慢同步波行波管特性研究

 在传统行波管的放大过程中，随着电子纵向群聚的加深，由于空间电荷效应导致的非线性效应，影响了行波管的输出性能。介绍了一种新型的行波管并分析了其作用机理，分析了这种新型行波管慢波系统的结构及其色散特性，讨论了T形齿结构和梯形齿结构慢波系统几何尺寸对工作性能的影响。通过圆极化行波的高频横向电场调制电子束，导致电子束在空间的扭转，使得电子在放大过程中没有经过纵向群聚而处于减速场中，消除了非线性效应，提高了行波管的输出性能。     

Laser doppler anemometry in hydrodynamic testing

Classical methods and the laser-Doppler-anemometry (LDA) application for the test section calibration of a water-cavitation tunnel is presented. The results show that the LDA is the best method for calibration and the classical methods satisfy the needs of standard tests. The advantages of the LDA are illustrated by determination of the pressure coefficient C _p for a hydrofoil of a high-speed axial pump under the stationary and nonstationary conditions, as well. The velocity vector distribution is measured around the central hydrofoil of a straight grid for angles of attack δ = 0° and 25° and undisturbed velocity v _∞ = 5.32 m/s. The results of the LDA measurements are used as a basis for the definition of the boundary conditions for numerical flow simulation and C _p calculation by the Fluent program. A flow visualization is made by aniline dyes and air bubbles.     

The autoignition of cyclopentane and cyclohexane in a shock tube

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 C₅ compound than for the C₆. 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.     

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

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 (? = 0.5), through stoichiometric to rich (? = 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 ?H₃ + O₂/HO˙₂ chemistry were very important in reproducing the correct kinetic behavior.     

Shock-tube and modeling study of ethyl methyl ether pyrolysis and oxidation

Pyrolysis and oxidation of ethyl methyl ether (EME) were studied behind reflected shock waves in the temperature range 900-1750 K at total pressures between 0.9 and 3.1 atm. The study was carried out using following methods, (1) time-resolved IR-laser absorption at 3.39 μm for EME decay and CH-compound formation rates, (2) time-resolved UV absorption at 216 nm for mainly CH₃ radical formation rate, (3) time-resolved UV absorption at 306.7 nm for OH radical formation rate, (4) time-resolved IR emission at 4.24 μm for CO₂ formation rate and (5) a single-pulse technique for product yields. The pyrolysis and oxidation of EME were modeled using a reaction mechanism including the sub-mechanisms for methane, acetylene, ethylene, ethane, formaldehyde, acetaldehyde and ketene oxidation. The reaction mechanism used in this study could reproduce almost all of experimental results. The sub-mechanisms of methane, ethylene, ethane, formaldehyde, and acetaldehyde were found to play an important role in EME oxidation.     

Laser peening of aluminum alloy 7050 with fastener holes

The fatigue properties of laser peened aluminum alloy 7050 specimens with fastener holes were investigated.The surface profile and residual stress induced in the shock affected zone were characterized.Then,the fatigue specimens with notch were treated by laser peening(LP),and the fatigue lifes of LP-treated specimens were measured and compared with base materials without LP.The results indicated that LP improved the fatigue lifes of all tested specimens.The average fatigue lifes of specimens treated by LP before hole-drilling were 173% longer than those of untreated samples and had better effects than those specimens treated by LP after hole-drilling.     

液氮与液态一氧化碳混合物的冲击压缩特性研究

 利用二级轻气炮研究了等体积的液态一氧化碳（LCO）与液氮（LN₂）混合物的冲击压缩特性，在10～25 GPa压力范围内获得了5个Hugoniot数据点。混合物冲击绝热线位于单质一氧化碳和液氮Hugoniot拟合线之间，并靠近前者。这意味着混合物的冲击压缩特性不是其中各单质行为的简单平均。     

A new type of sample tube for reducing convection effects in PGSE-NMR measurements of self-diffusion coefficients of liquid samples

Pulsed gradient spin-echo (PGSE) NMR measurements of the self-diffusion coefficients of low viscosity liquids are greatly hampered by the effects of convection especially away from ambient temperature. Here we report on a new NMR tube designed to minimize the deleterious effects of convection. In this tube, which derives from a Shigemi symmetrical NMR tube, the sample is contained in an annulus formed from a concentric cylinder of susceptibility matched glass. The performance of this tube was demonstrated by conducting measurements on the electrochemically important LiN(SO3CF3)2 (LiTFSI)-diglyme (DG) system. Calibrations were first made using DG at column heights of 2, 3, and 4-mm in the temperature range between -40 and 100 degrees C. Measurements of the diffusion coefficients of the lithium, anion, and DG were then performed to probe the solvent-ion and ion-ion interactions in the DG doped with LiTFSI. Changes in the 1H, 7Li, and 19F PGSE-NMR attenuation curves at -40 degrees C provided clear evidence of interactions between the DG and lithium ion.     

Peptide antagonists of superantigen toxins

Superantigens produced by Staphylococcus aureus and Streptococcus pyogenes are among the most lethal of toxins. Toxins in this large family trigger an excessive cellular immune response leading to toxic shock. Superantigens are secreted by the bacteria as diverse natural mixtures, a complexity that demands development of broad-spectrum countermeasures. We used a rational approach to design short peptides with homology to various domains in a typical superantigen (staphylococcal enterotoxin B) and screened each peptide for its ability to antagonize, in human peripheral blood mononuclear cells, superantigen-mediated induction of the genes encoding T helper 1 cytokines that mediate shock: interleukin-2, interferon-gamma and tumor necrosis factor. A dodecamer peptide proved a potent antagonist against widely different superantigens. This peptide protected mice from killing by superantigens and it was able to rescue mice undergoing toxic shock. The antagonist peptide shows homology to a beta-strand-hinge-alpha-helix domain that is structurally conserved among superantigens, yet currently of unknown function and remote from the binding sites for the known ligands essential for T cell activation, the major histocompatibility complex class II molecule and T cell receptor. The antagonist activity of this peptide thus identifies a novel domain in superantigens that is critical for their toxic action. The antagonist peptide provides a new tool for understanding the mechanism of excessive human immune response activation by superantigens that occurs during toxic shock and for identification of a novel target ligand that may interact with this superantigen domain.     

Forced Burgers Equation in an Unbounded Domain

The inviscid Burgers equation with random and spatially smooth forcing is considered in the limit when the size of the system tends to infinity. For the one-dimensional problem, it is shown both theoretically and numerically that many of the features of the space-periodic case carry over to infinite domains as intermediate time asymptotics. In particular, for large time T we introduce the concept of T-global shocks replacing the notion of main shock which was considered earlier in the periodic case (1997, E et al., Phys. Rev. Lett. 78, 1904). In the case of spatially extended systems these objects are no anymore global. They can be defined only for a given time scale and their spatial density behaves as (T)T ^–2/3 for large T. The probability density function p(A) of the age A of shocks behaves asymptotically as A ^–5/3. We also suggest a simple statistical model for the dynamics and interaction of shocks and discuss an analogy with the problem of distribution of instability islands for a simple first-order stochastic differential equation.