Showing papers on "Shock tube published in 1989"

The Riemann problem and interaction of waves in gas dynamics

Abstract: The initial-value problem constructed by Riemann (1860) to describe the motion of an ideal gas in a shock tube is investigated analytically, with an emphasis on the mathematical aspects Topics addressed include the simplest Riemann model and the interactions of elementary waves (shock waves, centered rarefaction waves, and contact discontinuities), one-dimensional isothermal flow, one-dimensional adiabatic flow, and two-dimensional flow Particular attention is given to the Riemann problem for a scalar conservation law, the interaction of a shock wave overtaking another in steady two-dimensional flow, and the diffraction of a planar shock along a compressive corner 92 refs

Rate constants for the reaction O+D2→OD+D by the flash photolysis–shock tube technique over the temperature range 825–2487 K: The H2 to D2 isotope effect

Abstract: Rate constants for the reaction of O(3P) atoms with deuterium, O+D2→OD+D, have been measured over the temperature range 825–2487 K. The experimental method that has been used is the flash photolysis–shock tube (FPST) technique. This technique utilizes atomic resonance absorption spectroscopy (ARAS) to monitor O‐atom depletion in the presence of a large excess of reactant, D2. The measurement is made in the stagnant reflected shock wave region. Thus, shock heating simply serves to prepare the gas density and temperature for a flash photolytically induced absorption photometric experiment. The results that have been obtained between 825 and 2487 K can be represented by the Arrhenius expression: k=(3.22±0.25)×10−10 exp(−7293±98 K/T) cm3 molecule−1 s−1. The average deviation of the present data from this equation is ±17%. An alternative three parameter expression that represents the data to within ±16% is k=1.95×10−15 T1.45 exp(−5250 K/T) cm3 molecule−1 s−1. When the recent results of Zhu, Arepalli, and Gordo...

Nucleation and growth rates of homogeneously condensing water vapor in argon from shock tube experiments

Abstract: A special shock tube process combining a reflected expansion wave with a weak shock wave is analyzed and calibrated. The process is employed to transfer water vapor carried in argon into a known supersaturated state for a short period of time (0.5 ms). During that period steady state homogeneous nucleation takes place followed by condensational growth. Nucleation and growth rates are measured by a 90° Mie-light scattering technique in the temperature range 200–260 K. The results are compared with existing theoretical models.

Reaction kinetics of NH in the shock tube pyrolysis of HNCO

Abstract: The high temperature kinetics of NH in the pyrolysis of isocyanic acid (HNCO) have been studied in reflected shock wave experiments. Time histories of the NH(X3Σ−) radical were measured using a cw, narrow-linewidth laser absorption diagnostic at 336 nm. The second-order rate coefficients of the reactions: (1) were determined to be: cm3−mol−1−s−1, where f and F define the lower and upper uncertainty limits, respectively. The data for k1a are somewhat better fit by:

Quantitative NH2 concentration determination in shock tube laser-absorption experiments

Abstract: The adsorption coefficient of the A2A1←X2B1(090←000)ΣPQ1,N7 transition in NH2 at 16,739.90 cm-1 has been measured in a shock tube as a function of temperature using a cw narrowband absorption laser. For controlled NH2 generation, both NH3 photolysis at 193 nm and NH3 pyrolysis were used; both calibration methods are independent of thermochemical data. Rapid scanning of the laser in pyrolysis and laser-photolysis experiments provided spectroscopic data for the splitting and the intensity ratio of the two spin components of the doublet transition, as well as information on collisional broadening by argon. From these experiments, the oscillator strength of the transition was inferred to be (6.35 ± 1.90) × 10-5. The shock tube diagnostic method for NH2 developed here is well-suited for a variety of reaction kinetics experiments.

Ignition and Flame Propagation Modeling With an Improved Methane Oxidation Mechanism

Abstract: An improved chemical mechanism for the combustion of propane has been developed which describes experimental results for a broad range of ignition and combustion phenomena. Simulations of shock tube induction times, measurements of chemical component concentrations in flow reactor experiments, flame speeds, and flame compositions have been carried out with a reasonable degree of success. The rate-determining reactions for some of the experimental simulations have been determined with “brute force” sensitivity analysis Minimum ignition energies were also calculated for spherical ignitions where the ignition energy was added in the form of internal energy which served only to increase the temperature of the gas (heat). The calculated minimum ignition energy gave reasonable agreement with the experimental value for near-stoichiometric mixtures. The H = 02 = OH + O reaction was shown to play a dominant role in determining the minimum ignition energy. This paper also attempts to correct errors which a...

Separation shock dynamics in Mach 5 turbulent interactions induced by cylinders

Abstract: Wall pressure fluctuations have been measured under the unsteady separation shock in interactions generated by unswept circular cylinders. Models were tested in the turbulent boundary layers on the tunnel floor and on a full-span flat plate. The freestream unit Reynolds number was 53×10 6 m -1 , and the wall temperature approximately adiabatic. Distributions of the shock frequency, shock period, and shock speeds in the upstream and downstream directions have been calculated

Shock wave generator

Abstract: A shock wave generator for the fragmentation of concrements, having a light-pulse-transmiting light guide and a converter arranged at the distal light guide end and having an ionization surface which, when a light pulse impinges, initiates a shock wave in the surrounding fluid, as well as having a shock wave outlet zone. In order to increase its efficiency and durability, the generator is constructed such that the ionization surface extends obliquely sloped with respect to the beaming axis of the impinging light pulse and the shock wave outlet zone is arranged in the direct shock wave beaming area of the ionization surface, thereby permitting a largely unhindered, low-loss propagation for the shock wave to the application point.

Phase-resolved heat-flux measurements on the blade of a full-scale rotating turbine

Abstract: This paper presents detailed phase-resolved heat-flux data obtained on the blade of a Teledyne 702 HP full-stage rotating turbine. A shock tube is used as a short-duration source of heated air, and platinum thin-film gages are used to obtain the heat-flux measurements. Results are presented along the midspan at several locations on the blade suction and pressure surfaces from the stagnation point to near the trailing edge. For these measurements, the turbine was operating at the design flow function and at 100 percent corrected speed. Results are presented for the design vane/blade spacing (0.19 Cs) and at a wide spacing (0.50 Cs). Data are also presented illustrating the phase-resolved blade heat-flux distribution with upstream cold gas injection from discrete holes on the vane surface. The results illustrate that several successive passages can be superimposed upon each other and that a heat-flux pattern can be determined within the passage.

Phase and Time-Resolved Measurements of Unsteady Heat Transfer and Pressure in a Full-Stage Rotating Turbine

Abstract: This paper presents detailed phase-resolved heat-flux data obtained on rotor blades and a comparison of simultaneously obtained time-resolved heat-flux and static pressure data obtained on the stationary shroud of a Garrett TFE 731-2 HP full-stage rotating turbine. A shock tube is used to generate a short-duration source of heated and pressurized air and platinum thin-film gages are used to obtain heat-flux measurements. Blade results are presented at several selected blade locations. Shroud surface pressure and heat-flux time histories are presented for comparable locations relative to the blade position.

A shock tube study of the reaction of the hydroxyl radical with H2, CH4, c-C5H10, and i-C4H10

Abstract: Reactions of the hydroxyl radical, OH, with several reactants have been studied near 1200 K in shock tube experiments in which UV absorption was used to monitor the OH concentration. The values of the rate coefficients were found to be 2.7 × 1012, 2.6 × 1012, 2.8 × 1013, and 1.26 × 1013 cm3/mol-s for the reactions of OH with hydrogen, methane, cyclopentane, and isobutane, respectively. These measured values are compared with previous experimental results and transition-state theory calculations.

Development of a laser absorption diagnostic for shock tube studies of CH

Abstract: A sensitive and quantitative diagnostic for CH based on cw laser absorption has been developed for CH kinetics experiments in a shock tube. A number of transitions in the A2 δ(ν = 0) ← X2Π(ν = 0) band and C2Σ+(ν = 0)← X2Π(ν = 0) band which are promising for accurate CH detection were considered, leading to the selection of 431.1311 nm (vac.) as the wavelength of maximum absorption over a range of temperature from 1500 to 4000 K. This corresponds to the coincidental overlap of the Q1d(7) and Q2c(7) transitions. Reflected shock experiments involving the pyrolysis of highly dilute mixtures of ethane or methane in argon were used to verify details of the spectroscopic modelling and to investigate the sensitivity of the method for monitoring CH. The detection limit for single-pass absorption (14.3 cm) was found to be below 0.2 ppm at 2000 K for post-shock pressures of 1 atm. This new diagnostic is well suited for studies of elementary reactions involving CH.

An experimental study of non-equilibrium vapour condensation in a shock-tube

Abstract: Filmwise non-equilibrium condensation of methanol vapour is investigated on a shock-tube endwall behind a reflected shock wave. The theoretical prediction of a transition phenomenon during the growth of a liquid film is experimentally demonstrated: the film grows approximately in proportion to the time at the early stages after the reflection of the shock wave and, after a transition period, it grows in proportion to the square root of the time. The condensation parameter of the vapour is obtained from the conformity between experiment and theory. It is found that the condensation parameter of the methanol vapour is of the magnitude of one tenth of the value for the complete capture of the molecules on the liquid surface.

Formation of silicon carbide particles behind shock waves

Abstract: The formation of silicon carbide powder in a 3% SiH4‐3% CH4‐Ar mixture was studied in a shock tube. The experiments were conducted at the temperature of 800–3650 K, pressures of 0.46–4.16 atm, and reaction times of 1–2 ms. The progress of reaction was monitored by the attenuation of a He‐Ne (633 nm) laser beam. Powders collected at the end wall of the tube were identified to be β‐SiC and at high temperatures contained particles having sizes up to 0.5 μm. The large particles had the form of thin single‐crystal platelets with hexagonal and truncated triangular shapes. The transmitted laser light intensity as a function of reaction time exhibited a pronounced minimum at incident shock temperatures above 1700 K. A reaction model is proposed that explains the experimental observations. It is postulated that SiC particles are nucleated homogeneously, along with Si particles. The latter are etched by the products of methane pyrolysis and the chemical species formed by the etching add to the growth of SiC particles.

Silane pyrolysis in a piston reactor

Abstract: A piston reactor has been developed to examine some aspects of particle formation during the pyrolysis of silane (SiH4). This reactor generates conditions intermediate between a static pyrolysis reactor and a shock tube reactor. It effectively excludes contributions of wall reactions to the pyrolysis experiments. The apparent kinetics for this reactor do not conform to values published in the literature. A model that incorporates silane-particle reactions can account for the deviations. In addition, emission of visible light accompanies the sooting reactions. The source of this emission seems to be the sum of incandescence from the hot particles and continuum radiation from another source.

Comparison with experiment for tvd calculations of blast waves from a shock tube

Abstract: Harten's second-order-accurate total-variation-diminishing (TVD) scheme is applied to calculation of flow from the open end of a shock tube. Comparison of numerical results with available experimental data for overpressure at selected points around the shock tube exit shows good agreement. Numerically indicated positions of the moving shock front and Mach stem also compare well with flow shadowgraph data. Where the problem geometry is sufficiently simple and rectangular gridding can be used, Harten's method affords a good choice for blast wave calculations.

Dynamic Measurements in Combustible and Detonable Aerosols

Abstract: We present all the methods used at the ISL for determining the principal parameters of rapid fuel air aerosol dispersions and of their reactions, i.e. combustion and detonation. In a shock tube or in the free atmosphere. we measure the velocity of the dispersed fuel, the breaking of the droplets, their size distribution and their velocities. The investigation of the thermal variations due to the fuel evaporation allows to obtain the composition of the two-phase mixture air/combustible gaz – fuel.

Laser photolysis shock tube for combustion kinetics studies

Abstract: An excimer laser photolysis method has been developed for shock tube kinetics studies of elementary combustion reactions. The excimer laser provides a high intensity source of UV photons well suited to the rapid production of reactive radicals, while shock wave heating provides a convenient means of varying the reaction environment over a wide temperature range. An ArF excimer (193.3 nm) was used to photolyze H 2 O behind reflected shock waves to produce H and OH radicals. The temporal variation of the OH concentration was monitored using a cw, narrow-linewidth laser absorption diagnostic, and the rate coefficient of the dominant reaction after photolysis, H+H 2 O→OH+H 2 , was determined. The measured rate coefficient, k =2.4(±.25)×10 14 exp (−10750 (±500)/T,K) cm 3 mole −1 s −1 , T=1600–2500 K, is in good agreement with other recent studies of this reaction in both the forward and reverse directions.

A shock tube and modeling study of the CH3 + CH2O reaction at high temperatures

Abstract: The reaction CH{sub 3} + CH{sub 2}O {yields} CH{sub 4} + CHO has been studied in a shock tube in the temperature range 1,170-1,630 K, and its rate constant has been determined by kinetic modeling of the observed CO formation: k{sub 1} = 10{sup 15.0 {plus minus} 0.4} exp({minus}11,600 {plus minus} 1,260/T). The results confirm earlier observations that the Arrhenius plot curves upward rather sharply at temperatures above 1,000 K, although the effect appears less dramatic than reported previously. This nonlinearity cannot be rationalized by a standard transition-state model. However, the inclusion of a correction for quantum mechanical tunneling leads to an acceptable fit of the observed data over the entire experimental range (300-1,700 K). A nonlinear least-squares fit to the calculated values covering 300-2,000 K gives rise to k{sub 1} = 10{sup {minus}12.05}T{sup 7.4} exp(483/T) cm{sup 3}/mol {times} s.

On the Interaction of Shock Waves with Contact Surfaces Between Gases of Different Densities

Abstract: The interaction of shock waves with a contact surface between gases of different densities has been studied experimentally and theoretically. The basic mechanism for the instability of perturbations at the interface is baroclinic vorticity generation resulting from the misalignment of the pressure gradient of the shock and the density gradient of the interface. In the present study, the effects of interface density contrast and initial thickness, and incident wave strength on the development of the instability at the interface are investigated. The experiments were performed in a new vertical shock tube facility where the interaction of a shock wave with either a discontinuous interface, formed by a thin (0.5 µm) plastic membrane, or a continuous interface, created by retracting a metal plate initially separating the two gases, was studied. Air was used on one side of the interface and either helium, carbon dioxide, refrigerant-22 or sulphur hexafluoride was used on the other side as the test gas. Experiments to study the time evolution of quasi-sinusoidal perturbations on a continuous interface have shown that the growth rates are reduced as the interface thickness is increased. It has been observed that growth rates of perturbations of wavelength λ ~ 25 mm on interfaces of thickness δ ~ 10 mm are about three times smaller than those predicted by the linear theory for the impulsive acceleration of discontinuous interfaces. A new model that accounts for the growth rate reduction caused by the presence of a finite density gradient on the interface has been proposed, and good agreement was obtained with the present experimental results. Experiments were also performed to observe the schlieren visual thickness of plane discontinuous or continuous interfaces with random small-scale perturbations after interaction with the incident shock wave and its reverberations. The interface was initially located near the end wall of the shock tube to permit the observation of the development of the interface phenomena after the arrival of the incident shock and its reverberations. It is found that the interaction of a shock wave with a discontinuous interface causes the appearance of a turbulent mixing zone between the two gases, whose growth rate slows down as time increases, owing to a decrease in turbulence intensity and the action of viscosity. Because of the large uncertainty associated with the measurements a short time after the interaction with the incident shock, the accurate determination of a possible universal power law governing the thickening of the interface is not feasible. Results for the interaction of the first reverberation of the primary wave with the already turbulent interface have demonstrated that this growth is sensitive to the initial pre-growth state of the interface. It also appears that the thickening of the turbulent mixing zone is accomplished by the merging of large structures within the interface. However, since the energy available for the turbulent motions at the impulsively accelerated interface remains constant after the interaction with the shock and also depends on the wavelength of the initial perturbation, it is not certain whether the development of mixing at the interface achieves an asymptotic stage of self-similar turbulence independent of initial conditions, as has been observed for the gravity-driven interfaces. Also, it has been found that the growth rates measured in the present experiments with discontinuous interfaces are nearly an order of magnitude lower than those reported by previous investigators. The continuous interfaces formed by the retracting plate are smoothed by molecular diffusion, and thus the combination of low density gradient and small initial perturbations is such that they exhibit growth only after being perturbed by acoustic noise introduced by the reverberation of waves between the interface, the side walls and the end of the shock tube. The development of viscous boundary layers on the side walls of the test section can cause the bifurcation of waves reflected from the end wall of the shock tube, and, thereafter, the formation of wall bubbles and interface contaminating jets. Moreover, the generation of vortical structures by the baroclinic instability excited by the interaction of reflected waves with the distorted interface within the boundary layer has been demonstrated. Significant contamination of the test gas can by achieved by these structures, even if reflected-wave bifurcation is absent. Moreover, the strain induced by the vorticity in these wall structures tends to thin the interface; the magnitude of this effect on the growth rates in the present plane interface experiments is estimated to be of order 10% for discontinuous interfaces and 50% for continuous interfaces.

Pressure wave propagation in a partially water-saturated porous medium

Abstract: Reflection and transmission of a stepwise pressure wave incident to a partially water‐saturated porous medium is investigated. The strongly dispersive character of the phase velocities and damping due to air bubble resonance causes pressure over‐ and undershoot in reflection and a fast oscillatory disturbance propagating into the porous medium. Theoretical results are compared with new results from shock tube experiments. A qualitative agreement is found.

Nonequilibrium shock layer temperature profiles from arc jet radiation measurements

Abstract: Shock layer temperature profiles are obtained through analysis of radiation from shock layers produced by a blunt body inserted in arc jet flow. Spectral measurements have been made in a nitrogen flow of 54.4 gm/s at an enthalpy of 8.72 MJ/kg. Vibrational temperatures for N2+ are obtained by matching spectral regions from arc jet spectra with spectra generated using the NEQAIR code. Temperature profiles obtained from the radiation layers show a vibrational temperature higher than the rotational temperature near the front of the shock and both temperatures decrease as the flow approaches the body. The spectral measurements are made and analysis completed for four distances, from the surface of the blunt body. The corresponding shock layer thickness is approximately 3.6 cm. Although the shock layer appears to be in thermal nonequilibrium, the measured rotational temperature approaches the single temperature results of viscous shock layer calculations at this test condition.

The Determination of Rate-Limiting Steps during Soot Formation

Abstract: : A single-pulse shock tube has been used to examine the pyrolysis and rich oxidation of cyclopentadiene as well as its pyrolysis in the presence of acetylene, biacetyl, and benzene. In addition, mixtures of benzene and hydrogen have been copyrolyzed. These fuels have been diluted in argon and shock heated over the temperature range of 1200 to 2000K and at total pressures of ten to thirteen atmospheres. Dwell times were about 500-600 microseconds. Collected gas samples were analyzed using gas chromatography for hydrogen, carbon oxides and C1 to C14- hydrocarbons. Experimental results and preliminary chemical kinetic modeling lead to mechanistic proposals on the decomposition of C5-rings as well as isomerization processes between C5- and C6-rings. In addition, data on cyclopentadiene indicates that this fuel has a sooting tendency comparable to that of benzene. Keywords: Pyrolysis, Cyclopentadiene, Benzene, Hydrogen addition, Ring formation, Ring isomerization, Soot formation, Soot modeling, Single-pulse shock tube.

A numerical study of shock wave refraction at a gas interface

Abstract: This paper describes the numerical simulation of a shock wave refracting at a gas interface. In this work we duplicate shock tube experiments performed by Abd-el-Fattah and Henderson using a multifluid, adaptive mesh refinement algorithm. We report on the results of four of these calculations and compare them to the shock tube experiments. The goal of this paper is to validate the numerical method by demonstrating that the numerical results are in excellent agreement with the shock tube experiments. Future work will be concerned with using our numerical method to explore the phenomenon of shock wave refraction and with examining the discrepancy between existing theory and experiment.