Single Pulse Shock Tube Studies Research Articles

Single-pulse shock-tube study on the pyrolysis of small esters (ethyl and propyl propanoate, isopropyl acetate) and methyl isopropyl carbonate

Single-pulse shock-tube experiments were used to study the thermal decomposition of selected oxygenated hydrocarbons: Ethyl propanoate (C2H5OC(O)C2H5; EP), propyl propanoate (C3H7OC(O)C2H5; PP), isopropyl acetate ((CH3)2HCOC(O)CH3; IPA), and methyl isopropyl carbonate ((CH3)2HCOC(O)OCH3; MIC) The consumption of reactants and the formation of stable products such as C2H4 and C3H6 were measured with gas chromatography/mass spectrometry (GC/MS). Depending on the considered reactant, the temperatures range from 716–1102 K at pressures between 1.5 and 2.0 bar. Rate-coefficient data were obtained from first-order analysis. All reactants primarily decompose by six-center eliminations: EP → C2H4 + C2H5COOH (propionic acid); PP → C3H6 + C2H5COOH; IPA → C3H6 + CH3COOH (acetic acid); MIC → C3H6 + CH3OC(O)OH (methoxy formic acid). Experimental rate-coefficient data can be well represented by the following Arrhenius expressions: k(EP → products) = 1013.49±0.16 exp(−214.95±3.25 kJ/mol/RT) s−1; k(PP → products) = 1012.21±0.16 exp(–191.21±2.79 kJ/mol/RT) s−1; k(IPA → products) = 1013.10±0.31 exp(–186.38±5.10 kJ/mol/RT) s−1; k(MIC → products) = 1012.43±0.29 exp(–165.25±4.46 kJ/mol/RT) s−1. The determination of rate coefficients was based on the amount of C2H4 or C3H6 formed. The potential energy surface (PES) of the thermal decomposition of these four reactants was determined with the G4 composite method. A master-equation analysis was conducted based on energies and molecular properties from the G4 computations. The results indicate that the length of a linear alkyl substituent does not significantly influence the rate of six-center eliminations, whereas the change from a linear to a branched alkyl substituent results in a significant reactivity increase. The comparison between rate-coefficient data also shows that alkyl carbonates have higher reactivity towards decomposition by six-center elimination than esters. The results are discussed in in the context of reactivity patterns of carbonyl compounds.

Shock tube study of natural gas oxidation at propulsion relevant conditions

Global climate change and rising crude oil prices have motivated search of clean, sustainable and economic fuel. Methane, having the highest hydrogen to carbon ratio is ideally the best option towards carbon neutral emissions and reduce greenhouse gas emissions. However, there the economic benefits are minimal when using pure methane because of purification costs. Natural gas primarily consists of methane but with appreciable amounts of C2C4 alkanes which significantly impact oxidation performance, especially when compared to pure methane at high pressures and temperatures, studies of natural gas have been scarce and there is new interest in studying natural gas for its application to propulsion devices. This single pulse shock tube study investigates the effect of natural gas composition on species yields as a function of temperature at various stoichiometric conditions (φ ≈ 0.5, 1.0, 2.0) and high pressure (240 atm) over a nominal reaction time of 2.5 ms. The experimentally measured speciation results for one real natural gas sample from Boise, ID (USA) and a reference natural gas sample are compared to species yield predictions using well established chemical kinetic mechanisms, to check the feasibility of using these mechanisms in the design of propulsion devices. The chemical kinetic simulations are carried out using the constant pressure approach as well as the changing pressure approach which considers the exact measured pressure change in the shock tube. The experiments provide a large speciation data set for optimizing chemical kinetic mechanisms for natural gas applications and to understand the effect of composition on natural gas chemical kinetics. The predictions vary significantly between models tested and effect of composition variation was evident. Aramco Mech 3.0 could predict the experiments almost perfectly, especially at fuel rich conditions.

Single-Pulse Shock Tube Pyrolysis Study of RP-3 Jet Fuel and Kinetic Modeling.

A single-pulse shock tube study of the pyrolysis of two different concentrations of Chinese RP-3 jet fuel at 5 bar in the temperature range of 900–1800 K has been performed in this work. Major intermediates are obtained and quantified using gas chromatography analysis. A flame-ionization detector and a thermal conductivity detector are used for species identification and quantification. Ethylene is the most abundant product in the pyrolysis process. Other important intermediates such as methane, ethane, propyne, acetylene, butene, and benzene are also identified and quantified. Kinetic modeling is performed using several detailed, semidetailed, and lumped mechanisms. It is found that the predictions for the major species such as ethylene, propene, and methane are acceptable. However, current kinetic mechanisms still need refinement for some important species. Different kinetic mechanisms exhibit very different performance in the prediction of certain species during the pyrolysis process. The rate of production (ROP) is carried out to compare the differences among these mechanisms and to identify major reaction pathways to the formation and consumption of the important species, and the results indicate that further studies on the thermal decomposition of 1,3-butadiene are needed to optimize kinetic models. The experimental data are expected to contribute to a database for the validation of mechanisms under pyrolytic conditions for RP-3 jet fuel and should also be valuable to a better understanding of the combustion behavior of RP-3 jet fuel.

A single pulse shock tube study of pentene isomer pyrolysis

A single-pulse shock tube study of the four pentene isomers is carried out at 2 ± 0.16 bar and 900–1600 K. C1 to C6 species profiles were recorded using gas chromatography mass spectrometry analyses. The species are identified using mass spectrometry and quantified by flame ionization detection. High-pressure limiting and pressure-dependent rate constants for 2M1B, 2M2B and 3M1B + Ḣ were calculated using RRKM theory with a Master Equation (ME) analysis using the Master Equation System Solver, MESS. A mechanism was formulated based on rate rules and theoretical calculations. Comparisons between experimental results and model simulations are provided for all of the five pentene isomers investigated with satisfactory agreement. Furthermore, an insight is provided into the influence of molecular structure on the reactivity of pyrolysis chemistry. Interestingly, it is found that the HACA mechanism is much less prominent for benzene formation compared to the role of cyclopentadienyl radical recombination with methyl radicals and also the recombination of propargyl radicals.

A hierarchical single-pulse shock tube pyrolysis study of C2–C6 1-alkenes

A single-pulse shock tube study of the pyrolysis of 2% C2–C6 1-alkenes is presented at 2 bar in the temperature range 900–1800 K in the current study. Reactant, intermediate and product species are obtained and quantified using gas chromatography-mass spectrometry (GC–MS) analysis. MS is used for species identification and a flame ionization detector is used for quantification. The experiments show the effect of carbon chain length on the production of smaller C1–C3 fragments. A new detailed kinetic mechanism, NUIGMech1.0, is used to simulate the data and the predictions for the major species are satisfactory. The improvement in predictions of the current mechanism, which includes C6 and C7 species, is substantial compared to AramcoMech3.0. Kinetic analyses are conducted with the current mechanism to identify the formation and consumption pathways of the quantified species. The experimental data are expected to contribute to a database for the validation of mechanisms at pyrolytic conditions. Additionally, the mechanism aims to provide a fundamental understanding of the pyrolytic chemistry of olefins.

Temperature approximations in chemical kinetics studies using single pulse shock tubes

Simulations of chemical kinetics processes in single pulse shock tubes are typically modelled by a constant pressure approximation. However, concerns have been raised by the use of such an approximation in the case when the post-reflected-shock-wave pressure profiles in a shock tube do not have perfect square wave profiles. This article discusses the validity of using the constant pressure approximation in such cases for modelling purposes. The constant pressure simulations are compared with changing pressure simulations. They show that the constant pressure simulations can be used as a sufficient modeling approximation for the shock tube chemistry of single reaction processes, complex reaction network models, highly diluted fuel mixtures and highly concentrated mixtures. Any discrepancies between the model predictions from the constant pressure approach and the changing pressure approach are minimal across a wide temperature range. A set of different types of chemical kinetic models and different physical conditions were tested for the comparison of the two approaches. The results justify using species measurements of quenched post rarefaction wave samples and chemical thermometer calibration methods with the constant pressure approximation in reporting reaction product data in single pulse shock tube studies. The study further confirms that the speciation results from the quenched samples obtained from single pulse shock tube experiments that have non-ideal pressure profiles can be well approximated as a constant pressure reaction process even for complex reaction networks and facilitate the reporting of modeling results as pertinent to single pressures, temperatures and times.

ChemInform Abstract: Unimolecular Rate Expression for Cyclohexene Decomposition and Its Use in Chemical Thermometry under Shock Tube Conditions

AbstractReview: 18 refs.

Ignition Delay of 3-Carene:Single Pulse Shock Tube Study

Ignition delay experiments of 3-carene, a biofuel, have been carried out in a single-pulse shock tube for three equivalence ratios, 0.5, 1 and 2. The temperature was varied from 1140 to 1606 K. In the above-mentioned conditions, ignition delay was found to vary from 1.180 ms to 144 μs. The ignition delay values of 3-carene were found to be lower than those of JP-10, a kerosene-based fuel being considered for hypersonic applications.

Unimolecular Rate Expression for Cyclohexene Decomposition and Its Use in Chemical Thermometry under Shock Tube Conditions.

The methods used in deriving the rate expressions from comparative rate single-pulse shock tube studies, recent direct shock tube studies, and high-pressure flow experiments bearing on the data for the reverse Diels-Alder decomposition of cyclohexene to form ethylene and 1,3-butadiene are reviewed. This current interest is due to the increasing need for accurate kinetics and physical data (particularly the temperature) for realistic simulations in practical areas such as combustion. The rate constants derived from the direct shock tube studies and high-pressure flow experiments are somewhat larger than those used in comparative rate single-pulse shock tube experiments. For the latter, it is shown that they have been derived from a variety of independent experiments that include rate constants for unimolecular decomposition and isomerization processes that are considered to be well understood. The possibility of non-Arrhenius behavior in the unimolecular rate constants as a consequence of the large range covered in rate constants (as much as 12 orders of magnitude) for the comparative rate experiments has been examined and ruled out as a source of the discrepancy. Our analysis shows that there is the need to consider the possibility of radical-induced decompositions for verifying the correctness of the reaction mechanisms in studying unimolecular reactions. In the case of cyclohexene decomposition, recent experiments demonstrating the presence of residual amounts of H atoms in shock tube experiments suggest that addition to the double bond can also lead to the formation of ethylene and 1,3-butadiene and hence to rate constants larger than the true values. This possibility is even more likely to occur in high-pressure flow experiments. As a result, the internal standard method must be used with care and a radical inhibitor should always be present in sufficiently large quantities to suppress possible chain reactions. The present analysis results have important implications for the determination of temperatures in shock tubes.

Shock Tube Study on the Thermal Decomposition of n-Butanol

Dilute concentrations of normal-butanol has been decomposed in single pulse shock tube studies in the presence of large quantities of a chemical inhibitor that suppresses contributions from chain decomposition. Reaction temperatures and pressures are in the range of [1126-1231] K and [1.3-6.5] bar. Ethylene and 1-butene are the only products. The mechanism of the initial decomposition steps involves direct elimination of water and C-C bond cleavage. The fundamental high pressure unimolecular decomposition rate expressions are k(C(4)H(9)OH → CH(3) + CH(2)CH(2)CH(2)OH) = 10(16.4±0.4) exp(42410 ± 800 [K]/T) s(-1); k(C(4)H(9)OH → CH(3)CH(2) + CH(2)CH(2)OH) = 10(16.4±0.4) exp(-41150 ± 800 [K]/T) s(-1); k(C(4)H(9)OH → CH(3)CH(2)CH(2) + CH(2)OH) = 10(16.4±0.4) exp(-41150 ± 800 [K]/T) s(-1); and k(C(4)H(9)OH → CH(3)CH(2)CH═CH(2) + H(2)O) = 10(14.0±0.4) exp(-35089 ± 800 [K]/T) s(-1), where the rate expressions for C-C bond cleavage are based on assumptions regarding the relative rates of the three processes derived from earlier studies on the effect of an OH group on rate expressions. All reactions are in the high pressure limit and suggest that the step size down in the presence of argon is at least 1300 cm(-1). These rate expressions are consistent with the following H-C bond dissociation energies: BDE(H-CH(2)CH(2)CH(2)OH) = 417.2 ± 7 kJ/mol, BDE(H-CH(2)CH(2)OH) = 419.2 ± 7 kJ/mol, and BDE(H-CH(2)OH) = 401.7 ± 9 kJ/mol, with an estimated uncertainty of 6 kJ/mol. The kinetics and thermodynamic results are compared with estimates used in the building of combustion kinetics databases.

Standard reactions for comparative rate studies: Experiments on the dehydrochlorination reactions of 2-chloropropane, chlorocyclopentane, and chlorocyclohexane

Single pulse shock tube studies of the thermal dehydrochlorination reactions (chlorocyclopentane → cyclopentene + HCl) and (chlorocyclohexane → cyclohexene + HCl) at temperatures of 843–1021 K and pressures of 1.4–2.4 bar have been carried out using the comparative rate technique. Rate constants have been measured relative to (2-chloropropane → propene + HCl) and the decyclization reactions of cyclohexene, 4-methylcyclohexene, and 4-vinylcyclohexene. Absolute rate constants have been derived using k(cyclohexene → ethene + butadiene) = 1.4 × 1015 exp(−33,500/T) s−1. These data provide a self-consistent temperature scale of use in the comparison of chemical systems studied with different temperature standards. A combined analysis of the present results with the literature data from lower temperature static studies leads to k(2-chloropropane) = 10(13.98±0.08) exp(−26, 225 ± 130) K/T) s−1; 590–1020 K; 1–3 bar k(chlorocylopentane) = 10(13.65 ± 0.10) exp(−24,570 ± 160) K/T) s−1; 590–1020 K; 1–3 bar k(chlorocylohexane) = 10(14.33 ± 0.10) exp(−25,950 ± 180) K/T) s−1; 590–1020 K; 1–3 bar Including systematic uncertainties, expanded standard uncertainties are estimated to be about 15% near 600 K rising to about 25% at 1000 K. At 2 bar and 1000 K, the reactions are only slightly under their high-pressure limits, but falloff effects rapidly become significant at higher temperatures. On the basis of computational studies and Rice–Ramsperger–Kassel–Marcus (RRKM)/Master Equation modeling of these and reference dehydrochlorination reactions, reported in more detail in an accompanying article, the following high-pressure limits have been derived: k∞ (2-chloropropane) = 5.74 × 109T1.37 exp(−25,680/T) s−1; 600–1600 K k∞ (chlorocylopentane) = 7.65 × 107T1.75 exp(−23,320/T) s−1; 600–1600 K k∞ (chlorocylohexane) = 8.25 × 109T1.34 exp(−25,010/T) s−1; 600–1600 K © 2011 Wiley Periodicals, Inc.* Int J Chem Kinet 44: 351–368, 2012

Ring-Expansion Reactions in the Thermal Decomposition of tert-Butyl-1,3-cyclopentadiene

The thermal decomposition of tert-butyl-1,3-cyclopentadiene has been investigated in single-pulse shock-tube studies at shock pressures of 182-260 kPa and temperatures of 996-1127 K. Isobutene (2-methylpropene), 1,3-cyclopentadiene, and toluene were observed as the major stable products in the thermolysis of dilute mixtures of the substrate in the presence of a free-radical scavenger. Hydrogen atoms were also inferred to be a primary product of the decomposition and could be quantitatively determined on the basis of products derived from the free-radical scavenger. Of particular interest is the formation of toluene, which involves the expansion of the ring from a five- to a six-membered system. The overall reaction mechanism is suggested to include isomerization of the starting material; a molecular elimination channel; and C-C bond fission reactions, with toluene formation occurring via radical intermediates formed in the latter pathway. These radical intermediates are analogous to those believed to be important in soot formation reactions occurring during combustion. Molecular and thermodynamic properties of key species were determined from G3MP2B3 quantum chemistry calculations and are reported. The temperature dependence of the product spectrum was fit with a detailed chemical kinetic model, and best-fit kinetic parameters were derived using a Nelder-Mead simplex minimization algorithm. Our mechanism and rate constants are consistent with and provide experimental support for the H-atom-assisted routes to the conversion of fulvene to benzene that have been proposed in the literature on the basis of theoretical investigations.

Single-pulse shock-tube study on the decomposition of 1-pentyl radicals

1-Pentyl radicals have been decomposed in single-pulse shock-tube experiments. The precursor is n-pentyl iodide, and the reaction conditions are 850–1000 K and 1.5–5 bar pressure. With 1-pentyl radicals being released with the rate expression k(n-C5H11I=1-C5H11+I)=4×1014 exp(−25,820/T)s−1 we find the branching ratios for the decomposition to form olefin products to be k(1-C5H11=2C2H4+CH3)/k(1-C5H11)=2.3 exp(−2018/T) k(1-C5H11=C3H6+C2H5)/k(1-C5H11)=0.27 exp(803/T) k(1-C5H11=1-C4H8+CH3)/k(1-C5H11)=0.068 exp(310/T) where k(1-C5H11) is the total rate constant for 1-pentyl radical decomposition. The results from the last two channels are interpreted in terms of radical isomerization through 1,4- and 1,3-hydrogen migration to form 2-pentyl and 3-pentyl radicals. From kinetic modeling, we find k(1-C5H11→2-C5H11)=1×1012 exp(−11,330/T)s−1 k(1-C5H11→3-C5H11)=3×1011 exp(−12,100/T)s−1 Under the present conditions, pentyl radicals are not being equilibrated. Some of the problems in the interpretation and extrapolation of experimental results on the decomposition of complex radicals are considered. It appears that the isomerization of secondary pentyl radicals will not be competitive with β C-C bond fission. The molecular elimination of HI from n-pentyl iodide is characterized by the rate expression (n-C5H11I=1-C5H10+HI)=1.05×1013 exp(−25,730/T)s−1 and is relatively unimportant in the present context.

Single-Pulse Shock Tube Studies of the Decomposition of Ethoxy Compounds

Tetraethyl orthocarbonate (TEOC), diethyl carbonate (DEC), and diethoxymethane (DEM) have been decomposed in single-pulse shock tube experiments. TEOC decomposes to give DEC, ethylene, and ethanol as organic products, while DEC results in only the latter two. In both cases the ethylene to ethanol ratio is equal to 1, and the mechanisms appear to involve molecular eliminations. The rate expressions for the initial processes are the following: k(TEOC → products) = 1013.91±0.14 exp((−27 529 ± 348)K/T) s-1, T = 1005−1180 K; k(DEC → C2H5OCO2H + C2H4) = 1013.03±0.11 exp((−23 290 ± 26 7 K/T) s-1, T = 955−1095 K. The listed uncertainties are one standard deviation. DEM decomposes more slowly than the other two compounds. For each DEM reacted, 1.2 ethylene and 0.5 ethanol are produced. Methane and ethane are also observed as products. The mechanism is postulated to involve both molecular and bond-breaking channels. It is concluded that ethanol arises only through the molecular channel, and on this basis the following rate expressions have been derived: k(DEM → products) = 1015.93±0.15 exp((−36 179 ± 403 K/T) s-1, T = 1150−1260 K; k(DEM → ethanol + products) = 1015.07±0.45 exp((−34 517 ± 1090 K/T) s-1; k(DEM → ethyl + OCH2OC2H5) = 1016.32±0.45 exp((−38 214 ± 1160 K/T) s-1. The results are compared with those dealing with the stability of analogous ethers, esters, and silicon compounds. For carbon compounds the addition of ethoxy groups to the compound destabilizes the molecule. It is further concluded that rate data on the molecular decompositions of ethoxy carbon compounds cannot be easily extrapolated to silicon-containing species.

Single-Pulse Shock Tube Study of the Decomposition of Tetraethoxysilane and Related Compounds

Tetraethoxysilane (TEOS) has been decomposed in single-pulse shock tube experiments over the temperature range 1160−1285 K and pressures of about 150 kPa (1.5 bar). The main observed products are ethylene and ethanol. The yields of these products as a percentage of decomposed TEOS increase with temperature. Studies have also been carried out with tetra-n-propoxysilane (TPOS), dimethyldiethoxysilane (DMDEOS), and trimethylethoxysilane (TMEOS). Evidence is presented that in all cases the main initial reaction is a 1,2-elimination to form the olefin and the corresponding silanol. A smaller contribution from C−C bond-breaking channels is also observed. In combination with lower temperature results and the thermochemistry, the following rate expressions for the elementary processes are recommended: k[TEOS → C2H4 + HOSi(OC2H5)3] = 1.04 × 1010T1.1 exp(−30 950 K/T) s-1; k[TEOS → CH3 + CH2OSi(OC2H5)3] = 4 × 1017 exp(−43 300 K/T) s-1. The observed ethanol product is postulated to arise from decomposition of the silanol in a gas phase reaction. A kinetic model which quantitatively accounts for the observed products in the decomposition of TEOS, DMDEOS, and TMEOS has been developed. The model includes radical reactions as well as molecular reactions of the silanol and subsequently formed products, including silicates and silyl acids. The model requires an activation energy of ≤200 kJ mol-1 for the reaction which forms ethanol from the silanol. Such a low barrier is apparently at odds with recently calculated values for the thermochemistry of some silicon compounds.

Single-pulse shock tube studies on the thermal decomposition of n-butyl phenyl ether, n-pentylbenzene, and phenetole and the heat of formation of phenoxy and benzyl radicals

Single-pulse shock tube studies on the thermal decomposition of n-butyl phenyl ether, n-pentylbenzene, and phenetole and the heat of formation of phenoxy and benzyl radicals

Gas pyrolsis in combustion single-pulse shock tube studies of N- and O-atom-containing heterocyclics

Studies on the pyrolysis of heterocyclic compounds behind reflected shocks using the single-pulse shock tube technique are reviewed. Special attention is paid to oxygen- and nitrogen-containing heterocyclics. Compounds that can be considered as prototypes of fuel constituents and can simulate larger, more complex molecules are chosen. The reactions that take place during the pyrolysis of these compounds are very similar to the reactions that occur under combustion conditions. Distributions of reaction products in the pyrolysis of these compounds, measured over a wide temperature range, are shown for selected molecules and Arrhenius parameters for the rates of their production is given. The experimental techniques and analytical procedures for the identification and the quantitative determination of the reaction products are reviewed. Methods for determining unimolecular initiations versus free radical reactions, such as the use of isotope labeling and the introduction of free radical scavengers, are discussed. General pyrolysis mechanisms for several heterocyclic compounds are suggested.

Single-pulse shock tube studies on the stability of 1-phenyl-2-butene and the strength of the vinyl-H bond in propene

Single-pulse shock tube studies on the stability of 1-phenyl-2-butene and the strength of the vinyl-H bond in propene

Single-pulse shock-tube studies on C-NO2 bond cleavage during the decomposition of some nitro aromatic compounds

Single-pulse shock-tube studies on C-NO2 bond cleavage during the decomposition of some nitro aromatic compounds

Shock-tube pyrolysis of chlorinated hydrocarbons: Formation of soot

Soot formation in pyrolysis of chlorinated methanes, their mixtures with methane, and chlorinated ethylenes were studied behind reflected shock waves by monitoring the attenuation of an HeNe laser beam. An additional single-pulse shock-tube study was conducted for the pyrolysis of methane, methyl chloride, and dichloromethane. The experiments were performed at temperatures 1300–3000K, pressures 0.4–3.6 bar, and total carbon atom concentrations (1–5) × 10 17 atoms/cm 3. The amounts of soot produced in the pyrolysis of chlorinated hydrocarbons are larger than that of their nonchlorinated counterparts. The sooting behavior and product distribution can be generally explained in terms of chlorine-catalyzed chemical reaction mechanisms. The pathway to soot from chlorinated methanes and ethylenes with high H : Cl ratio proceeds via the formation of C 2H, C 2H 2, and C 2H 3 species. For chlorinated hydrocarbons with low H : Cl ratio, the formation of C 2 and its contribution to soot formation at high temperatures becomes significant. There is evidence for the importance of CHCl radical and its reactions in the pyrolysis of dichloromethane.