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http://www.scielo.org.ar/rss.php?pid=0327-079320110004&lang=es
vol. 41 num. 4 lang. es<![CDATA[SciELO Logo]]>http://www.scielo.org.ar/img/en/fbpelogp.gif
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<![CDATA[Particle transport in turbulent flows along horizontal ducts]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400001&lng=es&nrm=iso&tlng=es
The present contribution describes three-dimensional Euler/Lagrange calculations of confined horizontal gas-particle flows (i.e. channel and pipe flows) emphasizing the importance of elementary processes, such as particle collisions with rough walls and inter-particle collisions, on the predicted overall flow pattern and pressure drop along the duct.<![CDATA[Influence of catalyst treatments on the decomposition of hydrogen peroxide on supported palladium catalysts]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400002&lng=es&nrm=iso&tlng=es
The decomposition of hydrogen peroxide was studied on Pd catalysts between 293 K and 313 K. The apparent activation energy value was calculated for all the catalysts and it varied between 27 and 55 kJ/mol. For all the catalysts tested, treatment with hydrogen resulted in a significant increase in the reaction rate, as well as a decrease in the apparent activation energy for the reaction. All the reactions were well represented by a first order rate law with respect to H2O2. The effect of the treatment with H2O2 was studied in successive reactions without catalyst exchange. A small deactivation was observed in the 5.0% Pd/AC and 0.5% Pd/AC catalysts after four H2O2 decomposition reactions.<![CDATA[Factorial design applied to corrosion of superduplex stainless steel]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400003&lng=es&nrm=iso&tlng=es
Steels employed in offshore oil and gas production are subject to a very corrosive environment. Especially the new oilfields located in pre-salt layers imply the contact of steels with high brine concentration, high temperature and presence of corrosive gas such as CO2. Besides these facts, stainless steels have to present higher mechanical properties obtained from an optimized heat treatment. In order to take into account these factors and their synergisms, on the present paper, we have chosen a factorial experimental design to study the corrosion behavior of superduplex steel UNS S32750 by electrochemical tests. The results of open circuit potential, polarization curves and electrochemical impedance were analyzed with statistical methods considering a confidence level of 95%. The factors that significantly affect the corrosion potential are the carbon dioxide and heat treatment; the corrosion current is sensitive to carbon dioxide, and the resistance of polarization is strongly affected by the CO2 content.<![CDATA[Optimization of the leaching conditions of an autoclave: Application to the dissolution of ferrocolumbite from san luis province, Argentina]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400004&lng=es&nrm=iso&tlng=es
The optimum working conditions of a Parr laboratory autoclave for minerals dissolution were studied and the results obtained were applied to the leaching with HF of a Nb and Ta mineral, from the San Luis, Argentina. The optimized variables were heating rate, liquid volume, stirring speed, inert solid addition and cooling rate. Changes in the liquid volume, stirring speed and presence of an inert solid did not produce modifications in the actual heating rate. The optimum heating programs ranged between 5 and 100C/min and depended on the programmed final temperature. The cooling period was 25 min. The results indicated that, using the optimum working conditions, more than 90% of mineral might be dissolved at 2200C. It was also observed that the dissolution of mineral was slightly affected by the stirring speed and the particle size whereas temperature, HF concentration, solid-liquid relation and reaction time strongly influenced the extraction of elements.<![CDATA[On the use of continuous distribution models for characterization of crude oils]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400005&lng=es&nrm=iso&tlng=es
Crude oil characterization plays a key role in upstream as well downstream operations of petroleum supply chain It is usually carried out by a batch distillation process known as true boiling point (TBP) distillation, which represents a "footprint" of the crude oil composition profile, once its shape depends on the amount and volatility of components in a given crude oil. In the last decades, crude oil characterization methods by continuous distribution models have been proposed, as an option to the classic (discrete) pseudo component approach. The comparative performance of five continuous distribution models - Beta, Gamma, Riazi, Weibull and Weibull extreme - in characterizing the TBP crude oil distillation curve is presented in this work. A large TBP database of different types of Brazilian crude oil is used to identify the optimal characterization parameters of these models by a least-squares statistical criterion. The modeling performance of each continuous distribution model was measured using statistical estimators. The Weibull extreme model presented the most adequate performance in terms of the root mean squared error (RMSE) for all crude oils. In general, the model parameters uncertainties increase with the crude oil API density, despite the reversed behavior shown by Gamma model.<![CDATA[Soret and dufour effects on heat and mass transfer due to a stretching cylinder saturated porous medium with chemically-reactive species]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400006&lng=es&nrm=iso&tlng=es
The diffusion-thermo and thermal-diffusion effects on heat and mass transfer by boundary layer flow over a stretching cylinder embedded in a porous medium have been studied numerically in the a presence of chemical reaction effect. The governing nonlinear partial differential equations are transformed into a set of coupled ordinary differential equations, which are solved numerically by using Runge-Kutta method with shooting techniques. Numerical results are obtained for the velocity, temperature and concentration distributions, as well as the skin friction coefficient, local Nusselt number and local Sherwood number for several values of the parameters, namely, the Reynolds number, Darcy number, chemical reaction parameter, Dufour and Soret numbers. The obtained results are presented graphically and the physical aspects of the problem are discussed.<![CDATA[Approximating the solution to lqr problems with bounded controls]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400007&lng=es&nrm=iso&tlng=es
New equations involving the unknown final states and initial costates corresponding to families of LQR problems are shown to be useful in calculating optimal strategies when bounded control restrictions are present, and in approximating the solution to fixed-end problems. The missing boundary values of the Hamiltonian equations are obtained by (offline) solving two uncoupled, first-order, linear partial differential equations for two auxiliary n W n matrices, whose independent variables are the time-horizon duration T and the eigenvalues of the final-penalty matrix S. The solutions to these PDEs give information on the behavior of the whole (T, S)-family of control problems. Illustrations of numerical results are provided and checked against analytical solutions of the cheapest stop of a train' problem.<![CDATA[Soret and dufour effects in a mixed convection couple stress fluid with heat and mass fluxes]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400008&lng=es&nrm=iso&tlng=es
An analysis is presented to investigate the Soret and Dufour effects on the mixed convection heat and mass transfer along a semi-infinite vertical plate embedded in a couple stress fluid with flux distributions. The governing non-linear partial differential equations are transformed into a system of ordinary differential equations using similarity transformations and then solved numerically. Profiles of dimensionless velocity, temperature and concentration are shown graphically for various values of Dufour number, Soret number and Couple stress parameter.<![CDATA[Robust stability test for uncertain discrete-time systems: a descriptor system approach]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400009&lng=es&nrm=iso&tlng=es
This paper presents a new and less conservative condition for the robust stability test of discrete-time polytopic systems by using a descriptor system approach. The stability condition is formulated in terms of a set of linear matrix inequalities and can be easily adapted for robust controller synthesis. The developed results can be viewed as a discretetime counterpart of the continuous-time one proposed by Cao and Lin (2004). This also enables us to offer a unified framework, namely the so-called descriptor system approach, for the analysis and synthesis of both discrete-time and continuous-time uncertain linear systems. Simulation examples are given to illustrate the theoretical results we established.<![CDATA[Using hybrid parallel programming techniques for the computation, assembly and solution stages in finite element codes]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400010&lng=es&nrm=iso&tlng=es
The so called "hybrid parallelism paradigm", that combines programming techniques for architectures with distributed and shared memories using MPI (Message Passing Interface) and OpenMP (Open Multi-Processing) standards, is currently adopted to exploit the growing use of multi-core computers, thus improving the efficiency of codes in such architectures (several multi-core nodes or clustered symmetric multi-processors (SMP) connected by a fast net to do exhaustive computations). In this paper a parallel hybrid finite element code is developed and its performance is evaluated, using MPI for communication between cluster nodes and OpenMP for parallelism within the SMP nodes. An efficient thread-safe matrix library for computing element/cell residuals (or right hand sides) and Jacobians (or matrices) in FEM-like codes is introduced and fully described. The cluster in which the code was tested is the CIMEC's 'Coyote' cluster, which consists of eight-core computing nodes connected through Gigabit Ethernet.<![CDATA[Influence of topography on wind pressures in tanks using CFD]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400011&lng=es&nrm=iso&tlng=es
The influence of topography on wind pressures acting on structures has been of interest to the civil engineering community for some time; however, because of the complexity of the problem, only few cases have been solved. The evaluation of pressures in tanks located in hills is one of those complex problems and has not been addressed in the literature. This paper presents a computational fluid dynamics simulation of the problem, in which the domain including the hill and the tank is discretized using finite elements. The results show that the actual location of the tank with respect to a hill has a significant influence on the pressures, so that tanks located at the top of a hill undergo severe increases in pressure coefficients and also changes in pressure distributions around the tank.<![CDATA[Molecular weight distributions in ideal polymerization reactors: An introductory review]]>
http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0327-07932011000400012&lng=es&nrm=iso&tlng=es
The ultimate aim of polymerization reaction engineering is the production of polymers with tailor-made properties. An introductory review into this field is presented, with emphasis on the effects on the molar mass distribution (MMD), of the sought combination of polymerization mechanism, reactor type, and reactor control. Three ideal polymerization mechanisms are analyzed: free-radical, "living" anionic, and step-growth. "Living" anionic and step-growth polymerizations are similar in that their growing chains remain reactive while inside the reactor; and for these systems the narrowest MMDs are produced in reactors with narrow residence time distributions (RDT); i.e.: batch or continuous tubular reactors. In contrast, in conventional free-radical polymerizations, the polymer molecules grow in a fraction of a second and thereafter remain inactive while inside the reactor. In this case, the RTD does not affect the MMD, and the homogeneous continuous stirred-tank reactors provide the narrowest MMDs. Representative mathematical models of polymerization reactors are useful for: a) quantifying the interrelationships between their numerous inputs and outputs; and b) developing open- and closed-loop strategies for increasing reactor productivity and product quality.