Showing papers on "Shock tube published in 1975"

Thermoacoustic effects in a resonance tube

Abstract: New experiments with a gas-filled resonance tube have shown that not only heating, but also cooling of the tube wall is possible and that these phenomena are not restricted to oscillation amplitudes that generate shocks. The present paper concentrates on amplitudes outside the shock region. For this case, an extended acoustic theory is worked out. The results show cooling in the section of the tube with maximum velocity amplitude (and thus dissipation) and marked heating in the region of the velocity nodes. A strong dependence of these effects on the Prandtl number is noted. The results are in good agreement with experiments. Although the theory is not valid for proper resonance conditions, it nevertheless sheds some light on what happens when nonlinear effects dominate.Closely related to the limit of validity of the thermoacoustic theory is the question of transition from laminar to turbulent flow in the viscous boundary layer (Stokes layer). This problem has also been investigated; the results are given in a separate paper (Merkli & Thomann 1975). In the present article laminar flow is assumed.

Rotational energy relaxation in molecular hydrogen

Abstract: Theoretical studies of rotational relaxation in para‐hydrogen are presented. By using a set of theoretically deduced state‐to‐state rotational rate constants the master kinetic equations and the relevant fluid mechanical equations are solved numerically. Calculations are performed over the range 100⩽T⩽1100 °K to simulate conditions for free jet expansion, shock tube, and sound absorption experiments. The over‐all rotational energy relaxation rates extracted from these calculations are compared with available experimental data and are shown to be in good qualitative agreement. The magnitudes and the temperature dependence of these rates depend critically on the degree and direction of the initial departure from equilibrium as well as the multilevel nature of the relaxation process. The apparent discrepancies in the measured rates from the different experiments are shown to be qualitatively self‐consistent in light of the present calculations.

Carrier gas effects on homogeneous nucleation of water vapor in a shock tube

Abstract: Condensation of water vapor was studied in the expansion wave of the driver section of a shock tube by light scattering and pressure measurements. By arranging the onset of condensation to occur at the terminating characteristic of the unsteady expansion wave, the gasdynamic analysis could be simplified. The composition of the test gas was varied from pure vapor (steam) to small mole fractions of H2O in different carrier gases, yielding onset conditions in the temperature and pressure ranges of 225 to 282 °K and 2 to 62 Torr, respectivly. Steam condensation is well predicted by the classical rate expression for homogeneous nucleation, modified to account for nonisothermal effects and a variable amount of carrier gases. Results for the transition region from the pure vapor to small mole fractions of water vapor in argon, helium, and air reveal a dependence of the computed nucleation rates of several orders of magnitude on the mole fraction of the carrier gas, which is not explained by the nonisothermal theory. Possible reasons for this effect are discussed.

A new technique for the study of test time and rarefaction wave effects in hypersonic shock tubes

Abstract: A knowledge of test time in shock tubes is important. Calculated values are unreliable because of the large role played by non-ideal effects such as turbulent flow through the opening diaphragm and by boundary layer phenomena. Moreover, in the case of combustion-driven and arc-driven shock tubes the driver gas state is usually only poorly known. For these reasons the test time must be evaluated experimentally. The author has developed a technique which employs a smear camera to measure the test time. The technique also provides a method for determining that regime of shock tube operation where the rarefaction wave reflected from the driver section plays a dominant role.

Experiments and Analyses on Shock Waves Propagating through a Gas-Particle Mixture

Abstract: The structures and the behaviors of shock waves propagating through a gas and solid particle mixture are studied by shock-tube experiments and by two methods of wave analysis. The shock waves concerned are incident on the mixture dispersed uniformly in downstream part of the driven section. Pressures and shock velocities are measured under the condition that the particle loading ratio and the shock Mach number are both less than two. The final equilibrium pressures behind the waves and the velocities of the fully decayed shock fronts agree well respectively with the results of the usual shock theory on the mixture and those of the model analysis on a perfect "effective" gas. The analysis by the method of characteristics is satisfactorily applied to give a good explanation of the observed process whereby a shock wave decays to a weak wave with continuous wave form. And, the authors point out some problems relating to the relaxation process and some inconsistencies of the "effective" gas theory when analyzing the unsteady wave motions.

Shock tube study of ionization rates of NaCl‐contaminated argon

Abstract: Electron density, electron temperature, and concentration of excited sodium atoms are measured in the weakly ionized regime behind a shock wave in impure argon in a shock tube using microwave techniques and spectrally resolved radiometry. Evidence is presented to show that an apparent increase in the rate of ionization is due to electron detachment of negative chlorine ions produced from sodium chloride vapor contained as an impurity. To be consistent with this chemical model, rate coefficients are found in the temperature range between 5500 and 8600 K for the dissociation of NaCl into an ion pair, dissociation of NaCl into a neutral pair, and electron detachment of a negative chlorine ion. Electron temperature is lower than heavy-particle temperature by roughly 1000 K. The electron-argon impact-ionization rate coefficient is a weak function of electron temperature in contradiction to expectation.

Behavior of burst diaphragms in shock tubes

Abstract: Analytical and experimental studies have been carried out on the behavior of strong shock tube diaphragms. These diaphragms exhibit large moments of resistance to opening due to stresses at the hinge line of the diaphragm petal. High speed shadowgraph photographs of the detailed opening process have been obtained and have been shown to compare well with the analytical model derived for flowfield‐diaphragm interaction. A nondimensional resistance parameter is shown to control the behavior of the thick diaphragm and the detailed flow process which controls the opening is identified.

Numerical simulation of regular and Mach reflections

Abstract: The results of an M∞=2 shock wave propagating in air down a shock tube and impinging on a wedge were calculated using both a two‐dimensional Eulerian code and a two‐dimensional Lagrangian code. The air was considered to be an ideal gas with γ=1.4. Both 2:1 and 1:2 wedges were used; these angles are known to generate regular and Mach reflected shock waves, respectively. The calculated results agreed quite well in both shock angle and overpressure with the appropriate theoretical or experimental results. A comparison of the two methods showed that the Eulerian calculation was easier and cheaper, gave equally good shock and expansion wave results, but ’’washed out’’ the slip line whose approximate position was still observable in the Lagrangian calculation.

Shock tube decomposition of dilute mixtures of nitrogen trifluoride in argon

Abstract: The decomposition of 0.25%, 0.50%, and 1.00% NF3 in argon was studied in a reflected shock wave. In these dilute mixtures the unimolecular decomposition pathway can be studied independently of other modes of decomposition. The temperature range of the study was 1500–2400 °K and the total pressure range was 0.8–60 atm. The total gas concentration behind the reflected shock ranged from 5×10−6 to 3.6×10−4 mol/cc. Kinetic measurements were made by following the decay of emission of the ν3+ν1 combination band of NF3 at 5.18 μ. The unimolecular rate constants of the Lindemann mechanism were obtained from the emission decay data and were converted to second order rate constants by dividing by the total gas concentration. The Arrhenius parameters calculated by a least squares analysis of the second order rate constants are given as k=1014.7 exp(−56 300/RT) cc/mol sec. The activation energy for the reaction is very close to the first N–F bond energy of 57 kcal/mol. Calculations using the Slater integral were in go...

Shock shapes on blunt bodies in hypersonic-hypervelocity helium, air, and CO2 flows, and calibration results in Langley 6-inch expansion tube

Abstract: Shock shape results for flat-faced cylinders, spheres, and spherically blunted cones in various test gases, along with preliminary results from a calibration study performed in the Langley 6-inch expansion tube are presented. Free-stream velocities from 5 to 7 km/sec are generated at hypersonic conditions with helium, air, and CO2, resulting in normal shock density ratios from 4 to 19. Ideal-gas shock shape predictions, in which an effective ratio of specific heats is used as input, are compared with the measured results. The effect of model diameter is examined to provide insight to the thermochemical state of the flow in the shock layer. The regime for which equilibrium exists in the shock layer for the present air and CO2 test conditions is defined. Test core flow quality, test repeatability, and comparison of measured and predicted expansion-tube flow quantities are discussed.

Incident shock-wave characteristics in air, argon, carbon dioxide, and helium in a shock tube with unheated helium driver

Abstract: Incident shock-wave velocities were measured in the Langley 6-inch expansion tube, operated as a shock tube, with air, argon, carbon dioxide, and helium as test gases. Unheated helium was used as the driver gas and most data were obtained at pressures of approximately 34 and 54 MN/sq m. A range of pressure ratio across the diaphragm was obtained by varying the quiescent test-gas pressure, for a given driver pressure, from 0.0276 to 34.5 kN/sq m. Single- and double-diaphragm modes of operation were employed and diaphragms of various materials tested. Shock velocity was determined from microwave interferometer measurements, response of pressure transducers positioned along interferometer measurements, response of pressure transducers positioned along the driven section (time-of-arrival gages), and to a lesser extent, measured tube-wall pressure. Velocities obtained from these methods are compared and limitations of the methods discussed. The present results are compared with theory and the effects of diaphragm mode (single or double diaphragm), diaphragm material, heating of the driver gas upon pressurization of the driver section, diaphragm opening time, interface mixing, and two-dimensional (nonplanar) flow are discussed.

Measurements of a Simulated Rocket Exhaust Plume near the Prandtl-Meyer Limiting Angle,

Abstract: : Direct measurements of the mass flux level and estimates of the mass velocity have been made in the side flow field of a small axisymmetric rocket nozzle operated in hard vacuum. A shock tube provided nitrogen and simulated rocket propellant (nitrogen tetroxide and Aerozene 50) sources for the nozzle. Data were recorded in the plume far field, for angles ranging from 0 to 90 degrees relative to the nozzle centerline. In all cases, substantial mass flux levels were observed in the neighborhood of the limiting Prandtl-Meyer characteristic. At an angle of 90 degrees to the nozzle centerline, the density levels were typically three orders of magnitude below centerline values and the mass velocities were approximately one-half of the centerline values. (Modified author abstract)

Single-pulse shock tube studies of hydrocarbon pyrolysis. Part 4.—Isomerization of allene to methylacetylene

Abstract: The isomerization of allene to methylacetylene has been studied in a single-pulse shock tube over the range 1440–1810 K and a first order rate constant of 3.02 × 1014 exp(–388 kJ mol–1/RT) s–1 has been obtained for the temperature range 1440–1700 K. Experiments with deuterated allenes, combined with the absence of side products, demonstrate that below 1700 K the mechanism is predominantly (> 95 %) molecular. RRKM calculations show that it is possible to generate the experimental rate constants using physically-realistic models and suggest a limiting high pressure rate constant of ∼1.6 × 1015 exp(–400 kJ mol–1/RT) s–1.

An experimental study of ultraviolet radiation behind incident normal shock waves in CO2 at Venusian entry speeds

Abstract: Radiation intensity profiles behind incident normal shock waves in pure CO2 have been measured spectroscopically in the Langley Arc-Driven Shock Tube. These profiles, which were obtained for shock velocities between 9 and 13 km/sec and ambient densities corresponding to Venus altitudes between 100 and 80 km, were measured in the vacuum ultraviolet regime. Wavelengths of 127.7, 158.0, 177.5, and 195.0 nm were monitored simultaneously using a four-channel vacuum spectrograph equipped with sodium-salicylate-coated photomultipliers, thereby including the CO(4+) band system which is the most prominent radiator. Measured nonequilibrium overshoots are modeled to provide a means of estimating the effect of nonequilibrium radiation heating to the stagnation region of proposed aero-shells for Venusian entry. These results indicate a significant increase in radiative heating due to nonequilibrium effects. The measurements are believed to represent the most accurate data available on the effect of nonequilibrium radiative heat transfer for Venus entry. This accuracy is primarily due to improved spectrographic instrumentation, which is discussed in some detail regarding its application in related studies.

Shock tube study of the thermal decomposition of O3 from 1000 to 3000°K

Abstract: Measurements have been made of the thermal decomposition of O3 behind incident shock waves Both uv absorption and Schumann−Runge emission measurements were made in separate experiments to follow the O3 and O−atom concentrations, respectively The high temperature data are interpreted to indicate a departure from the Arrhenius form for the rate constant of the unimolecular reaction O3 + M ? O + O2 + M The rate constant for this reaction is measured to be approximately a factor of 2 smaller at 3000°K than predicted by the Arrhenius fit to the low temperature data (200°K ? T ⩽ 900°K)

A Shock Tube Study of the Thermal Decomposition of Nitric Oxide

Abstract: Nitric oxide decomposition has been studied in a shock tube, time-of-fiight mass spectrometer system at 2700 to 4700 K and 1.5 to 3.5 atm using neon as diluent. The overall decomposition rate was found to be second order in NO concentration and in good agreement with previously reported rates. N2, O2 and O-atoms were the only observed reaction products. The concentration-time profiles of the observed species lend support to a mechanism of primary NO decomposition to N2 and O2 with slower decomposition to N2O and O-atoms. A mathematical simulation of the reaction consisting of eight elementary reactions was deduced by fitting experimental data to simulated concentration-time profiles.

A multiply‐pulsed double‐pass laser schlieren system for recording the movement of shocks and particle tracers within a shock tube

Abstract: A system has been developed for the photographic study of the flows produced by nonplanar shocks generated in a shock tube. An array of particle tracers is injected into the flow being examined and the movement of the tracers, as well as the shock fronts, is recorded using a double‐pass schlieren system. A repetitively pulsed ruby laser, synchronized with a high‐speed 16‐mm framing camera, is used as a light source. Analysis of the photographic record yields the time and space variation of the physical properties of the observed flow field.

Autoignition of Flowing Hydrogen-Air Mixtures

Abstract: A shock tube was used to determine the cause of autoignitions occurring at low temperatures in flowing hydrogen-air mixtures. For flow speeds above 750 m/sec, ignitions were observed at temperatures 400K below that established in static mixtures by the thermal explosion limit method. Gasdynamic processes which can produce high local static temperatures were absent. Therefore, the classical concept of "ignition temperature" does not seem to apply to these spontaneous ignitions. The onset of ignition has been experimentally correlated to the flow speed, the density in the boundary layer, the wall temperature, and the stagnation temperature of the flow. It has been concluded that the low temperature ignition originates in the boundary layer and is caused by friction induced ionization of the hydrogen molecule at the surface of the tube. When the boundary layer is turbulent, the low temperture ignitions do not occur.

The mechanism of the H2 + CL2 reaction: Ignition behind reflected shocks

Abstract: A detailed shock-tube investigation of the ignition in H2 + Cl2 + Ar mixtures in a shock tube is presented, and the mechanism of the reaction is discussed. Ignition delay times were determined from pressure and heat flux measurements behind reflected shock waves. The induction times measured ranged between 35 and 2100 μsec over the temperature range of 830–1260°K. The experimental results of close to seventy tests can be correlated by the relationship where the concentrations are expressed in mole/cm3. The above relationship served as a basis for a computer modeling of the ignition delay times. Ten calculations, simulating typical laboratory experiments, were run by the computer for each reaction scheme and the obtained temperature and composition dependence of the induction times were compared with the ones observed experimentally. A reaction scheme based on a simple exothermal chain propagation could not reproduce the experimental relationship. When the energy branching reaction HCl*(ν) + Cl2 (HCl3) HCl + Cl + Cl was added to the reaction scheme, a much better agreement with the experiment was obtained. It is believed that the above reaction does take place and that it is the main supplier of atoms to the system.

Incident Shock Interactions with Boundary Layers

Abstract: NCIDENT shock wave interactions with laminar and turbulent boundary layers were investigated experimentally. The work extends the existing data base used for comparisons with results of analytical methods for estimating: free interaction pressure distributions; extent of the pressure rise upstream of the incident shock location; and peak heat transfer rates. Pressure and heating rate distributions were obtained for wedge-generated shock waves interacting with Mach 8 laminar boundary layers, and for wedge- and sphere-generated shock waves interacting with Mach 6 turbulent layers. The Mach 6 data were obtained for Reynolds numbers up to 440 million, considerably larger than those obtained in most other similar investigations. Contents Shock waves that impinge on a surface can greatly amplify the local heat transfer and pressure loads on the surface. A sufficiently strong shock wave causes the boundary layer to