Showing papers on "Shock tube published in 2003"

Interpreting shock tube ignition data

Abstract: Chemical kinetic modelers make extensive use of shock tube ignition data in the development and validation of combustion reaction mechanisms. These data come from measurements using a range of diagnostics and a variety of shock tubes, fuels, and initial conditions. With the wide selection of data available, it is useful to realize that not all of the data are of the same type or quality, nor are all the data suitable for simple, direct comparison with the predictions of reaction mechanisms. We present here a discussion of some guidelines for the comparison of shock tube ignition time data with reaction mechanism modeling. Areas discussed include definitions of ignition time; ignition time correlations (with examples taken from recent n-heptane and isooctane measurements); shock tube constant-volume behavior; shock tube diameter and boundary layer effects; carrier gas and impurity effects; and future needs and challenges in shock tube research. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36:510–523, 2004

Shock waves at microscales

Abstract: We present a model for the effects of scale, via molecular diffusion phenomena, on the generation and propagation of shock waves. A simple parametrization of the shear stresses and heat flux at the wall leads to the determination of new jump conditions, which show that, for a given wave Mach number at small scales, the resulting particle velocities are lower but the pressures are higher. Also, the model predicts that the flow at small scale is isothermal and that the minimum wave velocity can be subsonic. Experiments with a miniature shock tube using low pressures to simulate the effects of small scale have shown qualitative agreement with the proposed model. In fact, the effects of scale appear even more important than what has been incorporated in the model.

Interaction of a shock with a sphere suspended in a vertical shock tube

Abstract: Shock wave interaction with a sphere is one of the benchmark tests in shock dynamics. However, unlike wind tunnel experiments, unsteady drag force on a sphere installed in a shock tube have not been measured quantitatively. This paper presents an experimental and numerical study of the unsteady drag force acting on a 80 mm diameter sphere which was vertically suspended in a 300 mm x 300 mm vertical shock tube and loaded with a planar shock wave of M s = 1.22 in air. The drag force history on the sphere was measured by an accelerometer installed in it. Accelerometer output signals were subjected to deconvolution data processing, producing a drag history comparable to that obtained by solving numerically the Navier-Stokes equations. A good agreement was obtained between the measured and computed drag force histories. In order to interpret the interaction of shock wave over the sphere, high speed video recordings and double exposure holographic interferometric observations were also conducted. It was found that the maximum drag force appeared not at the time instant when the shock arrived at the equator of the sphere, but at some earlier time before the transition of the reflected shock wave from regular to Mach reflection took place. A negative value of the drag force was observed, even though for a very short duration of time, when the Mach stem of the transmitted shock wave relfected and focused at the rear stagnation point of the sphere.

Shock tube measurements of branched alkane ignition times and OH concentration time histories

Abstract: Ignition times and hydroxyl (OH) radical concentration time histories were measured behind reflected shock waves during the oxidation of three branched alkanes: iso-butane (2-methylpropane), iso-pentane (2-methylbutane), and iso-octane (2,2,4-trimethylpentane). Initial reflected shock conditions ranged from 1177 to 2009 K and 1.10 to 12.58 atm with dilute fuel/O2/Ar mixtures varying in fuel concentration from 100 ppm to 1.25% and in equivalence ratio from 0.25 to 2. Ignition times were measured using endwall CH emission and OH concentrations were measured using narrow-linewidth ring-dye laser absorption of the R1(5) line of the OH A-X (0,0) band at 306.7 nm. The ignition times and OH concentration time histories were compared to modeled predictions of seven branched alkane oxidation mechanisms currently available in the literature and the implications of these comparisons are discussed. These data provide a unique database for the validation of detailed hydrocarbon oxidation mechanisms of propulsion related fuels. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 67–78 2004

Nano-particle sizing by laser-induced-incandescence (LII) in a shock wave reactor

Abstract: . Laser-Induced Incandescence (LII) is a relatively new optical diagnostic for particle sizing which is currently used in combustion science. Its advantage against light extinction and light scattering methods is the possibility of getting size information with high time and space resolution even for nano-particles. LII is mostly applied to particle formation or particle removal in reactive stationary flows, but it can also be used in shock-induced reactive flows. This is demonstrated in three examples: soot particle formation during high temperature pyrolysis of benzene, iron particle formation from iron pentacarbonyl, and formation of carbon-coated iron particles. From the principles of LII, it is not possible to obtain a complete particle growth curve from one individual shock tube experiment. Therefore, the kinetics of particle growth evolution must be determined from several “identical” shock tube experiments with a delayed triggering of the heat-up laser. The principles of LII, the in-situ measurement of particle size, and the comparison to beam-collected particles, which were visualized by a high resolution transmission electron microscope (HRTEM), are demonstrated. It was found that the energy accommodation coefficient during the particle cooling is $\alpha = 1$ for a soot surface but is significantly lower e.g. for an iron surface.

The Interaction of Supernova Remnants with Interstellar Clouds: Experiments on the Nova Laser

Abstract: The interaction of strong shock waves, such as those generated by the explosion of supernovae with interstellar clouds, is a problem of fundamental importance in understanding the evolution and the dynamics of the interstellar medium (ISM) as it is disrupted by shock waves. Here we present the results of a series of scaled Nova laser high energy density experiments investigating the evolution of a high-density sphere embedded in a low-density medium after the passage of a strong shock wave, thereby emulating the supernova shock-cloud interaction. The Nova laser was utilized to generate a strong (~Mach 10) shock wave that traveled along a miniature beryllium shock tube, 750 μm in diameter, filled with a low-density plastic emulating the ISM. Embedded in the plastic was a copper microsphere (100 μm in diameter), emulating the interstellar cloud. The morphology and evolution as well as the shock wave trajectory were diagnosed via side-on X-ray radiography. We describe here experimental X-ray radiographic results of this interaction out to several cloud crushing times and compare them to detailed two- and three-dimensional radiation hydrodynamic simulations using both arbitrary Lagrangian and Eulerian hydrodynamics (ALE), as well as high-resolution adaptive mesh refinement (AMR) hydrodynamics. A key result is the first experimental evidence that the cloud is destroyed by a three-dimensional nonlinear bending-mode instability (Widnall instability), confirming earlier predictions with high-resolution three-dimensional calculations.

Distortion of a spherical gaseous interface accelerated by a plane shock wave.

Abstract: The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces.

Onset of turbulence in accelerated high-Reynolds-number flow.

Abstract: A new criterion, flow drive time, is identified here as a necessary condition for transition to turbulence in accelerated, unsteady flows. Compressible, high-Reynolds-number flows initiated, for example, in shock tubes, supersonic wind tunnels with practical limitations on dimensions or reservoir capacity, and high energy density pulsed laser target vaporization experimental facilities may not provide flow duration adequate for turbulence development. In addition, for critical periods of the overall flow development, the driving background flow is often unsteady in the experiments as well as in the physical flow situations they are designed to mimic. In these situations transition to fully developed turbulence may not be realized despite achievement of flow Reynolds numbers associated with or exceeding stationary flow transitional criteria. Basically our transitional criterion and prediction procedure extends to accelerated, unsteady background flow situations the remarkably universal mixing transition criterion proposed by Dimotakis [P. E. Dimotakis, J. Fluid Mech. 409, 69 (2000)] for stationary flows. This provides a basis for the requisite space and time scaling. The emphasis here is placed on variable density flow instabilities initiated by constant acceleration Rayleigh-Taylor instability (RTI) or impulsive (shock) acceleration Richtmyer-Meshkov instability (RMI) or combinations of both. The significant influences of compressibility on these developing transitional flows are discussed with their implications on the procedural model development. A fresh perspective for predictive modeling and design of experiments for the instability growth and turbulent mixing transitional interval is provided using an analogy between the well-established buoyancy-drag model with applications of a hierarchy of single point turbulent transport closure models. Experimental comparisons with the procedural results are presented where use is made of three distinctly different types of acceleration driven instability experiments: (1) classical, relatively low speed, constant acceleration RTI experiments; (2) shock tube, shockwave driven RMI flow mixing experiments; (3) laser target vaporization RTI and RMI mixing experiments driven at very high energy density. These last named experiments are of special interest as they provide scaleable flow conditions simulating those of astrophysical magnitude such as shock-driven hydrodynamic mixing in supernova evolution research.

Spontaneous Ignition of Methane–Air Mixtures in a Wide Range of Pressures

Abstract: The ignition delay in methane–air mixtures ϕ = 0.5) within the range of temperatures of 1200–1700 K and pressures of 3–450 atm behind reflected shock waves in a shock tube is measured on the basis of emission of the electron‐excited OH radical (transition A2Σ+ – X2Π) at the wavelength of 306.4 nm and on the basis of absorption corresponding to the component F1 (2) (ν3 = 1) ← F2 (2) (ν3 = 0) of the P(7) line of the ν3 mode of the CH4 molecule at the wavelength of 3.3922 μm. The measured ignition delays are compared with those calculated by the GRI‐Mech 3.0 mechanism; good qualitative agreement of results is obtained in a wide range of pressures.

Flowfield Around Spike-Tipped Bodies for High Attack Angles at Mach 4.5

Abstract: The requirements for the design of a new short-range high-velocity missile are both the drag reduction and the correct information acquisition for the optoelectronic sensors embedded in the hemispherical nose. High anglesof attack must be studied to fulfill the maneuverability requirements of present and future missiles. A supersonic missile generates a bow shock around its blunt nose, which causes rather high surface pressure and temperature and, as a result, the development of high drag and damage of embedded sensors. The pressure and the temperature on the hemispherical nose surface can be substantially reduced if an oblique shock is generated by a forward-facing spike. Both the experiments and the computations are carried out to study the flowfield around three-dimensional blunt bodies equipped with forward-facing spikes for a large range of attack angles at a Mach number of 4.5. A blunt body, a classical disk-tip spike, a sphere-tip spike, and a biconical-tip spike are studied. The experiments involve high-pressure shock tunnel investigations using a shock tube facility. The differential interferometry technique is applied to visualize the flowfield around the different missile spike geometries. The differential interferogram pictures as well as surface pressure measurements are compared with numerical results. Numerical simulations based on steady-state three-dimensional Navier-Stokes computations are performed to predict the drag, the lift, and the pitching moment for the blunt body and for each spike-tipped missile. The computations allow one to bring out the advantages of each spike geometry in comparison to the blunt body.

Experimental and modeling study of shock‐tube oxidation of acetylene

Abstract: Nine mixtures of acetylene and oxygen diluted in argon were studied behind reflected shock waves at temperatures of 1150–2132 K and pressures of 0.9–1.9 atm. Initial compositions were varied from very fuel-lean to moderately fuel-rich, covering equivalence ratios of 0.0625–1.66. Two more mixtures with added ethylene were used to boost the sensitivity to reactions of vinyl oxidation. The progress of reaction was monitored by laser absorption of CO molecules. The collected experimental data were subjected to extensive detailed chemical kinetics analysis. The initial kinetic model was assembled based on recent literature data and then optimized using the solution mapping technique. The analysis was extended to include recent experimental observations of Hidaka and co-workers (Combust Flame 1996, 107, 401). The derived model reproduces closely both sets of experimental data, the result obtained by modifying nine rate coefficients and three enthalpies of formation of intermediate species. The identified parameter tradeoffs and justification for the changes are discussed. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 391–414, 2003

Burning Velocities of Real Gasoline Fuel at 353 K and 500 K

Abstract: Burning velocities for unleaded conventional gasoline (CR-87) and air mixtures were determined experimentally over an extensive range of equivalence ratios at 353 K and 500 K and at atmospheric pressure. Nitrogen dilution effects on the laminar flame speed were also studied for selected equivalence ratios at these same conditions. Experimental measurements employed the stagnation jet-wall flame configuration and Particle Image Velocimetry (PIV). The laminar burning velocity was obtained using linear extrapolation of stretched flame data to zero stretch rate. The measured flame speeds were compared with numerical predictions using a minimized detailed kinetic model for primary reference fuel (PRF) mixtures, which was developed based on stirred reactor, shock tube and flow reactor data.

A one-dimensional meshfree particle formulation for simulating shock waves

Abstract: In this paper, a one-dimensional meshfree particle formulation is proposed for simulating shock waves, which are associated with discontinuous phenomena. This new formulation is based on Taylor series expansion in the piecewise continuous regions on both sides of a discontinuity. The new formulation inherits the meshfree Lagrangian and particle nature of SPH, and is a natural extension and improvement on the traditional SPH method and the recently proposed corrective smoothed particle method (CSPM). The formulation is consistent even in the discontinuous regions. The resultant kernel and particle approximations consist of a primary part similar to that in CSPM, and a corrective part derived from the discontinuity. A numerical study is carried out to examine the performance of the formulation. The results show that the new formulation not only remedies the boundary deficiency problem but also simulates the discontinuity well. The formulation is applied to simulate the shock tube problem and a 1-D TNT slab detonation. It is found that the proposed formulation captures the shock wave at comparatively lower particle resolution. These preliminary numerical tests suggest that the new meshfree particle formulation is attractive in simulating hydrodynamic problems with discontinuities such as shocks waves.

Interferometric CT measurement of three-dimensional flow phenomena on shock waves and vortices discharged from open ends

Abstract: Three-dimensional flow phenomena are observed in a shock tube experiment for shock waves and vortices discharged from a square open end and a pair of circular open ends by using an interferometric CT (Computed Tomography) technique. To take multidirectional finite-fringe interferograms for a three-dimensional flow field, we introduce a rotating duct model in the test section of the shock tube. The experiments are performed for incident shock Mach numbers 1.50 and 1.30 in nitrogen gas of 98 kPa initial pressure. Good quality CT images of the density distribution are obtained by carefully selecting the projection images for a reproducible flow within a prescribed accuracy. The three-dimensional flow features are clearly visualized for a vortex ring, a secondary shock wave, shock-vortex interaction, and shock-shock interaction through the pseudo-color image of density distribution and the isopycnic surface. Their meanings in gas dynamics and shock dynamics are discussed. We demonstrate that studies of various three-dimensional problems in shock dynamics are possible using the present CT technique.

Wave dynamic processes induced by a supersonic projectile discharging from a shock tube

Abstract: A numerical study on wave dynamic processes occurring in muzzle blast flows, which are created by a supersonic projectile released from the open-end of a shock tube into ambient air, is described in this paper. The Euler equations, assuming axisymmetric flows, are solved by using a dispersion-controlled scheme implemented with moving boundary conditions. Three test cases are simulated for examining friction effects on the muzzle flow. From numerical simulations, the wave dynamic processes, including two blast waves, two jet flows, the bow shock wave and their interactions in the muzzle blasts, are demonstrated and discussed in detail. The study shows that the major wave dynamic processes developing in the muzzle flow remain similar when the friction varies, but some wave processes, such as shock-shock interactions, shock-jet interactions and the contact surface instability, get more intensive, which result in more complex muzzle blast flows.

A shock tube, laser-schlieren study of the pyrolysis of isobutene: Relaxation, incubation, and dissociation rates

Abstract: Dissociation, vibrational relaxation, and unimolecular incubation have all been observed in shock waves in isobutene with the laser-schlieren technique. Experiments covered a wide range of high-temperature conditions: 900–2300 K, and post-incident shock pressures from 7 to 400 torr in 2, 5, and 10% mixtures with krypton. The surprising observation is that of vibrational relaxation, well resolved over the full temperature range. The resolved process is completely exponential, with relaxation times in the range 20–120 ns atm. Relaxation and dissociation are clearly separated for T > 1850 K, with estimated incubation times near 200 ns atm. Incubation is essential for modeling of the very low-pressure decomposition. Modeling of gradients with a chain mechanism initiated by CH fission produces an excellent fit and accurate dissociation rates that show severe falloff. A restricted-rotor, Gorin-model RRKM analysis fits these rates quite well with the known bond-energy as barrier and 〈ΔE〉down = 680 cm−1. The extrapolated k∞ is log k∞(s−1) = 19.187–0.865 log T −87.337 (kcal/mol)/RT, in good agreement with previous work. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 381–390, 2003

Shock-tube experiments on Richtmyer–Meshkov instability growth using an enlarged double-bump perturbation

Abstract: This article reports on the latest experiments in the series of Richtmyer–Meshkov instability (RMI) shock-tube experiments. Previous work described a double-bump experiment that evidenced some degree of unrepeatability. The present work features an enlarged perturbation introduced to improve repeatability. In common with the previous work, the experiments were conducted at shock Mach number 1.26 (70 kPa overpressure), using the Atomic Weapons Establishment 200 × 100 mm shock tube with a three-zone test cell arrangement of air/sulphur hexafluoride/air. The sulphur hexafluoride gas (SF6) was chosen for its high density (5.1 relative to air) providing an Atwood number of 0.67. Gas separation was by means of microfilm membranes, supported by fine wire meshes. A double-bump perturbation of two-dimensional geometry was superimposed on the downstream membrane representing a 0.6% addition to the dense gas volume. Visualization of the turbulent gas mixing was by laser sheet illumination of the seeded SF6 gas using a copper vapor laser pulsing at 12.5 kHz. Mie scattered light was recorded using a 35-mm rotating drum camera to capture a sequence of 50 images per experiment. Sample experimental results shown alongside corresponding three-dimensional hydrocode calculations highlight the problems in both analysis and comparison caused by multiple scattering arising from the necessary use of a high seeding concentration. Included is a demonstration of the effectiveness of introducing into the hydrocode a Monte Carlo-based simulation of the multiple scattering process. The results so derived yield greatly improved qualitative agreement with the experimental images. Quantitative analysis took the form of deriving relative intensity data from line-outs through experimental images and their code equivalents. A comparison revealed substantial agreement on major features.

Numerical Investigation of Transient Nozzle Flow

Abstract: . The starting process of two-dimensional and axisymmetric nozzle flows has been investigated numerically. Special attention has been paid to the early phase of the starting process and to the appearance of a strong secondary shock wave. For both cases, shock intensities and velocities are obtained and discussed. The flow evolution in the axisymmetric case is proved to be more complex and the transient starting process is slower than in the plane case. Finally, the effects of changing the nozzle angle and the incident shock wave Mach number on the transient flow are addressed. It is shown that a faster start-up can be induced either by decreasing the nozzle angle or increasing the Mach number of the incident shock wave.

Attenuation of shock waves by granular filters

Abstract: Attenuation of shock waves in granular filters has been studied. Both pressurized air and solid explosives have been used for generating shock waves in a shock tube. The shock tube had a total length of $\sim 22$ m, and an internal diameter of 355 mm. Two large scale experiments have also been carried out in a tunnel with a cross-sectional area of 6.5 m2. The results are compared with results found in the literature (Zloch, 1976; Medvedev et al., 1990; Britan et al., 2001) and previous experiments in a smaller scale by Slungaard (2002). A simple correlation based on the work of Zloch (1976) for shock wave attenuation in tube bundles and an extensive amount of experiments, is proposed. The correlation $p_{2}/p_{1}=1/(1+\theta /B)$ can be used to estimate the attenuation of the shock wave through a granular filter with filter characteristic $\theta$ . Setting B=6 will give a conservative estimate of the attenuation, while setting B=3 will give the best fit to all the results from this study and the results found in the literature. The correlation is independent of the type of driver (pressurised air or solid explosives) and upstream shock strength.

Detonation driver for enhancing shock tube performance

Abstract: The development of a shock-induced detonation driver for enhancing the performance of a shock tube is described. The detonation wave is induced by the expansion of helium or air. Various gaseous fuel-oxidizer combinations are examined. This method produces a detonation wave which propagates downstream that transitions into a shock wave in the driven section. High-enthalpy flows with a maximum total temperature of 4200 K and a maximum total pressure of 34 atm in the driven tube are achieved. The problems of achieving the so-called perfectly-driven mode as well as those of inadequate fuel-oxidizer mixing are discussed.

Experimental Investigation of Two Cylindrical Water Columns Subjected to Planar Shock Wave Loading

Abstract: Two water columns with identical initial diameters of 4.8 mm were placed 30 mm apart inside a shock tube test section and were loaded by a shock wave of Mach number 1.47 in atmospheric air. The Weber and Reynolds numbers corresponding to these flow conditions are 6900 and 112.000, respectively. Double-exposure holographic interferometry was used to visualize the shock/water columns interaction. The process of the water columns deformation, displacement, and acceleration was well visualized and hence the drag coefficient of shock loaded water columns was evaluated. The front water column behaved virtually the same as a single water column under the same flow conditions. However, the displacement and acceleration of the rear water column was less significant than that of the front one

RFNC-VNIITF multifunctional shock tube for investigating the evolution of instabilities in nonstationary gas dynamic flows

Abstract: The design, operation, and functionality of the multifunctional shock tube (MST) facility at the Russian Federal Nuclear Center-VNIITF are described When complete, the versatile MST consists of three different driver sections that permit the execution of three different classes of experiments on the compressible turbulent mixing of gases induced by the (1)Richtmyer-Meshkov instability (generated by a stationary shock wave with shock Mach numbers <5), (2) Rayleigh-Taylor instability (generated by compression wave such that acceleration of the interface is <10 5 g 0 , where go = 98 m/s 2 ), and (3) combined Richtmyer-Meshkov and Rayleigh-Taylor instability (generated by a nonstationary shock wave with initial pressure at the front 5 X 10 6 Pa and acceleration of ≤10 6 go of the interface) For each of these types of experiments, the density ratio of the gases is ρ 2 /ρ 1 ≤ 34 Perturbations are imposed on a thin membrane, embedded in a thin wire array of microconductors that is destroyed by an electric current In addition, various limitations of experimental techniques used in the study of interfacial instability generated turbulent mixing are also briefly discussed

A shock tube study of the reaction NH2 + CH4 → NH3 + CH3 and comparison with transition state theory

Abstract: The rate coefficient for NH2 + CH4 NH3 + CH3 (R1) has been measured in a shock tube in the temperature range 1591–2084 K using FM spectroscopy to monitor NH2 radicals. The measurements are combined with a calculation of the potential energy surface and canonical transition state theory with WKB tunneling to obtain an expression for k1 = 1.47 × 103T3.01e−5001/T(K) cm3 mol−1 s−1 that describes available data in the temperature range 300 –2100 K. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 304–309, 2003

Electron Density Measurements Behind Strong Shock Waves by H-ß Profile Matching

Abstract: Emission spectroscopy was performed in a free-piston double-diaphragm shock tube to measure the electron density behind a strong shock wave, using nitrogen as the test gas. Time-frozen spectra from behind the shock wave were taken by an image-intensie ed charge-coupled device camera with a gate width of 100 ns. The laser schlieren diagnostics was used to detect the shock arrival and to correlate observed spectra with the distance from theshock front accurately. Theelectron density wasmeasured by meansof a lineproe le matching technique, using the H-¯ line broadened by the Stark effect. Using this measurement system, the electron density distribution was obtained with a high spatial resolution of §0:6 mm at a shock velocity of 12 km/s. Experimental results show that, especially at high shock velocities, the measured electron density increases more quickly behind the shock front than predicted by the thermal and chemical nonequilibrium models widely used. Several drawbacks in the conventional ionization model at high shock velocities are pointed out.

Driver gas contaminationin a detonation-driven reflected-shock tunnel

Abstract: A computational study has been carried out to examine the effects of driver gas contamination in the NASA HYPULSE facility at GASL when operating with a detonation driver in reflected-shock tunnel mode. Unlike high-enthalpy shock tunnels which use helium as a driver gas, the driver gas in a detonation driver consists of a mixture of water vapour and argon, which has very different chemical and thermodynamic properties than those of helium. The purpose of the present work is to quantitatively evaluate the effects of driver gas contamination on the flow properties in the test section. Two computational analyses have been performed. The first analysis examined the nozzle flow under the influence of a prescribed level of driver gas contamination. In the second analysis, the transient development of the driver gas leakage in the reflected-shock region in the shock tube is studied. The unique flow features brought about by the detonation-driver gas and the method for detecting the contamination are discussed.

Shock train and pseudo-shock phenomena in supersonic internal flows

Abstract: When a normal shock wave interacts with a boundary layer along a wall surface in supersonic internal flows and the shock is strong enough to separate the boundary layer, the shock is bifurcated and a series of shocks called “shock train” is formed. The flow is decelerated from supersonic to subsonic through the whole interaction region that is referred to as “pseudo-shock”. In the present paper some characteristics of the shock train and pseudo-shock and some examples of the pseudo-shocks in some flow devices are described.

Chemical Kinetic Study of Toluene Oxidation Under Premixed and Nonpremixed Conditions

Abstract: A study was performed to elucidate the chemical-kinetic mechanism of combustion of toluene. A detailed chemical-kinetic mechanism for toluene was improved by adding a more accurate description of the phenyl + O{sub 2} reaction channels, toluene decomposition reactions and the benzyl + O reaction. Results of the chemical kinetic mechanism are compared with experimental data obtained from premixed and non-premixed systems. Under premixed conditions, predicted ignition delay times are compared with new experimental data obtained in shock tube. Also, calculated species concentration histories are compared to experimental flow reactor data from the literature. Under non-premixed conditions, critical conditions of extinction and autoignition were measured in strained laminar flows in the counterflow configuration. Numerical calculations are performed using the chemical-kinetic mechanism at conditions corresponding to those in the experiments. Critical conditions of extinction and autoignition are predicted and compared with the experimental data. Comparisons between the model predictions and experimental results of ignition delay times in shock tube, and extinction and autoignition in non-premixed systems show that the chemical-kinetic mechanism predicts that toluene/air is overall less reactive than observed in the experiments. For both premixed and non-premixed systems, sensitivity analysis was used to identify the reaction rate constants that control themore » overall rate of oxidation in each of the systems considered. Under shock tube conditions, the reactions that influence ignition delay time are H + O{sub 2} chain branching, the toluene decomposition reaction to give an H atom, and the toluene + H abstraction reaction. The reactions that influence autoignition in non-premixed systems involve the benzyl + HO{sub 2} reaction and the phenyl + O{sub 2} reaction.« less