Scielo RSS <![CDATA[Latin American applied research]]> vol. 40 num. 2 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Mixed-signal design of biopotential front-ends]]> High resolution Sigma-Delta Analog-to-Digital Converters (SDADC) have drastically changed traditional analog signal processing stages. However, as the boundary between analog and digital worlds becomes diffuse, a "mixed-signal processing" approach arises. For instance, analog filters, traditionally implemented as independent processing stages can be easily incorporated in the design of the Sigma-Delta converter, resulting in a more compact approach, with important advantages regarding size and power consumption. In this context, a design technique for mixed-signals front-ends intended for biomedical signals is presented here. As an example, the design of an ECG front-end is presented. It accepts DC offsets of 1300mV, presents an AC input range of 110mV, a -3 dB bandwidth of 100Hz and a total noise less than 10µVp-p, operating at a clock frequency of 57kHz. The front-end provides "fast baseline recovery" features and its transient response fulfills the AAMI standard. A functional prototype was built and tested, validating the design procedure. <![CDATA[Dufour and Soret effects on unsteady MHD convective heat and mass transfer flow due to a rotating disk]]> An unsteady three dimensional MHD convective heat and mass transfer flow in an incompressible fluid due to a rotating disk is studied by taking into account the Dufour and Soret effects. The system of axial symmetric non-linear partial differential equations governing the unsteady flow, heat and mass transfer is written in cylindrical polar coordinates and reduced to nonlinear ordinary differential equations by similarity transformations. The resulting system of ordinary nonlinear differential equations is then solved numerically by a shooting method using Range-Kutta six order integration scheme. The flow, temperature and concentration fields are affected by the magnetic interaction parameter M, Rotational parameter R, Soret Number S0 and Dufour number Df respectively. The results of the numerical solution are presented graphically in the form of velocity, temperature and concentration profiles. The results for the wall flow, temperature and concentration gradients obtained are presented in tabular form for various values of the parameters M, R, S0 and Df. <![CDATA[Co-catalytic effect of nickel in Pt-Ru/C and Pt-Sn/C electrocatalysts for ethanol electrooxidation]]> In the present study, we examined the effect of adding nickel to Pt-Ru and Pt-Sn catalysts for ethanol electrooxidation. The alcohol-reduction process with ethylene glycol was used to prepare ten electrocatalysts. These were microchemically and physically characterized by EDX and XRD analysis. The electrocatalysts were evaluated at mini-electrodes with cyclic voltammetry at 25 and 50 °C in sulfuric acid and ethanol solutions, and as anodes in fuel cell tests. Nickel addition to Pt-Ru mixtures significantly increased the catalytic activity toward ethanol electrooxidation. For Pt-Ru-Ni catalysts, the current density was around five times greater than all other investigated mixtures. Nickel also duplicated the catalytic activity of the Pt-Sn catalyst. Furthermore ethanol electrooxidation increased with temperature for most catalytic mixtures, and the electrocatalytic activity of mixtures containing nickel, demonstrate a considerable increase with temperature. Pt-Ru-Ni catalytic activity was higher than that of all other investigated mixtures. The fuel cell test demonstrated that the addition of nickel to Pt-Ru and Pt-Sn catalysts enhances the performance of the DEFC. <![CDATA[New stability criteria for discrete-time systems with interval time-varying delay and polytopic uncertainty]]> This paper is considered with the robust stability problem for linear discrete-time systems with polytopic uncertainty and an interval time-varying delay in the state. On the basis of a novel Lyapunov-Krasovskii functional, new delay-range-dependent stability criteria are established by employing the free-weighting matrix approach and a Jensen-type sum inequality. It is shown that the newly proposed criteria can provide less conservative results than some existing ones. Numerical examples are given to illustrate the effectiveness of the proposed approach. <![CDATA[Convection in a rectangular channel with a flow of water in laminar-turbulent transition with high heat fluxes]]> In this work we examine the phenomenon of laminar-turbulent transition in a heated rectangular channel under high heat fluxes and water as a working fluid. We use an experimental device that allows the electric heating of the walls and the measurement of its temperature through thermocouples housed beneath the surface. The results of this initial exploration clearly show the dependence of the convection coefficient on the heat flux through different mechanisms. In first place we can see an increase in the convection coefficient with the heat flux that is correctly predicted for laminar and turbulent flows by the Sieder-Tate correction. In second place, there was a clear change of behaviour that corresponds to the start of the transition to turbulence. The Reynolds number that corresponds to the departure from laminar behavior was independent of the heat flux in the walls within the conditions covered in this study. <![CDATA[Application of peleg model to study effect of water temperature and storage time on rehydration kinetics of air dried potato cubes]]> Potato cubes (1×1×1 cm) were dried at 65ºC in hot air oven. Samples were rehydrated by immersion in water during different periods of time and temperatures (2312ºC and 10012ºC). Rehydration kinetic was monitored by measuring samples' weights at regular intervals. All measurements were repeated after two months to study effect of shelf life on rehydration. Peleg’s model was successfully applied to experimental data and the corresponding parameters were obtained and correlated with temperature. The parameters of the model were found to be greatly affected by the water temperature during rehydration. Rehydration of samples was also dependent to water temperature. In particular, temperature increase led to higher final moisture contents in the samples after two months storage. On the other hand, higher temperatures decreased the moisture content of the samples at the end of first month which could be related to the structural changes in potato due to higher temperatures. <![CDATA[Stochastic optimization for the simultaneous synthesis and control system design of an activated sludge process]]> This work presents two stochastic optimization methods to perform the integrated synthesis and design of an activated sludge process. The process synthesis and design are carried out simultaneously with the control system design to obtain the most economical plant which satisfies the desired control system performance. The mathematical formulation of this objective translates the process superstructure into a mixed-integer optimization problem with non-linear constraints and dynamical-performance-indices evaluations. The proposed stochastic optimization algorithms, namely simulated annealing and a real-coded genetic algorithm, are valid alternatives to classical optimization techniques for the solution of such complex problems. The results are encouraging for future applications, because the easy implementation and the quality of the solutions obtained make not only possible but practical the solution of the integrated synthesis and design problem. <![CDATA[Basic design of lyophilization protocols for human bone tissues]]> For lyophilization protocols, the shelf temperature, chamber total pressure, and process time must be selected to maximize equipment throughput while maintaining highly reliable product characteristics. A general method to estimate these operation parameters was devised and applied to the production protocols of human cancellous chips, cortical struts, and morselized bone to be used as osteoconductive allografts. The sample freezing point was estimated utilizing thermodynamic principles and the shelf temperature was set just below this value. The total chamber pressure was adjusted according to the water vapor pressure above ice, and the process time during main and final drying were estimated considering both the amount of water to be removed from each sample and the rate of heat transferred by contact and radiation from the surroundings. The initial pressure-shelf temperature-time surfaces projected for all the drying stages were tuned experimentally to yield optimized freeze-drying protocols for each type of sample. <![CDATA[Thermophoresis and chemical reaction effects on mhd mixed convective heat and mass transfer past a porous wedge in the presence of suction]]> The effects of thermophoresis and MHD mixed convection flow with heat and mass transfer over a porous wedge are presented here, taking into account the homogeneous chemical reaction of first order. The governing fundamental equations are approximated by a system of nonlinear ordinary differential equations and are solved numerically by using the Runge Kutta Gill and shooting methods. The steady-state velocity, temperature and concentration profiles are shown graphically. It is observed that due to the presence of first-order chemical reaction, the concentration decreases with increasing values of the chemical reaction parameter. The results also showed that the particle deposition rates were strongly influenced by thermophoresis and the strength of the magnetic field in the presence of buoyancy force, particularly for opposing flow and hot surfaces. Numerical results for the skin-friction coefficient, wall heat and mass transfer are obtained and reported graphically for various parametric conditions to show interesting aspects of the solution. <![CDATA[Effect of operating conditions on Fischer-Tropsch liquid products produced by unpromoted and potassium-promoted iron catalyst]]> The dependencies of Fischer-Tropsch synthesis liquid hydrocarbon product distribution on operating pressure and temperature have been studied for unpromoted and potassium-promoted iron catalyst. The study followed an experimental planning and the results were analyzed based on surface response methodology. The effects of different operating conditions on the distribution of the liquid product were compared based on number-average carbon number and distribution dispersion. Results showed that high pressures (25 to 30 atm) favor the production of heavy waxes that can be converted into liquid fuels through hydrocracking, while greater selectivity towards liquid fuels are favored by low temperature (240ºC) and low pressure (20 atm). The liquid product distribution produced using potassium promoted iron catalyst presented higher number-average number of carbons, lower dispersion and lower skewness when compared to product distribution obtained with the unpromoted iron catalyst. The use of potassium as a promoter resulted in the production of longer hydrocarbon chains with higher selectivity of a specific narrow carbon number range. <![CDATA[Free and forced convective MHD oscillatory flow over an infinite porous surface in an oscillating free stream]]> This paper deals with the mixed convection hydromagnetic oscillatory flow and periodic heat transfer of a viscous incompressible and electrically conducting fluid past an infinite vertical porous plate. The plate is subjected to a constant suction velocity and heat absorbing sinks, while the free stream is oscillating with time. A magnetic field of uniform strength is applied in the direction normal to the plate. The transient, nonlinear and coupled governing equations are solved using multi-parameter perturbation technique. Approximate solutions have been derived for the velocity and temperature fields as well as mean skin-friction and mean rate of heat transfer. It is found that, the increase in magnetic field strength leads to decrease transient velocity as well as temperature. Further, the amplitude (|H|) as well as phase (tan_) of the mean rate of heat transfer increases with increasing magnetic field strength (M) for electrolytic solution (Pr=1.0), while a reverse phenomenon is observed for mercury (Pr=0.025). <![CDATA[Validation of a 0D/1D computational code for the design of several kind of internal combustion engines]]> A code for computational simulation of internal combustion engines is presented. One- dimensional gas dynamics equations are used for model the flow through pipes and manifolds, and the remaining components in the engine (cylinders, valves, etc.) are modeled by using thermodynamic or 0D models. The numerical code developed is able to simulate sparkignition and compression-ignition, two-stroke and four- stroke, multi-cylinder and multi-valve engines, naturally aspirated or turbo-charged, and different geometries of the combustion chamber. The code was implemented in the scripting language Python, which is a dynamic object-oriented programming language that offers strong support for integration with other languages and tools. The numerical methods used in the discretization of the equations and implementation details are presented. Several test cases are included in order to show the performance of the code. <![CDATA[Preparation of ?-alumina foams of high surface area employing the polyurethane sponge replica method]]> Single phase ?-Al2O3 ceramic foams with high surface area (~180 m²/g) and porosity (67%) were prepared by the polyurethane sponge method from slurries containing ?-Al2O3 (~40 wt % of solids). Different commercial polyurethane sponges were tested as templates for the ceramic foams, and the better results were obtained with a hydrophilated polyester polyurethane with pore density of 122 ppi, and thermal behavior that suggested the presence of fire retardant and smoke suppressor additives. The mechanical strength of the ceramic foams depended on several factors, such as the chemical nature of the sponge employed as template, composition of the slurry, and the heating rate during calcination. It was found that at least four cycles of re-coating followed by drying were required before calcination to prevent collapse of the ceramic foams. <![CDATA[Identification of biochemical reactors using fractional differential equations]]> High production meeting product quality, process safety and environmental regulation provide to control systems a key role in biochemical plants operation. As a suitable mathematical model is essential for process control, this work reports an alternative tool, based on the use of fractional order differential equations, for biochemical reactor identification using previously reported experimental data. Three different approaches were considered: i) solving the nonlinear set of algebraic equations obtained from the derivatives of the objective function with respect to the parameters; ii) solving a multivariable nonlinear deterministic optimization problem; iii) solving a multivariable nonlinear heuristic optimization problem. All identified models were submitted to statistical fitting tests and the second approach (ii) proved to be the most efficient for process identification, satisfying all statistical tests. Common integer order models were identified, leading to poorer data fit when compared to the fractional model, proving the usefulness and success identification tool proposed.