Wave dynamic processes induced by a supersonic projectile discharging from a shock tube

A numerical study on wave dynamic processes occurring in muzzle blast flows, which are created by a supersonic projectile released from the open-end of a shock tube into ambient air, is described in this paper. The Euler equations, assuming axisymmetric flows, are solved by using a dispersion-controlled scheme implemented with moving boundary conditions. Three test cases are simulated for examining friction effects on the muzzle flow. From numerical simulations, the wave dynamic processes, including two blast waves, two jet flows, the bow shock wave and their interactions in the muzzle blasts, are demonstrated and discussed in detail. The study shows that the major wave dynamic processes developing in the muzzle flow remain similar when the friction varies, but some wave processes, such as shock-shock interactions, shock-jet interactions and the contact surface instability, get more intensive, which result in more complex muzzle blast flows.

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.

Experimental Investigation of the Velocity Effect on Adhesion Forces with an Atomic Force Microscope Atomic Force Microscope

Capillary forces are significantly dominant in adhesive forces measured with an atomic force microscope (AFM) in ambient air, which are always thought to be dependent on water film thickness, relative humidity, and the free energy of water film. We study the nature of the pull-off force on a variety of surfaces as a function of tip velocity. It is found that the capillary forces are of relatively strong dependence on tip velocity. The present experiment is expected to provide a better understanding of the work mechanism of AFM in ambient air.

What Do We Know about Long Laminar Plasma Jets?

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

Growth of liquid bridge in AFM

Capillary forces are dominant in adhesive forces measured with an atomic force microscope (AFM) in ambient air, which are thought to be dependent on water film thickness, relative humidity and the free energy of the water film. In this paper, besides these factors, we study the nature of the 'pull-off' force on a variety of atmospheres as a function of the contact time. It is found that capillary forces strongly depend on the contact time. In lower relative humidity atmosphere, the adhesion force is almost independent of the contact time. However, in higher relative humidity, the adhesion force increases with the contact time. Based on the experiment and a model that we present in this paper, the growth of the liquid bridge can be seen as undergoing two processes: one is water vapour condensation; the other is the motion of the thin liquid film that is absorbed on the substrate. The experiment and the growth model presented in this paper have direct relevance to the working mechanism of AFM in ambient air.

Microstructure and high-temperature wear and oxidation resistance of laser clad gamma/W2C/TiC composite coatings on gamma-TiAl intermetallic alloy

There are very strong interests in improving the high-temperature wear resistance of the y-TiAl intermetallic alloy, especially when applied as tribological moving components. In this paper, microstructure, high-temperature dry sliding wear at 600 degrees C and isothermal oxidation at 1000 degrees C on ambient air of laser clad gamma/W2C/TiC composite coatings with different constitution of Ni-Cr-W-C precursor mixed powders on TiAl alloy substrates have been investigated. The results show that microstructure of the laser fabricated composite coatings possess non-equilibrium microstructure consisting of the matrix of nickel-base solid solution gamma-NiCrAl and reinforcements of TiC, W2C and M23C6 carbides. Higher wear resistance than the original TiAl alloy is achieved in the composite coatings under high-temperature wear test conditions. However, the oxidation resistance of the laser clad gamma/W2C/TiC composite coatings is deceased. The corresponding mechanisms resulting in the above behaviors of the laser clad composite coatings are discussed. (c) 2006 Elsevier B.V. All rights reserved.

What Do We Know about Long Laminar Plasma Jets?

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

Effect of interfacial heat transfer on the onset of oscillatory convection in liquid bridge

In present study, effect of interfacial heat transfer with ambient gas on the onset of oscillatory convection in a liquid bridge of large Prandtl number on the ground is systematically investigated by the method of linear stability analyses. With both the constant and linear ambient air temperature distributions, the numerical results show that the interfacial heat transfer modifies the free-surface temperature distribution directly and then induces a steeper temperature gradient on the middle part of the free surface, which may destabilize the convection. On the other hand, the interfacial heat transfer restrains the temperature disturbances on the free surface, which may stabilize the convection. The two coupling effects result in a complex dependence of the stability property on the Biot number. Effects of melt free-surface deformation on the critical conditions of the oscillatory convection were also investigated. Moreover, to better understand the mechanism of the instabilities, rates of kinetic energy change and "thermal" energy change of the critical disturbances were investigated (C) 2009 Elsevier Ltd. All rights reserved.

Selection and ultrastructural observation of a high-CO2-requiring mutant of cyanobacterium Synechococcus sp PCC7942

A high-CO2-requiring mutant of Synechococcus sp. PCC7942 las been isolated after chemical mutagenesis of ethyl methane sulphonate (EMS). It was able to grow at 4% CO2, but not under ambient CO2. The initial screening of the mutant showed that the genetic reversion rate was about 10(-7) and death occurred 2 -3 days after being transferred from 4% CO2 to the ambient air. Its photosynthetic dependence on external dissolved inorganic carbon was higher than that of the wild type cells, but its carbonic anhydrase activity was comparatively low. In the ultrastructural level, various types of aberrant carboxysomes appeared in the mutant cells: rod-shaped carboxysomes, irregular carboxysomes and the "empty-inclusion carboxysomes" with increasing number of glycogen granules surrounding the thylakoids. All these alterations indicated that the mutant was defective in utilizing the external CO2. The induction of carboxysomes by lower levels of CO2 and the biogenesis of carboxysomes are herein discussed.

Ultra-dry oxygen atmosphere to protect tellurite glass fiber from surface crystallization

In this study we report on surface crystallization phenomena and propose a solution for the fabrication of long and robust tellurite glass fibers. The bulk tellurite glasses of interest were prepared by melting and quenching techniques. Tellurite glass preforms and fibers were fabricated by suction casting and rod-in-tube drawing methods, respectively. The surfaces of the tellurite bulk glass samples and of the drawn fibers prepared under different controlled atmospheres were examined by X-ray diffraction. When the tellurite glass fibers were drawn in ambient air containing water vapor, four primary kinds of small crystals were found to appear on the fiber surface, alpha-TeO(2), gamma-TeO(2), Zn(2)Te(3)O(8) and Na(2)Zn(3)(CO(3))(4)center dot 3H(2)O. A mechanism for this surface crystallization is proposed and a solution described, using an ultra-dry oxygen gas atmosphere to effectively prevent surface crystallization during fiber drawing. (C) 2010 Elsevier B.V. All rights reserved.

Novel trichromatic phosphor codoped with two rare earth ions in a single matrix

The luminescence properties of CaBPO5: Eu, Tb phosphor and the sensitization of Ce3+ were investigated. The CaBPO5: Eu, Tb phosphors were synthesized in the ambient air and the emission spectra of Eu3+, Tb3+ and Eu2+ were Observed in the phosphor. The result shows that there is electron transfer between conjugate rare earth ions. Sensitization of Ce3+ can improve the intensity of emission of Tb3+ and Eu2+. A novel trichromatic lamp phosphor codoped with Eu3+-Tb3+ in matrix CaBPO5 is then predicted.

利用激光诱导击穿光谱对铝合金成分进行多元素同时定量分析

利用激光诱导击穿光谱定量分析了铝合金中多种元素的成分。采用Nd∶YAG脉冲激光器,在空气环境下烧蚀铝合金固体样品获得等离子体。利用多通道光栅光谱仪和CCD检测器对200～980nm波长范围的光谱进行同时检测。研究了检测时延、激光脉冲能量、元素深度分布对光谱强度的影响,考虑这些因素之后对实验参数进行了优化。在优化的实验参数下对国家标准铝合金样品中的八种元素Si,Fe,Cu,Mn,Mg,Zn,Sn及Ni进行了定标,并利用定标曲线对一种铝合金样品进行了定量分析。实验结果表明,测量结果的相对标准偏差(RSD)最大为5.89%,相对误差在-20.99%～15%范围内,说明对铝合金样品成分进行定量分析,激光诱导击穿光谱是一种有效的光谱分析工具,但是分析结果的准确度仍需要提高。

贡嘎山地区大气中不同形态汞的分布规律及影响因素

汞是一种有毒的人体非必需重金属元素，是一种全球性污染物。大气在全球汞生物地球化学循环中占极其重要的地位，20世纪80年代末和90年代初，在没有人为和自然汞污染源的北欧和北美偏远地区的大片湖泊中发现的鱼体高甲基汞负荷就是由人为排放的汞通过大气长距离迁移后的沉降造成的。我国被认为是全球汞排放最多的国家，大量的汞排放到大气中势必会造成不同形态大气汞浓度以及大气汞沉降通量的升高，然而就目前而言，国内关于长时间分辨率的大气汞分布规律及汞沉降通量的系统研究还比较缺乏。 2005年5月到2007年5月，采用高时间分辨率大气自动测汞仪、镀KCl石英扩散管和颗粒汞微型捕汞管对贡嘎山地区大气中的气态总汞（TGM）、活性气态汞（RGM）和颗粒汞（TPM）进行了为期2年的系统监测；同时，系统的采集分析了贡嘎山地区的大气降水、植物叶片和河流水体样品，对大气汞的干、湿沉降进行了研究，并初步探讨了贡嘎山地区汞的输入和输出。主要得到以下结论： （1） 贡嘎山大气本底站两年的大气气态总汞的几何平均浓度为3.94±1.16 ng m-3，显著高于欧洲和北美同类型地区的大气气态总汞浓度，但和亚洲一些同类型地区的监测结果较为一致；和国内城市等受人为污染区域的大气气态总汞浓度相比，贡嘎山的浓度明显偏低，但略高于长白山大气本底站的气态总汞年均浓度。以上说明，贡嘎山地区的监测结果基本上反映了亚洲特别是我国西南偏远地区的大气汞的含量水平，说明我国人为活动的汞排放，已经造成了一定程度的大气汞污染。 （2） 活性气态汞和颗粒汞表现出和气态总汞不一致的分布特征，贡嘎山地区的活性气态汞平均浓度为6.2±3.9 pg m-3，颗粒汞的平均浓度为30.7±32.1 pg m-3，分别占到大气总汞的0.2%和0.8%，和国外偏远地区的研究结果一致。而活性气态汞和颗粒汞较高的沉降速率和较短的大气迁移距离是导致活性气态汞和颗粒汞偏低的主要原因。 （3） 贡嘎山不同功能区（特别是取暖季节）的大气气态总汞浓度明显高于全球背景区的平均浓度，且具有显著的空间分布特征：城区（8.63~22.5 ng m-3）>乡镇（4.74 ng m-3）>村落（2.55~8.83 ng m-3）>区域参照点（1.65~3.57 ng m-3）。 （4） 西南地区较高的汞释放背景是贡嘎山地区大气气态总汞偏高的一个重要原因；另外，石棉地区的金属冶炼活动、城市等人口集中地区的汞释放是造成该地区大气气态总汞浓度升高的区域性污染源。 （5） 气态总汞和颗粒汞表现出冬季>秋季>春季>夏季的季节性分布规律，而活性气态汞的季节性分布规律为春季>秋季>夏季>冬季。影响气态总汞和颗粒汞季节性变化的因素有释放源强度、气象条件（特别是风向）、光化学反应和干沉降速率的季节性变化；而活性气态汞的季节性变化则可能与大气光化学反应速率以及湿沉降的季节性变化有关。 （6） 贡嘎山地区海拔1600米和3000米空旷处的大气降水的湿沉降通量分别为9.1和26.1 μg m-2 yr-1，而森林地区的湿沉降通量可达57.0 μg m-2 yr-1；贡嘎山地区大气汞的干沉降通量51.9 μg m-2 yr-1，其中约15.9 μg m-2 yr-1(31 %)来自于植物落叶的沉降。 （7） 贡嘎山地区是大气汞的汇，仅泸定县的年均大气汞净沉降量就达142 kg yr-1，且绝大部分（>95%）进入了森林生态系统，而非森林地区的大气汞输入和输出则基本平衡。

Characterization of Air-Water Two-Phase Vertical Flow by Using Electrical Resistance Imaging

The characterization of air-water two-phase vertical flow in a 12 m flow loop with 1.5 m of vertical section is studied by using electrical resistance tomography (ERT). By applying a fast data collection to a dual-plane ERT sensor and an iterative image reconstruction algorithm, relevant information is gathered for implementation of flow characteristics, particularly for flow regime recognition. A cross-correlation method is also used to interpret the velocity distribution of the gas phase on the cross section. The paper demonstrates that ERT can now be deployed routinely for velocity measurements and this capability will increase as faster measurement systems evolve.

Laminar Burning Velocities and Markstein Lengths of Premixed Methane/Air Flames near the Lean Flammability Limit in Microgravity

Effects of flame stretch on the laminar burning velocities of near-limit fuel-lean methane/air flames have been studied experimentally using a microgravity environment to minimize the complications of buoyancy. Outwardly propagating spherical flames were employed to assess the sensitivities of the laminar burning velocity to flame stretch, represented by Markstein lengths, and the fundamental laminar burning velocities of unstretched flames. Resulting data were reported for methane/air mixtures at ambient temperature and pressure, over the specific range of equivalence ratio that extended from 0.512 (the microgravity flammability limit found in the combustion chamber) to 0.601. Present measurements of unstretched laminar burning velocities were in good agreement with the unique existing microgravity data set at all measured equivalence ratios. Most of previous 1-g experiments using a variety of experimental techniques, however, appeared to give significantly higher burning velocities than the microgravity results. Furthermore, the burning velocities predicted by three chemical reaction mechanisms, which have been tuned primarily under off-limit conditions, were also considerably higher than the present experimental data. Additional results of the present investigation were derived for the overall activation energy and corresponding Zeldovich numbers, and the variation of the global flame Lewis numbers with equivalence ratio. The implications of these results were discussed. 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.