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Recent 48

Vol. 17 (2016), No. 2 (48), July 2016

 

This issue of the RECENT® journal was edited with the support of:

CONTENTS



  • Tibor BEDO, Marcel DĂIAN, Bela VARGA, Maria STOICĂNESCU, Aurel CRIȘAN, Ioan CIOBANU

A Simulation and Experiment Concerning the Solidification of an Al-Zn Alloy Casting

» Abstract

A software product was developed to simulate the solidification of rotationally symmetrical castings. It applies to gravity and centrifugally cast alloys that solidify within a temperature interval. The software uses a mathematical model with finite differences and cylindrical coordinates. An experimental verification of this software is presented. The simulated cooling curves obtained by this software are compared to those obtained experimentally by thermal analysis. The verification was carried out for the case of a cylindrical hollow part cast from an aluminium-zinc alloy. The experimental results are very close to those obtained by simulation. This confirms the validity of software.

» Keywords

solidification simulation, casting, thermal analysis, aluminium-zinc alloy

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  • Ioan CIOBANU, Marcel DĂIAN, Ion Sorin MUNTEANU, Aurel CRIȘAN, Tibor BEDO

Aspects Regarding the Solidification Kinetics of a Cast Iron Centrifugal Casting

» Abstract

There are showing the results of a study on cooling and solidification kinetic of a tubular cylindrical part of eutectic cast iron. The study was conducted by computer simulation. Six variants were compared three by centrifugal casting and three variants of gravity casting. There were analysed: the average speed of the cooling in liquid state (in the range To - Ts), momentary cooling rate at the beginning of solidification (at Ts = solidus temperature) and the thickness of the white iron at the interface casting - mold. Comments are made on the whitening trend of parts cast by centrifugal castings compared to gravity casting. It is analysed the influence of the external sand cores and outer coating of metal forms in centrifugal casting on the tendency whitening the structure.

» Keywords

solidification, casting, centrifugal casting, solidification simulation, cast iron

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  • Virgil GEAMĂN, Mihai Alin POP, Irinel RADOMIR

Liquid Hot Isostatic Processing Applied to AtSi5Cu1 Aluminum Alloy

» Abstract

The paper presents some experimental data about the liquid hot isostatic processing (L.H.I.P.) process applied to ATSi5Cu1 aluminum alloy, most utilized in the Romanian machines building industry. L.H.I.P. treatment reduced the pores mean size by about 1/3, the volume fraction by one order of magnitude and the shrinkage voids in number and volume. All experimental data is obtained in the specific testing laboratory from Transilvania University of Brasov.

» Keywords

liquid isostatic pressing, rheoforming, semi-solid forming

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  • OBBU CHANDRA SEKHAR, ANOOP CHOKKA ANAND, LALITHESH AKULA, SAIKUMAR MUNIKANTLA

Simulation of Three-Level Diode Clamped Multilevel Inverter Using SPWM Technique, Space Vector Strategy & SHE Technique

» Abstract

The multilevel began with the three level converters. The fundamental thought of a multilevel converter to fulfil higher power to use a progression of power semiconductor switches with a few lower voltage dc source to perform the power transformation by combining a staircase voltage waveform. Regardless, the output voltage is smoother with a three level converter, in which the output voltage has three conceivable values. This results in less harmonics, yet on the other hand, it has more sections and is more unpredictable to control. In this project, three level inverter topologies and Sinusoidal pulse width modulation, Selective harmonic elimination and Space vector strategy has been connected to characterize the exchanging example for three level inverter that minimize the harmonic distortion at the inverter output. Mat lab Simulation result will be analysed.

» Keywords

SPWM (sinusoidal pulse width modulation), THD (total harmonic distortion), Space vector, SHE (selective harmonic elimination)

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  • Petko YANGYOZOV, Adile DIMITROVA

Approach of Assessment the Overall Necessity of Supply Logistics Processes Improvement

» Abstract

In the present paper an approach has been presented, through which it can determine the necessity of improvement of supply logistics in the organization. The need for developing and implementation of this approach is revealed. Through it starts the process of optimization of supply logistics and passes through the remaining steps. The paper continues with explanation how the comparison between the set forth goal of improvement and the actual state of the current logistics processes is made. The essence of functioning of the approach is presented. The formulas through which it can be calculated coefficients of „efficiency of maximized measures” and coefficients of “efficiency of minimized measures” are described. Algorithm and methodology of calculating the coefficient of overall efficiency of the supply logistics, coefficient of single-dimension efficiency of the supply logistics and coefficient of multi-dimension efficiency of the supply logistics that are necessary for determining the general necessity of improvement are reviewed.

» Keywords

supply logistics, optimization, necessity of improvement, coefficient of efficiency, algorithm, deviation, methodology

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  • Vadim ZHMUD, Lubomir DIMITROV

Calculation of Desired Coefficients for the Characteristic Equation of Closed Automatic Control System

» Abstract

The localization method has been developed to control linear and nonlinear objects. It is widely used, because it has adaptive properties, which contribute to its widespread use. One of the essential steps of this method consists in setting of the desired equation of the closed system. This equation determines the dynamic error of the system, i.e. the deviation of its output from the prescribed value during the transient process. The order of the system and the controller together define the desired order of this equation. The coefficients should be set based on the requirements for dynamic errors, which most often are typical. Therefore, it is logical to expect that the coefficients of the desired equations are also typical, but they are not. Each researcher uses his method of selecting these coefficients. The reasons for these methods are not always sufficiently justified. Development of science-based and, therefore, a reliable way of solving this problem would solve it once for the all times, so that not resolve it further. It would have saved the researchers of excess labor for this choice and would enhance the quality of the result, if the choice would be optimal. This paper offers not only the methodology of the choice with numerical optimization of the corresponding filter coefficients based on adequate criterion, but also gives the results of this procedure for the most frequent cases of the characteristic equations corresponding to the order from the second to the sixth. If necessary, this technique can be extended to higher order. Numerical simulations confirm the results.

» Keywords

control, regulation, dynamic accuracy, static accuracy, characteristic polynomial of a system, polynomial of desired dynamics

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