Showing papers by "Xi'an Jiaotong University published in 1986"

Simple Expression of Field Nonuniformity Factor for Hemispherically Capped Rod-Plane Gaps

Abstract: This communication compiles published computed data of the field nonuniformity factor for hemispherically capped rod-plane gaps, and presents two simple expressions for gaps with the ratio of gap length to rod radius ranging from 1 to 500.

Stability of predation models with time delay

Abstract: A computational method for determining regions of local stability of a system with time delay is presented. The procedure first looks for the regions of stability in terms of regions of unconditional stability, then the transition curves between stable and unstable sectors are computed. Two types of predator-prey models are used to illustrate the method

High frequency losses in multi-turn coils using a boundary element model

Abstract: Skin and proximity effect problems in single and multiple conductors are formulated using an integro-differential expression and are solved using Boundary Element techniques. The formulation is in terms of the vector potential and its normal derivative together with an average potential A 0 to account for integral constraint conditions. The solution accuracy offered by simple constant and linear elements is examined, as is the effect of discretization. The losses in single and multiple turn, solid and hollow conductor coils are examined as a function of turn spacing and separation for values of a/δ as high as 40; a is a characteristic dimension of the coil conductor.

Effect of Wavefront on Impulse Breakdown Voltages of Slightly Nonuniform Field Gaps in Nitrogen, Air and SF6

Abstract: The influence of the wavefront on impulse breakdown voltages for enclosed slightly-nonuniform-field gaps is reported. A 25 mm sphere-plane gap and a 20/50 mm diameter coaxial electrode system were used to obtain different fielt nonuniformity factors and different critical volumes. The wavefronts used in this work were 0.45, 1.35, 20, 50 and 90 ?s while the wavetail was around 450 , ?s for all cases. A 30 W UV lamp and 2 mCi caesium isotope were employed as irradiation sources. Experimental results show that strongly wavefront-dependent breakdown characteristics for small enclosed gaps reported by some other investigators were probably due to insufficient irradiation conditions. On the other hand, for the coaxial cylinder gap having a large critical volume, artificial irradiation was not necessary.

Breakdown characteristics of insulator strings stressed by short tail waves

Abstract: Traditionally, the withstand voltage of an insulator string to lightning impacts is checked using the standard impulse 12/50 μs First, the authors show that when a lightning stroke hits the tower or a ground wire, the resulting overvoltages are characterised by much shorter tail durations than the tail of incident lightning currents The analysis of wave shapes, in particular tail durations of the overvoltages affecting the insulator strings, is carried out in the form of a detailed parametric study Secondly, the authors describe the routine tests carried out in a HV laboratory to evaluate the withstand voltage of various insulator strings as a function of tail duration of the applied impulses Five strings of different lengths were tested, with impulse tail durations between 50 μs and 4 μs In conclusion, it clearly appears, when considering the mechanisms resulting in back flashover, that the withstand voltage of insulator strings is much higher than their conventional value for the 12/50 μs impulse

The Finite Element Method of Viscoelastic Large Deformation Plane Problem with Kirchhoff Stress Tensors-Green Strain Tensors Constitutive Relation

Abstract: The stress analysis of viscoelastic large deformation plane problem had been completed in [1]. Because the Eular stress tensors are used in the viscoelastic differential constitutive relation, when the rigid-body rotation can not be negligible the accuracy of results is relatively poor.

Boundary Element Method for Two-dimensional Transient Elastodynamics

Abstract: A boundary element method for two-dimensional transient elastodynamics is investigated. The method is combining the Laplace transform about time with the BEM. The results with respect to time can then be obtained by a numerical inversion. The singularities of the kernels is showed and the accuracy of numerical inversion is discussed. Some examples are given and compared with known solutions.

Stroboscopic Laser Speckle Interferometry Used for Measuring Dynamic Behaviour of Structures

Abstract: In this paper a new technology for measuring the dynamic behaviour of structures using the stroboscopic laser speckle interferometry is introduced. A good quality of whole-field speckle fringe patterns and Young's fringe patterns can be obtained when this method is used. Moreover, the methods of identifying the phase information of the vibratory structure and choosing the width of He-Ne laser pulses are discussed. Lastly, it presents the experimental results of comparing the stroboscopic laser speckle interferometry with the time-average speckle interferometry.

Isotropic/orthotropic two-layered strips containing cracks terminating at and going through an interface

Abstract: Crack problems for isotropic/orthotropic two-layered strips have been investigated. A system of two singular integral equations can be derived by using Fourier integral transformation and boundary conditions of crack problems. After stress singularities at crack tips or other special points are determined for internal and edge cracks, and for cracks terminating at and going through the interface, the system of singular integral equations is solved numerically by Gauss-Jacobi or Gauss-Chebyshev integration formulas for stress intensity factors at the tips and other singular points of cracks. Finally, possible crack growth behavior for cracks approaching and going through the interface is discussed.

A finite element analysis for the temperature field produced by a moving heat source

Abstract: Using moving mesh finite element method we discuss the temperature field produced by a moving heat source with the variable thermal conductivity and with the radioative and convective boundary conditions in a wide range of the velocity. The temperature-time relationships at various velocities in the static and moving coordinate systems are studied. The steady-state temperature distributions at various velocities in the moving coordinate systems are given. The temperature field produced by the plastic deformation at the process region (a region very near the crack tip) is also studied, and the results show that the highest temperature at the process region is lower than 1000°C or 1832°F.

Fatigue dislocation structure in alloy ferrite steel

Abstract: The fatigue dislocation structure in an alloy ferrite steel was studied by the method of electron diffraction contrast analysis. The dislocation structure features in annealed 12CrMo steel variate after 10~7 cycles under cyclic tensile stresses. The main features of fatigue dislocation structure are dislocation jog, dipole, Ioop, deformation band, tangle and banded dislocation structure. Alloy ferrite grains are broken into blocky-like microstructure regions.

Fatigue Thresholds of Isothermally Transformed Cast Steel and Nodular Cast Iron

Abstract: Silicon containing cast steel, plain nodular cast iron and Cu-Mo nodular cast iron were isothermally transformed at 360°C and 280°C respectively. Fatigue crack propagation behaviour at near threshold region of these materials were investigated. Particular emphasis has been devoted to the influence of bainite micro structure on fatigue threshold δKth. Cu-Mo nodular cast iron has the lowest δKth. Si cast steel and plain nodular iron have superior δKth associated with adequate mechanical properties. The aggregate of bainite ferrite and retained austenite instead of coarse carbides were found in Si cast steel. An explanation based on these results was proposed.

A New Analytical Model of Necking Phenomenon

Abstract: A finite element program based on elasto-plastic finite deformation theory is used to research the necking problem of titanium alloy. A new analytical model recommended in this paper is that an inclusion is first broken to form a void in the specimen, then neck is formed, void extendes and finally the specimen is broken. In the whole process the calculated and experimental nominal stress strain curves are coincided well. For the shape of neck a cubic curve is found more appropriate than the hyperbolic one.

Optimal control of xenon oscillations in load follow of a large nuclear reactor

Abstract: The problem of xenon spatial oscillations control during the load follow of large pressure water reactors is formulated as an optimization problem. The state equations of the system are composed of the one-group diffusion equation with temperature and xenon feedbacks, the iodine-xenon dynamics equations, and an energy balance equation for the core. The resulting distributed parameter model is first coverted into a lumped one by eigenfunction expansion and then a combination method, based on Differential Dynamic Programming and Matrix Riccati Method, is developed to obtain the optimal solution which can closely follow the desired power demand and maintain the desired flux distribution without too much control effort. Computational results show that the algorithms used in the proposed method converge for a broad spectrum of load follows. With the same precision, the combination method, comparing with DDP alone, is fairly computationally efficient.

A proposal for the establishment of nuclear-hydrogen energy system in china

Abstract: China is a country where its main energy source is coal. According to the rough estimate of the author, the exploitable and usable time of the coal is about 170 years. Then, what energy system we shall transit to before the ending of our coal era is an important problem for consideration. The Author suggests that a new energy-power system depending chiefly on nuclear and hydrogen energy, e.g. NHE system should be adopted. In this paper, the advantages and economics of NHES are illustrated. The urgency and feasibility to establish such a system are emphasized. The key nuclear-hydrogen energy conversion technologies as well as important measures for obtaining a high conversion efficiency of the NHES are discussed. Thus, the proposal advanced in the paper may be regarded as a tentative project for the establishment of a NHES in China.