Showing papers on "Shock tube published in 1981"

Physical optics of the laser-schlieren shock tube technique.

Abstract: The physical optics of the laser-schlieren technique for the measurement of rate processes in shock waves is examined in detail. The method utilizes the Kirchhoff-Huygens integral with the usual thin lens, paraxial, and Fresnel approximations, all of which are appropriate for the typical laser schlieren experiment. The resolution and sensitivity of the technique are defined for all detector separations, and a reliable method for locating the time origin in the schlieren signal is provided. Diffraction is found to have a significant effect on the shock front generated signal, and geometrical optics treatments of this signal are shown to be inadequate.

On the transition between regular and Mach reflection in truly non-stationary flows

Abstract: Shock reflections over a convex and a concave wedge were investigated by using a 5 × 7 cm ordinary pressure-driven shock tube. Dry air was used for both the driving and driven gases. The large difference between the transition from regular (RR) to Mach reflection (MR) and that from MR to RR was observed, confirming the results obtained by Ben-Dor, Takayama & Kawauchi (1980). These results contradict all of the previous theoretical transition criteria. A new theory on the transition between RR and MR was developed by applying Whitham's ‘ray shock’ theory. This new theory agrees quite well with the experimental results.

Shock tube measurements of the reaction behaviour of acetylene with O‐atoms

Abstract: By the direct measurement of the time dependent concentrations of H- and O-atoms in the reaction zone behind shock waves using the ARAS-technique, the reaction behaviour of acetylene with O-atoms in the temperature range 1500 K ≤ T ≤ 2570 K at pressures of about 2 bars was investigated The gas mixtures used consisted of argon with small initial concentrations of N2O (10 ppm to 100 ppm) and C2H2 (10 ppm to 25 ppm) Under the present conditions the results indicate a dominant influence of the reaction channels Which can be directly followed by the experiment A good interpretation of the measurements is possible using the reaction rate coefficients Extensive computer simulations with different models of secondary reactions show the dominant influence of the two given reaction channels for the present conditions and yield a practically perfect fit of the experimental values

An Experimental and Analytical Investigation of High Temperature Ignition of Ethanol

Abstract: An experimental and analytical investigation of the ignition of ethanol-oxygen-argon mixtures behind reflected shock waves was carried out over the temperature range 1300-1700K at pressures of 1.0 and 2.0 atm and equivalence ratios of 0.5, 1.0 and 2.0. In the experimental investigation, ignition was identified by the emission of visible light in a shock tube. The time interval between the arrival of the incident shock at the end flange and the subsequent ignition indicated by the first visible light emission was reckoned as the ignition delay. The experimental ignition delay (t) data were found to correlate with initial ethanol and oxygen concentrations (mole/cc) and with initial temperature (K). A 56 step kinetic model for ethanol oxidation in the above temperature range was assembled using published rate coefficient data wherever available and estimated data for the reactions which have not been studied. Theoretical ignition delays were evaluated from the concentration profiles.

Shock tube studies of the acetylene and ethylene pyrolysis by UV absorption.

Abstract: Absorption measurements at 216 and 230 nm of shock heated C2H2, C2H2+H2, and C2H4 diluted in Ar were made over the temperature range 1800 K to 2600 K at half of atmospheric pressure. Absorptivities were evaluated for C4H2, which is the main product of C2H2 pyrolysis, as well as for C2H2 and C2H4. The relative value of C4H2 absorption was confirmed to decrease with the presence of H2 in accordance with a computer modeling result using a 25-reaction mechanism. The time profile of C4H2 absorption could be modeled well with k=1012.4 cm3 mol−1 s−1 for the reaction H2+C2H=C2H2+H.

Optical absorption of carbon and hydrocarbon species from shock heated acetylene and methane in the 135-220 nm wavelength range

Abstract: Absorption spectroscopy of carbon and hydrocarbon species has been performed in a shock tube at an incident shock condition for a wavelength range of 135-220 nm, in order to obtain information needed for calculating radiation blockage ahead of a planetary probe. Instrumentation consisted of high frequency response pressure transducers, thin-film heat transfer gages, or photomultipliers coupled by light pipes. Two test-gas mixtures, one with acetylene and the other with methane, both diluted with argon, were used to provide a reliable variation of C3 and C2H concentration ratio. Comparison of tests results of the two mixtures, in the temperature range of 3750 + or - 100 K, showed the main absorbing species to be C3. The wavelength for maximum absorption agrees well with the theoretical values of 7.68 eV and 8.03 eV for the vertical excitation energy, and a value of 0.90 for the electronic oscillator strength, obtained from the measured absorption band, is also in good agreement with the predicted value of 0.92.

The propagation of cylindrical detonations in monodisperse sprays

Abstract: A detailed experimental study has been made of the propagation and structure of cylindrical detonations propagating through monodisperse droplet clouds of low and high vapor pressure fuels. The experiments were conducted in a 140 cm long pie-shaped shock tube using an explosive charge at the vertex as the initiator. The low vapor pressure systems considered were 400 μm droplets of decane, kerosene, and kerosene with 25% NPN in atmospheres of oxygen. A 400 μm heptane spray in oxygen was the high vapor pressure fuel. Both radius-time trajectories of the outward propagating waves and high speed Schlieren streak photographs of the reaction zone were obtained. Very different results were obtained for the low and high vapor pressure fuels. The low vapor pressure fuels had long reaction zones governed by the droplet breakup process and the detonations propagated at velocities appreciably below the theoretically computed CJ value. In the high vapor pressure case a gaseous detonation was observed to propagate through the fuel vapor already present followed by an extended region of droplet burning. A simple theory which explains the observed behavior of the two phase detonations in terms of the interaction between the outward propagation and the induction length has been developed. A new definition of induction length suitable to spray detonations has been introduced.

On a dusty-gas shock tube

Abstract: Analytical and numerical methods were used to investigate the flow in­duced by a shock wave in a shock-tube channel containing air laden with suspended small solid particles. Exact results are given for the frozen and equilibrium shock-wave properties as a function of diaphragm-pressure ratio and shock-wave Mach numbers. The driver contained air at high pressure. A modified random-choice method together with an operator-splitting technique show clearly both the decay of a discontinuous frozen shock wave and a contact discontinuity, and the formation of a stationary shock structure and an effective contact front of finite thickness. The effects of particle diameter, particle-number density and diaphragm- pressure ratio on the transitional behaviour of the flow are investigated in detail. The alteration of the flow properties owing to the presence of particles is discussed thoroughly and compared with classical shock-tube flows.

Shock Initiated Ignition in Heptane-Oxygen-Argon Mixtures,

Abstract: : The ignition of heptane in mixtures containing oxygen and argon was studied behind reflected shock waves in 56 mm i.d. and 26 mm i.d. single-pulse shock tubes. The measurements covered the temperature range 1100-1700 K and reflected shock pressures varying from 2 to 12 atmospheres.

Experiments on the stability of converging cylindrical shock waves

Abstract: S of converging cylindrical shock waves is investigated experimentally in an annular shock tube. Initially symmetric shocks are found to retain their symmetry during implosion except at small radius where a breakdown in shock front curvature eventually occurs. Artificially generated shock front perturbations are observed to promote this type of instability. In all cases, instability is manifest by the appearance of vortex pairs during the expansive shock motion which follows the implosion.

Resonance absorption measurements of atom concentrations in reacting gas mixtures. VI. Shapes of the vacuum ultraviolet oxygen (3S–3P) resonance triplet from microwave sources and empirical calibration in a shock tube

Abstract: Spectral line profiles of the atomic oxygen triplet 3S1–3P2,1,0 emitted from microwave discharges in O2–He mixtures containing 0.1% O2 were measured in the 43rd order using a 2 m vacuum Czerny‐Turner scanning spectrometer under three different lamp conditions. The profiles varied from nearly Gaussian to highly self‐absorbed. The line shapes were accounted for by the amount of light absorbed by ground state oxygen atoms within the lamp. Experiments were carried out to calibrate the microwave discharge lamp for analysis of oxygen atoms using oxygen resonance triplet absorption. Known concentration of O atoms were produced in the temperature range 1950–2600 K in a shock tube by the complete dissociation of dilute (1–20 ppm) mixtures of N2O in Ar and related to the fraction of light absorbed. It was found that the ratios of the intensities of the oxygen components, measured using a low‐resolution monochromator, can be used to obtain resonably accurate calibration curves. Kinetic data obtained on N2O dissociat...

Breakdown experiments in air and nitrogen above 1500K

Abstract: The breakdown voltage in nitrogen and air was studied at temperatures between 1400 and 4500K at a pressure of a few atmospheres in a shock tube. The measurements were carried out in the physically clean situation behind the reflected shock wave. Below a certain temperature the character of breakdown is dielectric; above this limit the character changes into a thermal one (ohmic heating). The transition occurs in nitrogen at about 2500K and in air at 3100K. The influence of gas relaxation on breakdown was also considered.

Cross-Sectional Concentration of Particles during Shock Process Propagating through a Gas-Particle Mixture in a Shock Tube,

Abstract: : Shock wave-forms in a gas-particle mixture are discussed according to the following investigations A shock tube of 70 mm internal diameter is used for the experiments by dispersing glass spheres into atmosphere air in the driven section Pressure, particle velocity and particle number are measured Mach number of the shock wave is less than 15, mass flow ratio of the phases ranges from 01 to 05, and diameters of the particles are 30 and 50 microns

Real gas and wall roughness effects on the bifurcation of the shock reflected from the end wall of a tube

Abstract: : Several aspects of the interaction between the boundary layer on the shock tube side wall and the shock reflected from the end wall were investigated experimentally in the free piston shock tube T3. The experiments were conduted at sufficiently high shock speed to produce vibrational excitation and dissociation in the case of nitrogen and carbon dioxide, while the experiments in argon were restricted to the ideal gas range.

Shock Wave Ignition of Pulverized Coal.

Abstract: : Heating and ignition delay times behind an incident shock of small clouds of coal dust-particles of three different size ranges were measured in a shock tube. Four different compositions of O2-N2 mixtures were used as the oxidizer and the incident shock Mach number varied from 4.1 to 4.8. Observed delay times were in the range 10 to 150 microsec, decreasing with increasing Mach number and O2 concentration. The theoretical analysis is based on the numerical solution of the one-dimensional transient heat equation for a single spherical particle. Particle acceleration has an important effect on heat transfer and is considered in the model. Heating due to gas phase and heterogeneous reactions is represented with appropriate volumetric or surface source terms. The results show that volatilization is insufficient to account for the short delay times observed. The main mechanism for ignition in these short times appears to be heterogeneous surface reaction both on the outside and within the porous particle.

A Single Pulse Shock Tube Study of Soot Formation from Benzene Pyrolysis

Abstract: : Vapor phase benzene diluted in argon has been pyrolyzed in a single pulse shock tube fitted with an in-line, fullport ball valve to facilitate post shock gas sampling. The disappearance of benzene has been shown to be second order with respect to benzene concentration. Product analyses have shown the major products to be acetylene and styrene with trace amounts of diacetylene, methane, vinylacetylene and toluene also detected. Gravimetric analysis of the solid residue has been achieved using a removable liner in the end section of the shock tube. Recovery of solid and gas species was found to be 85-95% of the original reactant mass with increasing temperature until c. 1900 K at which time the fractional soot yields remain constant at 0.8 to 2700 K.

Experiments in Shock Liquefaction

Abstract: : The liquefaction shock wave, a compression shock which converts vapour into liquid, has been experimentally produced as the reflected shock at the closed end of a shock tube in test fluids with many molecular degrees of freedom Measurements of pressure, temperature, index of refraction and shock velocity confirm the existence of the shock and its general conformity to classical Rankine-Hugoniot conditions Normal and stereoscopic photography confirms the existence of a clear liquid phase and reveals small two-phase torus-form vortex rings, which are formed in or near the shockfront and move in the same direction as, but less rapidly than the shock wave, ie, the rings move away from the closed end of the shock tube If the fluid behind the shock is a mixture of droplets and vapour (partial liquefaction) evidence of shock splitting at the phase boundary is found

Pseudo-stationary oblique shock-wave reflection in carbon dioxide: Domains and boundaries

Abstract: : Transition boundaries for a triatomic gas between various types of shock-wave reflection configurations--regular reflection (RR), single Mach reflection (SMR), complex Mach reflection (CMR), and double Mach reflection (DMR)--exhibited by a plane shock impinging on a compressive corner in a shock tube were studied experimentally using CO2 as the test gas. The shock Mach numbers ranged from 1.8 to 10.2, and the initial pressure from 3.5 to 80 torr at room temperature. The regions corresponding to the four types of reflection were obtained in terms of the shock Mach number, Ms, and the wedge angle, theta sub w. The resultant transition map in the Ms - theta sub w plane will be of value to future researchers in the field as an aid in designing their experiments. Excellent agreement was obtained between the transition boundary from CMR to DMR according to the criterion proposed by Ben-Dor & Glass that the flow downstream of the reflected shock should be sonic relative to the kink of the reflected shock for the transition to occur and the results of the present experiments. This fact will further confirm the validity of their criterion. The density contours of the flow fields were obtained by means of infinite-fringe interferograms, some of which, particularly those of CMR and DMR, were hitherto unavailable in the literature. They may prove valuable for comparison with the results of numerical simulation of the phenomena, which is currently the subject of intense research at various institutions.

Weak and strong ignition. II. Sensitivity of the hydrogen-oxygen system. Memorandum report

Abstract: This paper identifies the physical and chemical mechanisms which cause certain mixtures of hydrogen, oxygen and argon to be very sensitive to sound wave or entropy (temperature) perturbations. Detailed simulations of the effects of sound waves in typical mixtures are used to show explicity the effects of such fluctuations on the chemical induction time. A quantity delta TAU max, is defined which represents the maximum variation produced in the chemical induction time of a system given the amplitude and frequency of a perturbation. It is observed that these perturbations may cause ignition to occur unevenly in such mixtures and this leads to ignition which appears spotty. Using detailed numerical simulations and a generalized induction parameter model derived from it, delta TAU max is evaluated and a criterion is developed for spotty and smooth ignition behind reflected shock waves. These effects are related to weak and strong ignition observed in shock tube experiments.

Hot‐electron production and resonance absorption of laser light in the shock‐front targets

Abstract: Laser targets, created by fast optical breakdown of shock fronts produced in an electrothermal shock tube filled with D2 or H2, have been used for studies of resonance absorption of CO2 laser light In these targets, the critical‐surface density‐gradient vector always points along the shock‐tube axis, leading to reproducible hot‐electron emission by plasma wave breaking The angular distribution and energy spectrum of this emission have been studied For 200 psec risetime laser pulses, the measured scaling of hot‐electron temperature with laser pulse energy agrees with predictions of models of profile steepening by the ponderomotive force of the laser light For 20 psec risetime laser pulses, the results are consistent with a model in which the detected electron emission is dominated by a ’’chirped’’ monoenergetic burst lasting only 1 to 2 psec In this model, the electron energy spectrum is due to the integrated temporal variation of the enery of the burst, and the hot‐electron temperature is practically independent of laser pulse energy

Influence of Surface Roughness on the Shock Transition in Quasi-Stationary and Truly Non-Stationary Flows,

Abstract: : The effect of surface roughness on the shock transition over flat wedges in quasi-stationary flows is examined experimentally. For flat wedges saw-toothed roughnesses a mesh No. 40 sand paper and a mesh No. 320 sand paper were used. Experiments were conducted on the Institute of High Speed Mechanics 40 mm x 80 mm shock tube equipped with a double exposure holographic interferometer. The incident shock Mach number range was 1.04 to 4.0 for nitrogen or dry air. In the case of curved wedges, the shock transition angles were determined by means of streak camera technique with curved slits.