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

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/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.     

Principles of Pyrex® glass chemistry: structure–property relationships

Pyrex^® glass was one of the first commercial boroaluminosilicate glass compositions, selected in 1915 from thousands of compositions due to its ability to sustain mechanical and thermal shock. While the microscopic structure of Pyrex^® glass has recently been investigated, the microscopic origins of its macroscopic properties are not well understood, i.e., the atomic scale foundation of the original empirical invention of Pyrex^® glass has yet to be established. In this work, we have tackled this problem by investigating the effects of varying Si/Al and Na/B ratios on the boron and aluminum speciation and a range of physical and rheological properties in the Pyrex^® glass family. We show that the canonical Pyrex^® boroaluminosilicate composition is indeed optimal for attaining relatively high values of glass transition temperature and elastic moduli and a low coefficient of thermal expansion, while simultaneously maintaining a high glass-forming ability.     

Application of Diode Lasers to Determine Excitation Temperature in Hollow Cathode Discharges by Optogalvanic Spectroscopy

The excitation temperatures of sputtered gadolinium and uranium atoms in an argon hollow cathode discharge have been determined by diode laser-excited optogalvanic spectroscopy. These results have been compared to those determined by conventional emission spectroscopy. It was found that the temperatures derived from each method do not differ very much, but the optogalvanic method revealed a better standard deviation uncertainty due to the good signal-to-background ratios and excellent spectral resolutions. Temperature variations with discharge currents ranging from 15 to 50 mA have been examined.     

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.     

Thermal expansion in various phases of Nitinol using TMA

Phase transformation in the near-equiatomic, cold-worked Nitinol, exhibiting shape memory effect, has been investigated through the study of thermal expansion by employing thermomechanical analyser (TMA). The characteristic transformation temperatures determined by this method are compared with those obtained through DSC thermograms. The reliability and sensitivity of the thermal expansion probe have been discussed. The variation of thermal expansion coefficient with temperature in different heat treated samples has been studied in the range 30–120°C. Thermal expansion coefficient is found to be −ve during M↔A transformation and +ve in R↔A transformations. Thermal expansion coefficient in martensitic region in the presence and absence of R-phase has been determined.     

Simulation of positronium plasma by the molecular dynamics method

In this paper, the relaxation properties of a fully ionized, hot, ideal plasma have been studied using the molecular dynamics method. As an example, the classical problem of equalization of the electron and ion temperatures for various mass ratios is considered, the relaxation times for temperatures is determined, and the influence of the number of particles and the type of boundary conditions on the simulation results is studied. The simulation results are compared with the available theoretical results.     

Thermal expansion of nanocrystalline and coarse-crystalline silver sulfide Ag<Subscript>2</Subscript>S

In situ studies of the thermal expansion of polymorphic phases of coarse-crystalline and nanocrystalline silver sulfide, namely, monoclinic acanthite α-Ag₂S and cubic argentite β-Ag₂S, have been performed for the first time by high-temperature X-ray diffraction. The temperature dependences of the unit cell parameters of acanthite and argentite have been measured from temperatures in the range of 300–623 K, and the thermal expansion coefficients of acanthite and argentite have been determined. The observed difference between the thermal expansion coefficients of nano- and coarse-crystalline acanthite is shown to be due to a small size of nanocrystalline silver sulfide particles, which leads to an increase in the anharmonicity of atomic vibrations.     

Untersuchungen der kontrahierten Edelgassäule bei mittleren Drücken II. Meßergebnisse in Argon

In this work plasma parameters of the constricted argon discharge have been determined by an iterative method. For the conditions 20 ? Rp ? 300 cm Torr and 0,05 ? i/R ? 2,5 A/cm the electron density, the electron temperature, and the gas temperature are obtained. These results are compared with measurements and calculated values of other authors. With the exception of the determination of gas temperatures at 2,5 A/cm the agreement between our results and the measurements of some other authors is satisfactory.     

High-pressure shock tube studies on graphite oxidation reactions with carbon dioxide and water

Heterogeneous reactions involving graphite particles reacting with gaseous carbon dioxide and steam were studied under high-pressure and high-temperature conditions. The high-pressure shock tube facility at the University of Illinois at Chicago was utilized to perform the heterogeneous experiments with post-shock pressures ranging from 209 to 363 atm, temperatures ranging between 1546 and 2692 K, and nominal reaction times of 1 ms. Graphite particles were injected into the shock tube, by means of a particle injector, containing mixtures of varying concentrations of H₂O or CO₂ in Ar as balance. The overall reaction rate coefficients from these graphite experiments have been calculated and compared to previous work performed with carbon black particles as well as theoretical and experimental values from prior investigators.     

Conditions for laser-induced plasma to effectively remove nano-particles on silicon surfaces

Particles can be removed from a silicon surface by means of irradiation and a laser plasma shock wave.The particles and silicon are heated by the irradiation and they will expand differently due to their different expansion coefficients,making the particles easier to be removed.Laser plasma can ionize and even vaporize particles more significantly than an incident laser and,therefore,it can remove the particles more efficiently.The laser plasma shock wave plays a dominant role in removing particles,which is attributed to its strong burst force.The pressure of the laser plasma shock wave is determined by the laser pulse energy and the gap between the focus of laser and substrate surface.In order to obtain the working conditions for particle removal,the removal mechanism,as well as the temporal and spatial characteristics of velocity,propagation distance and pressure of shock wave have been researched.On the basis of our results,the conditions for nano-particle removal are achieved.     

Characterization of supersonic beams by time-of-flight technique

Time-of-flight technique has been used to characterize supersonic beams of polyatomic molecules in terms of translational and vibrational temperatures, various velocity parameters and speed ratio. Collision effectiveness and effective specific heat ratio of polyatomic gases pertinent to jet expansion have also been determined and interpreted.     

发射光谱法研究甲基环己烷燃烧时激发态自由基时间历程

采用三组单色仪探测系统,测量了甲基环己烷在高温反射激波作用下瞬态燃烧反应过程中三种激发态自由基OH*,CH*和C*₂的特征光辐射,得到了激发态自由基时间历程和光辐射相对强度随温度的变化规律。反射激波温度1 200～1 700 K,激波压力1.5 atm,甲基环己烷摩尔分数0.1%,当量比1.0。在点火燃烧初始阶段三种自由基几乎同时产生,自由基持续时间随着温度的升高而变短。相同温度下CH*和OH*自由基持续时间大于C*₂自由基,在1 400 K以下C*₂自由基发光消失。OH*和CH*自由基发光强度在T<1 400 K时对温度变化不敏感,而在T>1 400 K时CH*自由基峰值随温度快速增长,C*₂和OH*峰值随温度增大比较平缓。将实验结果和化学反应机理模拟结果进行了对比,实验获得的OH*自由基时间历程在低温时和机理预测结果吻合较好,但在高温时有一定差异。CH*自由基时间历程在高温与机理结果吻合较好,在低温时机理预测结果CH*自由基持续时间要长于实验结果。实验测得的结果为含激发态物种化学反应动力学机理的验证和优化提供了依据。     

Solitons and other solutions of the nonlinear fractional Zoomeron equation

In order to investigate the nonlinear fractional Zoomeron equation, we propose three methods, namely the Jacobi elliptic function rational expansion method, the exponential rational function method and the new Jacobi elliptic function expansion method. Many kinds of solutions are obtained and the existence of these solutions is determined. For some parameters, these solutions can degenerate to the envelope shock wave solutions and the envelope solitary wave solutions. A comparison of our new results with the well-known results is made. The methods used here can also be applicable to other nonlinear partial differential equations. The fractional derivatives in this work are described in the modified Riemann–Liouville sense.     

On the dense phases of He3 at very low temperatures

A class of physical properties of the dense phases of He³ are dominated at low temperatures by their nuclear spin systems. The latter determine the lower limits of these properties, and yield thereby approximately their behavior and limiting values at very low temperatures and at the absolute zero. The theory of the spin system allows one to estimate the lowest temperatures at which the various thermal properties of this class have to be determined experimentally to approach with a preset deviation their limiting values in the ground state of these dense phases. These determinations, with a common relative value of this difference, of the nuclear paramagnetic susceptibility, heat capacity, expansion coefficient, and their temperature rates of variation require decreasingly lower temperatures, with the latter finite quantities imposing temperatures one order of magnitude lower than the susceptibility. These studies suggest that in order to obtain satisfactory, experimentally inaccessible, extrapolated very low temperature behaviors and finite limits of the above class of properties, careful prior analysis of their finite temperature values will have to be performed in terms of the partial properties of the spin systems as well as those of the degrees of freedom other than spin of the dense phases of He³.     

Technique for determining the channel expansion rate at the stage of electrical breakdown using a grounded intercepting ring

A method is described that allows one to study the conductivity dynamics of a channel produced by explosion of a wire at the stage of electrical breakdown. Experimental data have been presented for the expansion rate of the conductivity channel in extended (up to 1.9 m long) arbitrarily shaped gapes that were produced by an exploding copper wire 90 µm in diameter. The initial stored energy and applied voltage were, respectively, 2.7–3.7 kJ and 8–10 kV. It has been shown that the expansion rate of the conductivity channel coincides with the propagation rate of a shock wave and is inversely proportional to the square root of its radius and propagation time. The radius of the shock wave is a linear function of the square root of its propagation rate. Experimental data are in satisfactory agreement with the calculated results obtained by Lin [18] in terms of the model of an intense shock wave. It has been shown that the diameter of the conductivity channel depends on the position of the trailing edge of the shock wave.     

Reflection coefficient of low-energy ions as a universal function of the scaled transport cross section

Multiple collision theory of heavy ion ranges in an infinite medium has been used to calculate the reflection coefficient from the penetration profile, by a method first described by Bottiger et al. Interaction with target electrons has been neglected and nuclear collisions have been described by the power cross sections. Calculations are restricted to low reduced ion energies )₀А.1 and to target/ion mass ratios 7⢪, when electronic stopping is much less than the nuclear stopping. The Gaussian approximation of the penetration profile and the reflection coefficient are found in a form of simple analytic formulas. Furthermore, the penetration profile was constructed by using Edgeworth expansion and moments of the distribution up to the fourth order, and the reflection coefficient was determined from the profile. Good agreement between the analytical results and the reflection coefficient obtained from Edgeworth expansion was found, for target atom/ion mass ratios 7̿.3. It is shown that the reflection coefficient is a universal function of the scaled transport cross section. The scaling is fulfilled for v̀, when ion reflection is determined by large-angle multiple collisions, and breaks down for v<1 i.e. 7<1, when scattering angles are small. Results are compared with TRIM computer simulation data.     

Observation of multichannel collisions of cold metastable calcium atoms

Recent theoretical work indicates that collisions between metastable alkaline-earth atoms (AEAs) in the presence of external magnetic fields should be largely determined by partial waves with large angular momenta even at very low temperatures. Unusually large inelastic collision cross sections were predicted and doubts have been raised regarding the feasibility of evaporative cooling of metastable AEAs in magnetic traps. Here we present experimental data for 40Ca[4s4p]3 P2 clearly confirming the asserted multichannel character of the collision mechanism. While elastic cross sections are found to be similar to the predicted values, inelastic cross sections exceed the calculations by an order of magnitude. Our results substantiate the expectation of inefficient evaporative cooling.     

Effect of strong electron correlations and electron-lattice interaction on the ordering of electron orbitals

The interactions between electrons in degenerate orbitals and between electrons and the lattice degrees of freedom have been treated by the Monte Carlo method taking into account the anharmonicity of ionic vibrations in the framework of the phenomenological model. The existence regions of the ferromagnetic and antiferromagnetic orbital orders have been found. Two critical temperatures have been determined at which the long-range orbital order disappears and the distribution of electrons over the orbitals becomes uniform. The correlation between the orbital ordering types and the thermal expansion coefficient has been established. It has been found that an increase in anharmonicity leads to the stabilization of the antiferromagnetic orbital ordering.     

Determination of the rotational temperature and the molecular density in an expanding NH3 jet by infrared absorption

Rotational temperatures are determined by measuring the absorption of infrared laser radiation. The possibilities of this method are critically examined and tested. As a result the molecular density in the expansion could be determined, too. Color-center laser radiation has been absorbed by a molecular jet of NH₃. An anomalous line shape has been observed, related to a Doppler shift from molecules moving along the various streamlines. No deviations from a thermal rotational distribution have been observed.Work supported by Stichting voor Fundamenteel Onderzoek der Materie (F.O.M.)     

Molecular Dynamic Simulation of High Speed Deformation Effect on Microstructure Change in Thermal Heated Metals

In the present paper computer simulation of high-speed deformation (shock wave propagation) by molecular dynamic method is performed in thin copper sample, having the form of rectangular parallelepiped (10 a ‐ 10 a ‐ 20 a , where a is the lattice constant) with 8000 atoms. On the surfaces Z 0 =0 and Z max =20 a the mirror boundary conditions with rigid walls and the periodic boundary conditions along X and Y directions corresponding to short sides of deformed crystal are used, which allows to investigate the reflection of shock wave from the surfaces in Z direction. The changes of microstructure have been investigated up to 12 ps. The numerical calculations of microstructure changes have been performed here taking into account the effect of thermal heating of crystal lattice before shock wave front. The numerical results show that comparing with the propagation of shock waves under room temperature in thermal heated structure additional displaced atoms (vacancies and interstitials) are produced. The obtained results show that the production of point defects during high-speed deformation is determined by the thermal softening of microstructure and generation rate of point defects very strong increases with an increasing of high speed deformation rate. The peculiarities of microstructure changes in deformed copper are analyzed here at the different initial temperatures and various high-speed deformations (average ion velocities behind shock wave).