Showing papers on "Shock tube published in 1994"

Diode-laser absorption technique for simultaneous measurements of multiple gasdynamic parameters in high-speed flows containing water vapor

Abstract: A distributed-feedback InGaAsP diode laser, emitting near 1.38 μm, was used to acquire spectrally resolved absorption profiles of H2O lines in the ν1 + ν3 band at a repetition rate of 10 kHz. The profiles were used for simultaneous measurements of flow parameters in high-speed, one-dimensional (1-D) transient flows generated in a shock tube. Velocity was determined from the Doppler shift, which was measured with a pair of profiles simultaneously acquired at different angles with respect to the flow direction. Temperature was determined from the intensity ratio of two adjacent lines. Pressure and density were found from the fractional absorption. From these primary gasdynamic variables, the mass and momentum fluxes were determined. Experiments were conducted with three different gas mixtures in the shock tube: pure H2O at initial pressures lower than 3 Torr, up to 6% of H2O in O2 at initial pressures below 120 Torr, and up to 8% of H2 in O2 at initial pressures below 35 Torr. In the third case, pyrolysis of H2/O2 behind incident shocks produced known yields of H2O. With all three mixtures, results compare well with 1-D shock calculations. This H2O diagnostic strategy shows promise for applications in both ground and flight testing.

Acetylene Oxidation in a JSR From 1 to 10 Atm and Comprehensive Kinetic Modeling

Abstract: Acetylene oxidation in a jet-stirred reactor has been investigated at high temperature (800-1100 K) in the pressure range 1 to l0atm. Molecular species concentration profiles of H2, CO, CO2, CH4, C2H2, C2H4, C2H6 were obtained by probe sampling and GC analysis. Acetylene oxidation was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C1,—C6 hydrocarbons. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet-stirred reactor, ignition delay times measured in shock tube, atmospheric pressure C2H2/Air flame speeds, and C2H2/O2/Ar premixed flame structures obtained at low pressure on flat flame burners. The mechanism has also been used successfully to represent the kinetics of the oxidation of CH4, C2H4, C2H6, C3H6, C3H3, mixtures of CH4 with C2H6 and/or C3,H8 in the same conditions

Nonstationary flows and shock waves

Abstract: 1. Introduction 2. Shock waves on earth and in space 3. Transition fronts 4. One-dimensional flows in a simple shock tube 5. Shock tubes with area change 6. Boundary-layer effects 7. Two-dimensional studies of oblique shock-wave reflection and diffraction 8. Spherical and cylindrical shock-tube analogues and flow simulation 10. Dusty-gas shock tube 11. Real-gas effects on shock-tube flows 12. Implosion waves and applications 13. Shock-tube construction and instrumentation 14. Closing comments Index

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

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

Experimental and computational studies focusing processes of shock waves reflected from parabolic reflectors

Abstract: This paper describes experimental and numerical studies of the focusing process of shock waves reflected from various shapes of a parabolic reflector. The effect of incident shock strength on the focusing process was also investigated. Experiments were carried out in a conventional shock tube and a test gas was air for incident shock Mach numbers ranging from 1.1 to 2.0. In the experiments, the process of shock focusing was visualized by schlieren method. Numerical simulations were conducted for incident shock Mach numbers up to 3.0 by solving the two-dimensional unsteady Euler equations. The numerical results were compared with experiment for various parabolic reflector shapes and for various incident shock Mach numbers. Based on the experimental and computational results, the pattern of shock focusing and shock focusing mechanism are discussed.

Gasdynamic enhancement of nonpremixed combustion

Abstract: To promote efficient performance of very high speed air-breathing propulsion systems, the combustor Mach number must be of the order of six for a flight Mach number of 18. Because of this high gas speed through the combustor, mixing rates of hydrogen fuel with air must be very rapid in order to allow a combustor of reasonable length. It is proposed to enhance the rate of mixing and combustion of hydrogen and air, and thereby reduce combustor length, through the introduction of streamwise vorticity generated by the interaction of a weak oblique shock wave with the density gradient between air and a cylindrical jet of hydrogen. Because of the high Mach number flow in the combustor, the oblique shock traverses the jet at a small angle with respect to the free stream direction, and the principle of slender body theory allows one conceptually to replace the three-dimensional steady flow with a two-dimensional unsteady flow. As a consequence, two-dimensional time-dependent computational studies and an extensive experimental shock tube investigation were employed to assess mixing rates for the steady flow in the combustor. The results indicated that under realistic conditions, adequate mixing could be accomplished within 1 ms, a rate that was technologically interesting. Encouraged by these experiments, a “practical” injector, utilizing shock-enhanced mixing, was designed for a combustor having a free stream Mach number of 6.0. A detailed aerodynamic and mixing investigation was carried out in the Mach 6 High Reynolds Number Tunnel at the NASA-Langley Research Center. The results confirmed both the details and the overall effectiveness of the shock-enhanced mixing concept.

Drop deformation and breakup due to shock wave and steady disturbances

Abstract: An experimental study of drop deformation properties induced by both shock wave and steady disturbances is described. Three test facilities were used, as follows: a shock tube facility for measurements of effects of shock wave disturbances on drops in gases, a 10 m high drop tube facility for measurements of effects of steady disturbances on drops in gases and a 1 m high drop tube facility for measurements of effects of steady disturbances on drops in liquids. Various dispersed and continuous phase gases and liquids were considered to provide dispersed/continuous phase density ratios of 1.15–12,000, Ohnesorge numbers of 0.0006–600, Weber numbers of 0.004–700 and Reynolds numbers of 0.03–16,000. At low Ohnesorge numbers (

Development of a Primary Standard for the Measurement of Dynamic Pressure and Temperature

Abstract: The National Institute of Standards and Technology is developing a primary standard for the measurement of dynamic pressure and temperature. Our method requires a dynamic source and a technique for calibrating it. The source is a shock tube; the calibration technique is based on the properties of diatomic gas molecules measured by laser spectroscopic methods.

Numerical Study of Transient Flow Phenomena in Shock Tunnels

Abstract: Computational fluid dynamics (CFD) was used to study some transient flow features that can occur during the startup process of a shoch tunnel. The investigation concentrated on two areas: (1) the flow near the endwall of the driven tube during shock reflection and (2) the transient flow in the nozzle. The driven tube calculations were inviscid and focused on the study of a vortex system that was seen to form at the driven tube's axis of symmetry. The nozzle flow calculations examined viscous and inviscid effects during nozzle startup. The CFD solutions of the nozzle flows were compared with experimental data to demonstrate the effectiveness of the numerical analysis.

Development of shock wave and vortex structures in unsteady jets

Abstract: The paper deals with the formation of a gaseous jet behind a sonic nozzle. The nozzle was located at the end wall of a shock tube. A specially made two-direction shadow system, which ensured a simultaneous recording of side-on and head-on images of the jet, as well as a diffraction interferometer were used. On the basis of the data obtained, an analysis of the vortex structure of the jet was carried out, the amplitude of the azimuthal instability was measured and the spatial distribution of the density in the gaseous flow was obtained. Comparison between the experimental density distribution, numerical results and a nonself-similar point source blast wave model was conducted.

Detonator-to-shock tube ignition transfer connector

Abstract: A detonator-to-shock tube ignition transfer connector for bi-directional explosive transfer from a detonator to one or more shock tubes is disclosed wherein the connector has a housing defining a barrel portion for enclosing a detonator assembly, the barrel portion having an opening at both ends, and a coupling attached to one end of the barrel portion. The coupling and the barrel portion form a coupling channel for holding shock tubes within the path of an explosive force created when a detonator assembly positioned in the barrel portion is ignited, thereby initiating the shock tubes. The invention also includes a collar lock for holding the detonator assembly within the barrel portion and preventing the detonator assembly from being accidentally removed.

Ignition of aluminum droplets behind shock waves in water

Abstract: Gram quantities of molten aluminum droplets at temperatures up to 1973 K are forced to interact with water under sustained pressure pulses of up to 40.8 MPa in a hydrodynamic shock tube. Conditions are identified under which the thermal interaction (physical regime) develops into chemical ignition and total combustion events. This, and the absence of catastrophic breakup in the physical regime, raises some very interesting questions with regard to the sequence of events during large‐scale aluminum–water explosions.

A multiple-reflection cell suited for absorption measurements in shock tubes

Abstract: The improvement of the detection limit in optical concentration measurements by means of multiple-reflection cells (MRC) has become a well established method in absorption spectroscopy. To apply such an experimental method to shock tubes, several technical problems have to be solved. The use of special optical cells showing mechanical ruggedness is required. In this report we present such a MRC designed for a cylindrical shock tube with an inner diameter of 100 mm. By means of 8 mirrors (optical coated stainless steel) the absorption length has been increased by almost one order of magnitude (863 mm) while the optical path is kept in one plane. The use of fixed flat mirrors provides a very high mechanical stability, which facilitates the application to shock tubes. The first experimental results with this MRC were obtained by measuring the thermal decomposition of O3 behind incident shock waves using an UV laser beam. A direct comparison to the signals simultaneously detected by the use of the conventional absorption length (i.e. the tube diameter of 100 mm) documents the suitability of this MRC for applications in chemical kinetics.

Application of the space-time conservation element and solution element method to shock-tube problem

Abstract: An Euler solver based on the method of space-time conservation element and solution element is in this paper to simulate shock-tube flows involving shock waves, contact discontinuities, expansion waves and their intersections. Seven test problems are considered to examine the capability of this method. The numerical results, when compared with exact solutions and/or numerical solutions by other methods, indicate that the present method can accurately resolve strong shock and contact discontinuities without using any ad hoc techniques which are used only at the neighborhood of a discontinuity.

Design and Performance of Quick Opening Shock Tube Using Rubber Membrane for Weak Shock Wave Generation.

Abstract: In order to generate weak shock waves with repeatability of better than 99%, a shock tube was constructed which, instead of rupturing conventional diaphragms, utilizes 0.5 mm to 1.0 mm thick rubber membrane to separate the driver and driven gases. Sudden leakage of the high-pressure gas which bulged the rubber membrane subsequently drove a weak shock wave into the low-pressure channel within a relatively short formation length. The rubber membrane was operated within its elastic limit and moved very quickly, giving good repeatability. With identical initial conditions for a series of experiments, the scatter of shock Mach number was found to be at most ±0.25% for shock Mach number ranging from Ms=1.02 to 1.55 throughout more than 100 runs.

Explosion simulator and system for generating audio and visual effects

Abstract: An explosion simulator generates bang, smoke and flash cues. Pressurized gas is released into a shock tube to generate a shock wave and, thereby produce the bang cue. The pressurized gas is released from a gas reservoir into the shock tube through a diaphragm which is broken by a firing pin in response to a control circuit. A smoke powder packet containing a smoke powder is positioned in the shock tube. The pressurized gas travelling in the shock tube bursts the smoke powder packet and the smoke powder is dispersed to produce a smoke cue, or cloud. A flashtube generates the flash cue to illuminate the smoke cloud. An audio/visual explosion simulation system is also disclosed for generating multiple bang, smoke and flash cues from a single shock tube. Immediately prior to generating a bang cue, a gas supply fills a gas reservoir with pressurized gas. A reusable valve opens to release the pressurized gas from the reservoir into the shock tube and closes to seal the reservoir for refilling by the gas supply. A smoke generator repeatedly releases smoke powder into the shock tube to generate smoke clouds for each successive smoke cue.

Reflection of Shock Waves

Abstract: The study of reflection of shock waves from rigid boundaries has been a subject of considerable interest. Reflection phenomenon can be divided into three sub-heads.

Isomerization of 2-methyl-4,5-dihydrofuran. Studies with a single-pulse shock tube

Abstract: The isomerization of 2-methyl-4,5-dihydrofuran was studied behind reflected shock waves in a pressurixed driver single-pulse shock tube over the temperature range 805-1030 K and densities of approximately ∼3×10 -5 mol/cm 3 . Two isomerization products, acetylcyclopropane and 3-penten-2-one, are obtained in the isomerization. Acetylcyclopropane is formed in an irreversible process from 2-methyl-4,5-dihydrofuran. It further isomerizes, at higher temperatures, to cis- and trans-3-penten-2-one. At high temperatures where the conversion of 2-methyl-4,5-dihydrofuran is high, the main source for 3-penten-2-one is acetylcyclopropane. At lower temperatures 3-penten-2-one is formed mainly by a direct isomerization of 2-methyl-4,5-dihydrofuran

Interaction of a shock wave with a two-phase interface

Abstract: The acceleration by an incident shock of a planar interface between a gas and a particle-gas mixture has been investigated experimentally and numerically. The experiments were conducted in a newly developed vertical shock tube in which the planar interface of the particle-gas mixture was generated and its particle concentration history was measured. Polydisperse corn starch particles with a mean diameter of 10μm were used. We recorded the motion of the interface, as well as of the incident and reflected shock by using a 4 channel spark shadowgraph. The experimental conditions were Mach numberM s=5.15 and initial pressurep 1=50kPa for various particle concentrations in nitrogen. The reflected shock appears with a delay after the incident shock enters the particle-gas mixture. Numerical methods were employed to solve the two-phase governing equations. Experiments and numerical solutions are in good agreement.

Interaction of a normal shock wave with a compressible turbulent flow

Abstract: A speckle photographic method, which is sensitive to changes of gradients in fluid density, is applied for analyzing a compressible turbulent air flow with density fluctuations. Spatial correlation coefficients, turbulent length scales, and energy spectra are determined under the assumption of homogeneous isotropic turbulence. The experiments are performed in a shock tube where the flow is passed through a turbulence grid. Measurements are taken before and after the turbulent regime interacts with the normal shock wave reflected from the tube's end wall. Amplification of the turbulence intensity by the shock interaction process is verified quantitatively and is shown to be restricted to the lower wave numbers in the spectrum.

Signal tube and detonator cord connector

Abstract: The present invention is directed to an improved connector comprising a first and second holding means for pressure fitting a detonating cord and shock tube in a substantially orthogonal pressure fitting relationship.

Effects of Blowing Ratio on Heat Transfer to the Throat Region of a Porous-Walled Nozzle.

Abstract: : This experiment analyzed the effects of blowing ratio on heat transfer to the throat region of a porous-walled nozzle, using the AFIT low speed shock tube. Heat flux data were taken from both sides of a two-dimensional Mach 2.0 (Re/m=5.2x10(exp 7) nozzle using thin film resistance thermometers. One side was transpiration-cooled by secondary air injection through a sintered wall, while the other side served as a control. Control results were validated using empirical relations, and cooled side results showed up to a 14% reduction in heat transfer coefficient at blowing ratios of only 0.51%. The linear nature of cooling effectiveness at these low blowing ratios allowed a modification of nozzle heat transfer equations to include a blowing ratio parameter. Disturbance of primary flow was also minimized, causing no measurable reduction of nozzle performance.

V.U.V. absorption diagnostic for shock tube kinetics studies of C2H4

Abstract: A sensitive and quantitative absorption diagnostic has been developed for measurement or C2H4 in shock tube kinetics experiments. This diagnostic has been calibrated from 300 to 2571 K at 1.2 atm in shock tube experiments, resulting in an absorption coefficient at 174.4 nm of kabs = 3.11 × 105/T[K]-51.0 cm-1 atm-1. The absorption coefficient for C2H2 at this wavelenght was also measured over the same temperature and pressure range. Kinetics experiments were performed to measure the rate of ethylene decomposition at reflected shock conditions of 1600–2300 K at 1.2 atm, with mixtures of 189–444 ppm C2H4 in argon. Sensitivity analysis based on a comprehensive hydrocarbon mechanism confirmed that the rate of ethylene decomposition was significantly affected by the rate of only one reaction: C2H4 + M → C2H2 + H2 + M. The rate coefficient for this reaction near the low-pressure limit was determined to be k = 9.17 × 1016 exp(- 37780/T) cm3 · mol-1 · s-1 + 20% over the entire temperature range, in good agreement with past measurements.