CFD Modeling of Bubble Column Reactor Including the Influence of Gas Contraction

In this paper a CFD model for a bubble column reactor undergoing a first order reaction A → B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter D_T = 1 m and height H_T = 5 m. The incoming gas stream contains inerts, varying in proportion from 10 % to 90 %. Three‐dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity U_G = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts. The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.     

Validating the Darken Relation for Diffusivities in Fluid Mixtures of Varying Densities by Use of MD Simulations

Molecular Dynamics (MD) simulations have been carried out to determine the self‐diffusivities, D_i,self, of the pure hydrocarbons methane (C1), ethane (C2), propane (C3), and n‐hexane (nC6) at various fluid densities. The MD simulations are in reasonable agreement with published experimental data. The influence of fluid density on both D_i,self and the Maxwell‐Stefan (M‐S) diffusivities, ?_ij, in binary C1‐C2, C1‐C3, C2‐C3, and C1‐nC6 mixtures was also investigated. The MD simulations show that the M‐S diffusivities in binary fluid mixtures can be estimated with good accuracy using the Darken relation.     

Thermal stability and elevated temperature mechanical properties of electroslag remelted Fe-16wt.%Al-(0.14–0.5)wt.%C intermetallic alloys

Addition of carbon in the range of 0,14–0.5 wt.% to the Fe₃Al-based intermetallic Fe-16wt.%Al (Fe-28at.%Al) alloy results in the formation of a thermally stable dispersion of Fe,AIC carbide phase. The volume fraction of these precipitates increases with increase in carbon content. Processing of these alloys through a combination of air induction melting and electroslag remelting leads to enhanced elevated temperature mechanical properties compared to those reported for the low (< 0.01 wt.%) carbon alloys with similar Al contents. Enhancement of up to 30% in elevated temperature yield strength was observed at the test temperatures (600, 700 and 800°C) used. The improvement in mechanical properties may be attributed to the presence of strengthening Fe₃AlC phase as well as the interstitial carbon present in the alloy matrix. The addition of carbon also leads to improved room temperature mechanical properties in contrast with other alloying additions (such as Mo, Ti and Si) used for enhancing elevated temperature properties of Fe₃Al-based intermetallic alloys. It is suggested that carbon may be an important alloying addition to these alloys.     

Active gate control of series connected IGBTs using positive current feedback technique

Controllable devices like insulated gate bipolar transistors (IGBTs) are very popular on account of their highly desirable features. This prompts their use in high-voltage applications. But high-voltage IGBT development is facing constraints and there is a requirement for series connection of these devices to enhance their voltage withstanding capability. The series connection of IGBTs, however, has its own problems (e.g., dynamic voltage imbalances leading to device break down etc.). Active gate control offers a good solution to this problem. In this brief, a novel scheme of "active gate control using a positive current feedback" is proposed. The system uses a current feed back network where the gate voltage is dependent on the current through the device but not on the overvoltage across the device (the existing trend). The scheme has been simulated using PSPICE and validated experimentally. Analytical and control aspects are discussed. All the results are included. It is concluded that the proposed technique leads to a simple modular control circuit, wider range of operating currents, and increased system stability.     

N and p doped poly(3,4-ethylenedioxythiophene) electrode materials for symmetric redox supercapacitors

A symmetric redox supercapacitor has been fabricated based on n and p doped Poly(3,4-ethylenedioxythiophene)(PEDOT) coated on stainless steel (SS) electrodes. The characterization and performance of the supercapacitor has been studied by FTIR, Cyclic Voltammetry and AC Impedance spectroscopy. The supercapacitor showed a maximum specific capacitance of 121 F g⁻¹ at a scan rate of 10 mV s⁻¹. The time constant calculated for the supercapacitor through the active–reactive power behavior measurement was 12 milliseconds indicating the suitability of the system for efficient use at low frequency range.     

Replacement metal-gate NMOSFETs with ALD TaN/EP-Cu, PVD Ta, and PVD TaN electrode

This work reports the first replacement (damascene) metal gate NMOSFETs with atomic layer deposition (ALD) TaN/PVD and electroplated Cu as the stacked gate electrode. Transistors with PVD TaN and PVD Ta electrode are also fabricated. Our data show that ALD TaN has the right work function for the N-MOSFETs. The Cu damascene process can reduce the gate resistivity. The ALD process has the advantage of reducing the stress and radiation damage to the gate oxide. The damascene process flow bypasses high temperature steps (>600/spl deg/C)-critical for metal gate and high-k materials.     

Distributed key management for dynamic groups in MANETs

Most existing solutions to group security in Mobile Ad Hoc Networks (MANETs) rely on a multicast Core Based Tree (CBT) for key distribution. Such solutions, although suitable for systems with low mobility and static characteristics, are unsuitable for dynamic and sparse groups with changing neighborhoods. In this paper, we propose an entirely decentralized key generation mechanism, employing a central trusted entity only during initialization. Using our approach, keys can be established between group members with absolutely no prior communication. The solution relies on threshold cryptography and introduces a novel concept of Node-Group-Key (NGK) mapping. We have provided an extensive analytical model for the computations involved and communication costs and have also provided a lie detection mechanism. Simulation results show appreciable performance improvement and enhanced robustness.     

Noninvasive Procedure for Improving Efficiency of Three-Phase Induction Motors

In developing countries, three-phase squirrel cage induction motors, even though with load equal to their name plate rating, may operate at low efficiencies. This may be because their winding parameters are not optimized for lower running cost. To ascertain the cause, it is necessary to know the stator winding data and compare it with the results of the design. If a deviation exists, the winding is improper. However, it may be possible to improve efficiency by rewinding the motor. We propose a noninvasive procedure to determine the following winding data: number of parallel circuits in a phase, number of turns per coil, and conductor area. The procedure includes a new iterative algorithm for determining these winding data. We discuss experimental investigations to validate the proposed procedure and to test its effectiveness in energy conservation. We outline the usefulness of the procedure for designers and the advantages it offers to service providers and educational institutions.     

Optimisation of machining parameters for milling operations using a scatter search approach

This paper describes the application of a newly developed metaheuristic, the scatter search, for optimising the machining parameters of milling operations. An objective function based on maximum profit is used while considering the practical constraints, such as allowable speed and feed, maximum machine power, surface finish requirement and maximum cutting speed permitted by the rigidity of the machine tool. An example is taken from the literature for comparing the results of the proposed method with other heuristics and handbook recommendations.