Shock Tube Investigations Research Articles

Isopropylcyclohexane pyrolysis at high pressure and temperature: Part 1- theoretical study

The thermal decomposition of isopropylcyclohexane (IPCH) has been investigated at the CBS-QB3 and CASSCF/MRCI levels of theory. The pyrolysis of IPCH follows a radical chain mechanism, which mainly includes the C–C bond scission, H-atom abstraction, dissociation of alkene and alkyl-cyclohexene, decomposition, and isomerization of alkyl-cyclohexane radicals, decomposition and isomerization of alkenyl radicals, and stepwise dehydrogenation of cyclic intermediates. The rate constants for all elementary reactions have been evaluated with conventional transition state theory (TST/VTST) in the temperature range of 800–2000 K. The energy barriers and rate constants have been compared with previous theoretical reports of other cyclohexanes (like cyclohexane, methylcyclohexane, and ethylcyclohexane) to see the effect of isopropyl group in the cyclohexane dissociation. The final products of IPCH thermal decomposition are methane (CH4), ethylene(C2H4), propylene (C3H6), 1,3-butadiene (1,3-C4H6), and 1,3-pentadiene (1,3-C5H8), benzene, cyclopentadiene etc. The main goal of this work has been to come up with a detailed description of the IPCH thermal decomposition using high-level quantum chemical methods and provide reliable kinetic information to help in understanding the results from shock tube investigations discussed in part 2 of this work.

Evaluation of the SBT-TAS collision scheme on treating unsteady flows

The aim of the current work is to evaluate the accuracy and efficiency of a newly suggested collision scheme in the Direct Simulation Monte Carlo (DSMC) method; Simplified Bernoulli Trials implemented over transient adaptive subcells, SBT-TAS, for two different unsteady flow test cases. First, the performance of the SBT-TAS scheme is evaluated via simulating the shock-tube test case through a broad range of Knudsen numbers. The shock-tube investigations indicated that the SBT-TAS scheme could predict solutions as accurate as the nearest neighbor (NN) scheme. We show that the implemented SBT-TAS scheme requires at least 3–4 particles per each subcell to predict intermolecular collision frequency accurately. Then, a two-dimensional cylinder at Mach number of 0.6 is considered to investigate the accuracy and efficiency of the SBT-TAS in the prediction of the unsteady vortexes shedding. On the same grid and using the same number of particles per cell (PPC), the SBT and No Time Counter (NTC) collisions schemes fail to predict the correct vortex shedding behind the cylinder accurately and the structure of vortices are smeared out while the SBT-TAS and NN collisions schemes predict the strength of the vortices and their oscillation frequency with the same level of accuracy. Our results indicate that SBT-TAS preserves its accuracy in prediction of the aforementioned phenomenon while it uses relatively the same sample sizes and number of particles compared with its NN counterpart.

Shock tube investigations of ignition delays of n-butanol at elevated pressures between 770 and 1250 K

The ignition delays of n-butanol, a potential bio-fuel candidate, have been determined in a high-pressure shock tube. Conditions behind the reflected shock are approximately between 10–42bar and 770–1250K. To our knowledge, the ignition delay measurements of butanol at these high pressures are the first of their kind. CH emission and pressure time histories have been probed to determine ignition delay times for all experiments. For stoichiometric fuel–air-mixtures the influence of the temperature and pressure has been characterized. Interestingly the experimental data deviate from the Arrhenius behavior for temperatures lower than 1000K. This is in contrast to simulation results which have been obtained by employing the simulation tool CANTERA with different reaction mechanisms applying the typical assumption of isochoric conditions. It has been found out that a positive pressure and temperature gradient behind the reflected shock has a significant influence on the ignition delay below 1000K causing a pronounced decrease in the ignition delay times. This change of the conditions behind the reflected shock is attributed to the shock attenuation and probably from pre-ignition. Including the measured pressure gradients and assuming an isentropic compression behind the reflected shock, the simulation data and the experimental results show a same trend in the temperature dependence of the ignition delay. Nevertheless, striking differences between experiment and simulation persist especially for higher pressures. By performing sensitivity analysis at different temperatures some critical reactions could be identified and their role under our experimental conditions is discussed. In summary it can be stated that the employed reaction mechanisms may not be fully applicable to high-pressure conditions and it seems plausible that the lack of more detailed low temperature fuel specific reactions could be the probable cause for the discrepancies which calls for detailed investigations at elevated pressures.

Determination of the rate coefficient for the C 3H 3 + C 3H 3 reaction at high temperatures by shock-tube investigations

The kinetics of the C 3H 3 + C 3H 3 reaction was investigated behind incident shock waves at temperatures ranging from 995 to 1440 K and at pressures between 600 and 1000 mbar with argon as bath gas. The C 3H 3 radicals were generated by thermal decomposition of propargyl iodide with initial concentrations between 3 × 10 15 and 6 × 10 15 cm −3 corresponding to initial mole fractions between 750 and 800 ppm. Measurement of the UV absorption at 332.5 nm was used to monitor the C 3H 3 concentration, and the rate coefficients for the C 3H 3 + C 3H 3 reaction were determined by fitting a second-order rate law to the absorption–time profiles. Values between 2 × 10 −11 cm 3 s −1 near 1000 K and 7.5 × 10 −12 cm 3 s −1 near 1400 K were obtained with no significant pressure dependence. The negative temperature dependence can be expressed in the form k 1 = 5.8 × 10 −13 exp (3534 K/ T) cm 3 s −1 with an estimated maximum error of ±30% in k 1. The reaction channel leading to C 6H 5 + H is estimated to contribute with less than 10% to the overall reaction at temperatures between 1000 and 1300 K and, pressures ranging from 80 to 500 mbar.

Overview of diagnostic methods used in shock-tube investigations of mixing induced by Richtmyer-Meshkov instability

We present an overview of the diagnostic methods used in shock-tube investigations of mixing induced by Richtmyer–Meshkov instability. The different diagnostic techniques are first briefly presented, and then reviewed in a simple single table, which lists their advantages and disadvantages, their technological characteristics and domain of validity, the physical parameters measured, the laboratory in which they were developed and an assessment of their mean cost.

Shock-tube investigations on the self-ignition of hydrocarbon-air mixtures at high pressures

The self-ignition behaviour of various fuel-air mixtures has been investigated without inert gas dilutionusing a high-pressure shock tube. In order to obtain data directly applicable to the modeling of engine combustion, the shock-tube facility was designed to handle fuel-air mixtures up to initial pressures of more than 40 bar and to achieve measuring times up to 10 ms. As typical representatives of engine fuel components, n-heptane, benzene, iso-octane, methanol, and methyl-tert-butylether (MTBE) were investigated. Two pressure levels for the investigation, 13 bar and nearly 40 bar, have been chosen. The ignition of n-heptane begins with a rapid pressure increase, especially at higher temperatures. Benzene, iso-octane, methanol, and MTBE show a slow initiation of the ignition without distinct pressure peak (mild ignition) at low temperatures, which at higher temperatures changes to a rapid pressure increase after a variable time lag (strong ignition). The strong ignition limit depending on temperature, pressure, and fuel was determined. An investigation was made of the dependence of the ignition delay times of iso-octane and benzene on equivalence ratio and temperature at nearly 40 bar. A comparison of the ignition delay times of all the fuels investigated is presented for stoichiometric mixtures and at two pressure levels.

Heterogeneous detonation in a rigid porous medium

Shock-tube investigations of the detonation process in a gas--film system (film of liquid fuel on the walls--gaseous oxidizer occupying the tube interior) have shown that, despite important differences, heterogeneous detonation in such a system has many features in common with homogeneous detonation in gases [1-3]. In [4] the possibility of detonation propagating in a rigid porous medium (quartz sand) whose pores are filled with an acetylene--oxygen mixture of stoichiometric composition was experimentally established. It is also of interest to investigate the conditions of initiation and propagation of detonation in a porous medium consisting of inert particles whose surface is wetted with a film of liquid fuel, while the pore volume is filled with gaseous oxidizer. Below we present the results of an experimental study of heterogeneous detonation of this type.

Rate coefficients for H 2 + NO 2 = HNO 2 + H derived from shock tube investigations of H 2O 2NO 2 ignition

Rate coefficients for H 2 + NO 2 = HNO 2 + H, the initiation reaction of the H 2NO 2 chain, were evaluated from experimental induction times of H 2-Air-NO 2 mixtures measured in a shock tube, and yielded. k 1 = (2.4 ± 1.0) × 10 13 exp [ −(29000 ± 1000) RT ] cm 3/mole sec over the temperature range 760°K to 1000°K, and at pressures from 1 to 4 atm. Uncertainties in related rate coefficients lead to an estimated overall uncertainty in log k 1 of ± 0.4. An analysis, invoking the steady state assumption, of the H 2NO 2 chain related k 1 to experimental induction times and accounted for the main features of the experimental data. Analytic approximations to k 1 were then refined by computer modelling. Subsequent numerical predictions of induction times and NO 2 concentrations prior to ignition were in good agreement with experimental data on H 2-Air-NO 2 mixtures.

Vibrational Relaxation of CO by Fe Atoms

Abstract : Shock tube investigations on Fe(CO)5 + Ar mixtures are described. Behind incident shocks we observe immediate decomposition of the iron pentacarbonyl to yield Fe and vibrationally cold CO; the CO is then very efficiently relaxed by the Fe-atoms. The results are interpreted to yield a relaxation time for CO infinitely dilute in Fe at 1 atm of P(tau)(CO, Fe) of about 0.06 atm micro sec for T = 1400-2900K. This is believed to be the first measurement of vibrational relaxation by metal atoms. (Author)

Absolute gf-values for CrI and CrII

Absolute gf-values for 38 lines of CrI and 13 CrII have been measured with a shock tube as a source of radiation. The results are in agreement with other shock tube investigations, but show serious disagreement with the results of Corliss and Bozman. The magnitude of this disagreement appears to be wavelength dependent.

Shock-Tube Study of Nitrogen Dissociation using Vacuum-Ultraviolet Light Absorption

A shock-tube investigation of the dissociation of nitrogen diluted in argon is described which utilizes vacuum-ultraviolet absorption at 1176 Å to monitor the disappearance of molecular nitrogen. Measurements of the dissociation rate coefficients for the process kd(M)N2 + M→2N + M were obtained over the temperature range 8000° to 15 000°K where the collision partner, M, was Ar, N2, and N. The temperature dependence of the individual rate coefficients was found to be the same and was obtained with good precision. The magnitudes of the rate coefficients were generally lower than the currently accepted values obtained in previous shock-tube investigations, especially the value for kd(N) which, although it was found to be about 10 times greater than kd(Ar), was six times less than two previous measurements. The results of the investigation may be summarized by the following expressions for the dissociation rate coefficients: kd(Ar) = (2.3 ± 0.2) × 10−3T−1.6exp( − 1.132 × 105 / T) cm3sec−1, kd(N2) = (6.2 ± 1.6) × 10−3T−1.6exp( − 1.132 × 105 / T) cm3sec−1, kd(N) = (2.7 ± 1.0) × 10−2T−1.6exp(−1.132 × 105 / T) cm3sec−1. Encouragingly good agreement between the recombination rate coefficients, kr(Ar), inferred from the measurements of kd(Ar), with predictions of kr(Ar) obtained from the theories of Benson and Fueno and Keck and Carrier, were obtained.

Kinetics of O 2 dissociation and recombination

Shock tube investigations of relaxation processes in O2 and O2-Ar mixtures have been carried out for several years at our laboratory. Herewe briefly review the techniques utilized, the results obtained, and give more detailed comparisons with other work. Three experimental programs have been carried out employing the uv absorption technique to measure the O2 concentration. The first experiment dealt with the dissociation rate of O2 by Ar catalysts from 5000 to 18,000°K. It was found that there exists an incubation time (of the order of a vibrational relaxation time) before the O2 dissociates. When this data is used in conjunction with the subsequently obtained recombination data, a T−1/2 pre-exponential factor was clearly indicated for the rate constant expression which is kd=4.2(±14%)×10−8T−1/2 exp (−D/RT) cm6/particle2·sec. The second experiment measured the recombination rate of O atoms by Ar catalysts between 1500 and 3000°K. The excess O atoms were produced by shocking dilute O3-Ar mixtures. At 2000°K the measured recombination coefficient kr is about 0.7×10−34 cm6/particle2·sec. This study, with the previously mentioned dissociation work, showed that Keq=kd/kr. The third investigation involved the details of the shock front structure in O2 from Mach 4–21. Between Mach 4 and 10 translational and rotational processes occurred simultaneously within the shock front. Between Mach 10 and 14 vibrational relaxation moved up to and finally was swallowed by the “shock front.” Around Mach 16 the dissociation process started within the “shock front.” The shock tube kd results for Ar catalyst, when extrapolated to 300°K, compare well with recombination studies. But the high catalytic efficiencies for O and O2 found around 5000°K (Ar:O2:O≈1:18:50) are not apparent at 300°K. For O2 the extrapolated high temperature work is about 40 times that measured at 300°K, while that for O atoms is 100–800 times that reputed to be measured at 300°K, depending on how the extrapolation is made. Although there is some doubt as to whether the catalytic efficiency of the O atom has been correctly measured at 300°K, estimates of its value by reinterpretation of other work in the literature still fails to close this gap to better than a factor of 10. It is suggested that for the chemically-active catalyst O and O2 the temperature dependence changes markedly between 300°K and approximately 3000°K.

Some Evidence Justifying the Representation of Blasts as Traveling Gusts

BECAUSE OF THE COMPLEXITY of predicting the forces on an airfoil resulting from penetration of a traveling blast wave, a simple approximation tha t is frequently made is to represent the blast wave as a traveling gust, neglecting the associated overpressure. The resulting lift-growth function can be conceived of as resulting from two causes: growth of circulation and a noncirculatory part which, in incompressible flow at least, can be attributed to mass. The circulatory lift reaches a steady-state value whereas the apparent mass effect has a finite impulse. Those who have used the traveling gust representation of the blast wave have invariably been aware of the associated overpressure and the diffraction of the blast wave about the airfoil. The force associated with the undiffracted overpressure is proportional to the area of the airfoil subtended by the blast a t any instant, and thus yields a finite impulse which is proportional to the volume. For a thin airfoil this impulse is negligible. The force associated with the diffraction of the blast wave, on the other hand, is not negligible, and it is not clear how much of this diffraction is accounted for b}^ the traveling-gust representation. I t is the purpose of this note to show tha t when a blast wave of any magnitude is represented by a traveling gust and the impulse is computed using apparent-mass concepts, the result is equivalent to accounting for diffraction of the blast wave. Since the familiar lift-growth functions for a traveling gust include apparent-mass effects as well as the circulatory lift growth, these functions are applicable directly to the blast-penetration problem because the diffractive loading has already been taken into account. Criscione and Hobbs, using a simplified model based on shocktube investigations estimate the diffractive loading on a thin airfoil by a shock wave (see Fig. 1). Inasmuch as the airfoil is thin and stationary, forces both due to undiffracted overpressure and growth of circulation are absent. The model of Criscione and Hobbs has subsequently been partially justified theoretically by Ehlers and Shoemaker. > Assuming for simplicity tha t all shock and rarefaction waves travel at the speed of the incident wave (an assumption slightly different from tha t made in reference 1), the average pressure differential is

Investigation Into the Operating Cycle of a Two-Dimensional Supersonic Wind Tunnel

The operating cycle of an intermittent supersonic wind tunnel, while probably recognized, has never been denned rigorously, In this paper, the wind-tunnel operating cycle is divided into five distinct phases based on the character of the flow along the windtunnel axis. These phases are, in cyclic order, the nonstationary starting phase, the quasi-stationary starting phase, the stationary phase, the quasi-stationary closing phase, and the nonstationary closing phase. The first of the starting phases is treated both theoretically and experimentally for the case of the opening valve located downstream from the working section and experimentally for the case of the valve located upstream from the nozzle throat. These theoretical studies are based on an assumed wave pattern (subsequent to the breaking of the diaphragm) determined from results of shock-tube investigations and by application of an approximate theory for the solution of two-dimensional nonstationary flow problems due to Guderley. The quasi-stationary starting phase is studied theoretically by extending the stationary supersonic-subsonic jet theory in which the jet exit static pressure is less than the pressure in the region into which the jet is issuing. The theoretically postulated state of affairs during these phases is verified by obtaining experimentally a measured series of shadowgraphs and, thereby, determining the time history of the wind-tunnel starting process. The effect of condensation of water vapor on these phases is studied experimentally.