Sharif Digital Repository

The aim of this study is to compare the synergistic impact of like and opposite-charged nanoparticle and surfactant systems on enhancing stability and mobility of foam in the absence and presence of oil which is not sufficiently discussed in the available literature. Stability of nanoparticle dispersion is examined at various levels of pH when nanoparticle and surfactant are in like and opposite-charged conditions. In situ modification of silica nanoparticle by opposite-charged surfactant is analyzed. Static stability tests as well as Hele–Shaw cell are employed to understand how nanoparticle and surfactant charges may control the stability and mobility of foam when oil is present/absent.... 

An optimal fuel guidance strategy with acceleration limit is derived for interception of maneuvering targets. The guidance/control system is assumed as a linear time-varying arbitraryorder and is identical in each channel. An approximate model for drag acceleration is also considered in the solution. The optimal fuel strategy has some detrimental effects in most practical situations, if it is used entirely to the end. This strategy can be used at the beginning of the guidance phase for large initial heading errors in order to quickly reduce it with minimum effort. An explicit guidance law is then developed for minimum and nonminimum phase autopilots in order to modify the undesirable effects... 

Various fracture geometrical properties such as orientation, extension, discontinuity, dispersion, location, and networking were investigated. Results indicate improved oil recovery and sweep efficiency in the presence of vertical fractures. Longer vertical fractures seemed to have even more beneficial effects. Horizontal fractures revealed detrimental effects on oil recovery and the performance became worse for longer horizontal fractures. Discontinuous horizontal fractures caused a better performance especially when combined with continuous vertical fractures (networking). Vertical fractures helped the growth of the steam chamber in the vertical direction, which resulted in higher oil... 

The aim of the present work is to study the effect of reinforcement content and sintering temperature on the densification and microstructural development during sintering of Al6061/SiC composite compacts. Prealloyed Al6061 powder was mixed with various amounts of SiC particles up to 27 vol.-% and compacted at 350 MPa. Sintering was carried out in a dry nitrogen atmosphere at 580-620°C. It was shown that the presence of SiC particles retards the densification of the prealloyed powder during solid phase sintering and liquid phase sintering. This effect is particularly noticeable at SiC fractions higher than ∼9 vol.-%, at which continuous networks of the hard particles are formed. Increasing... 

NiTi shape memory alloy (SMA) ingots were produced by vacuum induction melting/casting unit. The effect of microstructure on the hot (1000C) and cold (room temperature) rolling workability of cast samples was investigated. Microstructure of the cast samples was related to the type of crucible which used for melting and the average vacuum pressure of VIM furnace. Optical microscope, scanning electron microscope (SEM), and energy dispersive technology (EDS) methods were used to inspect the origins of observed cracks after rolling processes. The results showed that the formation of both Ti4Ni2O and TiC impurities formed due to inappropriate melting conditions had detrimental effects on... 

This paper presents a powerful application of genetic algorithm (GA) for the minimization of the total harmonic current distortion (THCD) in high-power induction motors fed by voltage source inverters, based on an approximate harmonic model. That is, having defined a desired fundamental output voltage, optimal pulse patterns (switching angles) are determined to produce the fundamental output voltage while minimizing the THCD. The complete results for the two cases of three and five switching instants in the first quarter period of pulse width modulation (PWM) waveform are presented. Presence of harmonics in the stator excitation leads to a pulsing-torque component. Considering the fact that... 

Welding process is widely used in manufacturing of many important engineering components. For such structures, the most important problem is the development of residual stresses and distortion due to welding. Welding tensile residual stresses have a detrimental effect and play an important role in an industrial environment. Crack initiation and propagation in static or fatigue loading, or in stress corrosion can be greatly accelerated by welding tensile stresses. Practically, however, it is often very difficult to characterize the residual stress state completely, while the knowledge of the complete residual stress distribution in structures is essential for assessing their safety and...