alkanes studied behind reflected shock waves. • Laser absorption spectroscopy diagnostics employed to track key intermediates. • Measurements showed clear differences in product distribution among different fuels. • Over 80–95% of initial carbon atoms recovered within test time for all fuel classes. Decarbonization of energy systems that drive the transportation industry is imperative for effectively mitigating global greenhouse gas emissions. In this pursuit, the transition towards scalable and sustainable fuels will play a key role. These fuels are predominantly composed of a diverse array of hydrocarbons spanning various molecular classes. To better understand the complex chemistry of such fuels, we studied the impact of molecular structure on the distribution of pyrolysis products of C 6 -C 12 hydrocarbons, encompassing both normal and branched alkanes, in a shock tube at combustion-relevant temperatures. An extensive suite of laser absorption spectroscopy-based diagnostics was used to simultaneously measure the time histories of key intermediates (methane, ethylene, and larger alkenes) and fuel during the pyrolysis of ten neat hydrocarbons: n-hexane, n-heptane, n-octane, n-decane, n-dodecane, 2-methylhexane, 2-methylheptane, 2-methylnonane, triptane (or 2,2,3-trimethylbutane), and iso -octane (or 2,2,4-trimethylpentane). These experiments were conducted behind reflected shock waves using 2% Fuel/Argon test mixtures at a nominal pressure of 2 atm over the temperature range of 1100–1500 K. Conducting these experiments under similar test conditions facilitated the observation of clear trends in the distribution of intermediates formed during the pyrolysis of these fuels. The current paper delineates the experimental strategy and presents the measured mole fraction time histories of the stable intermediates. A companion paper [1] delves into the observed trends and effects of molecular structure, in particular, the impact of increasing carbon number and the impact of varying degree of branching on the yields of these pyrolysis products. We also investigate the impact of fuel blending and provide insights into the role of inter-coupling on the distribution of stable intermediates. The authors anticipate that the trends and findings presented in these two papers will guide the design of sustainable fuels and play a pivotal role in modeling the combustion chemistry of the next generation of fuels. To the best of our knowledge, this work presents the first systematic, detailed pyrolysis study of several transportation-relevant hydrocarbon fuels, involving simultaneous measurements of multiple species in a shock tube using laser absorption spectroscopy. [ABSTRACT FROM AUTHOR]

• Measured intermediate yields during shock-tube pyrolysis of ten n- and iso -alkanes. • Characterized distinct trends in species' yields with varying molecular structure. • Investigated the impact of fuel blending on intermediate product distribution. • Conducted preliminary analysis to correlate species' yields with infrared spectra. Transitioning to scalable and sustainable fuels produced from renewable feedstocks will be essential for decarbonizing the transportation sector. As these fuels typically encompass a wide range of molecular classes, it is important to understand the impact of varying molecular structure on their overall combustion behavior. To that end, experiments were conducted to investigate the thermal decomposition of ten neat hydrocarbons with varying carbon numbers and degree of branching in a shock tube at engine-relevant temperatures. A multi-wavelength, laser absorption spectroscopy-based approach was used to measure key stable intermediates formed during the pyrolysis process. The experiments revealed clear trends in intermediate formation and distribution, influenced by the molecular structure of each fuel. Additional experiments were conducted with a 2% composite fuel (consisting of a 50% n-heptane/50% iso -octane mixture)/Ar mixture to assess the impact of fuel blending on the pyrolysis process. A companion paper (Boddapati et al., 2024) delineates the experimental strategy and provides comprehensive mole fraction time histories of the major products, along with thorough analysis of carbon atom recovery. This paper establishes empirical trends in the measured intermediate yields with varying carbon number and degree of branching, and provides a discussion on the chemical phenomena governing the thermal decomposition of these fuels. In addition, this paper also lays the foundation for developing compact, robust chemical kinetic models for conventional and alternative fuels based on compositional information obtained through infrared spectra. The insights gained from these studies have the potential to guide sustainable fuel design and contribute to modeling the combustion chemistry of next-generation fuels. [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]

• Developed a rotational temperature sensor for non-equilibrium flows containing nitric oxide. • Acquired spectrally-resolved nitric oxide absorption cross-section measurements at high temperatures at two different wavelengths. • Compared spectroscopic model predictions of the NO gamma-bands to high-temperature cross-section data. • Provided empirical adjustments to line-positions modeled for the NO gamma-bands. • Inferred translational and rotational temperature by applying the temperature diagnostic developed in this work. In high-enthalpy air flows, nitric oxide (NO) formation and removal involves multiple, complex energy transfer processes that occur under nonequilibrium conditions, thus motivating the need for diagnostic methods to probe such flows in laboratory settings. In this work, a rotational temperature diagnostic for NO was developed using a spectroscopic model (Stanford NO model) of the γ -bands of NO to inform optimal wavelength candidates for absorption using two continuous-wave (CW), ultraviolet (UV) lasers. Absorption cross-sections of shock-heated NO were measured with two CW UV lasers over a range of temperatures and wavelengths, and results were compared to the Stanford NO model. All measurements were completed behind reflected shocks in 0.4% and 2% mixtures of NO in argon (Ar). Fixed-wavelength cross-section measurements from 2000 K to 6000 K between 1 atm to 0.085 atm were made at nominal wavelengths of 226.1026 nm and 224.8155 nm. Additionally, fixed-temperature measurements were made over the wavelength range 226.1013 nm to 226.1035 nm at 0.8 atm. The combination of experiments provided temperature and wavelength sweep cross-sections to compare against the Stanford NO model. The formulation for the collisional broadening parameter, 2 γ , was divided into two temperature regimes, transitioning at 2500 K. Additionally, line positions of the R 11 (26.5) , R 12 (34.5) , Q 21 (26.5) , and Q 22 (34.5) transitions around 224.8155 nm and the Q 11 (12.5) , R 12 (19.5) , P 21 (12.5) , and Q 22 (19.5) transitions around 226.1026 nm were adjusted to best fit the cross-section data. A vibrational equilibrium assumption was used to determine the translational and vibrational temperatures at the point of cross-section determination, while a coupled vibration-dissociation (CVDV) model characterized the extent of dissociation at the point of cross-section determination. Data from both temperature-sweep and wavelength-sweep measurements agree to within ± 10% of the Stanford NO model, with most cross-sections falling within ± 5% of model predictions. Oxygen (O 2) cross-sections measurements were also taken behind reflected shocks in dilute 2% O 2 in Ar mixtures at 2000 K and 6000 K to quantify the interfering O 2 absorbance at the chosen diagnostic wavelengths. [ABSTRACT FROM AUTHOR]

The overall rate constants for the reactions of hydroxylradicals (OH) with four small methyl esters, namely methyl formate(CH3OCHO), methyl acetate (CH3OC(O)CH3), methyl propanoate (CH3OC(O)C2H5), and methyl butanoate (CH3OC(O)C3H7), were investigated behind reflected shock waves using UV laserabsorption of OH radicals near 306.69 nm. Test gas mixtures of individualmethyl esters and tert-butyl hydroperoxide (TBHP),a fast source of OH at elevated temperatures, diluted in argon wereshock-heated to temperatures spanning from 876 to 1371 K at pressuresnear 1.5 atm. The overall rate constants were determined by matchingthe measured OH time-histories with the computed profiles from thecomprehensive chemical kinetic mechanisms of Dooley et al. (2010)and Dooley et al. (2008), which were originally developed for theoxidation of methyl formate and methyl butanoate, respectively. Thesemeasured values can be expressed in Arrhenius form as kCH3OCHO+OH= 2.56 × 1013exp(−2026/T) cm3mol–1s–1, kCH3OC(O)CH3+OH= 3.59 × 1013exp(−2438/T) cm3mol–1s–1, kCH3OC(O)C2H5+OH= 6.65 × 1013exp(−2539/T) cm3mol–1s–1, and kCH3OC(O)C3H7+OH= 1.13 × 1014exp(−2515/T) cm3mol–1s–1overthe temperature ranges studied. Detailed error analyses were performedto estimate the overall uncertainties of these reactions, and theestimated (2σ) uncertainties were found to be ±29% at 913K and ±18% at 1289 K for kCH3OCHO+OH, ± 29% at 930 K and ±17% at 1299 K for kCH3OC(O)CH3+OH, ±25% at 909 K and ±17% at 1341 K for kCH3OC(O)C2H5+OH, and ±24% at 925 K and±16% at 1320 K for kCH3OC(O)C3H7+OH. We believe these are the first directhigh-temperature rate constant measurements for the reactions of OHwith these small methyl esters. These measured rate constants werealso compared with the estimated values employed in different comprehensivekinetic mechanisms. Additionally, the structure–activity relationshipfrom Kwok and Atkinson (1995) was used to estimate these four rateconstants, and the estimations from this group-additivity model arein good agreement with the measurements (within ∼25%) at thepresent experimental conditions. [ABSTRACT FROM AUTHOR]

The overall rate constants for the reactions of hydroxylradicals (OH) with a series of ketones, namely, acetone (CH3COCH3), 2-butanone (C2H5COCH3), 3-pentanone (C2H5COC2H5), and 2-pentanone (C3H7COCH3), were studied behind reflected shock waves over the temperaturerange of 870–1360 K at pressures of 1–2 atm. OH radicalswere produced by rapid thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP) and were monitored by thenarrow line width ring dye laser absorption of the well-characterizedR1(5) line in the OH A–X (0, 0) band near 306.69nm. The overall rate constants were inferred by comparing the measuredOH time histories with the simulated profiles from the detailed mechanismsof Pichon et al. (2009) and Serinyel et al. (2010). These measuredvalues can be expressed in Arrhenius form as kCH3COCH3+OH= 3.30 × 1013exp(−2437/T) cm3mol–1s–1, kC2H5COCH3+OH = 6.35 × 1013exp(−2270/T) cm3mol–1s–1, kC2H5COC2H5+OH= 9.29 × 1013exp(−2361/T) cm3mol–1s–1, and kC3H7COCH3+OH= 7.06 × 1013exp(−2020/T) cm3mol–1s–1. The measured rateconstant for the acetone + OH reaction from the current study is consistentwith three previous experimental studies from Bott and Cohen (1991),Vasudevan et al. (2005), and Srinivasan et al. (2007), within ±20%.Here, we also present the first direct high-temperature rate constantmeasurements of 2-butanone + OH, 3-pentanone + OH, and 2-pentanone+ OH reactions. The measured values for the 2-butanone + OH reactionare in close accord with the theoretical calculation from Zhou etal. (2011), and the measured values for the 3-pentanone + OH reactionare in excellent agreement with the estimates (by analogy with theH-atom abstraction rate constants from alkanes) from Serinyel et al.Finally, the structure–activity relationship from Kwok andAtkinson (1995) was used to estimate these four rate constants, andthe estimated values from this group-additivity model show good agreementwith the measurements (within ∼25%) at the present experimentalconditions. [ABSTRACT FROM AUTHOR]

Abstract: An updated H2/O2 reaction mechanism is presented that incorporates recent reaction rate determinations in shock tubes from our laboratory. These experiments used UV and IR laser absorption to monitor species time-histories and have resulted in improved high-temperature rate constants for the following reactions: The updated mechanism also takes advantage of the results of other recent rate coefficient studies, and incorporates the most current thermochemical data for OH and HO2. The mechanism is tested (and its performance compared to that of other H2/O2 mechanisms) against recently reported OH and H2O concentration time-histories in various H2/O2 systems, such as H2 oxidation, H2O2 decomposition, and shock-heated H2O/O2 mixtures. In addition, the mechanism is validated against a wide range of standard H2/O2 kinetic targets, including ignition delay times, flow reactor species time-histories, laminar flame speeds, and burner-stabilized flame structures. This validation indicates that the updated mechanism should perform reliably over a range of reactant concentrations, stoichiometries, pressures, and temperatures from 950 to greater than 3000K. [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]

In shock-heated high-enthalpy air flows, the vibrational temperature of oxygen (O 2) provides critical insight into the non-equilibrium chemistry. Here, a two-color O 2 vibrational temperature diagnostic was developed by utilizing spectroscopic models to inform optimal wavelength candidates for both a continuous-wave (CW), ultraviolet (UV) laser and a picosecond pulsed, UV laser. Cross-sections of shock-heated O 2 were measured using a CW UV laser, and results over a range of wavelengths and temperatures are compared against a Stanford model and Specair, a spectroscopic model for high temperature air species developed by Laux et al. All measurements were completed behind reflected shocks in 2% and 5% O 2 in argon (Ar) mixtures. Vibrational temperatures for cross-section measurements were calculated for plateaus and peaks in experimental absorbances using a Bethe-Teller relaxation model up to 6,000 K and a steady-state approach above 6,000 K. Temperature sweep measurements were fixed around 223.237 nm, while wavelength sweep measurements were taken around 4550 K and ranged between 223.23 nm to 223.27 nm. Temperature sweep cross-sections agree to within 15% of Specair modeled cross-sections, with most measurements falling within 10% of Specair predictions. Wavelength sweep cross-sections agree at shorter wavelengths with Specair cross-sections, but longer wavelength features are offset from both the Stanford model and Specair predictions. Changing the spin-splitting equations used by the Stanford model from the Herzberg formulation to the Nicolet formulation also brought the Stanford model to within 15% of all temperature sweep data, with most measurements falling within 10% of the Stanford model predicts. The spin-splitting adjustment also improved the agreement between the Stanford model and the wavelength sweep data at shorter wavelengths. [ABSTRACT FROM AUTHOR]

• Spectroscopic model developed for the Schumann-Runge system. • Spectroscopic model extends to high temperatures and non-equilibrium conditions. • UV pulsed laser absorption spectroscopy acquired O 2 absorption cross-sections. • Developed steady-state model for vibrational temperature determination above 6000 K. • Developed spectroscopic models agree with cross-section data up to 8400 K. Accurate spectroscopic modeling is critical when measuring time-resolved, state-specific chemical kinetics of diatomic molecules. Here, a spectroscopic model (Stanford model) was developed to accurately simulate oxygen absorption cross-sections in the Schumann-Runge system for non-equilibrium conditions. Cross-sections of shock-heated oxygen (O 2) have been measured using a picosecond pulsed ultraviolet (UV) laser, and the viability of two spectroscopic models has been demonstrated. Measurements were taken behind reflected shocks in 2% and 5% O 2 in argon (Ar) mixtures around 211.2 nm and 236.9 nm up to initial post-reflected shock temperatures of 10,700 K. Cross-sections were plotted against vibrational temperature and compared to calculated cross-sections from the Stanford model and the Adjusted Spectrum model. Vibrational temperatures for cross-section measurements were calculated for plateaus and peaks in experimental absorbances using a Bethe-Teller relaxation model up to 6,000 K and a steady-state approach above 6,000 K. Vibrational temperatures calculated using the steady-state approach were 3–5% higher than coupled vibration-dissociation (CVD) calculations. The experimental cross-sections agree to within 15% of the Stanford model for both wavelength regimes. [ABSTRACT FROM AUTHOR]

The uniquely strong radiative properties of the cyano-radical (CN) make it an important species to model in high-enthalpy, nonequilibrium flows containing carbon and nitrogen. In this work, spectroscopic models for the Violet and Red systems of CN have been developed and validated at wavelengths in both the ultraviolet (UV) and near-infrared (NIR) regimes that would provide quantitative CN rovibrational temperature and concentration. The ultraviolet wavelengths chosen probe high-lying ( J ′ ′ = 30. 5 − 40. 5) and low-lying ( J ′ ′ = 0. 5 − 5. 5) rotational states in the ground vibrational level ( v ′ ′ = 0) of CN along with high-lying rotational states in the third excited vibrational level ( v ′ ′ = 3) of CN. Meanwhile, the NIR wavelengths cover a range of lower rovibrational content, including high-lying and low-lying rotational levels from the ground up to the fourth excited vibrational level of CN. Dilute cyanogen (C 2 N 2) in argon (Ar) mixtures were shock heated to produce stable quantities of CN that could be spectroscopically studied. In the UV, fixed-temperature absorption cross-section measurements at different wavelengths and a fixed-wavelength temperature sweep for the chosen diagnostic wavelengths enabled an evaluation of the different spectroscopic models and their ability to correctly infer the post-shock temperature in mixtures with high Ar dilution. In the NIR, the lasers were scanned at 100 kHz to probe multiple transitions over different rovibrational states over a wide range of temperatures, and both Einstein A's and relative line positions of observed Red transitions of CN were experimentally quantified. • Developed spectroscopic models for cyano-radical (CN) Violet and Red systems. • Acquired spectrally-resolved CN Violet absorption cross-sections at four wavelengths. • Acquired Einstein A and line position information about CN Red transitions. • Provided empirical adjustments to line-positions for the CN Violet and Red systems. • Accurately inferred CN rotational and vibrational temperature using UV diagnostics. [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 A new spectroscopic predictor for estimating ignition characteristics including derived cetane number is proposed for hydrocarbon fuels. This spectroscopic predictor is the ratio of room temperature absorbance of unreacted fuel vapor at 3.41 and 3.39 μ m, termed here as the "3.41/3.39 absorption ratio." Its wide availability and applicability are demonstrated for a range of pure hydrocarbons, mixtures of pure hydrocarbons, and distillate and synthetic jet fuels. Quasi-linear calculation methods are provided for practical use. Spectroscopic and kinetic interpretations are provided based on the fraction of –CH 2 – hydrogen relative to all carbon-bonded hydrogen. In addition, the correlations between the proposed predictor and ignition delay time and C 2 H 4 yield are presented and discussed to exhibit the predictor's potential as a fuel screening tool. [ABSTRACT FROM AUTHOR]

Abstract The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin. In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel. The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model. Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels. The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products. The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels. [ABSTRACT FROM AUTHOR]

Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach. [ABSTRACT FROM AUTHOR]

We propose and test an alternative approach to modeling high-temperature combustion chemistry of multicomponent real fuels. The hy brid chem istry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel pyrolysis products. The pyrolysis (or oxidative pyrolysis) process is modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients are derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three conventional jet fuels and two rocket fuels as examples. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all five fuels. Sensitivity analysis shows that for conventional, petroleum-derived real fuels, the uncertainties in the experimental measurements of C 2 H 4 and CH 4 impact model predictions to an extent, but the largest influence of the model predictability stems from the uncertainties of the foundational fuel chemistry model used (USC Mech II). In addition, we introduce an approach in the realm of the HyChem approach to address the need to predict the negative-temperature coefficient (NTC) behaviors of jet fuels, in which the CH 2 O speciation history is proposed to be a viable NTC-activity marker for model development. Finally, the paper shows that the HyChem model can be reduced to about 30 species in size to enable turbulent combustion modeling of real fuels with a testable chemistry model. [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]

Laminar flame speeds of premixed propane in 21 % O 2 -79 % Ar (so-called "airgon") were experimentally measured in a shock tube at atmospheric pressure (1 atm ± 2 %) for a range of equivalence ratios (0.5, 0.75, 1, and 1.2) and unburned-gas temperatures (644–1,044 K). Simultaneous side-wall schlieren and end-wall CH* emission imaging was utilized to track the propagation of outwardly expanding laminar flames initiated by laser-induced spark ignition behind reflected shock waves. The linear curvature model with an area-averaged formulation was used to extrapolate laminar flame speeds to zero-stretch. The high-temperature laminar flame speed results were compared with 1D simulations performed using two detailed kinetic models, NUIG 1.3 and Aramco 3.0, and two skeletal models, USC Mech II and San Diego 2016. The experimental results agree with the NUIG 1.3 model within 3.5 % , while the predictions of the models disagree by up to 16 %. Sensitivity analyses conducted using the models revealed six chemical reactions that strongly impact the prediction of laminar flame speed. In order for the present data to be compatible with practical combustion systems using air as the oxidizer, the propane/airgon measurements were scaled to obtain laminar flame speeds of propane/air using ratios calculated from 1D simulations. The scaled experimental measurements showed good agreement with literature propane/air results at 650 K. Empirical correlations of laminar flame speed as a function of unburned-gas temperature are then reported for the studied equivalence ratios. [ABSTRACT FROM AUTHOR]

The impact of chain length on the time histories of key intermediate species that form upon first-stage ignition was studied experimentally in a shock tube using multi-color laser absorption diagnostics. CO, CH 2 O, OH, and composite time histories of heavier carbonyls were measured simultaneously during the low temperature oxidation of n-octane & n-decane in the temperature range of 720–860 K, and pressure range of 4.1–5.8 atm. The test mixtures were 0.64% & 0.6% fuel in oxygen, respectively. A two-color diagnostic was also developed and used in tandem with other diagnostics to quantify the evolution of temperature during oxidation. To our knowledge, this work reports the first measurement of CH 2 O, OH, and temperature during oxidation of C8 or heavier hydrocarbon fuels at low temperatures. The measured time histories were also compared against the predictions of two recent detailed kinetic models. Significant differences in the measured and predicted first-stage ignition delay times, absolute concentration of species post-ignition and the maximum rate of formation of species were observed. Our measurements will serve as targets for further refinement of these detailed kinetic models and in the development of rate rules for similar classes of fuels. The measurements also revealed significant differences in the relative reactivity of n-octane and n-decane. Taken together with previous speciation studies conducted in n-heptane, we observe a strong correlation between the species time histories, reactivity, and chain length of straight chain alkanes. This observation lends further support to the selection of CO and CH 2 O as target species for development of Low-Temperature Hy brid Chem istry (HyChem) models for real transportation and aviation fuels in future. [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]

The rate constant of the H-abstractionreaction of formaldehyde(CH2O) by hydrogen atoms (H), CH2O + H = H2+ HCO, has been studied behind reflected shock waves withuse of a sensitive mid-IR laser absorption diagnostic for CO, overtemperatures of 1304–2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxaneas the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintainrelatively constant H atom concentrations, the current study significantlyboosted the measurement sensitivity of the target reaction and suppressedthe influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304–2006 K, 1 atm)= 1.97 × 1011(T/K)1.06exp(−3818 K/T) cm3mol–1s–1, with uncertainty limits estimated to be +18%/–26%.A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12level of theory, is in good agreement with the shock tube measurementand extended the temperature range of the current study to 200–3000K, over which a modified Arrhenius fit of the rate constant can beexpressed as kCH2O+H(200–3000K) = 5.86 × 103(T/K)3.13exp(−762 K/T) cm3mol–1 s–1. [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: This study addresses the influences of residual radical impurities on the computation and experimental determination of ignition times in H2/O2 mixtures. Particular emphasis is made on the often-times encountered problem of the presence of H-atoms in the initial composition of H2/O2 mixtures in shock tubes. Two methods are proposed for quantifying experimentally H-residual impurities in shock tubes, namely, an a priori method that consists of detecting OH traces upon shocking unfueled mixtures, and a posteriori method in which the amount of impurities is inferred by comparing fueled experimental autoignition data with calculations. A stochastic Arrhenius model that describes the amount of H-radical impurities in shock tubes is proposed on the basis of experimental measurements as a function of the test temperature. It is suggested that this statistical model yields a probability density function for the residual concentration of hydrogen radicals in standard shock tubes. Theoretical quantifications of the uncertainties induced by the impurities on autoignition times are provided by using the 5-step short chemistry of Del Álamo et al. [1]. The analysis shows that the relative effects of H-impurities on delay times above crossover become more important as the dilution increases and as the temperature and pressure decrease. Below crossover, the effects of H-impurities on the ignition delay vanish rapidly, and are negligible compared to the departures produced by the non-ideal pressure rise that is seen in some shock-tube experiments at such low temperatures. The influences of kinetic uncertainties on the ignition time are typically negligible compared to the effects of the uncertainties induced by H-impurities when the short mechanism is used, except for air at high temperatures for which kinetic uncertainties dominate. Furthermore, calculations performed with the short mechanism show that correlations between the uncertainties in the rates of branching and termination steps have only some small influences on the ignition-time variabilities near crossover, where a global sensitivity analysis shows an increasing importance of the recombining kinetics. Computational quantifications of uncertainties are carried out by using numerical simulations of homogeneous ignition subject to Monte-Carlo sampling of the concentration of impurities. For the conditions analyzed, these computations show that the variabilities produced in ignition delays by the uncertainties in H-impurities are comparable to the experimental data scatter and to the effects of typical uncertainties of the test temperature when the Stanford chemical mechanism [2] is used. The calculations also unveil that the utilization of two other different chemical mechanisms, namely San Diego [3] and GRI v3.0 [4], yields variations in the ignition delays which are within the range of the uncertainties induced by the H-impurities. Finally, the effects of residual impurities in kinetic-isolation experiments and in supersonic-combustion ramjets are briefly discussed. [Copyright &y& Elsevier]

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]

The rate constants of two unimolecular decomposition channels of ethanol, C 2 H 5 OH(+Ar) = CH 3 + CH 2 OH(+Ar), and C 2 H 5 OH(+Ar) = C 2 H 4 + H 2 O(+Ar) were measured near 1 atm and 10 atm behind reflected shock waves between 1190 and 1550 K by tracking the evolution of CO and C 2 H 4 , respectively. Use of sensitive laser diagnostics in conjunction with carefully selected test mixtures enabled the suppression of sensitivity of CO and C 2 H 4 time-histories to secondary reactions, resulting in a significant reduction of uncertainty in the measured rate constants. Additionally, CH 3 CHO time-histories measured during the pyrolysis of 2% C 2 H 5 OH/Argon mixtures were used to determine the rate constant of the most dominant H-abstraction channel, C 2 H 5 OH + H = sC 2 H 4 OH + H 2 between 1290 and 1530 K. The low temperature CH 3 CHO time-histories were also used to infer the rate constant of sC 2 H 4 OH = CH 3 CHO+H. The measured rate constants were used to generate an updated kinetic model, whose performance was evaluated against the species time-history data reported in recent shock tube pyrolysis studies. The performance of the updated model was also evaluated against ignition delay times (IDTs) of stochiometric C 2 H 5 OH/Oxygen/Nitrogen mixtures, measured between 1000 and 1500 K near 20 atm in this study. The updated kinetic model was found to show excellent agreement with both shock tube pyrolysis and IDT studies. Additionally, the reduction in the uncertainty of the rate constants studied in this work resulted in a significant reduction in the uncertainty of the model predictions. The rate constant expressions listed below were derived from the measurements in this work, and are valid in the temperature range of 1200–1600 K. k C 2 H 5 OH (+ Ar) ↔ CH 3 + CH 2 OH (+ Ar) , 1 a t m = 2.125 × 10 13 e − 34070 T s − 1 (± 21 %) k C 2 H 5 OH (+ Ar) ↔ CH 3 + CH 2 OH (+ Ar) , 10 a t m = 3.079 × 10 14 T 0.23 e − 38030 T s − 1 (± 28 %) k C 2 H 5 OH (+ Ar) ↔ C 2 H 4 + H 2 O (+ Ar) , 10 a t m = 6.062 × 10 36 T − 6.45 e − 42180 T s − 1 (± 31 %) k sC 2 H 4 OH ↔ CH 3 CHO + H , 1 a t m = 5.817 × 10 9 e − 11372 T s − 1 (± 55 %) k C 2 H 5 OH + H ↔ sC 2 H 4 OH + H 2 = 1.780 × 10 7 T 1.94 e − 1835 T s − 1 (± 28 %) [ABSTRACT FROM AUTHOR]

Time-histories of the concentration of major products of ethanol pyrolysis at 1 atm and 10 atm in the temperature range of 1200–1600 K were measured behind reflected shock waves using fixed-wavelength laser absorption spectroscopy. Measurement of absorbance at nine wavelengths enabled the determination of the evolution of CH 4 , C 2 H 4 , C 2 H 5 OH, CO, CH 2 O and CH 3 CHO during the pyrolysis of 2% C 2 H 5 OH/Argon mixtures. These measurements enabled almost complete tracking of oxygen and carbon during pyrolysis. The performance of five recently developed kinetic models was evaluated against the measurements. It was found that none of these models could reconcile all the measured time-histories simultaneously. Sensitivity and Rate-of-production analysis revealed that the thermal decomposition and H-abstraction reactions of ethanol are the key reactions that have a controlling influence on the species time-histories. Additional experiments were designed and conducted to infer the rate constants of these reactions. The details of these experiments, the rate constant determination methodology, and the implications of the updated rate constants on the model performance are discussed in the companion paper [1]. To the best of our knowledge, this work represents the first detailed study of ethanol pyrolysis involving simultaneous measurement of multiple species in a shock tube using laser absorption spectroscopy. [ABSTRACT FROM AUTHOR]

Ethane pyrolysis chemistry plays a critical role in the combustion behavior of natural gas and gives insight into the decomposition patterns of larger alkanes. In this work, ethane pyrolysis was studied behind reflected shock waves with a convex optimization-based laser absorption speciation technique. Species time-histories of ethane, ethylene, methane, and acetylene were quantified in the decomposition of 1% and 2% ethane in argon at conditions between 1178 and 1527 K and 3.1–4.2 atm. Six laser wavelengths, simultaneously probing sensitive regions of each species' spectra, enabled the sensitive detection of all major pyrolysis products and confirmed the negligible formation of other species. The time-history data presented in this work paint a coherent picture of ethane decomposition that motivates specific refinements to state-of-the-art detailed kinetic models. [ABSTRACT FROM AUTHOR]