高背压超声气体团簇喷流中团簇平均尺寸沿喷流方向演化研究

本文通过对高背压(50 bar, 1 bar = 1.0×10⁵ Pa)氩气经长锥型喷嘴(长度L=30 mm)向真空绝热膨胀所形成的超声气体团簇喷流的数值模拟, 分析比较了由喷嘴喉口起沿喷流方向在喷流中心轴线上团簇平均尺寸的演化情况. 结果表明: 沿喷流方向团簇平均尺寸显示先增长后趋于饱和的变化趋势, 具有较大尺寸团簇的区域出现在距离喷嘴喉口大约20 mm. 据此本文再结合关于喷流中原子密度沿喷流方向变化的模拟结果开展了锥形喷嘴长度的优化研究. 针对由常见构型的锥形喷嘴(喉径～ 0.5 mm, 半张角～ 8.5°)在高背压下形成的团簇喷流, 20 mm左右的长度为锥形喷嘴的适宜长度.     

Spectroscopic measurements of non-equilibrium nitrogen plasmas at moderate pressures

A local vibro-rotational analysis of the excited species, produced in a 35 MHz discharge reactor in flowing nitrogen, has been carried out by measuring radial and axial emission intensities of some vibrational sequences and selected rotational lines of the (0,2) band of the second positive system (SPS) of N₂ (C³Π_u - B³Π_g), in the pressure range 5–35 torr.Radial from lateral band or line emission intensities have been obtained by applying Abel's inversion technique to derive the corresponding vibrational and rotational temperatures with the use of Boltzmann plots. General maps of emission intensities and of vibrational and rotational temperature distributions within the reactor have been drawn.It is shown that a decrease of a factor 10³ in the emission intensities from the core to the periphery of the discharge corresponds to a variation in T_V and T_R of only a factor ?2. This result has been interpreted on the basis of the kinetic mechanisms for the excitation and deexcitation of chemical species under discharge conditions. The decrease of _V and the increase of T_R with increasing pressure are interpreted according to V-R (vibration-rotation) energy-transfer mechanisms.     

Gel forming behaviour of milk protein concentrates at moderate pressures

Abstract

Gel formation is a time dependent process, which is a function of many external variables. Here we report the effect of pressure on the gelation of certain milk proteins. It is found that hydrostatic pressure (1000 bar) decreases the gelation time of concentrated reconstituted milk (13%w/w protein) at around 50°C by more than sevenfold.     

A cluster algorithm for jet studies

A procedure is described which determines the number of jets in hadronic final states by means of a cluster algorithm. In addition it yields a measurement of the energy and the direction of each jet. The properties of this method are studied using Monte Carlo simulations of different types ofe ⁺ e ^?-annihilation final states. It is shown that in the case of 3-jet events direct comparison with the underlying parton structure can be made. Possible further applications of this method are discussed.     

Optical emission spectrometric diagnosis of laser-induced plasma and shock front produced at moderate pressure

We present a non-contact optical investigation of laser-induced plasma at moderate Ar pressure ranging from 1 to 100 Pa.The significant shock front and spatial fractionation among the different charged ions are demonstrated at the pressure of 20 Pa. The collisions between Si IV ions and ambient Ar atoms generate distinct and excited Ar Ⅱ ions, fresh Si Ⅲ ions, and electrons at the dense layer. The electron density peaks at the position of the shock front, indicating that the collision that yield...     

Nanostructures produced by cluster beam lithography

2 clusters with about 1000 molecules per unit charge are accelerated to up to 120 keV kinetic energy for mask projective surface bombardment. Patterning is achieved via physical as well as chemical surface erosion. Very smooth eroded surfaces result for bulk natural diamond, silicon, and glass. Polycrystalline, strongly faceted CVD diamond films are effectively planarized. Submicron structures with various wall inclinations can be generated. Atomic force microscopy of individual impact structures reveals nanometer-sized hillocks instead of craters. The collective motion of the impacted surface material is considered crucial for the cluster impact-induced nanomodifications. Atomic ion beam lithography is considered for comparison. Received: 3 July 1998     

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利用时空分辨的测量技术,测定了XeCl紫外激光烧蚀金属Cu消融粒子的发射光谱随时间与空间的强度分布。通过在不同的氢气压强下拍摄到的XeCl308nm紫外激光烧蚀金属Cu消融粒子的发光羽照片,发现激光消融粒子发光羽的颜色在不同区域有不同的颜色,不同区域的发光羽颜色随环境气压的改变而变化。随环境气压的增大,发光羽不仅逐渐变小,而且逐渐变淡。对激光消融粒子发光羽的机理进行了探讨,激光消融粒子发光羽的发光动力学模式在不同区域有不同的发光模式,且随环境气压的变化而改变。定性地解释了所观察的实验现象。     

Experimental and computational investigation of autoignition of jet fuels and surrogates in nonpremixed flows at elevated pressures

Experimental and computational investigations are carried out to elucidate the fundamental mechanisms of autoignition of surrogates of jet-fuels at elevated pressures up to 6 bar. The jet-fuels tested are JP-8, Jet-A, and JP-5, and the surrogates tested are the Aachen Surrogate made up of 80 % n-decane and 20 % 1,3,5-trimethylbenzene by mass, Surrogate C made up of 60 % n-dodecane, 20 % methylcyclohexane and 20 % o-xylene by volume, and the 2nd generation Princeton Surrogate made up of 40.4 % n-dodecane, 29.5 % 2,2,4-trimethylpentane, 7.3 % 1,3,5-trimethylbenzene and 22.8 % n-propylbenzene by mole. Using the counterflow configuration, an axisymmetric flow of a gaseous oxidizer stream, made up of a mixture of oxygen and nitrogen, is directed over the surface of an evaporating pool of a liquid fuel. The experiments are conducted at a fixed value of mass fraction of oxygen in the oxidizer stream and at a fixed value of the strain rate. The temperature of the oxidizer stream at autoignition, T_ig, is measured as a function of pressure, p. Experimental results show that the critical conditions, of autoignition of the surrogates are close to that of the jet-fuels. Overall the critical conditions of autoignition of Surrogate C agree best with those of the jet-fuels. Computations were performed using skeletal mechanisms constructed from a detailed mechanism. Predictions of the critical conditions of autoignition of the surrogates are found to agree well with measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition for all surrogates. The low-temperature chemistry, of the component of the surrogate with the greatest volatility, was found to have the most influence on the critical conditions of autoignition.     

Swelling properties and intrinsic reactivities of coal chars produced at elevated pressures and high heating rates

A high-temperature, high-pressure flat-flame burner reactor was developed to prepare char at different pressures. This system achieves particle heating rates of 10⁵ K/s, which better mimics industrial conditions than conventional drop tube or radiative flow reactors. Previous data at atmospheric pressure demonstrated a significant decrease in particle swelling during devolatilization as heating rates increased from 10⁴ K/s (the typical drop tube heating rate) to 10⁵ K/s. Pyrolysis experiments were performed at pressures from 1 to 15 atm at 1300 °C for two bituminous coals and a lignite. Average swelling was determined from a combination of the mass release and the average density. The results indicate significantly lower swelling ratios at elevated pressures than reported in the literature. Scanning electron micrographs show that the bubbles in the bituminous coal particles popped before significant swelling at these elevated heating rates. Lignite particles exhibited shrinkage rather than swelling, but still showed a small effect of pressure. TGA oxidation reactivities were determined for the Pitt #8 and Knife River lignite char samples at their respective char preparation pressures. The oxidation reactivities of both the bituminous and lignite chars decreased with increasing pressure.     

Boron at tera-Pascal pressures

The study of boron structure is fascinating because boron has various allotropes containing boron icosahedrons under pressure. Here, we propose a new boron structure (space group $Fmoverline{3}m$) that is dynamically stable at 1.4 tera-Pascal (TPa) using density functional theory and an evolutionary algorithm. The unit cell of this structure can be viewed as a structure with a boron atom embedded in the icosahedron. This structure behaves as a metal, and cannot be stable under ambient pressure. Furthermore, we found electrons gather in lattice interstices, which is similar to that of the semiconductor Na or Ca$_{2}$N-II under high pressure. The discovery of this new structure expands our comprehension of high-pressure condensed matter and contributes to the further development of high-pressure science.     

Tomography of high harmonic generation in a cluster jet

Tomographic measurement of high harmonic generation in a cluster jet was demonstrated by programming the cluster density distribution with a laser machining technique. The growth of harmonic energy with the propagation of the pump pulse was resolved by scanning the end of the argon cluster distribution in the path of the pump pulse. A downstream shift of the position of rapid growth and a decrease of the slope with increasing backing pressure as results of changes in the phase-matching condition were observed, which explains the presence of an optimal backing pressure.     

The steady-state kinetics of the hydrogen-oxygen reaction on a Pt(111) surface at low and moderate pressures

The steady-state kinetics of the hydrogen-oxygen reaction on a Pt(111) surface is studied at low (10⁻⁵−10⁻⁴ Pa) and moderate (5–100 Pa) pressures of the stoichiometric mixture of the reagents. The moderate-pressure kinetics is analyzed on the basis of data obtained in the transient studies at low pressures. It is found that the reaction rate, extrapolated from low to moderate pressures, exceeds the measured rate by about an order of ten.     

Iron at high negative pressures

With the object of verifying the presence of a region of anomalous iron compressibility at negative pressures, as predicted by the ab initio calculations, the reflection of compression pulses from the surfaces of iron single crystals was detected. No evidence of the expected formation of rarefaction shock waves was observed in the range of attained tensile stresses up to 7.6 GPa. The breaking stresses achieved 25-50% of the theoretical iron ultimate strength for a load duration of ～10^?8 s. The dependence of breaking strength on the extension rate did not reveal any singularities in the region of assumed anomaly in iron compressibility.     

FXR discharges at various pressures

The formation period time, current rise time, and photon energy of FXR discharges have been investigated at initial pressures of 5×10^?6 to 1×10^?2Torr. It has been found that the parameters studied are almost constant between 5×10^?6 and 1×10^?3Torr. The results obtained indicate that the upper limit of the interelectrode breakdown pressure is about 10^?3 Torr when the initial pressure is 10^?6 Torr.     

Infrared line narrowing and cluster absorption in a planar jet

Infrared laser absorption by a planar molecular jet is studied. The absorption lineshape is narrowed by a factor of 2 to 3, as compared to the absorption lineshape in bulk gas. The infrared absorption of CO₂ clusters is observed directly in a jet. The formation of clusters in the jet is monitored for different mixtures and distances from the nozzle.Work supported by Stichting voor Fundamenteel Onderzoek der Materie (FOM)