Showing papers on "Shock tube published in 2007"

Methane/propane oxidation at high pressures: Experimental and detailed chemical kinetic modeling

Abstract: Shock tube experiments and chemical kinetic modeling were performed to further understand the ignition and oxidation kinetics of various methane–propane fuel blends at gas turbine pressures. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of CH4/C3H8 in ratios ranging from 90/10% to 60/40%. Equivalence ratios varied from lean (ϕ = 0.5), through stoichiometric to rich (ϕ = 3.0) at test pressures from 5.3 to 31.4 atm. These pressures and mixtures, in conjunction with test temperatures as low as 1042 K, cover a critical range of conditions relevant to practical turbines where few, if any, CH4/C3H8 prior data existed. A methane/propane oxidation mechanism was prepared to simulate the experimental results. It was found that the reactions involving CH3O˙, CH3O˙2, and ĊH3 + O2/HO˙2 chemistry were very important in reproducing the correct kinetic behavior.

The autoignition of cyclopentane and cyclohexane in a shock tube

Abstract: Ignition delay times of cyclohexane–oxygen–argon and cyclopentane–oxygen–argon mixtures have been measured in a shock tube, the onset of ignition being detected by OH radical emission. Mixtures contained 0.5 or 1% of hydrocarbon for values of the equivalence ratio ranging from 0.5 to 2. Reflected shock waves allowed temperatures from 1230 to 1840 K and pressures from 7.3 to 9.5 atm to be obtained. These measurements have shown that cyclopentane is much less reactive than cyclohexane, as for a given temperature the observed autoignition delay times were about 10 times higher for the C 5 compound than for the C 6 . Detailed mechanisms for the combustion of cyclohexane and cyclopentane have been proposed to reproduce these results. The elementary steps included in the kinetic models of the oxidation of cyclanes are close to those proposed to describe the oxidation of non cyclic alkanes and alkenes. Consequently, it has been possible to obtain these models by using an improved version of the EXGAS software, a computer package for the automatic generation of detailed kinetic models for the gas-phase combustion of alkanes and alkenes. Nevertheless, the modeling of the oxidation of cyclanes requires new types of generic reactions to be considered, and especially to define new correlations for the estimation of the rate constants. Quantum chemical calculations have been used to improve the estimation of some sensitive rate constants in the case of cyclopentane. The main reaction pathways have been derived from flow rate and sensitivity analysis.

Primary Breakup of Turbulent Round Liquid Jets in Uniform Crossflows

Abstract: An experimental investigation of the deformation and breakup properties of turbulent round liquid jets in uniform gaseous crossflows is described. Pulsed shadowgraph and holograph observations were obtained for turbulent round liquid jets injected normal to air crossflow in a shock tube. Crossflow velocities of the air behind the shock wave relative to the liquid jet were subsonic (36-90 m/s) and the air in this region was at normal temperature and pressure. Liquid injection was done by a pressure feed system through round tubes having inside diameters of 1 and 2 mm and length-to-diameter ratios greater than 100 to provide fully developed turbulent pipe flow at the jet exit. Test conditions were as follows: water and ethyl alcohol as test liquids, crossflow Weber numbers based on gas properties of 0-282, streamwise Weber numbers based on liquid properties of 1400-32,200, liquid/gas density ratios of 683 and 845, and jet exit Reynolds numbers based on liquid properties of 7100-48,200, all at conditions in which direct effects of liquid viscosity were small (Ohnesorge numbers were less than 0.12). Measurements were carried out to determine conditions required for the onset of breakup, ligament and drop sizes along the liquid surface, drop velocities after breakup, liquid column breakup as whole, rates of turbulent primary breakup, and liquid column trajectories. Phenomenological theories proved to be quite successful in interpreting and correlating the measurements.

Near-infrared diode laser absorption sensor for rapid measurements of temperature and water vapor in a shock tube

Abstract: A fast-response (100 kHz) tunable diode laser absorption sensor is developed for measurements of temperature and H2O concentration in shock tubes, e.g. for studies of combustion chemistry. Gas temperature is determined from the ratio of fixed-wavelength laser absorption of two H2O transitions near 7185.60 cm-1 and 7154.35 cm-1, which are selected using design rules for the target temperature range of 1000–2000 K and pressure range of 1–2 atm. Wavelength modulation spectroscopy is employed with second-harmonic detection (WMS-2f) to improve the sensor sensitivity and accuracy. Normalization of the second-harmonic signal by the first-harmonic signal is used to remove the need for calibration and minimize interference from emission, scattering, beam steering, and window fouling. The laser modulation depth for each H2O transition is optimized to maximize the WMS-2f signal for the target test conditions. The WMS-2f sensor is first validated in mixtures of H2O and Ar in a heated cell for the temperature range of 500–1200 K (P=1 atm), yielding an accuracy of 1.9% for temperature and 1.4% for H2O concentration measurements. Shock wave tests with non-reactive H2O–Ar mixtures are then conducted to demonstrate the sensor accuracy (1.5% for temperature and 1.4% for H2O concentration) and response time at higher temperatures (1200–1700 K, P=1.3–1.6 atm).

Experimental investigation of primary and secondary features in high-mach-number shock-bubble interaction.

Abstract: Experiments to study the compression and unstable evolution of an isolated soap-film bubble containing helium, subjected to a strong planar shock wave (M=2.95) in ambient nitrogen, have been performed in a vertical shock tube of square internal cross section using planar laser diagnostics. The early phase of the interaction process is dominated by the formation of a primary vortex ring due to the baroclinic source of vorticity deposited during the shock-bubble interaction, and the mass transfer from the body of the bubble to the vortex ring. The late time (long after shock interaction) study reveals the presence of a secondary baroclinic source of vorticity at high Mach number which is responsible for the formation of counterrotating secondary and tertiary vortex rings and the subsequent larger rate of elongation of the bubble.

Experimental and modelling study of gasoline surrogate mixtures oxidation in jet stirred reactor and shock tube

Abstract: The oxidation of several mixtures of surrogate for gasoline was studied using a jet stirred reactor and a shock tube. One representative of each classes constituting gasoline was selected: iso-octane, toluene, 1-hexene and ethyl tert-butyl ether (ETBE). The experiments were carried out in the 800–1880 K temperature range, for two different initial pressures (0.2 and 1 MPa), with an initial fuel molar fraction of 0.001. The equivalence ratio varied from 0.5 to 1.5. Each hydrocarbon sub-mechanism was validated using shock tube data. The full mechanism describing the surrogate fuel oxidation is constituted of the sub-mechanisms for each fuel components and by adding interaction reactions between different hydrocarbon fragments. Good agreement between the experimental results and the computations was observed under JSR and shock tube conditions.

Quantitative numerical and experimental studies of the shock accelerated heterogeneous bubbles motion

Abstract: This work deals with quantitative comparisons between experimental and numerical results for shock-bubbles interactions. The bubbles are filled with three different gases (nitrogen, krypton and helium) surrounded by air in order to investigate all kind of density jumps across the interface. For each case, three incident shock wave intensities are also studied. The experiments are led by using a shock tube coupled with a visualization diagnostic device: the T80 shock tube [G. Jourdan, L. Houas, L. Schwaederle, G. Layes, R. Carrey, and F. Diaz, “A new variable inclination shock tube for multiple investigations,” Shock Waves 13, 501 (2004)]. Considering the same initial and geometrical conditions, the numerical results are obtained with the help of a recent numerical method: the discrete equations method [R. Abgrall and R. Saurel, “Discrete equations for physical and numerical compressible multiphase mixtures,” J. Comput. Phys. 186, 361 (2003); R. Saurel, S. Gavrilyuk, and F. Renaud, “A multiphase model with internal degrees of freedom: Application to shock-bubble interaction,” J. Fluid Mech. 495, 283 (2003); A. Chinnayya, E. Daniel, and R. Saurel, “Modelling detonation waves in heterogeneous energetic materials,” J. Comput. Phys. 196, 490 (2004); O. Le Metayer, J. Massoni, and R. Saurel, “Modelling evaporation fronts with reactive Riemann solvers,” J. Comput. Phys. 205, 567 (2005)], devoted to the computation of interface problems as well as multiphase mixtures. For each configuration, the quantitative comparisons are in good agreement showing the capability of both methods (numerical and experimental) to describe complex physical flows.

Combustion of CO/H2 mixtures at elevated pressures ☆

Abstract: The high pressure oxidation of dilute CO mixtures doped with 150–200 ppm of H 2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500 K and pressures spanning 21–500 bars for stoichiometric ( Φ = 1) and fuel lean ( Φ = 0.5) oxidation. Stable species sampled from the shock tube were analyzed by standard GC, GC/MS techniques. The experimental data obtained in this work were simulated using a detailed model for H 2 /CO combustion that was validated against a variety of experimental observables/targets that span a wide range of conditions. These simulations have shown that within experimental error the model is able to capture the experimental trends for the lower pressure data sets (average nominal pressures of 24 and 43 bars). However the model under predicts the CO and O 2 decay and subsequent CO 2 formation for the higher pressure data sets (average nominal pressures of 256 and 450 bars). The current elevated pressure data sets span a previously unmapped regime and have served to probe HO 2 radical reactions which appear to be among the most sensitive reactions in the model under these conditions. With updated rate parameters for a key HO 2 radical reaction OH + HO 2 = H 2 O + O 2 , the model is able to reconcile the elevated pressure data sets thereby extending its capability to an extreme range of conditions.

The development of a detailed chemical kinetic mechanism for diisobutylene and comparison to shock tube ignition times

Abstract: Shock tube experiments and chemical kinetic modeling were carried out on 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, the two isomers of diisobutylene, a compound intended for use as an alkene component in a surrogate diesel. Ignition delay times were obtained behind reflected shock waves at 1 and 4 atm, and between temperatures of 1200 and 1550 K. Equivalence ratios ranging from 1.0 to 0.25 were examined for the 1-pentene isomer. A comparative study was carried out on the 2-pentene isomer and on the blend of the two isomers. It was found that the 2-pentene isomer ignited significantly faster under shock tube conditions than the 1-pentene isomer and that the ignition delay times for the blend were directly dependant on the proportions of each isomer. These characteristics were successfully predicted using a detailed chemical kinetic mechanism. It was found that reactions involving isobutene were important in the decomposition of the 1-pentene isomer. The 2-pentene isomer reacted through a different pathway involving resonantly stabilized radicals, highlighting the effect on the chemistry of a slight change in molecular structure.

Fluid-structure interaction effects in the dynamic response of free-standing plates to uniform shock loading

Abstract: The problem of uniform shocks interacting with free-standing plates is studied analytically and numerically for arbitrary shock intensity and plate mass. The analysis is of interest in the design and interpretation of fluid-structure interaction (FSI) experiments in shock tubes. In contrast to previous work corresponding to the case of incident blast profiles of exponential distribution, all asymptotic limits obtained here are exact. The contributions include the extension of Taylor's FSI analysis for acoustic waves, the exact analysis of the asymptotic limits of very heavy and very light plates for arbitrary shock intensity, and a general formula for the transmitted impulse in the intermediate plate mass range. One of the implications is that the impulse transmitted to the plate can be expressed univocally in terms of a single nondimensional compressible FSI parameter.

Reflected shock ignition and combustion of aluminum and nanocomposite thermite powders

Abstract: A comparison of the ignition and combustion characteristics of Al-Fe2O3 and Al-MoO3 nanocomposite powders and two sizes of aluminum powder in inert and oxidizing environments was performed in the region behind a reflected shock in a shock tube. Radiation intensity was monitored by photometry, and temporal information on the particle temperatures was obtained using high-speed pyrometry. In addition, emission spectra were collected to identify intermediate species produced during combustion. In inert environments, both thermite materials showed evidence of ignition within 1–2 ms at 1400 and 1800 K. Particle temperatures during reaction ranging from 2700–3350 K were observed, with Al-MoO3 having generally higher temperatures than Al-Fe2O3. Addition of oxygen in the ambient environment reduced ignition times and increased combustion temperatures to 3350–3800 K as well, suggesting that heterogeneous reactions can enhance the combustion performance of the thermite materials. In air at 3 atm, the nanocomposite t...

Controlling the form of strong converging shocks by means of disturbances

Abstract: The influence of artificial disturbances on the behavior of strong converging cylindrical shocks is investigated experimentally and numerically. Ring-shaped shocks, generated in an annular cross sectional shock tube are transformed to converging cylindrical shocks in a thin cylindrical test section, mounted at the rear end of the shock tube. The converging cylindrical shocks are perturbed by small cylinders placed at different locations and in various patterns in the test section. Their influence on the shock convergence and reflection process is investigated. It is found that disturbances arranged in a symmetrical pattern will produce a symmetrical deformation of the converging shockfront. For example, a square formation produces a square-like shock and an octagon formation a shock with an octagonal front. This introduces an alternative way of tailoring the form of a converging shock, instead of using a specific form of a reflector boundary. The influence of disturbances arranged in non-symmetric patterns on the shape of the shockfront is also investigated.

Shock tube/time-of-flight mass spectrometer for high temperature kinetic studies.

Abstract: A shock tube (ST) with online, time-of-flight mass spectrometric (TOF-MS) detection has been constructed for the study of elementary reactions at high temperature. The ST and TOF-MS are coupled by a differentially pumped molecular beam sampling interface, which ensures that the samples entering the TOF-MS are not contaminated by gases drawn from the cold end wall thermal boundary layer in the ST. Additionally, the interface allows a large range of postshock pressures to be used in the shock tube while maintaining high vacuum in the TOF-MS. The apparatus and the details of the sampling system are described along with an analysis in which cooling of the sampled gases and minimization of thermal boundary layer effects are discussed. The accuracy of kinetic measurements made with the apparatus has been tested by investigating the thermal unimolecular dissociation of cyclohexene to ethylene and 1,3-butadiene, a well characterized reaction for which considerable literature data that are in good agreement exist. The experiments were performed at nominal reflected shock wave pressures of 600 and 1300 Torr, and temperatures ranging from 1260 to 1430 K. The rate coefficients obtained are compared with the earlier shock tube studies and are found to be in very good agreement. As expected no significant difference is observed in the rate constant between pressures of 600 and 1300 Torr.

Richtmyer-Meshkov instability induced by the interaction of a shock wave with a rectangular block of SF6

Abstract: In this article the interaction of a shock wave with a rectangular block of sulphur hexafluoride (SF6), occupying part of the test section of a shock tube, is studied by experimental and numerical means. The difference between the ratios of the specific heats of the two gases (air and SF6) gives rise to numerical problems (generation of spurious waves at their interface). This necessitated the development of a multifluid algorithm (augmented Navier-Stokes formulation). The governing equations are based on a thermodynamically consistent and fully conservative formulation. A Riemann-problem-based scheme (the weighted average flux method) is used to integrate the hyperbolic part of the system. To this end, a new approximate Riemann problem solver has been formulated to account for the variable ratio of specific heats. The resulting algorithm was implemented in an adaptive mesh refinement code, which allowed high-resolution simulations to be performed on desktop computers. The evolution of the flow is well ca...

Open-ended shock tube flows : Influence of pressure ratio and diaphragm position

Abstract: The influence of the pressure ratio and the diaphragm location on the flow from open-ended shock tubes is investigated. In contrast to previous studies, in which attention was focused on the discharge of the shock wave from the shock tube, we consider also the influence of the contact discontinuity and the expansion fan. It is found that if the pressure ratio is large enough to lead to supersonic flow behind the contact discontinuity, the flow at the open end relaxes from the conditions behind the contact discontinuity to sonic conditions once the tail of the expansion fan arrives at the open end. Theory indicates that the time scale over which the flow relaxes to sonic conditions is nearly independent of the initial Mach number. Also, the time scale is much longer than that required by the acceleration of subsonic conditions behind the contact discontinuity to sonic conditions. The relaxation process is shown to influence the evolution of the Mach-disk shock, the barrel shock, and the reflected shock wave in an underexpanded jet.

Shock-Tube Determination of CN Formation Rate in a CO-N2 Mixture

Abstract: The chemical process of CN formation in a CO-N 2 mixture is studied through temporally resolved intensity measurement of CN violet radiation occurring in the reflected-shock region of a shock tube. A 78% CO-22% N 2 mixture is driven by cold hydrogen to a shock speed of up to 3.45 km/s, to produce a reflected-shock temperature corresponding to Martian entry flight of up to 6.4 km/s. Absolute calibration of spectrometer is performed using a standard lamp of radiance. A reaction model is constructed by combining four existing models and multiplying the C 2 dissociation rate by a factor of five.

Interaction of a shock wave with a loose dusty bulk layer

Abstract: The interaction of a planar shock wave with a loose dusty bulk layer has been investigated both experimentally and numerically. Experiments were conducted in a shock tube. The incident shock wave velocity and particle diameters were measured with the use of pressure transducers and a Malvern particle sizer, respectively. The flow fields, induced by shock waves, of both gas and granular phase were visualized by means of shadowgraphs and pulsed X-ray radiography with trace particles added. In addition, a two-phase model for granular flow presented by Gidaspow is introduced and is extended to describe such a complex phenomenon. Based on the kinetic theory, such a two-phase model has the advantage of being able to clarify many physical concepts, like particulate viscosity, granular conductivity and solid pressure, and deduce the correlative constitutive equations of the solid phase. The AUSM scheme was employed for the numerical calculation. The flow field behind the shock wave was displayed numerically and agrees well with our corresponding experimental results.

Visualization of Wave Rotor Inner Flow Dynamics

Abstract: The design of a wave rotor requires an understanding of the pressure wave dynamics in the cells (rotor passages). The present paper describes a two-dimensional numerical simulation and an experimental visualization of the wave rotor compression process. First, a unique experimental apparatus with fixed cells and rotating ports was constructed for the visualization and direct measurements; this arrangement is opposite to the conventional setup. Next, experimental and numerical results were compared to verify the simulation modeling, particularly with regard to the propagation velocity of pressure waves in the cells. Last, the effects of gradually opening the cell to the ports and leakage through the clearance, which are considered to be dominant factors in the wave rotor operation, on the pressure wave dynamics were carefully investigated. The results showed that the gradual passage opening greatly influences the primary shock wave, whereas leakage mostly influences the reflected shock wave. Moreover, it was revealed that the leakage generates an extra pressure wave during the compression process due to the interaction between adjacent cells.

Radiation Measurements in a Shock Tube for Titan Mixtures

Abstract: Experiments have been carried out in the frame of the Huygens mission preparation to evaluate the radiative heat flux during the entry of the space probe into the atmosphere of Titan. Time-resolved emissions of CN and C 2 molecules are studied behind a strong shock wave generated in a free piston shock tube. CN violet, C 2 Swan, and CN red radiative systems are recorded for pressures from 40 to 1100 Pa at a shock velocity equal to 5500 m/s. The calibration of the emission measurement is carried out and values of absolute heat flux density are given. The calibrated time profiles are commented and the evolution with pressure of the nonequilibrium radiation is underlined. An analysis of the CN violet spectra is performed by comparison with a spectroscopic code, taking into account self-absorption and able to determine rotational and vibrational temperatures as well as ground state density by an iterative least-squares process. Last, obtained data led to the proportions of radiation emitted by CN violet and red systems as well as C 2 Swan bands.

PIV Measurements in Shock Tunnels and Shock Tubes

Abstract: Shock tubes and shock tunnels generate compressible flows that are characterized by short time durations and large gradients. Therefore, these flows are a challenging application for all kinds of measurement systems. Additionally, a high information density is desirable for each experiment due to short measurement times. During recent years, particle image velocimetry (PIV) was therefore extensively tested and successfully applied to different flow configurations in the shock-tube department at ISL. It turned out that one difficulty common to all shock-tube or tunnel applications is a good timing and triggering of the PIV system. An appropriate seeding is another crucial factor. The latter is particularly difficult to manage because, in contrast to continuous facilities, no assessment of the seeding quality is possible before and during the experiment. Several measurement results are shown and explained in the chapter.

Diagnostic Modelling of an Expansion Tube Operating Condition for a Hypersonic Free Shear Layer Experiment

Abstract: Computational simulations of the AIR-1 test condition in the University of Illinois’ Hypervelocity Expansion Tube were conducted to verify facility operation and to obtain free stream properties that are otherwise difficult to measure. Two types of simulation were undertaken. The first was a one-dimensional simulation of the entire facility and the second was a hybrid simulation, combining a one-dimensional simulation of the shock tube section with a two-dimensional simulation of the acceleration tube. The one-dimensional simulation matched the experimental data well, however the two-dimensional simulation did not initially match the experimental measurements of shock speed and test gas pitot pressure. Further investigation showed the shock speed discrepancy was consistent with air contamination into the acceleration tube and subsequent two-dimensional simulations assuming 10% air contamination showed reasonable agreement with experimental data. Using data taken from the two-dimensional simulation of the expansion tube as a transient inflow condition, modelling was undertaken of a free shear layer experiment being conducted in the facility. Results from equilibrium, finite rate, and perfect gas models were compared. The finite rate simulation provides the best agreement with experimental Schlieren images, with the simulation capturing the major flow structures seen in experiments.