Sharif Digital Repository

Conventional defects in GTAW welding of Aluminum 5052 alloy are the decrease in weld zone strength because of grain growth, formation of columnar grains in the fusion line, solidification cracks and porosities. In this work, effect of strong field electromagnetic stirring on solidification structure of 5052 alloy and elimination of GTAW defects have been investigated. For arc welding processes (especially tungsten inert gas welding), a process called magnetic stirring was proposed, patented, and investigated in the 1970s. The essential aim was to improve weld quality and they indicated that the the Lurentz force is cause of weld stirring. In the earlier researches, alternating magnetic field... 

Analysis of hydrodynamic model of sea wave problems has very good conformity with boundary element method (BEM). Because, the governing equation of many sea wave problems is potential equation which is the base of BEM. In this research, after some explanation about BEM and its application in fluid mechanics, a new composition of elements on the discretized boundary is introduced in favor of increasing the precision. In this method, the constant elements are used in corner elements and the linear elements are used in other elements. This issue is validated by comparing with analytical solution. In addition, free surface flow with porous medium in the fluid is investigated in sloshing 2D... 

Dependency of human life to supplying of electricity energy, leads to different challenges for power system operators and planners. Demand growth and development of renewable energy sources have caused heavy investment costs to the transmission system. High price of construction of new transmission lines make planners to investigate effective factors on transmission expansion planning in order to make better decisions. On the other hand, the different amount of energy consumption during a day and the uncertainty of renewable energy result in transmission line congestion which cause electricity price gap between peak hours and off-peak hours. This price gap, also, necessitate the development... 

With rising oil prices and predictions about the future of hydrocarbon reserves, issues of enhanced oil recovery and optimal production of existing reservoirs are of particular importance. In this study, molecular dynamics simulation was used to study the absorption of silica nanoparticles with surface groups at the interface of water and decane. The effect of surface chemistry of silica nanoparticles on the interface properties of water-decane with changing the surface groups was simulated. The initial surface factor of nanoparticle is hydroxyl (OH), which is called a hydrophilic surface agent. Nanoparticles were modified by methyl and ethyl surface agents to increase the hydrophobicity and... 

Production of cellulosic ethanol from lignocellulosic biomass represents a alternative to the petroleum fuel due to its renewable nature and sustainable availability. The major strategy used for cellulosic ethanol production includes three main steps of biomass pretreatment, enzymatic hydrolysis and ethanol fermentation. The enzymatic hydrolysis contributes significantly to the cost of cellulosic ethanol and from the process economics perspective, the improvement in the enzymatic hydrolysis step is a prerequisite. The fiber is composed of cell wall fractions and residual starch which can be hydrolyzed to reducing sugar.A method of hydrolysis of polysaccharides in corn fiber, feed... 

Hydro power is a clean energy resource which has numerous advantages over other renewable energy systems including competitiveness with fossil fuel systems, high reliability, considerable potential, and availability of technology. These advantages have drawn attention to hydropower systems. Small Hydro Power (SHP) systems can be used for supplying the demand of rural and remote areas where electricity network rarely exists. SHP may also provide an investment opportunity. Comprehensive analysis of design features and operation of SHP and management of risks involved requires development and application of reliable analytical tools. The present research aims at developing a model for optimal... 

In this project, the Line-Modulated Fixed Off-Time method is used as the current control strategy for a Power Factor Corrector (PFC) pre-regulator. A zero voltage switching PWM (ZVS-PWM) auxiliary circuit is configured to perform ZVS in the switches. To reduce conduction losses, the rectifier circuit and the PFC are combined to create bridgeless PFC and a new zero-voltage-switching is introduced for reduse switching loss. Soft commutation of the main switch with zero voltage is achieved without additional current stress. When main switch is turned off ,the voltage across the switch rises very slowly. when the current of the switch decays to zero the voltage across the switch is very low.A... 

In this study, we examine all decisions related to operating room planning and scheduling problem by considering uncertainty in surgery durations. To achieve this purpose, first, a mixed integer linear program is developed to determine the master surgical schedule. Then, two different approaches are investigated to model the weekly planning and scheduling of elective patients problem considering uncertain surgery durations. In the first approach, a unified model is proposed and an effective and efficient method, based on the recently introduced anarchic society optimization algorithm, is developed to solve the problem. In the second approach, first an advance scheduling problem is formulated...