Expansion of clouds from pressurized liquids

This paper deals with the cloud formed by the expansion of a pressurized liquid as a result of tank rupture. Effects of combustion are ignored in this cloud model and the only energy for cloud formation is that of the pressurized liquid before tank rupture. The present cloud model is applicable to the rupture of a tank containing a volatile non-flammable or flammable liquid before ignition. The rupture of a tank of non-volatile flammable liquid can also result in the formation of a cloud if the liquid is heated before tank rupture. Although the latter case can be treated by the methods described here this paper is focussed on the type of cloud formed by liquids that are normally volatile at ambient temperature and are shipped pressurized at that temperature.     

The terrestrial particulate environment and its future evolution

In this paper an overview over the current space debris situation is given. With the help of ESA's new Meteoroid And Space Debris Terrestrial Environment Reference (MASTER) model, the spatial density of space debris is calculated as function of altitude and latitude. Statistical information about collision risk and collision velocities is presented. In order to estimate the ratio of debris to meteoroid impacts on the surfaces of returned hardware from space (LDEF, Hubble Space Telescope solar array and EuReCa) a state-of-the-art meteoroid model is required. At the Max-Planck-Institute in Heidelberg the meteoroid model of Neil Divine was enhanced and implemented in a software programme used at ESOC for meteoroid flux predictions on satellite surfaces. Finally, using the models of the current environment as initial conditions an attempt is made to predict the future evolution of the space debris environment. Two models using different approaches are considered: CHAINEE, a particle-in-the-box model and SDM, a semi-deterministic model. Both models are used to study the effectiveness of debris mitigation measures.     

Propagation of hypervelocity impact fragment clouds in pressure gas

This study investigated the propagation of hypervelocity impact fragment clouds in pressure gas. Fragment clouds were generated through perforation of thin aluminium bumper plates by spherical aluminium projectiles. A thick aluminium backwall plate, placed inside a pressure container at a given distance from the bumper plate, caught the fragments to act as a witness plate for the residual damage potential of the fragments. Crater depth statistics are presented as a function of container pressure. The fragment cloud was photographed by means of an image converter camera. The images showed a strong deformation of the fragment cloud for increased container pressures and were used to extract residual velocities until up to 50 μs after impact. The deceleration of the velocity as a function of time after impact suggested an exponential decay function as the best fit to the curve. Thus, maximum fragment impact velocities on the backwall plate could be extrapolated from the axial cloud velocities. The extrapolated curves were compared with experimental time-of-flight measurements, and proved a good match. Fragment impact velocities and maximum crater depths were used to calculate maximum fragment particle sizes as a function of the container gas pressure.     

Toxic gas clouds: effects and implications of dry deposition on concentration

The influence of the dry deposition process on concentration pertaining to toxic gas clouds was investigated by model calculations. Three main release scenarios were simulated, with nine micrometeorological cases considered for each. To compare and confirm the results, two model types, a stochastic particle model and a box-type model, were independently used to simulate many of the different cases. The results showed that the effects of dry deposition may be strong for releases at, or confined close to the ground, e.g. neutral or unstable stratification can cause higher concentrations than stable stratification after 10-15 km. Risk distances are in turn affected and may be substantially shortened, e.g. for a zero-height release like that from an evaporating pool, a 50% reduction in total airborne substance may occur within 500 m at a low wind velocity and neutral or stable stratification.     

重气效应研究进展

为了研究天然气泄漏后的重气效应,从易燃易爆有毒气体的重气扩散机理和研究方法出发,具体分析了重气云团的扩散过程,概述了国内外发展的模拟重气云团扩散的大量的数值模型和开展的大量实验.主要包括唯象模型、箱及相似模型、三维流体力学模型和浅层模型四大模型及其研究进展.最后提出了三维流体力学模型和现场数据采集相结合的方法是天然气重气效应未来的研究方向.     

Microstructural evolution and semisolid forming of SiC particulate reinforced AZ91HP magnesium composites

Abstract

The aim of this paper is to report the experimental results concerning the microstructural evolution and formability of semisolid forming SiC particulate reinforced AZ91HP Mg composites. The composite materials were produced in the form of cylindrical billets by a rotating cylinder method. The microstructural evolution was characterised by conventional parameters (liquid fraction, average size of globules, and number of globules per unit area) as functions of temperature and isothermal holding time in the semisolid state. In addition, special attention was paid to the shapes of the globules and the liquid droplets entrained within the globules and to the variation of hardness with regard to microstructural evolution. Finally, the formability of the composites was evaluated through thixocasting trials, and then correlated with the effective liquid fraction of the composites.     

衡阳市冬春季大气颗粒物中重金属的污染评价

采用潜在生态危害指数法和富集因子法,对衡阳市冬春季大气颗粒物中重金属污染进行评价。结果表明:重金属元素的潜在生态危害大小为CdCrPbCuNi,春季冬季,细颗粒物PM~2.5总悬浮颗粒物TSP,Pb、Cd、Cr的潜在生态危害程度均属于极强;重金属的富集程度为:CdCrPbCuNi,PM_2.5TSP;Pb、Cu、Cd冬季富集程度大于春季,Cr和Ni冬季富集程度小于春季;TSP和PM_2.5中重金属的富集系数EF值均大于1,说明重金属的相对富集受到人为活动的影响。     

Microstructure evolution through heavy compression aided by thermodynamic calculations

The induced martensite transformation in a dual-phase bainitic ferrite–austenite steel during heavy compression was studied by thermodynamic computations. Compression tests were conducted at temperatures of 298 and 573 K on the rectangle samples at the strain rate of 0.001 s⁻¹. The samples were deformed to 40 and 70% of their original thickness. It was found that 70% compression of the steel at room temperature resulted in transformation of retained austenite to martensite, which is in agreement with thermodynamic calculations. Additionally, heavy compression resulted in the formation of fine grains with high angle grain boundaries which confirms grain refinement.     

Micro-structural evolution of cubic zirconia irradiated with swift heavy ions

This article deals with the study of the micro-structural properties of cubic zirconia irradiated with swift heavy ions, based on the combination of complementary analysis techniques (TEM, AFM, XRD, RBS/C). At low irradiation fluences, cross-sectional TEM observations show the creation of latent tracks in the wake of incident ions by electronic excitation processes. The melt matter flowing from ion tracks leads to the formation of large hillocks at the surface of samples revealed by AFM. At high irradiation fluences, the overlapping of tracks results in the subdivision of crystals into slightly disoriented nano-crystallites detected by XRD. RBS/C data analyzed with a Monte-Carlo simulation code confirm the occurrence of this peculiar micro-structural transformation.     

Introduction of damage evolution in a scale transition approach for highly-filled particulate composites

The present paper deals with a non-conventional scale transition for modelling the behaviour of highly-filled particulate composites, starting from a methodology initially proposed by Christoffersen [Christoffersen J. Bonded granulates. J Mech Phys Solids 1983;31:55–83] and recently extended by Nadot et al. [Nadot C, Dragon A, Trumel H, Fanget A. Damage modelling framework for viscoelastic particulate composites via a scale transition approach. J Theor Appl Mech 2006;44(3):553–83] in presence of damage. The model thus obtained is here completed with several ingredients allowing to describe damage evolution and in particular a defect nucleation criterion as well as a closure criterion. These criteria are formulated in terms of displacement, and so as to ensure continuity in terms of macroscopic stress. They are finally introduced in an iterative numerical solving procedure which allows to follow damage evolution as a discrete sequence of interfacial debonding including also eventual closure of defects.     

Experimental and numerical study of heavy gas dispersion in a ventilated room

In order to better evaluate the consequences of an accidental release of heavy gas, such as uranium hexafluoride (UF(6)), in some installations in the nuclear fuel cycle, an experimental and numerical study was conducted by IRSN on heavy gas dispersion in a ventilated room. This study was based on about 20 injection configurations of a large quantity of a heavy tracer gas, sulphur hexafluoride (SF(6)), inside two ventilated rooms of different sizes. Stratification of the tracer gas was detected in all the configurations studied, even at low concentrations. Numerical simulations performed with the multidimensional CFX code enabled the stratification and the concentration levels reached in the rooms to be predicted overall, and the higher the air flow rate, the more satisfactory the comparison between simulation and experiment.     

Validation of turbulence models in heavy gas dispersion over obstacles

Accidental release and dispersion of toxic gases were always major hazards for public health and safety that process industries had to deal with. Real terrain dispersion simulation for risk analysis purposes poses specific difficulties related to complex turbulence phenomena development, especially when obstructions are found into the flow region. In this paper, several turbulence models are tested and compared against experimental data of dense vapor dispersion, in order to decide for their suitability for simulating such flows. Computations were carried out using the computational fluid dynamics (CFD) code CFX 5.6, while experimental data were obtained from Thorney Island large scale field trials. Computational results showed good agreement with experimental measurements indicating that CFDs provide a reliable means of estimating gas dispersion in real terrains.     

Numerical simulation of the macrostructure evolution of a heavy steel ingot

The macroscopic grain structure of a 234 t steel ingot was simulated by using cellular automaton and finite element method provided by ProCAST. The growing speed coefficient of Kurz–Giovanola–Trivedi model was modified to reflect the growing process of grains. Chilled fine grains, columnar grains and coarse equiaxed grains were obtained from surface to core of the ingot, which were roughly in agreement with the measured ones. The influences of the nucleation and growth parameters were investigated. It was found that the relationship between nucleation and growth determines the macrostructure profile. An index, the ratio of growing speed over the nucleated sites at certain undercooling was proposed to estimate the macrostructure tendency.     

顶空-气相色谱法测定卷烟主流烟气粒相物中吡啶

为研究气相色谱测定卷烟主流烟气粒相物中吡啶含量,采用碳酸钠水溶液作基质校正剂,通过顶空进样、DB-WAXETR色谱柱分离、火焰离子化检测器检测。结果表明:吡啶在0.20~16.20μg/mL质量浓度范围内线性拟合度为0.9998,加标回收率在98.4%~104.7%之间,定量限为0.08μg/支,RSD小于5%,该方法可以快速、准确地测定卷烟主流烟气粒相物中吡啶。     

Kinetics of shrinkage and shape evolution during liquid phase sintering of tungsten heavy alloy

A numerical model describing gravity-induced shape distortion and densification during solid and liquid phase sintering is proposed. The constitutive formulation is based on the continuum theory of sintering and implemented in commercial finite element software. Simulations under gravity-induced stress are attempted on the basis of the model parameters where viscosity is assumed to be temperature and porosity dependent. Viscosity is assessed through shrinkage and shrinkage rate data obtained experimentally from dilatometry over a controlled temperature regime. Effects of temperature, heating rate, and liquid phase formation on porosity evolution are analyzed. Additionally, sample studies on the influence of heating rate, gravity, friction coefficient, aspect ratio, and volume on the predicted distortion profiles after sintering of a tungsten heavy alloy are also presented. These numerical results are compared with experimental data from the literature.     

Determination of the parameters of atmospheric gas clouds by radar methods at frequencies in the rotational spectrum

An analysis is made of a method of determining the parameters and coordinates of atmospheric gas clouds radar systems which can measure the attenuation of electromagnetic radiation power at a resonant absorption frequency. A possible technical application of these systems is indicated. This approach to the problem of controlling and monitoring the state of the atmosphere can describe the propagation dynamics of contamination in real time, which is extremely important for ensuring safety in regions liable to chemical contamination. After suitable development, the proposed method may provide the basis for remote monitoring by radar techniques and in principle, may also be used to study various types of nonreflecting objects possessing the property of resonant absorption. Pis’ma Zh. Tekh. Fiz. 23, 1–5 (December 26, 1997)     

Study of microstructure degradation and rejuvenation evolution for F-class gas turbine blade

Service degradation of F-class gas turbine blade and its microstructure evolution during a simple rejuvenation process are investigated, and then microhardness of the rejuvenated blade is evaluated. The results show that the turbine blade suffers from significant service degradation such as spheroidising, coarsening and rafting of the γ′ phase. It is also verified that the γ′ precipitates microstructure could be recovered and even improved by a full solution followed by two-step aging. The proper solution temperature is necessary to dissolve the deformed γ′ precipitates and allows optimum reprecipitation during next aging. The microstructure of the rejuvenated blade all shows bimodal distribution with coarse and fine γ′ precipitates. The microhardness is higher and more uniformly distributed than that of the service-exposed blade.     

The evolution of instabilities in gas microjets under acoustic action

The results of an experimental investigation of the effect of external acoustic perturbations on the stability of millimeter- and submillimeter-scale gas jets flowing out into the atmosphere are presented. Data on flow visualization by the shadow method and the instantaneous velocity fields of the flow by the PIV method are obtained. The sound effect in the jet is shown to lead to the asymmetric mode of instability. The growth of this mode downstream leads to flow bifurcation. Frequency characteristics of the effect for jets of different geometries and jets of different gases are compared.     

A precision gas feeding system for a PIG heavy ion source

A precision gas feeding system aiming at stable and efficient operation of a small, cold-cathode PIG heavy ion source for the K = 50 MeV AVF cyclotron is described in detail. The system can regulate the supply gas pressure with a precision of 0.6 Torr and keep a constant gas flow rate with a stability of 0.010 ± 0.003 cm³/min. The ion source operating parameters such as the arc current, the arc voltage and the gas flow rate remain fairly constant through the lifetime of the source. By adopting this system in the routine accelerations of carbon or oxygen ions, the average beam current was increased by a factor of 2–3.