International Journal of Engineering
Volume:25 Issue: 3, Sep 2012

Flow channel design on bipolar plates has a direct effect on Proton Exchange Membrane (PEM) fuel cell performance. It has been found out that the flow field design has a deterministic role on the mass transport and water management, and therefore on the achieved power in PEM Fuel cells. This study concentrates on improvements in the fuel cell performance through optimization of channel dimensions and configuration. To find an optimized state, a two dimensional numerical model of the flow distribution based on the Navier-Stokes equations and by use of an individual computer code is presented. The simulated results showed a very good agreement with the experimental results obtained in previous works. Finally, numerical simulation has been conducted to investigate the advantages of a newly proposed pattern with inspiration from plant leaves. The main design criteria are focused on less pressure drop and more uniform pressure and velocity distributions throughout the flow channels. It was found that both velocity and pressure fields are much more homogeneous in the new channel design; therefore, it is expected to produce a more uniform dispersal of reactants over the GDL and the catalyst layer, which in turn causes the efficiency to enhance.

Question Answering (QA) is an emerging important field in Information Retrieval. In a QA system the archive of previous questions asked from the system makes a collection full of useful factual nuggets. This paper makes an initial attempt to investigate the reuse of facts contained in the archive of previous questions to help and gain performance in answering future related factoid questions. It models the role of facts in questions through discourse transition of user question answering process, and presents approaches to identify and extract these facts with the help of lexical semantic resources. Strategies to implement the reuse of facts to boost query generation in the passage retrieval stage of a QA system as well as ideas on system evaluation are discussed.

The non-convex behavior presented by nonlinear systems limits the application of classical optimization techniques to solve optimal control problems for these kinds of systems. This paper proposes a hybrid algorithm, namely BA-SD, by combining Bee algorithm (BA) with steepest descent (SD) method for numerically solving nonlinear optimal control (NOC) problems. The proposed algorithm includes the merits of BA and SD simultaneously. The motivation of presenting the proposed algorithm includes that BA is showed to converge to the region that global optimum is settled, rapidly during the initial stages of its search. However, around global optimum, the search process will become slowly. In contrast, SD method has low ability to convergence to local optimum, but it can achieve faster convergent speed around global optimum and the convergent accuracy can be higher. In the proposed algorithm, at the beginning step of search procedure, BA is utilized to find a near optimum solution. In this case, the hybrid algorithm is used to enhance global search ability. When the change in fitness value is smaller than a predefined value, the searching procedure is switched to SD to accelerate the search procedure and find an accurate solution. In this way, the algorithm finds an optimum solution more accurately. Simulations demonstrate the feasibility of the proposed algorithm.

The performance of Vector Controlled Induction Motor drive depends on the accuracy of rotor resistance which will vary with temperature and frequency. The MRAS approach using reactive power and flux as a state variable for rotor resistance estimation makes MRAS computationally simpler and easy to design. In this paper, Rotor Flux based MRAS (RF-MRAS) and Reactive Power based MRAS (RP-MRAS) for rotor resistance estimation is designed and validated through MATLAB simulation. The performance of these two methods are analyzed extensively and compared in terms of accuracy and settling time. The suitable method for rotor resistance estimation for Space Vector PWM Inverter fed induction motor drive is identified. The promising obtained results are presented.

This paper investigates the impact of provided service by the retailers and manufacturers on customers’ demand and members’ profit in a supply chain. It focuses on a supply chain structure with one manufacturer and a common retailer. The demand of customers depends on retailer price and service level. A game-theoretic framework is applied to obtain the equilibrium solutions for each entity in supply chain. In order to investigate the impact of service on the demand and supply chain members’ profit when the manufacturer is a leader, we derive and compare equilibrium solutions for the supply chain under three different scenarios. These scenarios include the case that manufacturer and retailer do not provide any service to customers; the case that retailer provides service to customers; and the case that manufacturer provides service to customers. We compare results from these three scenarios and provide the best scenario for the proposed problem.

An M/M/1 queueing system with second optional service and unreliable server is studied. We consider that the server works at different rate rather than being idle during the vacation period. The customers arrive to the system according to Poisson process with state dependent rates depending upon the server’s status. All customers demand the first essential service whereas only some of them demand the second optional service. A customer either may leave the system after the first essential service with probability (1-r) or at the completion of the first essential service go for second optional service with probability r (0≤r≤1). The server may breaks down according to Poisson process during the busy and working vacation duration. Both service times in vacation and in service period are exponentially distributed. The matrix geometric technique is used for the analysis of the concerned queueing system. The sensitive analysis is also performed to examine the variation of the system performance characteristics with various input parameters.

In the present investigation, we deal with the reliability characteristics of a repairable system consisting of two independent operating units, by incorporating the coverage factor. The probability of the successful detection, location and recovery from a failure is known as the coverage probability. The reboot delay and common cause shock failure are also considered. The times to failure of the components, time to failure due to common cause, time to repair and time to reboot are assumed to follow exponential distributions. The Markov model of the system is developed and the system state transition probabilities are determined which are further used to evaluate some reliability indices such as availability and mean time to failure. We use fuzzy logic approach for analyzing the system performance by assuming the trapezoidal membership functions of the system descriptors viz. failure rates and repair rates. The fuzzy mean time to failure and fuzzy availability have been established. A numerical experiment has been performed to validate the analytical results.

Energy and exergy concepts come from thermodynamics laws and are applicable to all fields of science and engineering. This study considers numerical simulations of combustion of hydrogen with air in a constant pressure environment and exergy terms according to first and second laws analysis using homemade code. Chemical kinetic model includes 20 reactions and 9 species. At the first stage of present study, the effects of airl-fuel ratio (φ) on energy and exergy terms are investigated for different φ (from 0·6 to 1·4) at constant temperature 1000 (K). The predictions show that the increase of φ from 0·6 to 1 increases average temperature, thermo-mechanical and chemical exergys and ireversibilty, while increase of φ from 1 to 1.4 decreases them, except for chemical and total exergys. Also second- law efficiency increases with increasing of φ. At the second stage, at φ=1.4, in which the second-law efficiency is maximum, initial temperature of mixture (is varied from1000 to 1400 (K) by 100 (K) step. The predictions show that increase of from 1000 to 1100 (K) increases average temperature, thermomechanical, total exergy and irreversibility while decreases the chemical exergy and second-law efficiency. Also, the increase of from 1100 to 1400 (K) increases the thermo-mechanical, chemical total exergys and second-law efficiency, while irreversibility decreases. It was found that second-law efficiency reaches a maximum value in the case of higher airl-fuel ratio and initial temperature.

Many researchers have been interested in application of mathematical methods to find analytical solutions of nonlinear equations and for this purpose, new methods have been developed. One of the newest analytical methods to solve nonlinear equations is Reconstruction of variational Iteration Method (RVIM) which is an accurate and a rapid convergence method in finding the approximate solution for nonlinear equations. By applying Laplace Transform, RVIM overcomes the difficulty of the perturbation techniques and other variational methods in case of using small parameters and Lagrange multipliers, respectively. In this study RVIM is applied for the effects of magnetic field and nano particle on the Jeffery-Hamel flow. The traditional Navier-Stokes equation of fluid mechanics and Maxwell’s electromagnetism governing equations are reduced to nonlinear ordinary differential equations to model the problem. The flow field inside the divergent channel is studied for various values of Hartmann number and angle of channel. Finally the effect of nano particle volume fraction in the absence of magnetic field is investigated, too. The validity of RVIM method is ascertained by comparing our results with numerical (Runge Kutta method) results.

Dispersion and deposition of aerosol particles over two square cylinders confined in a channel in laminar unsteady vortical flow were investigated numerically. Lattice Boltzmann method was used to calculate fluid characteristics and modify Euler method was employed as Lagrangian particle tracing procedure to obtain particle trajectories. Drag, Saffman lift, gravity, buoyancy and Brownian motion are forces that were included in particle equation of motion. Augmentation of total deposition efficiency at staggered arrangement started at lower stokes number in comparison with inline arrangement. Presence of second obstacle increased deposition efficiency for Stokes number greater than 1. Particles having Stokes number smaller than 0.1, followed streamlines and were not captured by squares. Deposition efficiency increased slightly between Stokes numbers 1 to 5 with respect to stokes numbers from 0.1 to 1. Deposition efficiency had bigger growth rate at HD=5 with respect to HD=3. Ultra fine particles with stokes number smaller than 0.1 acted like fluid particles at their positions and dispersed in the vortices, in the periodic asymmetric behind the obstacles. Particles tend to move on vortices boundary as Stokes numbers increased from 0.1 to 1. At HD=3, particles could not move in the region between two obstacles.