Showing papers on "Shock tube published in 2009"

A Shock Tube Study of the Ignition of n-Heptane, n-Decane, n-Dodecane, and n-Tetradecane at Elevated Pressures

Abstract: The ignition of n-heptane, n-decane, n-dodecane, and n-tetradecane has been investigated in a heated shock tube. n-Alkane/air mixtures at Φ = 0.25, 0.5, and 1.0 were studied in reflected shock experiments at 9−58 atm and 786−1396 K. Ignition times were measured using a combination of endwall electronically excited OH emission and sidewall pressure measurements. The measured ignition times are compared to previous data, where available, with good agreement and to several kinetic modeling predictions. The current data and the combination of the current data with previous shock tube and rapid compression machine measurements show that any differences in reactivity for C7 and larger n-alkanes is slight, within the experimental uncertainties, for n-alkane/air mixtures with common carbon content at a large range of temperatures (650−1400 K) and elevated pressures. To our knowledge, the n-tetradecane measurements presented here are the first ignition measurements to be reported for this compound. The complete da...

n-Dodecane oxidation at high-pressures: Measurements of ignition delay times and OH concentration time-histories

Abstract: Ignition delay times and OH concentration time-histories were measured during n-dodecane oxidation behind reflected shocks waves using a heated, high-pressure shock tube. Measurements were made over temperatures of 727–1422 K, pressures of 15–34 atm, and equivalence ratios of 0.5 and 1.0. Ignition delay times were measured using side-wall pressure and OH∗ emission diagnostics, and OH concentration time-histories were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH A–X (0, 0) system at 306.47 nm. Shock tube measurements were compared to model predictions of four current n-dodecane oxidation detailed mechanisms, and the differences, particularly in the low-temperature negative-temperature-coefficient (NTC) region where the influence of non-ideal facility effects can be significant, are discussed. To our knowledge, the current measurements provide the first gas-phase shock tube ignition delay times (at pressures above 13 atm) and quantitative OH concentration time-histories for n-dodecane oxidation under practical engine conditions, and hence provide benchmark validation targets for refinement of jet fuel detailed kinetic modeling, since n-dodecane is widely used as the principal representative for n-alkanes in jet fuel surrogates.

Experimental study and modeling of shock tube ignition delay times for hydrogen–oxygen–argon mixtures at low temperatures

Abstract: Recent literature has indicated that experimental shock tube ignition delay times for hydrogen combustion at low-temperature conditions may deviate significantly from those predicted by current detailed kinetic models. The source of this difference is uncertain. In the current study, the effects of shock tube facility-dependent gasdynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen–oxygen ignition delay times. Shock tube hydrogen–oxygen ignition delay time data were taken behind reflected shock waves at temperatures between 908 to 1118 K and pressures between 3.0 and 3.7 atm for two test mixtures: 4% H2, 2% O2, balance Ar, and 15% H2, 18% O2, balance Ar. The experimental ignition delay times at temperatures below 980 K are found to be shorter than those predicted by current mechanisms when the normal idealized constant volume (V) and internal energy (E) assumptions are employed. However, if non-ideal effects associated with facility performance and energy release are included in the modeling (using CHEMSHOCK, a new model which couples the experimental pressure trace with the constant V, E assumptions), the predicted ignition times more closely follow the experimental data. Applying the new CHEMSHOCK model to current experimental data allows refinement of the reaction rate for H + O2 + Ar ↔ HO2 + Ar, a key reaction in determining the hydrogen–oxygen ignition delay time in the low-temperature region.

A Shock Tube Study of n- and iso-Propanol Ignition

Abstract: An understanding of the ignition and oxidation characteristics of propanol, as well as other alcohols, is important toward the development and design of combustion engines that can effectively utilize bioderived and bioblended fuels. Building upon a database for “first-generation” alcohols including methanol and ethanol, the ignition characteristics of the two isomers of propanol (n-propanol and iso-propanol) have been studied in a shock tube. Ignition delay times for propanol/oxygen/argon mixtures have been measured behind reflected shock waves at temperatures ranging from approximately 1350 to 2000 K and a pressure of 1 atm. Equivalence ratios of 0.5, 1.0, and 2.0 have been used. Pressure measurements and CH* emissions were used to determine ignition delay times. The influences of equivalence ratio, temperature, and mixture strength on ignition delay have been characterized and compared to the behavior seen with a newly developed detailed kinetic mechanism. The overall trends are captured fairly well by...

Hydrogen–nitrous oxide delay times: Shock tube experimental study and kinetic modelling

Abstract: Hydrogen–nitrous oxide mixtures have been studied for decades. In addition to their fundamental interest, they might play an important role in semi-conductor industry safety. Indeed, in silane–nitrous oxide mixtures, widely used in this industry, the silane molecule readily decomposes into molecular hydrogen which can react violently with nitrous oxide. Despite numerous shock tube studies on H 2 –N 2 O delay times, the pressure effect has never been addressed. The present work aims at studying this effect and at developing a detailed kinetic mechanism able to accurately reproduce experimental delay time data. Delay times of H 2 –N 2 O–Ar mixtures have been measured behind reflected shock waves in the 1300–2000 K temperature range and at a pressure around 300 kPa. Mixtures equivalence ratios ranged between 0.5 and 2, and the dilution was 98 and 99 mol% Ar. The present results and those from a previous study carried out in our institute show, first that, in the studied conditions, the equivalence ratio has no influence on delay times, and second, that the pressure increase drastically reduces the delay times. A kinetic model has been constructed from previously published mechanisms and tested against the present data with a mean error of 29%. Moreover, other delay time data from the literature for H 2 –O 2 –Ar, NH 3 –Ar and H 2 –N 2 O–Ar mixtures are also correctly reproduced as well as macroscopic parameters such as reduced activation energies.

A shock tube study of the auto-ignition of toluene/air mixtures at high pressures

Abstract: The auto-ignition of toluene/air mixtures was studied in a shock tube at temperatures of 1021–1400 K, pressures of 10–61 atm, and equivalence ratios of Φ = 1.0, 0.5, and 0.25. Ignition times were measured using endwall OH∗ emission and sidewall piezoelectric pressure measurements. The measured pressure time-histories do not show significant pre-ignition energy release, in agreement with the rapid compression machine study of Mittal and Sung [G. Mittal, C.-J. Sung, Combust. Flame 150 (2007) 355–368] and disagreement with the shock tube study of Davidson et al. [D.F. Davidson, B.M. Gauthier, R.K. Hanson, Proc. Combust. Inst. 30 (2005) 1175–1182]. Kinetic modeling predictions from three detailed mechanisms are compared. Sensitivity analysis indicates that the reaction of toluene (C 6 H 5 CH 3 ) and the benzyl radical (C 6 H 5 CH 2 ) with molecular oxygen are important and examination of the rate coefficients for these reactions suggests that improved rate parameters for the multi-channel C 6 H 5 CH 2 + O 2 reaction may improve model predictions.

The use of driver inserts to reduce non-ideal pressure variations behind reflected shock waves

Abstract: Non-ideal shock tube facility effects, such as incident shock wave attenuation, can cause variations in the pressure histories seen in reflected shock wave experiments. These variations can be reduced, and in some cases eliminated, by the use of driver inserts. Driver inserts, when designed properly, act as sources of expansion waves which can counteract or compensate for gradual increases in reflected shock pressure profiles. An algorithm for the design of these inserts is provided, and example pressure measurements are presented that demonstrate the success of this approach. When these driver inserts are employed, near- ideal, constant-volume performance in reflected shock wave experiments can be achieved, even at long test times. This near-ideal behavior simplifies the interpretation of shock tube chemical kinetics experiments, particularly in experiments which are highly sensitive to temperature and pressure changes, such as measurements of ignition delay time of exothermic reactions.

Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube

Abstract: We report on new characterization capabilities recently implemented in the NASA Ames Electric Arc Shock Tube (EAST) facility. A new optical configuration, completely enclosed within a high vacuum chamber and attached to the shock tube, has enabled observation of spatially (and hence, temporally) resolved radiation through the shock layer. These imaging optics are coupled with four spectrometers covering the complete wavelength range of 1201700 nm, allowing for simultaneous measurement, at the same axial location, of spectral features over a broad range at various spectral resolutions. Measurements in the new system have addressed several of the discrepancies between model and experiment in prior EAST testing. The presence of CN impurity emission has been nearly eliminated, while atomic C and H emissions have been reduced. The presence of background continuum radiation has been confirmed as a real effect in the shock tube. Stark broadening measurements have been performed on Hydrogen Balmer-α line and show the electron number densities in the shock to be somewhat higher than model predictions. Experimental artifacts in the old configuration have been discovered and explain disagreements in the measured absolute magnitude of radiance.

Ignition time measurements for methylcylcohexane- and ethylcyclohexane-air mixtures at elevated pressures

Abstract: The ignition of methylcyclohexane (MCH)/air and ethylcyclohexane (ECH)/air mixtures has been studied in a shock tube at temperatures and pressures ranging from 881 to 1319 K and 10.8 to 69.5 atm, respectively, for equivalence ratios of 0.25, 0.5, and 1.0. Endwall OH* emission and sidewall pressure measurements were used to determine ignition delay times. The influence of temperature, pressure, and equivalence ratio on ignition has been characterized. Negative temperature coefficient behavior was observed for temperatures below 1000 K. These measurements greatly extend the database of kinetic targets for MCH and provide, to our knowledge, the first ignition measurements for ECH. The combination of the MCH measurements with previous shock tube and rapid compression machine measurements provides kinetic targets over a large temperature range, 680–1650 K, for the validation of kinetic mechanisms. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 82–91, 2009

Experimental validation of a Richtmyer–Meshkov scaling law over large density ratio and shock strength ranges

Abstract: A universal scaling law for the Richtmyer–Meshkov instability is validated with experimental results covering a wide range of density ratios and shock strengths. These results include the first membraneless, gas-phase, interface experiments for A>0.5 and M>1.5. The shock-accelerated, sinusoidal interface experiments are conducted in a vertical shock tube with a large square cross section and cover the experimental parameter space: 0.29

On Transonic Shocks in a Nozzle with Variable End Pressures

Abstract: In the book, Courant and Friedrichs (Supersonic Flow and Shock Waves. New York: Interscience Publishers, 1948) described the following transonic shock phenomena in a de Laval nozzle: Given the appropriately large receiver pressure p r , if the upstream flow is still supersonic behind the throat of the nozzle, then at a certain place in the diverging part of the nozzle a shock front intervenes and the gas is compressed and slowed down to subsonic speed. The position and the strength of the shock front are automatically adjusted so that the end pressure at the exit becomes p r . When the end pressure p r varies and lies in an appropriate scope, in general, it is expected that a curved transonic shock is still formed in a nozzle. In this paper, we solve this problem for the two-dimensional steady Euler system with a variable exit pressure in a nozzle whose divergent part is an angular sector. Both existence and uniqueness are established.

Shock Radiation Measurements for Mars Aerocapture Radiative Heating Analysis

Abstract: NASA's In-Space Propulsion Technology program is supporting the development of shock radiation transport models for aerocapture missions to Mars and Venus. Phenomenological models of nonequilibrium shock radiation will be incorporated into high-fidelity flowfield computations used to predict the aerothermal environments for a Mars or Venus aerocapture entry vehicle. These models are validated with shock radiance measurements obtained at flight-relevant conditions. A comprehensive test series in the NASA Ames Electric Arc Shock Tube facility at a representative freestream condition was recently completed. The facility's optical instrumentation enabled spectral measurements of shocked gas radiation from the vacuum ultraviolet to the near infrared. The instrumentation captured the nonequilibrium postshock excitation and relaxation dynamics of dispersed spectral features. A description of the shock tube facility, optical instrumentation, and examples of the test data are presented.

Recent advances in shock tube/laser diagnostic methods for improved chemical kinetics measurements

Abstract: Shock tubes combined with laser diagnostics provide state-of-the-art capabilities for studying the chemical kinetics of combustion processes. We report here several new concepts and methods designed to improve shock tube performance and modeling, extend shock tube operating regimes, provide access to low vapor pressure fuels, and quantitatively measure species time-histories using continuous wave laser absorption. These new methods are discussed in the context of studying ignition processes of hydrocarbon fuels at practical engine conditions; examples of the use of these methods to study the chemical kinetics of real fuels and to resolve current issues related to shock tube facility effects are given.

Hydrogen Peroxide Decomposition Rate: A Shock Tube Study Using Tunable Laser Absorption of H2O near 2.5 μm

Abstract: The thermal decomposition of hydrogen peroxide was measured behind reflected shock waves in hydrogen peroxide/inert gas mixtures using a sensitive laser diagnostic for water vapor. In these mixtures, the formation rate of water is predominantly controlled by the decomposition rate of hydrogen peroxide. Rate determinations were made over a temperature range of 1000−1200 K and a pressure range of 0.9−3.2 atm for both argon and nitrogen carrier gases. Good detection sensitivity for water was achieved using tunable diode laser absorption of water at 2550.96 nm within its v3 fundamental band. Hydrogen peroxide decomposition rates were found to be independent of pressure at 0.9 and 1.7 atm and showed only slight influence of pressure at 3.2 atm. The best fit of the current data to the low-pressure-limit rate for H2O2 dissociation in argon bath gas is k1,0 = 1015.97±0.10 exp(−21 220 ± 250 K/T) [cm3 mol−1 s−1] (1000−1200 K). Experiments conducted in a nitrogen bath gas show a relative collision efficiency of argo...

The high-pressure pyrolysis of saturated and unsaturated C7 hydrocarbons

Abstract: Pyrolysis experiments on n -heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High-Pressure Shock Tube (HPST) at pressures relevant to diesel combustion systems. The experimental pressures for these experiments ranged from 25 to 50 atm and temperatures varied from 1000 to 1350 K with reaction times ranging from 1 to 3 ms. Dilute reagent mixtures ∼100 ppm were prepared in bulk argon and shock heated to study the stable intermediates. The experimental data has been used to develop and validate an updated kinetic model for the pyrolysis of saturated and unsaturated C 7 hydrocarbons. The experimental results and their implication on increased NO emissions from biodiesel blends will also be discussed.

Interpreting Endwall and Sidewall Measurements in Shock-Tube Ignition Studies

Abstract: Chemiluminescence emission from exited species such as OH* or CH* as well as pressure can be convenient and effective diagnostics for monitoring ignition delay times in shock-heated mixtures. Ideally, the ignition delay time obtained from the radical-species emission signal should agree with ignition delay time as obtained from the pressure trace. Under ideal shock-tube conditions, ignition behind the reflected shock wave occurs first at the endwall, so the measurement of endwall pressure is often considered the best way to determine ignition delay time when such an increase in pressure is available. However, the signal-to-noise ratio of data from a pressure transducer mounted in the endwall can be relatively low when compared to that of an emission signal, so the latter technique provides a useful alternative to pressure. In the present paper, an analytical model of endwall emission measurements is presented, and recent experimental results are studied to determine whether or not endwall measurements are...

Shock tube experiments and numerical simulation of the single-mode, three- dimensional Richtmyer-Meshkov instability

Abstract: A vertical shock tube is used to perform experiments on the single-mode three-dimensional Richtmyer-Meshkov Instability. The interface is formed using apposed flows of air and SF6 and the perturbation is created by the periodic motion of the gases within the shock tube. Planar laser induced fluorescence is used for flow visualization. Experimental results were obtained with a shock Mach number of 1.2. A three-dimensional numerical simulation of this experiment was conducted the results of which are compared with the experimental images and measurements.

Numerical analysis on auto-ignition of a high pressure hydrogen jet spouting from a tube

Abstract: In this study, a direct numerical simulation based on compressible flow dynamics has been applied to the autoignition and extinction of a high-pressure hydrogen jet spouting from a tube. The diameter of the tube is 4.8 mm. The length of the tube is 71 mm. At the inlet, pressure is set at 3.6, 5.3 and 21.1 MPa, and temperature is set at 300 K for all cases. To explore the autoignition of hydrogen jet, two-dimensional axisymmetric Navier–Stokes equations with a detailed chemical kinetics and rigorous transport properties have been employed. The hydrogen jet through the tube is choked. The numerical results show that the high-pressure hydrogen jet produces a semi-spherical shock wave in the ambient air at the early time of jetting. The shock wave heats up the air to a high temperature and causes the autoignition of the hydrogen and air mixture in the tube as well as at the tube exit.

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Abstract: Shock tube ignition delay times were measured for DF-2 diesel/21% O 2 /argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.

Wall shocks in high-energy-density shock tube experiments

Abstract: The radiative precursor of a sufficiently fast shock has been observed to drive the vaporization of shock tube material ahead of the shock. The resulting expansion drives a converging blast wave into the gas volume of the tube. The effects of this wall shock may be observed and correlated with primary shock parameters. We demonstrate this process in experiments performed on the Omega Laser Facility, launching shocks propagating through xenon with speeds above 100 km/s driven by ablation pressures of approximately 50 Mbars. Wall shocks in laser experiments, in which the principal shock waves themselves should not be radiative, are also reported—in which the wall shocks have been launched by some other early energy source.

Shock Tube Study of Methylcyclohexane Ignition over a Wide Range of Pressure and Temperature

Abstract: Ignition delay times were measured for gas-phase methylcyclohexane (MCH)/O2/argon and MCH/air mixtures behind reflected shock waves Initial postshock conditions covered temperatures of 795−1560 K, pressures of 1−50 atm, fuel concentrations of 025−2%, and equivalence ratios (ϕ) of 05−20 Ignition delay times were measured using side-wall pressure and CH* and OH* emission measurements Current measurements complement past high-pressure rapid compression machine results, are in good agreement with past low-pressure shock tube data, and significantly extend the pressure range of available shock tube ignition time data Detailed comparisons of experimental data with predictions of available MCH mechanisms are presented, and comparisons of shock tube MCH ignition delay times to those of other important jet fuel surrogates and cyclo-alkanes are discussed

Comparisons of Air Radiation Model with Shock Tube Measurements

Abstract: This paper presents an assessment of the predictive capability of shock layer radiation model appropriate for NASA s Orion Crew Exploration Vehicle lunar return entry. A detailed set of spectrally resolved radiation intensity comparisons are made with recently conducted tests in the Electric Arc Shock Tube (EAST) facility at NASA Ames Research Center. The spectral range spanned from vacuum ultraviolet wavelength of 115 nm to infrared wavelength of 1400 nm. The analysis is done for 9.5-10.5 km/s shock passing through room temperature synthetic air at 0.2, 0.3 and 0.7 Torr. The comparisons between model and measurements show discrepancies in the level of background continuum radiation and intensities of atomic lines. Impurities in the EAST facility in the form of carbon bearing species are also modeled to estimate the level of contaminants and their impact on the comparisons. The discrepancies, although large is some cases, exhibit order and consistency. A set of tests and analyses improvements are proposed as forward work plan in order to confirm or reject various proposed reasons for the observed discrepancies.

Shock ignition: modelling and target design robustness

Abstract: Shock ignition of a pre-compressed deuterium tritium fuel is considered here. When properly timed, a converging shock launched in the target prior to stagnation time strongly enhances the hot spot pressure. This allows ignition to be reached in a nonisobaric configuration. We show in this work that the igniting mechanism is pressure amplification by shock convergence and shock collision. The shock ignition applied to the HiPER target allows one to study the robustness of this method. It is shown that the spike energy is not a critical parameter and that the spike power delivered on the target depends mainly on the shell implosion velocity. Finally, a family of homothetic targets ignited with a shock wave is studied.

High-pressure ethylene oxidation behind reflected shock waves

Abstract: Oxidation of ethylene/air mixtures has been investigated behind reflected shock waves in a shock tube of 76 mm in diameter. Experiments were performed within the temperature range of 1060–1520 K, pressures of 5.9–16.5 atm, and stoichiometries of ϕ = 0.5, 1.0, and 2.0. Emissions of OH (308.9 nm), CH (431.5 nm) and C 2 (516.5 nm) molecules, pressures and ion current records were implemented to measure ignition times of the mixture along the centreline of the tube and in the boundary layer. Empirical correlations for ethylene ignition times have been deduced from the experimental data. Auto-ignition modes (strong, transient and weak) and ignition limits of the mixtures were identified comparing velocities of reflected shock wave and reaction front at different locations from the reflecting wall. Extensive database for validations of high-temperature ethylene reaction mechanism and numerical methods for reaction flow simulations has been obtained from experimental observations.