Pressure jump across normal ionizing shock waves

The pressure jump across normal ionizing shock waves in hydrogen has been measured using piezoelectric probes. The observed pressure jumps exhibit substantial radial variations with average values higher than those predicted by theory.     

High-pressure ethylene oxidation behind reflected shock waves

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  = 0.5, 1.0, and 2.0. Emissions of OH (308.9 nm), CH (431.5 nm) and C₂ (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.     

Ignition of acetylene-oxygen mixtures behind shock waves

The delay time of ignition of C₂H₂-O₂-Ar mixtures of various compositions behind reflected shock waves were measured at 980–1300 K and 0.65 ± 0.05 MPa. A kinetic scheme of the ignition of acetylene based on the available rate constants of the key elementary reactions was developed. The scheme satisfactorily describes the experimental data from various works over wide temperature, pressure, and concentration ranges: 980–2400 K, 0.01–1.0 MPa, and 0.5–20.3 vol % acetylene and 1.25–20.4 vol% O₂.     

High-temperature UV absorption of methyl radicals behind shock waves

The absorption of ultraviolet narrow-line laser radiation by methyl radicals (CH₃) in the electronic system has been studied at high temperatures behind shock waves. Methyl radicals at high temperatures were generated by the shock heating of methyl precursors: azomethane, methyl iodide, and ethane. The spectral shape and intensity of the broadband absorption feature from 211.5 to 220 nm at high temperature (1565 K) has been measured. The absorption coefficient of CH₃ at 216.62 nm, the wavelength of peak absorption at high temperatures in the P+Q band, has been determined from 1200 to 2500 K. Additionally, the absorption coefficients of several interfering UV-absorbing combustion species (, and C₃H₆) have been determined at 216.62 nm.     

Some experiments on ionization behind reflected shock waves in argon

Until now, ionization phenomena behind reflected shock waves have been studied by means of microwave or optical techniques only. In order to get direct and local information on electron temperature and density a plasma eater has been used in this work. Stationary values of these parameters, consistent with thermodynamic equilibrium, were reached after a few microseconds. The time delay between the shock arrival and the onset of ionization was measured more exactly by miniaturized plasma eater systems.     

Multi-species time-history measurements during n-hexadecane oxidation behind reflected shock waves

Species concentration time-histories were measured during oxidation for the large, normal-alkane, diesel-surrogate component n-hexadecane. Measurements were performed behind reflected shock waves in an aerosol shock tube, which allowed for high fuel loading without pre-test heating and possible decomposition and oxidation. Experiments were conducted using near-stoichiometric mixtures of n-hexadecane and 4% oxygen in argon at temperatures of 1165–1352 K and pressures near 2 atm. Concentration time-histories were recorded for five species: C₂H₄, CH₄, OH, CO₂, and H₂O. Methane was monitored using DFG laser absorption near 3.4 μm; OH was monitored using UV laser absorption at 306.5 nm; C₂H₄ was monitored using a CO₂ gas laser at 10.5 μm; and CO₂ and H₂O were monitored using tunable DFB diode laser absorption at 2.7 and 2.5 μm, respectively. These time-histories provide critically needed kinetic targets to test and refine large reaction mechanisms. Comparisons were made with the predictions of two diesel-surrogate reaction mechanisms (Westbrook et al. [1]; Ranzi et al. [9]) that include n-hexadecane, and areas of needed improvement in the mechanisms were identified. Comparisons of the intermediate product yields of ethylene for n-hexadecane with those found for other smaller n-alkanes, show that an n-hexadecane mechanism derived from a simple hierarchical extrapolation from a smaller n-alkane mechanism does not properly simulate the experimental measurements.     

Differential characteristic of the flow behind a shock wave

Relationships between the derivatives on both sides of a discontinuity in a nonstationary shock wave moving with acceleration in a one-dimensional vortex flow of perfect gas are deduced. The problem of interaction between the shock wave and a weak discontinuity is solved based on these relationships.     

Observation of nonequilibrium radiation behind strong shock waves in low-density air

Nonequilibrium radiation phenomena behind strong shock waves in low-density air are observed by using a couple of CCD camera systems in a shock tube experiment. The simultaneous observation for total radiation and its spectral radiation is carried out in order to elucidate spaced-ependent contribution of an individual radiation spectrum to the total radiation intensity. The results are shown for the shock velocity range from 9.0 km/s to 12.1 km/s at the initial pressure 13.3 Pa. Wavelength range is selected from 300 nm to 445 nm to investigate mainly the contributions from UV radiation. It is found that the band spectra due to the molecular species N₂⁺ and CN mainly contribute to the first-peak, while the spectra due to the atomic species O⁺ and N mainly contribute to the formation of the second-peak. It is also found that the Balmer series in H spectra strongly contributes to the second-peak. The radiation along the tube wall surfaces is composed of the same constituents as those around the tube axis as well as the spectra coming from the impurities.     

Characterization of adsorbed species on soot formed behind reflected shock waves

The present work addresses the soot formation parameters behind reflected shock waves and the identification of adsorbed species on their surface. Soot induction delay times and yields have been experimentally determined in the case of toluene pyrolysis highly diluted in argon for the following conditions: the initial carbon atoms concentration was kept constant around 1 × 10¹⁸ C atoms cm⁻³, reflected shock pressure and temperature ranges of 1135-1600 kPa and 1470-2230 K, respectively. The decrease of the induction time, as the temperature is raised, was described using an Arrhenius type expression while, for the bell-shaped evolution of the soot yield versus the temperature, a modified Gaussian expression was derived. Using TEM analysis, the mean particle diameter was found to decrease from 35 to 20 nm as the temperature is raised from 1475 to 2135 K. The micro-texture of the soot sample was found to vary as the temperature is raised, leading to a more organised structure. The adsorbed species on these soot were characterized using laser desorption/ionization time of flight mass spectrometer. Results indicate that for temperatures below 1600 K, PAHs in the 178-572 atomic mass units (amu) range were identified. PAHs range was limited to 178-374 amu above 1900 K and they were of benzenoid type above 1600 K. The amount of species adsorbed on the soot surface was found to be inversely proportional to the soot yield with a maximum for the lower temperature domain.     

Effect of silane addition on acetylene ignition behind reflected shock waves

Ignition delay time and species profile measurements are reported for the combustion of C₂H₂/O₂/Ar mixtures with and without the addition of silane for temperatures between 1040 and 2320 K and pressures near 1 atm. Characteristic times, namely ignition time and time to peak, were determined from the time histories of CH* (A²Δ → X²Π) and OH* (A²Σ⁺ → X²Π) emission near 430 and 307 nm, respectively. For the cases without silane, there is good agreement between the present data and some recent acetylene oxidation results. Small SiH₄ additions (<10% of the fuel) reduced the ignition time in stoichiometric mixtures by as much as 75% for shocks near 1800 K. Similar reductions were seen in the fuel-lean mixture, although the effect was less temperature dependent. Several detailed chemical kinetics mechanisms of hydrocarbon oxidation were compared to the ignition delay-time data and species profiles for C₂H₂/O₂/Ar mixtures without silane. All models under-predicted ignition time for the 98% diluted stoichiometric mixture but matched the fuel-lean ignition data somewhat better. Two of the models displayed the shift in activation energy at lower temperatures seen in the data, although no one model was able to reproduce all ignition times over the entire range of mixtures and conditions.     

Autoignition of propane–air mixtures behind reflected shock waves

Ignition times and autoignition modes for propane–air mixtures have been studied behind reflected shock waves. Experiments were performed over temperatures between 1000 and 1750 K, pressures between 2 and 20 atm, and equivalence ratios of = 0.5, 1.0, and 2.0. Ignition delay times were determined using pressure measurements, C₂ emission profiles, and luminosity measurements in the visible spectrum (380–680 nm). Empirical correlations for ignition time for low temperature (1000–1300 K) and high temperature (1300–1800 K) ranges have been deduced from the experimental data. Different autoignition modes of the mixture (strong, transient, and weak) were identified by comparing velocities of reflected shock wave at different distances from the reflecting wall.     

Some experiments on relaxation behind reflected shock waves in argon-molecular gas mixtures

The limitation of the applicability of plasma eaters in mixtures of argon with molecular components is shown. We were able to measuren _e andT _e in argon-hydrogen mixture behind the reflected shock wave and find a strong reduction of both parameters caused by the molecular species. Similarly, the delay of the onset of ionization in argon-hydrogen and argon-nitrogen mixtures is substantially reduced. On the other hand, the molecular component reduces the velocity of thermodynamic equilibration, as has been shown by means of time resolved optical interferometry.     

Effects of NO2 addition on hydrogen ignition behind reflected shock waves

Ignition delay time measurements of H₂/O₂/NO₂ mixtures diluted in Ar have been measured in a shock tube behind reflected shock waves. Three different NO₂ concentrations have been studied (100, 400 and 1600 ppm) at three pressure conditions (around 1.5, 13, and 30 atm) and for various H₂–O₂ equivalence ratios for the 100 ppm NO₂ case. Results were compared to some recent ignition delay time measurements of H₂/O₂ mixtures. A strong dependence of the ignition delay time on the pressure and the NO₂ concentration was observed, whereas the variation in the equivalence ratio did not exhibit any appreciable effect on the delay time. A mechanism combining recent H₂/O₂ chemistry and a recent high-pressure NOx sub-mechanism with an updated reaction rate for H₂ + NO₂ ? HONO + H was found to represent correctly the experimental trends over the entire range of conditions. A chemical analysis was conducted using this mechanism to interpret the experimental results. Ignition delay time data with NO₂ and other NOx species as additives or impurities are rare, and the present study provides such data over a relatively wide pressure range.     

Dynamics of excited hydroxyl radicals in hydrogen-based mixtures behind reflected shock waves

The chemiluminescence originating from OH¹, the excited hydroxyl radical, is one of the most extensively used diagnostics to characterize auto-ignition delay time of gaseous mixtures behind reflected shock waves. We have carried out new experiments and modeling of this diagnostic as well as analyzed previous results for hydrogen-based mixtures, including H₂–O₂, H₂O₂–H₂O, H₂–N₂O and H₂–O₂–N₂O. The experiments were analyzed with a detailed chemical reaction model which included mechanisms for OH¹ creation, quenching and emission. Simulations of the reaction behind reflected shock waves were used to predict OH¹ emission profiles and compare this with measured results as well as profiles of temperature and the ground state concentrations of OH. Analysis of OH¹ rates of progress demonstrates that a quasi-steady state approximation is applicable and an algebraic model for OH¹ concentrations can be derived that relates emission to the product of concentrations of O and H for H₂–O₂ and H₂O₂ mixtures and an additional contribution by the product of H and N₂O when N₂O is an oxidizer.     

Heat release of carbon particle formation from hydrogen-free precursors behind shock waves

The process of heat release during carbon particle formation and growth after pyrolysis of carbon suboxide C₃O₂ behind shock waves was investigated. For this goal, temperature and optical density of gas-particle mixtures initially consisting of 3% C₃O₂ + 5% CO₂ in Ar were measured as a function of time. The temperature was determined by two-channel emission-absorption spectroscopy at λ = 2.7 ± 0.4 μm, corresponding to the CO₂ (1,0,1) vibrational band. In the range of initial temperatures behind the shock waves from 1600 up to 2200 K a significant heating of the mixture during particle formation and growth was observed that increased towards higher temperatures. The analysis of the obtained data in combination with previous results about the temperature dependence of the particle size shows a decrease of the heat release of condensation from ∼200 kJ/mol per atom for particles containing ∼1000 atoms to ∼50 kJ/mol per atom for particle containing ∼10⁶ atoms.     

Viscous effects on elliptic flow and shock waves

Fast thermalization and a strong buildup of the elliptic flow of QCD matter as found at RHIC are understood as the consequence of perturbative QCD (pQCD) interactions within the 3+1 dimensional parton cascade BAMPS. The main contributions stem from pQCD bremsstrahlung 2↔3 processes. By comparison to Au+Au data of the flow parameter v₂ as a function of participation number the shear viscosity to entropy ratio is dynamically extracted, which lies in the range of 0.08 and 0.2, depending on the chosen coupling constant and freeze out condition. Furthermore, first simulations on the temporal propagation of dissipative shock waves are given. The cascade can either simulate true ideal shocks as well as initially diluted, truly viscous shocks, depending on the employed cross sections or mean free path, respectively.     

Density waves in traffic flow model with relative velocity

The car-following model of traffic flow is extended to take into account the relative velocity. The stability condition of this model is obtained by using linear stability theory. It is shown that the stability of uniform traffic flow is improved by considering the relative velocity. From nonlinear analysis, it is shown that three different density waves, that is, the triangular shock wave, soliton wave and kink-antikink wave, appear in the stable, metastable and unstable regions of traffic flow respectively. The three different density waves are described by the nonlinear wave equations: the Burgers equation, Korteweg-de Vries (KdV) equation and modified Korteweg-de Vries (mKdV) equation, respectively.     

Thermally nonequilibrium processes occurring during the ignition of hydrocarbon-air mixtures behind shock waves

The effect of a nonequilibrium vibrational excitation of the reactants on the ignition of methane-air mixtures in a supersonic flow behind the front of an incline shock wave. It was demonstrated that the equilibrium kinetic models give incorrectly predict the induction zone length (within a factor of 3) and the final pressure in the combustion products. Even a moderate preliminary vibrational excitation of N₂ molecules makes it possible to substantially (by a factor 10 to 15) decrease the length of the induction zone behind the shock wave front (to ～1–2 m) at even moderate temperatures of the shocked gas (1400–1500 K).