Shock tube experiments

Figure B2.5.6. Temperature as a fiinction of time in a shock-tube experiment. The first r-jump results from the incoming shock wave. The second is caused by the reflection of the shock wave at the wall of the tube. The rise time 8 t typically is less than 1 ps, whereas the time delay between the incoming and reflected shock wave is on tlie order of several hundred microseconds. Adapted from [110].

In a more recent example, a shock-tube experiment was used to study the themial decomposition of methylamine between 1500 K and 2000 K [61, 62] ... 

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

Votsmeier M, Song S, Davidson D F and Hanson R K 1999 Sensitive detection of NH2 in shock tube experiments using frequency modulation spectroscopy int. J. Chem. Kinetics 31 445-53... 

Wagner, H. Gg. Gaseous Detonations and the Structure of a Detonation Zone (in Fundamental Data obtained from Shock Tube Experiments, Editor Ferri, A.). Pergamon Press, Oxford 1961... 

Although the concentrations of H202 in the shock-tube experiments were much... 

Shock-tube experiments on the decomposition of hydrogen sulphide have been performed but were unsuccessful because traces of oxygen and other oxidizers could not be removed from the reactant24. No data are available on the homogeneous decomposition of hydrogen polysulphides, nor have the kinetics of pyrolysis of selenium and tellurium hydrides been studied. 

Shock tube experiments by Jacobs27 have shown that it is essential to purify the ammonia and the diluent from oxygen or other oxidizing components, otherwise oxidation would seriously interfere with decomposition. Jacobs followed the decay of ammonia through its infrared emission at 3 n in the temperature range 2100-3000 °K. He argued that an assumed reaction order of in ammonia and of i in the inert gas would best fit the observed concentration-time records, i.e. 

Most of our knowledge about the kinetics of the homogeneous decomposition has come from shock-tube experiments. These have been performed in several laboratories under a variety of experimental conditions. However, their results are contradictory in some respects especially with regard to activation energy and on the question of the importance of chain reactions. In some cases the experimental conditions are such that consecutive reactions have to be taken into account or at least cannot be safely excluded. Until recently, one reason for the difficulty of reconciling the results of different investigators was that, if they were interpreted in terms of the unimolecular reaction48... 

At the high temperatures attained in shock-tube experiments (28) is followed by... 

The reason for the low activation energy was and remains not understood. Jacobs et al.52 found a similar effect in their shock-tube experiments at 2800-4600 °K using the same diluent as Fishbume. For HC1 and DC1 they obtained... 

The solution procedure to this equation is the same as described for the temporal isothermal species equations described above. In addition, the associated temperature sensitivity equation can be simply obtained by taking the derivative of Eq. (2.87) with respect to each of the input parameters to the model. The governing equations for similar types of homogeneous reaction systems can be developed for constant volume systems, and stirred and plug flow reactors as described in Chapters 3 and 4 and elsewhere [31-37], The solution to homogeneous systems described by Eq. (2.81) and Eq. (2.87) are often used to study reaction mechanisms in the absence of mass diffusion. These equations (or very similar ones) can approximate the chemical kinetics in flow reactor and shock tube experiments, which are frequently used for developing hydrocarbon combustion reaction mechanisms. 

The significant step is represented by reaction (8.84). One should recall that there can be appreciable amounts of H02 in the early parts of a flame. The appearance of the N02 is supported further by the fact that reaction (8.84) is two orders of magnitude faster than reaction (8.85). The importance of the hydroperoxy radical attack on NO appeared to be verified by the addition of NO to the cold-fuel mixtures in some experiments. In these tests, the NO disappeared before the visible region was reached in oxygen-rich and stoichiometric flames, that is, flames that would produce H02. The N02 persists because, as mentioned previously, its reduction to N2 and 02 is very slow. The role of H02 would not normally be observed in shock tube experiments owing to the high temperatures at which they usually operate. 

The reaction probabilities for O and OH with soot particles have been measured by Roth and co-workers in a series of shock tube experiments [58-60], They have found that both radicals react with soot particles with a collision efficiency of between 0.10 and 0.20. In contrast, the reaction probability with 02 is at least an order of magnitude lower [55], Of course, at lower temperatures and sufficiently lean mixtures, soot oxidation by radical species becomes small and oxidation by 02 is important (though slow). Consequently, soot that passes through or avoids the primary reaction zone of a flame (e.g., due to local flame quenching) may experience oxidation from 02 in the post-flame gases. Analysis of soot oxidation rates in flames [54-57] has supported the approximate value of the OH collision efficiency determined by Roth and co-workers. 

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

Comparison of results of single-pulse shock-tube experiments with those from an earlier study suggest that the existing rate expression for HF elimination from 1,1,1-trifluoroethane may need to be re-evaluated. ... 

Lifshitz, A. Tamburu, C. Suslensky, A. Dubnikova, F. Decomposition of Anthranil. Single Pulse Shock-Tube Experiments, Potential Energy Surfaces and Multiwell Transition-State Calculations. The Role of Intersystem Crossing. Phys. Chem. A 2006, no, 8248-8258. 

High speed infrared spectroscopy is also available and has been used to study explosions and shock-tube experiments. Specific information on vibration and rotation states can be obtained by this method. Again the spectral information would be complex, but at least the electronic transitions are hereby eliminated. 

The slowest process will be the vibration-translation activation to the (v = 1) level, which will be rate-determining, and the subsequent vibration-vibration transfers will occur at increasingly fast rates with increasing vibrational quantum number. (For harmonic oscillators 1 = n(m+ l) 1 .) Shock-tube experi-... 

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

Laser induced fluorescence is particularly well suited to combustion chemistry, as a sensitive "in-situ" probe for free radicals in flames or under more controlled conditions in laboratory flash photolysis, discharge flow tube, or shock tube experiments. Using laser-saturation fluorescence previous studies from this laboratory (J ) have shown that C2(a3n ) is present in high concentrations in the hot region of an oxy-acetylene flame. C2(a-,n and X1 ) reacts with 0 .(2,3 4) One of the products of this reaction (and/or the reaction of C2H+02) is CC0.(2) In the present study, we report C20()rn.-f, i 7 fluorescence excitation spectra, A"3 , lifetimes and quenching rate constants, and... 

Ha + Da->2HD.—The theoretical search for the probable shape of the transition state in the four-centre exchange reaction H2 + Da->2HD has been under way for many years (a review of the earlier work in this field may be found in ref. 245). The controversy is centred on the inability of thorough ab initio calculations to find a mechanism for the reaction with a barrier height as low as that suggested on the basis of shock-tube experiments.248 247 Three studies were done recently, two with full Cl245 248 and one explicitly using interparticle co-ordinates,249 a method described above in conjunction with the work of Conroy and Bruner207 on H3. 

The problem that arises in these works is that the shock-tube experiments indicate that the activation energy for the reaction is only 176 kJ mol-1. As can be seen... 

A series of experiments were performed by Exxon Research and Engineering Co., and the results were utilized in the current modeling effort. Although the global steps in the quasiglobal toluene and iso-octane kinetics model were developed to represent these data, the range of applicability of the model was further tested by analyzing kinetics data from shock-tube experiments. 

The general problem has been to extend the usefulness of the induction parameter model proposed by Oran et al. (1). This induction parameter model (IPM) is proposed as a means to enable one to estimate, relatively easily, the energy necessary to achieve ignition when using a thermal heating source Much of the calibration of this model, for example the effect of deposition volume (quench volume), can be done with one-dimensional models, and shock tube experiments. There are phenomena, however, which must be studied in two or three dimensions. Examples are turbulence and buoyancy. This paper discusses the effect of buoyancy and possible extensions to the IPM. 

In contrast to the relatively limited number of experimental approaches utilized to determine electron collisional information for C02 laser species, many different types of experiments have been employed in the determination of heavy particle rates as a function of temperature, for temperatures slightly below room temperature up to several thousand degrees. At room temperature, measurements have been obtained using sound absorption and/or dispersion as well as impact-tube and spectrophone techniques. High temperature rate data have been obtained primarily from shock tube experiments in which electron beam, infrared emission, schlieren, and interferometric diagnostic techniques are employed. For example, as many as 36 separate experiments have been conducted to determine the relaxation rate of the C02 bending mode in pure C02 [59]. The reader is referred to the review by Taylor and Bitterman [59] of heavy-particle processes of importance to laser applications for a detailed description and interpretation of available experimental and theoretical data. 

Frenklach et al. [78], who evaluated the sooting tendency of fuels by shock tube pyrolysis at various temperatures, found that the sooting rate increased with temperature, reached a maximum, and then decreased. The maxima occur in the range 1900-2300 K. The pyrolysis in cofiow diffusion flames is initiated at temperatures much lower than the stoichiometric temperature, so that the soot forms prior to reaching the maximum temperatures Frenklach et al. created in their shock tube experiments. In shock tubes the fuel is instantaneously exposed to very high temperatures thus the precursors that form decompose and the soot... 

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