Influence of source impedance on the performance of a fixed filter-thyristor controlled reactor (FF-TCR) type compensator

The influence of source impedance on the performance of a fixed filter-thyristor controlled reactor (FF-TCR) type compensator to improve the power factor of a thyristor controlled inductive load has been described. It is shown that the compensator gives better power factor improvement at higher source impedance, i.e. with a weak supply system. However, even with a small source impedance (i.e. a good supply system), a reasonable improvement in power factor is realized. The FF-TCR compensator keeps the distortion of the terminal voltage waveform within reasonable limits. The results obtained from the analysis of the lossy FF-TCR compare very well with the results obtained experimentally. This type of compensator is recommended strongly for industrial use.     

Elasto-plastic analysis of single-angle bolted-welded connections using the finite element method

The results of an extensive numerical elaslo-plastic study of single-angle connections subjected to cyclic loading consisting of end shear and bolt-moment are discussed. The thrust of the study is directed towards an improved understanding of the stress variations and the propagation of plastic zones in the vicinity of the weld.     

Selective leaching kinetics and upgrading of low-grade calcareous phosphate rock in succinic acid

Selective leaching kinetics of calcareous phosphate rock in succinic acid solution was investigated. Using a known particle size range of the sample, liquid/solid ratio and acid concentration, the influence of various reaction temperatures was studied in order to elucidate the kinetics of calcareous material in the rock. The leaching rate increased with increasing temperature. A kinetic model was suggested to describe the selective leaching process of calcareous material analyzing the kinetic data. The selective leaching curves were evaluated in order to check the validity of shrinking core models for liquid–solid systems. The experimental data were tested by graphical and statistical methods and it was found that the leaching of calcareous material in the rock was controlled by chemical reaction, i.e., 1 − (1 − x)^1/3 = 1.47 × 10⁶e^−64.92/RTt. The apparent activation energy of the leaching process was found as 64.92 kJ mol⁻¹ over the reaction temperature range from 313 to 353 K.     

Expanding Finance-Based Scheduling to Devise Overall-Optimized Project Schedules

Construction contractors often finance projects using bank credit lines that allow contractors to withdraw money up to certain credit limits. Finance-based scheduling provides schedules that ensure that the contractor’s indebtedness at any time during the construction stage does not exceed the credit limit. Generally, constricted credit limits tend to yield prolonged schedules. Provided that credit limits can be adequately relaxed, compressed schedules of compressed-duration activities can be attained. Devising a compressed schedule calls for the incorporation of time-cost trade-off (TCT) analysis to strike a balance between the decreased overhead costs and the increased direct costs of the activities. Since employing TCT analysis usually causes great fluctuations in the daily resource requirements by mixing compressed-duration activities of high resource demand with others of low resource demand, therefore, the need for resource management techniques becomes inevitable to ensure efficient utilization of resources. This note used genetic algorithms to expand finance-based scheduling to devise schedules for relaxed credit limits. A prototype system was developed and coded using VISUAL BASIC, then demonstrated using a five-activity example project. The prototype was validated by comparing the results with those obtained by using the integer programming. Expanding finance-based scheduling to handle the whole spectrum of credit limits helps devise overall-optimized schedules that consider cash, time, cost, and resources.     

岩石动力学中离散元程序处理波幅的新方法

顺应日益更新的计算机硬件及其外部设施，结合离散元程序UDEC，MATLAB和EXCEL，提出了一种在UDEC中全面处理各节点或单元中心波幅的数据处理新方法．该方法涉及修改UDEC源程序以扩展时间历程及表格上限、应用UDEC中的内嵌语言FISH在建模件中自动记录波的时间历程以及在MATLAB中处理各节点或单元中心波的幅值．给出了该方法在岩石动力学中波动问题研究的应用实例，结果表明：该方法优点多，在岩石动力学研究中具有较高的实用价值。     

Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.     

A two-dimensional mathematical model of a short vacuum arc in external magnetic field: results of numerical calculations

The paper deals with the investigation of the impact made by two-dimensional effects on the process of passage of current in a short vacuum arc in an axial magnetic field. A two-fluid mathematical model is used, which is based on hydrodynamic and electrodynamic equations. The axial magnetic field B _z affects significantly the magnitude of two-dimensional effects: the two-dimensional effects increase with decreasing B _z . The simulation results demonstrate that the contraction of plasma density exceeds that of current density. The distribution of anode drop of potential on the anode surface is nonuniform; in the case of certain (critical) values of current, the anode drop goes to zero on the external boundary of plasma. The dependence of the critical current on B _z is determined. The distribution of current density on the starting plane is nonuniform with a maximum on the axis, and the ion trajectories are inclined to the discharge axis. The possibility is discussed of matching the solution in the plasma region of vacuum arc with that for cathode flames.     

The influence of admixtures on the surface energy of Portland cement paste

This paper describes the effects of some admixtures on the strength and swelling characteristics of hardened cement paste. It is shown that the simple addition of admixtures changes the surface free energy of the highly dispersed phase of the hardened cement paste. Accordingly, the additional changes in strength and volume of the concrete system on the addition of admixtures can be partly attributed to the subsequent changes in the surface energy. The hypothesis that the increase in drying shrinkage and creep can be attributed to the influence of admixtures in altering the initial spacing and degree of layering of the tobermorite gel is qualitative whereas measurement of changes in the surface free energy provides a quantitative measure. Measurements of surface energy described in this paper are obtained from the measurements of dimensional changes on sorption and the use of Griffith's equation.

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Résumé On traite de l'influence de 4 combinaisons d'adjuvants, soit chlorure de calcium, lignosulfonate, lignosulfonate+chlorure de calcium et lignosulfonate+triéthanolamine, sur les caractéristiques de résistance et de gonflement de la pate de ciment durcie et sur la résorption après dessication. Les calculs, qui reposent sur les propriétés de la pate après résorption et l'utilisation de l'équation de Griffiths, indiquent que la simple addition d'adjuvants modifie l'énergie libre superficielle de la pate. L'eau et le méthanol formaient les phases absorbées et rien n'indiquait l'influence qu'aurait pu exercer un adjuvant spécifique. Ceci peut probablement être attribué aux déformations de retrait (et p. e. aux micro-fissures) engendrées durant le séchage. Enfin, l'évidence expérimentale suggère que la modification d'énergie superficielle après résorption est plus forte pour les éprouvettes qui renferment les adjuvants. Les recherches antérieures avaient pourtant indiqué que c'est durant le premier cycle de séchage que l'effet des adjuvants est le plus prononcé. D'autres techniques de mesure de l'énergie superficielle pourraient fournir une explication plus quantitative quant à l'influence des adjuvants, étant donné que les méthodes qui ont servi pour les recherches décrites ici prenaient en compte des propriétés des pates après le séchage.

     

Stress-strain properties of structural steel at strain rates of up to 105 per second at sub-zero,room and high temperatures

A testing technique and method of processing the displacement-time data have been developed following which the stress-strain characteristics of structural steel at strain rates between 10³ to 10⁵ per second over a strain of about 50% and at different temperatures have been determined. The steel under present test condition within this strain rate range showed a strong strain rate sensitivity. The material inertia and temperature rise during high speed deformation were found to have mutually cancelling effect on the deduced flow stress. In determining the results, appropriate friction correction was also made and the results presented in this paper are all converted to those under frictionless condition. Finally, a constitutive equation has been proposed for the steel incorporating the effects of work-hardening and strain rate sensitivity of the material.     

Leaching of radionuclides from uranium mill tailings and their flotation concentrates by hydrochloric acid and chloride salts

Hydrochloric acid leaching was conducted on Elliot Lake uranium mill tailings and their flotation concentrates (i.e., pyrite and radioactive concentrates), in order to obtain tailings low enough in radionuclides for environmentally safe disposal. The best leach residues in terms of radium grade are found to contain 66 pCi radium/g from the uranium mill tails, and 60 and 390 pCi radium/g, respectively, from the pyrite and radioactive concentrates. Furthermore, leach tests with 2 M solutions of sodium chloride and potassium chloride, respectively, were also conducted on the flotation concentrates. However, both acid and the chloride salt failed to yield tailings containing background level of radium (i.e., 2–3 pCi radium/g)