Showing papers on "Shock tube published in 1979"

Transition to Mach reflexion of shock waves in steady and pseudosteady flow with and without relaxation

Abstract: Experiments were conducted in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon to measure the transition condition in pseudosteady and steady flow. The transition condition in the steady flow, in which the wall was eliminated by symmetry, agrees with the calculated von Neumann condition. In the real gases this calculation assumed thermo-dynamic equilibrium after the reflected shock. In the pseudosteady flow of reflexion from a wedge the measured transition angle lies on the Mach-reflexion side of the calculated detachment condition by an amount which may be explained in terms of the displacement effect of the boundary layer on the wedge surface. A single criterion based on the availability of a length scale at the reflexion point explains the difference between the pseudosteady and steady flow transition condition and predicts a hysteresis effect in the transition angle when the shock angle is varied during steady flow. No significant effects on the transition condition due to finite relaxation length could be detected. However, new experiments in which interesting relaxation effects should be evident are suggested.

Comprehensive Mechanism for Methanol Oxidation

Abstract: A detailed chemical kinetic mechanism, involving 26 chemical species and 84 elementary reactions, is proposed for the oxidation of methanol Within the scope of this mechanism, turbulent flow reactor and shock tube experimental data are used to determine rate expressions for several of the important reactions involving CH3OH and its intermediate product species, CH2OH Calculations using the proposed mechanism and elementary reaction rates accurately reproduce experimental results over a combined temperature range of 1000-2180K, for fuel-air equivalence ratios between 005 and 30 and for pressures between 1 and 5 atmospheres The resulting chemical kinetic model is then employed, together with an unsteady, one-dimensional numerical model for flame propagation, to predict the laminar flame speed of a stoichiometric methanol-air mixture The calculated laminar flame speed is 44+ 2 cm/ sec and is in good agreement with experimentally observed values

An Analytical Study of the Shock Tube Ignition of Mixtures of Methane and Ethane

Abstract: An analytical study of the ignition of mixtures of methane and ethane has been carried out, using a numerical model consisting of 25 chemical species and 75 elementary chemical reactions. Computed ignition delay times and effective activation energies are compared with published experimental shock tube results. Initial conditions studied include temperatures from 1300-1900 K. and mixtures ranging from pure methane to pure ethane, with stoichiometric amounts of oxygen, diluted in argon. The analytical model reproduces experimental results for the effective activation energy and ignition delay time for both pure methane and pure ethane. Ignition delay times computed for mixtures of methane and ethane are found to lie between the pure methane and the pure ethane results. Addition of relatively small quantities of ethane to methane rapidly reduces the ignition delay time of the methane-ethane mixtures to values close to those of pure ethane. The chemical kinetic factors involved in the ignition of fu...

Domains and boundaries of non-stationary oblique shock-wave reflexions. 2. Monatomic gas

Abstract: Interferometric data were obtained in the 10 cm × 18 cm hypervelocity shock tube of oblique shock-wave reflexions in argon at initial temperatures and pressures of nearly 300 °K and 15 Torr. The shock Mach-number range covered was 2 [les ] Ms [les ] 8 over a series of wedge angles 2° [les ] θw [les ] 60°. Dual-wavelength laser interferograms were obtained by using a 23 cm diameter field of view Mach-Zehnder interferometer. In addition to our numerous results, the available data for argon and helium obtained over the last two decades were also utilized. It is shown analytically and experimentally that in non-stationary flows six domains exist in the (Ms, θw) plane where regular reflexion (RR), single-Mach reflexion (SMR), complex-Mach reflexion (CMR) and double-Mach reflexion (DMR) can occur. The transition boundaries between these regions were all established analytically. The experimental results from different sources substantiate the present analysis, and areas of disagreement which existed in the literature are now clarified and resolved. It is shown that real-gas effects have a significant influence on the size of the regions and their boundaries. In addition, isopycnics (constant density lines) are given for the four types of reflexion, as well as the density distribution along the wedge surface. This data should provide a solid base for computational fluid dynamicists in comparing numerical techniques with actual experimental results.

Shock tube determination of the rate coefficient for the reaction N2+O→NO+N

Abstract: The rate coefficient for the reaction N 2 + O → k 1 NO + N has been measured in the temperature range 2384–3850 K using a shock tube technique. Test gas mixtures consisting of N2, O2, N2O and Kr were heated by incident shock waves, and the concentration of NO in the post-shock region was monitored using two independent spectroscopic techniques: IR emission at 5.3 μm, and absorption of CO laser radiation at 5.17 μm. The two methods yielded excellent agreement. The importance of interferences from other reactions was minimized by careful tailoring of the test mixture composition, in particular by using N2O rather than O2 as a source of 0 atoms. The rate coefficient k1 was determined by comparing experimental and calculated NO profiles using a computer simulation in which k1 was an adjustable parameter. The results of 20 experiments indicate that k1=1.84×1014 exp (−76,250/RT) cm3/mol×s with an uncertainty of approximately ±35% in the temperature range investigated.

Experimental investigation of flow and heating in a resonance tube

Abstract: Experiments have been performed to determine the basic mechanism of heating in resonance tubes of square section with constant area excited by underexpanded jet flows. The jet flow between the nozzle exit and the tube inlet plays a key role in the performance of a resonance tube. A detailed and systematic investigation of the unsteady complex shock structure in this part of the flow region has led to a better understanding of the fundamental mechanisms associated with the gas heating in such tubes. A study of the effects of tube location in relation to free-jet shock location (without the presence of the resonance tube) has shed further light on the underlying mechanism of sustained oscillations of the flow in a resonance tube.

A shock-tube study of ammonia pyrolysis

Abstract: The pyrolysis of NH3 was studied behind reflected shock waves at temperatures 2500–3000 K, using mass spectrometric analysis of dynamically sampled gas. The initial mixtures contained 0.14% to 6% N...

Shock tube measurements of rate coefficients of elementary gas reactions

Abstract: The primary objective of this paper is to review the current status of rate coefficient determinations of elementary gas reactions using modern shock tube measurement technques. The major inaccuracies in these determinations arise from uncertainties in reaction conditions due to gas dynamic effects, uncertainties introduced by interfering rate processes such as vibrational relaxation, competing reactions, or reactions involving impurities, and uncertainties introduced by the diagnostics. Some new diagnostics and experimental methods which can improve accuracy of shock tube rate coefficient measurements are discussed. Included among these are tunable laser absorption spectroscopy for species concentration and temperature measurements, digital data recording, new techniques for controlled generation of radical species, and computer optimization of experimental conditions. Currently attainable levels of accuracy of shock tube rate coefficient determinations of elementary gas reactions are illustrated for several classes of reactions including unimolecular decomposition reactions, bimolecular atom--molecule exchange reactions, and termolecular recombination reactions. 5 figures, 1 table.

An experimental study of liquefaction shock waves

Abstract: The existence of a liquefaction shock wave, a compression shock which converts vapour into liquid, has recently been predicted on physical grounds. The liquefaction shock was experimentally produced as the reflected shock at the closed end of a shock tube. 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, with a discrepancy ∼ 10°C between measured and predicted liquid temperatures. Photographic observations confirmed the existence of a clear liquid phase and revealed the (unanticipated) presence of small two-phase torus-form rings. These rings are interpreted as vortices and are formed in or near the shockfront (∼ 50 rings/mm2 are visible near the shockfront at any given time). Separate experiments with the incident shock under conditions of partial liquefaction produced a fog behind the shock: measurements of laser-beam attenuation yielded the thickness of the condensation zone and estimates of the droplet size (∼ 10−7 m).

Rotational Raman calibration of Thomson scattering

Abstract: Anti-Stokes rotational Raman scattering from nitrogen gas at near atmospheric pressure and room temperature has been used to calibrate the intensity of ruby laser light Thomson scattered from a plasma in a transverse magnetohydrodynamic shock tube. The main limit to the accuracy of this calibration method is the uncertainty in the available Raman cross section for nitrogen.

Detonability of unconfined natural gas-air clouds

Abstract: The transport and use of considerable quantities of liquified natural gas leads to concern over the possibility of a large spill and the consequent formation and ignition of an unconfined combustible cloud (predominantly methane and air). While such a cloud can burn, it is not clear whether detonation can occur under these unconfined conditions. This study represents a combined experimental-theoretical effort directed towards answering this question. On the experimental side, stoichiometric mixtures of CH 4 +2O 2 +XN 2 (X=7.52 for air) have been tested in a special, laboratory scale, sectored shock tube. Using a condensed explosive for initiation, it was found that detonation was easily established for small amounts of N 2 and that the wave speed was very close to theoretical Chapman-Jouguet. However, with increasing N 2 the wave gave evidence of oscillatory behavior and the initiation energy required to establish detonation increased rapidly with X. The threshold or critical initiation-energy was determined for various values of X and then extrapolated to the “air” case. The very large value of 7.5×10 6 joules per cm was indicated. Also, with higher X the detonation velocity fell increasingly below the theoretical value. In the analysis a two front model in which the detonation is replaced by a shock and reaction front separated by an induction zone was used. The flow behind the wave was computed using algebraic profiles for the velocity and density. The Mach number-radius trajectory was then computed from the overall energy equation in integral form, using different values of the initial blast energy. The critical blast energy is then that value which divides decaying trajectories from those approaching the Chapman-Jouguet Mach number. There was good agreement between the theory and measured values of critical energy for the initiation of spherical detonations in CH 4 , O 2 , N 2 mixtures. The relation of the theory to cylindrical shock tube measurements is discussed.

Infrared measurements of sulfur dioxide thermal decomposition rate in shock waves

Abstract: The rate of thermal decomposition of sulfur dioxide dilute in argon has been measured in a shock tube, using IR emission from the ν3 band to monitor SO2. These results yielded a rate coefficient k=8(±2) ×1015 exp(−108 000/RT) cm3 mole−1 sec−1 for the reaction SO2+M→SO+O+M in the temperature range 2800 to 3880°K and the total concentration range 8×10−6 to 3.4×10−5 mole/cc. The measured rate coefficient agrees well with the results of Just and Kiefer, determined respectively from O‐atom formation rate observations and densitometric measurements, and the combined data yield a common rate coefficient between 2500 and 5200°K.

Evaluation of boundary‐layer effects in shock‐tube studies of chemical kinetics

Abstract: Particle times of flight in incident shock flow were determined experimentally by marking several positions of the test gas (mainly Ar) in the shock tube with an infrared emitting gas (NO or CO2). From the local particle velocity, derived from the particle flight times, temperature and pressure changes behind the shock front were evaluated. Several experimental data were found to be correctly described by Mirels's formulations when used properly. The limitations of the formulations are discussed. It is found to be advisable to evaluate boundary-layer effects on shock-tube flow by experiments, rather than theory, in carrying out chemical kinetics studies.

Shock Tube as a Device for Testing Transonic Airfoils at High Reynolds Numbers

Abstract: A performance analysis of gas-driven shock tubes shows that transonic airfoil flows with chord Reynolds numbers in the range of 100 million can be generated behind the primary shock in a large shock tube. A study of flow over simple airfoils has been carried out at low and intermediate Reynolds numbers to assess the testing technique. Results obtained from schlieren photos and airfoil pressure measurements show that steady transonic flows similar to those observed for the airfoils in wind tunnels can be generated within the available testing time in a shock tube with either properly-contoured test section walls or a properly-designed slotted-wall test section. The study indicates that the shock tube is a useful facility for studying two-dimensional high Reynolds number transonic airfoil flows.

Interaction of Weak Shock Waves with Screens and Honeycombs

Abstract: The interaction of weak shock waves with screens and honeycombs is examined to facilitate the design of a pulsed flowing gas laser system operating at a high repetition rate. Interactions with zero and finite base flow (M<0.3) are studied using a shock tube capable of generating weak shock waves. By reflecting the incident wave from the driven section end plate, interactions with and without base flow can be studied in a single experiment. A quasi-steady flow theory is developed to model shock interaction with screens, while a quasi-one-dimensional flow code is used for the interaction between shock waves and honeycombs. Comparisons between analytical and experimental results are made.

Acoustic Suppression in a Pulsed Chemical Laser

Abstract: The analytical and experimental studies aimed at developing an acoustic attenuator for a high-energy pulsed chemical laser are described. A numerical finite-difference code was developed to analyze the attenuation of strong pressure waves (shocks) in ducts. The attenuation concepts analyzed include variable flow area, vented walls, porous flow-through absorbers, and combinations of these. Experimental results were obtained for a vented duct attenuator on a shock tube. The code was used to design the acoustic attenuator of a 50-pulse/s D2 /F2 laser module. Code predictions for the laser device were compared with experimental data.

Experimental studies of the production of converging cylindrical shock waves

Abstract: AN improved experimental technique for the production of cylindrical converging shock waves is presented. This is accomplished with an annular shock tube fitted with an axisymmetric area contraction which redirects an annular incident shock into a cylindrical implosion chamber. The contraction profile is designed in accordance with Whitham's ray-shock theory, and a three-element profile is found to provide the best overall performance on the basis of twodimensional tests. Experiments with a three-element conical contraction demonstrate that highly symmetrical cylindrical implosions may be achieved by this method, and that the cylindrical shock amplification is well predicted by the Chester-Chisnell-Whitham (CCW) area Mach No. rule.

Shock tube coupled to the time‐of‐flight mass spectrometer via a molecular beam sampling system

Abstract: A method for continuous mass spectrometric analysis of high‐temperature reacting gas mixtures is described. The apparatus consists of a unique combination of three devices: the shock tube, the time‐of‐flight mass spectrometer, and the supersonic molecular beam. The driven section of the shock tube constitutes the reservoir of a supersonic molecular beam by which gas is continuously extracted from the reaction zone and introduced through a two‐stage high‐capacity vacuum system into the ionization region of the mass spectrometer. The shock tube and the mass spectrometer are coupled at right angles to one another. This configuration avoids excessive pressure buildup in the mass spectrometer system. The apparatus has an estimated mass resolution of 100 amu, a frequency range of 10–100 kHz, and can be operated over a wide range of shock conditions during the complete high‐temperature pulse.

A shock-tube study on the process of soot formation from acetylene pyrolysis

Abstract: The induction times for soot formation were measured for several mixtures of acetylene behind reflected shock waves over a wide temperature range (1500 to 3300 K) and it was found that there were three temperature ranges showing different trends of the induction time with temperature. Besides pure acetylene, mixtures containing a small amount of oxygen or water vapor were also used to elucidate the effects of combustion product species, and OH was found to be the species most likely to suppress soot formation. The carbon mass growth rates were also measured for these mixtures, and it was found that a large carbon mass growth rate corresponded to a short induction time.

Interaction between the Unsteady Boundary Layer and Inviscid Hot Flow in a Shock Tube

Abstract: A numerical calculation is presented for the flow in a low-pressure shock tube. The evolution of the hot flow quantities due to mutual interaction between the boundary layer and inviscid hot flow are taken into account. A better knowledge of the unsteady evolution of this hot flow is so obtained, including the distribution of all parameters at the limiting regime. The computational results, with Argon as a driven gas, are in agreement with the different experimental results at usual measurement stations.

Shock‐tube study of thermal decomposition of nitric oxide between 2700 and 3500 K

Abstract: The decomposition of nitric oxide at temperatures ranging from 2700 to 3500 K was studied by means of the shock tube. The experimental data were reduced by the method described in a preceding paper and explained consistently by a set of the elementary reactions. The rate constant of the initiation reaction 2NO N2O + O, which was not well known in this temperature range, was deduced precisely. k1 was one order of magnitude lower than that reported previously in similar shock-tube experiments, and was consistent with results obtained below 2000 K and from the reverse reaction.

The formation of an aqueous fog in a shock tube

Abstract: The extent to which a shock tube could be used for laboratory experiments on the fluid mechanics of two-phase mixtures of droplets in an inert carrier gas was determined. A shock tube high pressure section was charged with moist carrier gas and the flow produced a stationary cloud of water droplets behind the reflected expansion wave. The droplets were formed by homogeneous nucleation and preliminary experiments showed that their diameters were of the same order as the wavelength of light. Subsequently, two-colour light transmission measurements were made and the droplet diameter was determined by using the Mie scattering theory. Attempts to measure the size distribution indicated that the droplets had a narrow range of sizes. Depending on the initial conditions, diameters were in the range 0.3 mu m to 0.9 mu m. The fog persisted for several milliseconds before the arrival of the reflected shock wave from the opposite end of the shock tube. Preliminary observations indicate that the droplets cause considerable dispersion of the shock.

Transient wave propagation in bubbly liquids

Abstract: Solutions for the pressure profiles of transient plane waves in a bubbly liquid have been obtained by using a finite difference procedure. In contrast to previous analyses, the liquid was not treated as incompressible. However, relative motion between the bubbles and the liquid was neglected. Two waves were clearly observable in the solutions. The first was a precursor wave in the liquid which had very little effect on the pressure in the bubbles. It was followed by a second wave in which the compression or expansion of the bubbles took place. An important finding was that the compressibility of the liquid had a substantial effect on the pressure profile of the second wave, which implies that the liquid compressibility should be included in future analytical studies of transient pulses in bubbly liquids. Comparison of the numerical solutions with the results of shock tube experiments [L. Noordzij and L. van Wijngaarden, J. Fluid Mech. 66, 115–143 (1974)] showed good agreement except that the solutions did not predict oscillations that were observed near the head of the pulse in the experiments. [Work supported by U.S. Department of Energy.]

Fast-Acting Valves for Use in Shock Tubes : Part 2, Formation of shock waves

Abstract: In the previous paper, two kinds of fast-acting valves were developed to replace diaphragm-breaking as a means of generating shock waves in the conventional shock tubes. One of them, called a type-H valve is used in a shock tube in this paper, and the strength of shock wave generated by the valve and the correlation between the shock formation distance and the opening time of the valve are experimentally clarified. Furthermore, the experiments where a diaphragm is used are also performed, keeping all other operating conditions equal, and the results are compared with those by the valve. When the initial pressure ratio is comparatively small, valve produces as strong a plane shock wave as the conventional diaphragm shock tubes, although the shock Mach numbers are less than those produced by the breaking of diaphragm at a larger initial pressure ratio.

Kinematics of Mach Waves Inside and Outside Supersonic Jets

Abstract: Various interferometric techniques have been used for investigating the speeds of those structures present in the supersonic jet boundary layer which are accompanied by obviously long almost straight and nearly parallel Mach waves. The characteristics of the jet have been varied within wide limits. Besides the conventional blow-down method of generating cold jets three different shock tube methods of producing hot jets have been applied. Two families of long and strong Mach waves outside the jet as well as two families inside turned out to be moving at three of five preferred speeds of long living boundary layer structures. Mach numbers formed with the five speeds are neither dependent on a length ratio nor on a Reynolds number. They depend solely and in a simple manner on the jet Mach number and the ratio of the sound velocities inside and outside the jet. An equally straight forward consideration of the kinematics of sound waves on both sides of the jet boundary layer leads to the same formulae. The Mach waves can be suppressed.

Single Pulse Shock Tube Study of the Reaction Between Nitrogen Dioxide (NO2) and Carbon Monoxide (CO)

Abstract: The reaction between NO2 and CO has been studied in a single pulse shock tube in the temperature range 950-1500 °K and at a total concentration of approximately 2.5× 10−5 moles/cc. Both the measured ratios of ([CO2]/[CO])1 and ([NO]/[NO2])1 have been used in separate series of experiments to examine the reaction The Arrhenius frequency factor and activation energy for ka were found to be 1013.51±0.25 cc/mole-sec and 32000± 1300 cal/mole, respectively. This result for k 12 is compared to literature values and the favorable comparison is taken as an indication that the modified single pulse shock tube analysis employed in this study leads to more reliable chemical kinetic data. The analysis methods include a special test mixture filling technique and a relative concentration at time solution for determining the final species concentration. These techniques reduce variation in the shocked gas reaction time and avoid concentration measurement errors associated with volume expansion sampling in a shoc...

Shock-tube determination of absorption cross sections and A 2 Delta - X 2 Pi band transition moments of SiH

Abstract: The overall absorption cross sections and electronic transition moments of the A 2 Δ − X 2 Π band system of SiH have been determined by using an absorption technique with a shock tube at temperatures of 2600–3800 K over the wavelengths of 150–650 nm. Absorption cross sections are shown to be dominated by continua. The possible contributions to the overall cross sections by a low-lying 4 Σ - and a high-lying 4 Σ - state are discussed. At 200, 228, 340, 405, and 550 nm, the continuum cross sections are (2.9±1.0)×10 -17 , (2.0±0.5)×10 -17 , (3.2±0.6)×10 -18 , (3.8±0.6)×10 -18 , and (1.8±0.8)× -18 cm 2 , respectively. The overall emission intensity and the Si+H→SiH+ hv radiative recombination rate are (6.7± 2.3)×10 -35 (3500/ T ) 0.7 ( Si )( H ) watt - cm -3 and (1.3±0.4)×10 -17 (3500/ T ) 1.1 ( Si )( H ) cm -3 - s -1 , respectively. The A − X transition moments are 0.12±0.04a.u. for the (0, 0) and (1, 1) bands. The intensity of each branch in the A − X (0, 0) band follows approximately the prediction based on the Honl-London factors of Kovacs. The data are applied to the study of the flow field around a spacecraft entering the Jovian atmosphere.