Shock Tube Methods Research Articles

Dynamic characterisation of pressure transducers using shock tube methods

This paper describes the characterisation of the dynamic response of a range of pressure transducer systems. The transducers were subjected to virtually instantaneous pressure step inputs in the National Physical Laboratory’s shock tube facilities. The magnitudes of these pressure steps were derived from ideal gas theory, with prior commissioning tests having been performed to demonstrate the theory’s validity in this application. The results demonstrate a significant variation in response obtained from various combinations of transducer, instrumentation settings, and mounting arrangement.

Ignition delay times of ethane under O2/CO2 atmosphere at different pressures by shock tube and simulation methods

Pressurized oxy-fuel combustion is a promising oxy-fuel technology owing to its high efficiency and low emission. The ignition delay times of ethane under O2/CO2 atmosphere were determined in a shock tube at different pressures, equivalence ratios, and C2H6 and CO2 concentrations. The results suggested that the ignition delay times decrease with the increasing ethane concentration at 0.8, 2.0, and 10 bar, while the effect of the fuel concentration on the ignition delay times is not sensitive to the pressure. The ignition delay times increased with the increasing equivalence ratio at 0.8 and 2.0 bar, while the effect of the equivalence ratio decreased with the increasing pressure from 0.8 to 2.0 bar. At 10 bar, the effect of the equivalence ratio on the ignition delay times further weakened at high temperatures, while the ignition delay times decreased with the increasing equivalence ratio in the low-temperature range. An updated model (OXYMECH) was developed and updated on the basis of our previous work, providing yields in good agreement with the experimental data under all conditions, while Aramco 2.0 showed poor prediction of the experimental results at 10 bar. Analysis of the sensitivity and the rate of production indicated that updating the rate constants of the reactions C2H6 + HO2 ⇔ C2H5 + H2O2, H + O2 (+M) ⇔ HO2 (+M), CH3 + HO2 ⇔ CH3O + OH, 2HO2 ⇔ H2O2 + O2, C2H4 + H (+M) ⇔ C2H5 (+M), and H + O2 ⇔ O + OH improves the performance at 10 bar.

Dynamic characterisation of pressure transducers using shock tube methods

This paper describes the results obtained from a range of dynamic pressure transducers in response to a rapid step pressure input. The testing was carried out in a pair of instrumented shock tubes at NPL. The results demonstrate a wide variation in the results obtained from different combinations of transducer, instrumentation, and mounting arrangement.

Evaluated kinetics of terminal and non-terminal addition of hydrogen atoms to 1-alkenes: a shock tube study of H + 1-butene.

Single-pulse shock tube methods have been used to thermally generate hydrogen atoms and investigate the kinetics of their addition reactions with 1-butene at temperatures of 880 to 1120 K and pressures of 145 to 245 kPa. Rate parameters for the unimolecular decomposition of 1-butene are also reported. Addition of H atoms to the π bond of 1-butene results in displacement of either methyl or ethyl depending on whether addition occurs at the terminal or nonterminal position. Postshock monitoring of the initial alkene products has been used to determine the relative and absolute reaction rates. Absolute rate constants have been derived relative to the reference reaction of displacement of methyl from 1,3,5-trimethylbenzene (135TMB). With k(H + 135TMB → m-xylene + CH3) = 6.7 × 10(13) exp(-3255/T) cm(3) mol(-1) s(-1), we find the following: k(H + 1-butene → propene + CH3) = k10 = 3.93 × 10(13) exp(-1152 K/T) cm(3) mol(-1) s(-1), [880-1120 K; 145-245 kPa]; k(H + 1-butene → ethene + C2H5) = k11 = 3.44 × 10(13) exp(-1971 K/T) cm(3) mol(-1) s(-1), [971-1120 K; 145-245 kPa]; k10/k11 = 10((0.058±0.059)) exp [(818 ± 141) K/T), 971-1120 K. Uncertainties (2σ) in the absolute rate constants are about a factor of 1.5, while the relative rate constants should be accurate to within ±15%. The displacement rate constants are shown to be very close to the high pressure limiting rate constants for addition of H, and the present measurements are the first direct determination of the branching ratio for 1-olefins at high temperatures. At 1000 K, addition to the terminal site is favored over the nonterminal position by a factor of 2.59 ± 0.39, where the uncertainty is 2σ and includes possible systematic errors. Combining the present results with evaluated data from the literature pertaining to temperatures of <440 K leads us to recommend the following: k∞(H + 1-butene → 2-butyl) = 1.05 × 10(9)T(1.40) exp(-366/T) cm(3) mol(-1) s(-1), [220-2000 K]; k∞(H + 1-butene → 1-butyl) = 9.02 × 10(8)T(1.40) exp(-1162/T) cm(3) mol(-1) s(-1) [220-2000 K]. Analogous rate constants for other unbranched 1-olefins should be very similar. Despite this, a factor of three discrepancy in the branching ratio for terminal and nonterminal addition is noted when comparing the present values with recommendations from a recent model of the important H + propene reaction. This difference is suggested to be well outside of the possible experimental errors of the present study or the expected differences with 1-butene. There thus appear to be inconsistencies in the current model for propene. In particular the addition branching ratio from that model should not be used as a reference value in extrapolations to other systems via rate rules or automated mechanism generation techniques.

Thermal decomposition of 1,1,1-trifluoroethane revisited.

1,1,1-Trifluoroethane (CH3CF3) has been frequently used as a chemical thermometer or an internal standard in shock tube studies to determine relative rates of chemical reactions. The rate constants for the thermal decomposition of CH3CF3 were recently reported to have anomalous pressure dependence in the high-temperature falloff region. In the present study, the kinetics of the CH3CF3 decomposition were reinvestigated using shock tube/laser absorption (ST/LA) spectroscopy and single-pulse shock tube (SPST) methods over the temperature range 1163-1831 K at pressures from 95 to 290 kPa. The present rate constants are 2-3 times smaller than those reported in previous single-pulse experiments performed at near high-pressure limit conditions. The recommended rate constant expression, k = 5.71 × 10(46)T(-9.341) exp(-47073 K/T) s(-1), was obtained over the temperature range 1000-1600 K with uncertainties of ±40% at temperatures below 1300 K and ±32% at 1600 K. The rate constants at the high-temperature region showed clear falloff behavior and were in good agreement with recent high-temperature experiments. The falloff rate constants could not be reproduced by a standard RRKM/master-equation model; this study provides additional evidence for the unusual pressure dependence previously reported for this reaction. Additionally, the rate constants for the decomposition of 1,1-difluoroethylene (CH2CF2) were determined over the temperature range 1650-2059 K at pressures of 100 and 205 kPa, and were reproduced by the RRKM/master-equation calculation with an average downward energy transfer of 900 cm(-1).

Mathematical modelling to support traceable dynamic calibration of pressure sensors

This paper focuses on the mathematical modelling required to support the development of new primary standard systems for traceable calibration of dynamic pressure sensors. We address two fundamentally different approaches to realizing primary standards, specifically the shock tube method and the drop-weight method. Focusing on the shock tube method, the paper presents first results of system identification and discusses future experimental work that is required to improve the mathematical and statistical models. We use simulations to identify differences between the shock tube and drop-weight methods, to investigate sources of uncertainty in the system identification process and to assist experimentalists in designing the required measuring systems. We demonstrate the identification method on experimental results and draw conclusions.

Dynamics of the energy transfer process in non-ideal systems in an example of measurement of gas thermal conductivity in shock tubes

We study the dynamics of energy transfer process in non-ideal gas systems whose thermal conductivity is measured in a shock tube. Different modifications of shock tube methods and agreement between the experimental thermal conductivity data obtained by different authors are discussed. A mathematical model for the energy transfer process in high-temperature gases in developed.

Isomerization and Decomposition of Chloromethylacetylene

AbstractThe isomerization and thermal decomposition of chloromethylacetylene (CMA) has been studied with two shock tube techniques. The first experiment (Jerusalem) utilizes single‐pulse shock tube methods to measure the isomerization rate of CMA to chloroallene. In addition, equilibrium constants can be estimated at ∼1200 K. The second experiment (Argonne) monitors Cl‐atom formation at temperatures above ∼1150 K. Absolute yield measurements have been performed over the 1200–1700 K range and indicate that two decomposition channels contribute to CMA destruction, namely, Cl fission and HCl elimination. The results show that the branching fraction between processes is temperature dependent. Therefore, direct Cl‐atom fission is accompanied by molecular elimination, undoubtedly giving HCl and one or more isomers of C3H2.MP2 6–31G(d,p) ab initio electronic structure calculations have been used to determine vibration frequencies and moments of inertia for three C3H3Cl isomers. Using these quantities, the experimental equilibrium constants required that ΔH00(CH2Cl–C≡CH ⇌ CHCl=C=CH2) = −;0.24 kcal mole−1. A potential energy surface pertinent to the present system has been constructed, and RRKM calculations have been carried out in order to explain the isomerization rates. The isomerization data can be explained with E0 = 52.3 kcal mole−1 and 〈ΔEdown〉 = 225 cm−1. Subsequent semi‐empirical Troe and RRKM‐Gorin modeling of the Cl atom rate data require E0 = (67.5 ± 0.5) kcal mole−1 with a 〈ΔEdown〉 = (365 ± 90) cm−1. This suggests a heat of formation for propargyl radicals of (79.0 ± 2.5) kcal mole−1.

Rate of the retro‐Diels‐Alder dissociation of 1,2,3,6‐tetrahydropyridine over a wide temperature range

AbstractRate constants for the retro‐Diels‐Alder dissociation of 1,2,3,6‐tetrahydropyridine, to 1,3‐butadiene and methanimine, have been measured over 650–1450 K. To cover this range, three separate techniques were used at three laboratories: laser schlieren and single pulse shock tube methods, and a comparative rate flow system technique. The derived rate constants are extrapolated to the high‐pressure limit with an RRKM model parameterized to fit the falloff observed in the laser‐schlieren measurements. The resulting high‐pressure rate constants show a reduction in activation energy of about 10 kcal/mol, comparing the isoelectronic cyclohexene, but little change in A‐factor. There is an apparent increase in activation energy of 4 kcal/mol over the temperature range of these experiments, which is just outside probable error. Such a rise in activation energy is in marked contrast to the drop usually seen in simple bond fission, and may reflect a change from a concerted to a stepwise, biradical mechanism at high temperatures.

Ethylbenzene Pyrolysis: Benzyl CC or CH Bond Homolysis

AbstractThe thermal unimolecular decomposition of ethylbenzene has been studied by both the very‐low‐pressure pyrolysis and heated single‐pulse shock tube methods in an attempt to resolve a recent controversy as to whether the initial dissociation channel is benzyl C ‐ C, or benzyl C‐H homolysis, Observed decomposition products and mass balance studies lead to the conclusion that up tc 1288K the predominant decomposition channel is benzyl C‐C homolysis. Additional support of this conclusion is provided by a study of isopropylbenzene decomposition by these same methods. Arguments are given which strongly suggest that benzyl C‐H homolysis cannot be the major reaction channel even at temperatures as high as 1600K.

ChemInform Abstract: A SHOCK TUBE, LASER‐SCHLIEREN STUDY OF PROPENE PYROLYSIS AT HIGH TEMPERATURES

Laser-schlieren, shock tube methods have been used to determine the rate of initiation in propene pyrolysis. Given that the dominant initiation reaction is M + C/sub 3/H/sub 6/ ..-->.. CH/sub 3/ + C/sub 2/H/sub 3/ + M, the rate constant over 1650-2300 K and 1 x 10/sup -6/ .. C/sub 3/H/sub 5/ + H cannot be excluded.

Measurement of vibrational relaxation time of oxygen between 475° and 675°K using an acoustic resonant technique

Vibrational relaxation times of O2–He, O2–CO2, and O2–H2 mixtures have been measured using a resonant chamber method between 475° and 675°K. This temperature range lies in the gap between the bulks of previous data obtained using acoustic and shock tube methods. Data analyses were carried out by a novel computer techique. Data for pure O2 were obtained by extrapolating that of mixtures to zero impurity concentrations. Our data agree with previous acoustic results at 473° and 573°K and shock tube results at 700°K. The temperature dependence of the vibrational relaxation time for pure O2 was compared to the theories of Landau and Teller [Physik. Z. Sowjetunion 10, 34 (1936)]; Schwartz et al. (SSH) [J. Chem. Phys. 20, 1591 (1952)]; and Parker [J. Chem. Phys. 34, 1763 (1961); 41, 1600 (1964)]. The simple Landau–Teller theory has shown better agreement with experimental data than that of SSH and Parker for the case of O2–O2, O2–He, and O2–H2 collisions.

Ultrasonic relaxation and carbon-13 NMR studies of dialkylmethylammonium chloride micelles

The kinetic results of the dialkylmethylammonium chloride surfactants of relatively short chain length investigated in this study do not show drastic changes compared to the result obtained for monoalkyl surfactants. Perhaps the most striking results is that the relaxation times and the dissociation rate constants are close to those found for monoalkyl surfactants of about the same alkyl chain length. The critical micelle concentrations are in agreement with previous studies on diaklyldimethylammonium chlorides. The aggregation number determined by /sup 13/C NMR for dihexylmethylammonium chloride is in agreement with results obtained for monoalkylammonium surfactants. The kinetic study of dialkylammonium surfactants of longer chain length would perhaps show larger differences but should be performed by other techniques. Shock-tube methods could be used to look for relaxation processes in aqueous solution of dialkylammonium surfactants of at least 9 carbon atoms per alkyl chain. 48 references.