Sharif Digital Repository

Magnetron is one of the microwave-generating devices that operates based on the microwave-electron flow interaction in the active region of the tube, which is affected by the resonant cavities and It is used in a wide range of power (from kW range at home microwave ovens to several GW in relativistic high power magnetrons and the frequencies from the order of few too MHz to tens of GHz). Although nowday's low-power generators based on transistors have replaced microwave generating tubes and even MW radio transmitters are made based on the combination of semiconductor microwave generators, but in the production of RF pulses at higher frequencies and powers, the electron tubes are the main... 

With the rapid advances in microwave semiconductor devices and systems, the demand for implementation of nonlinear transmission lines (NLTLs) is rapidly increased. NLTLs support short pulses and wide band-widths. These networks provide cutoff frequencies over hundreds of GHz and picosecond rise times. Among all of the applications of NLTLs, the most attention has been paid to their application in the generation of high power RF pulses. In this application, a high power rectangular pulse is injected to the line, gradually evolved to an oscillatory pulse while propagating through the network, and finally, in the form of a high power RF pulse, is delivered to the load. NLTLs are the most... 

Selective oxidation of H2S to elemental sulfur in a fixed bed reactor over seven nanocatalysts has been investigated and compared to commercial claus catalyst. Among them, Al2O3-supported sodium oxide nanocatalyst prepared with wet chemical technique and Al2O3 nanocatalyst prepared with spray pyrolysis method were the most active catalysts for selective oxidation of H2S to elemental sulfur. The remaining nanocatalysts were deactivated, mainly due to the interaction with H2S and conversion into sulfides. For synthesis of Al2O3 with spray pyrolysis method we use statistical design of experiments (box-behnken design) in order to screen the effects of significant synthesizing parameters  

In this study, the influence of multi wall carbon nanotube on the thermal properties and microstructure of open cell flexible polyurethane (PU) foam was investigated. To do this untreated CNT (named here n-CNT) along with three different highly functionalized CNTs having carboxyl, hydroxyl and amide functional groups, abbreviated here by CNT-COOH, CNT-OH and CNT-NH-COR, respectively, were employed. A series of PU foam with and without CNTs (both treated and untreated ones) were prepared by cast molding. Prior molding, the CNTs (0.1 wt% loading) were first dispersed into the polyol (Mw = 3000 g/mol and OH No. 56) and then the mixture was subsequently added to the isocynate (TDI). The SEM... 

Prediction of axial flow turbine’s performance with different operating conditions due to the high costs and lack of reliable empirical data is always under developing. In this type of turbine, fluid flow is three-dimensional, viscous and highly turbulent. Nowadays, although a relatively detailed picture of the flow field can be obtained by the computational and numerical tools, but using of appropriate loss models in one-dimensional modeling for the performance prediction as well as optimization of the turbine in initial design phases, is a common trend.
In the current study, aerodynamic losses in the axial flow turbine are investigated. Several loss models for the axial flow turbines... 

Knew is the biggest and most complicated joint in the body.Morever knew creat mobility must bear the weight of body and have a stability during static and dynamic activities.Knew is exposed different forces that force to around ligaments and soft fibers.After knew injuries, administration of aid is the important part of rehabilitation program.Rehabilitation robots with ability of accuracy and repetitive practices are good method for reducing time and energy of physiotherapist.At this thesis a 3 DOF Cartesian robot is designed and constructed for rehabilitation of knew that can do every practices for rehabilitation.Steps of designing and constructing of robot are based on movments of knew... 

Evaluation of the seismic-induced settlement of an unsaturated soil layer depends on several variables, among which the small strain shear modulus, Gmax, and soil’s state of stress have been demonstrated to be of particular significance. Recent interpretation of trends in Gmax revealed considerable effects of the degree of saturation and hydraulic hysteresis on the shear stiffness of soils in unsaturated states. Accordingly, the soil layer is expected to experience different settlement behaviors depending on the soil saturation and seasonal weathering conditions. In this study, a semi-empirical formulation was adapted to extend an existing Gmax model to infer hysteretic effects along... 

Polymer/clay nanocomposites (PCNs) are multifunctional materials that have superior mechanical and thermal properties than polymer-based materials. These materials are obtained by adding small amounts of nanoclays to a polymer matrix. In this work, a molecular dynamics simulation was carried out to investigate the thermomechanical properties of thermosetting clay nanocomposites. The polymer matrix was composed of phenolic molecules with the crosslink density of 0.74, and different structures of PCNS were considered by varying the amount of polymer molecules. Molecular dynamics simulations of different nanocomposite structures were used to provide the atomistic insights, the molecular... 

In this research a novel palladium nanocatalyst was synthesized over single-wall carbon nanotube (SWNT) support and then applied in the selective hydrogenation of acetylene in an ethylene rich-flow stream. This nanocatalyst displayed a very promising selectivity toward ethylene production upon rising of the temperature, as well as suppressing oligomer formation during acetylene hydrogenation. It was rationalized that the overwhelming governing mechanism for the acetylene hydrogenation over the 0.48 wt% Pd/SWNT was the hydrogen transfer. Furthermore, new operating conditions for selective hydrogenation of acetylene in ethylene rich-flow were introduced through which the selective... 

The major obstacle in using natural gas as a clean and abundant fuel in transportation industries is the low density of energy obtained from its combustion. Adsorbed Natural Gas (ANG) is a useful method to increase the energy density and store the natural gas. The lower cost and higher safety have made it different from the other storage technologies. In addition, ANG can be useful to solve the lack of gas in the peak of consumption. Among all adsorbents, Activated Carbon (AC) has shown the highest adsorption capacity of natural gas. In the present project, walnut shell has been used as the precursor in the chemical synthesise of AC. The main effective factors in chemical activation, such as... 

This work numerically simulates pressure drop and heat transfer for flow in a pipe with porous wall. Incompressible two dimensional, ax symmetric laminar or turbulence flow with constant mass flow rate is considered. It is assumed that there is no thermal equilibrium in the porous media region between solid and fluid phase. Effect of porous wall thickness and Darcy number on the rate of heat transfer and pressure drop were investigated. The Darcy number varies from to and porous wall thickness can fill entire pipe. the turbulent model is used to calculate the fluid flow and heat transfer characteristics in turbulent flow. Nusselt number can enhance about ... 

Adobe structures are considered as masonry structures which are out of scope of 2800 Iranian code. Due to this code, since these structures are made of weak materials, they do not have a considerable resistance to earthquakes. Since these structures can be found all around Iran; therefore, seismic study of walls as an important component of these structures, is, indeed, necessary. During this study, cyclic tests on three types of adobe walls, without opening, with window-opening, and with door-and-window-opening with a scale of 2:3 were done to study their in-plane behavior and also their failure modes. Based on our observations, in these walls, because of small ratio of height to length,... 

A series of V2O5 catalysts supported on multiwall carbon nanotube (MWCNT), single wall carbon nanotube (SWCNT) and graphene were synthesized by hydrothermal and reflux methods for oxidative dehydrogenation of propane (ODHP) to propylene. The catalysts were characterized by techniques including the BET surface area measurements, XRD, FTIR, H2-TPR, NH3-TPD, FESEM and UV-vis diffuse reflectance. The performance of the catalysts and the supports were subsequently examined in a fixed bed reactor. The vanadium catalyst supported on graphene with C/V molar ratio of 1:1 synthesized through the hydrothermal method had the best performance under the reactor test conditions of 450°C, feed C3H8/Air... 

One of the ultimate goals in robotics is to design autonomous robots. Among other things, this means a robot has to be able to plan its own motion. To be able to plan a motion, a robot must have some knowledge about the environment in which it is moving. Some of this information can be provided by a floor plan. For other information the robot will have to rely on its sensors. In this thesis, we consider that the environment is unknown for the robot. So, the robot uses weak sensor to planning its motion. The sensor can distinguish between colors. The robot can follow the colors. Therefore, we need to specify the umber of colors in which the robot can navigate by them. We propose an efficient... 

An injector is one of the most important components of the two components chemical propulsion system used in satellite control. The design of an injector is a complicated and time-consuming process and is largely dependent on laboratory data. In this study, the spray characteristics obtained from a small scale swirl injector under different operating conditions have been experimentally measured in a laboratory environment, such as the injector discharge coefficient, spray cone angle, and the distribution of droplets in the field. To calculate the discharge coefficient, the flow rate of the injector has been measured for different operating pressures. The shooting method and image processing... 

In this thesis, elliptical liquid jet breakup in crossflow has been investigated. To do this, two elliptical nozzle with different aspect ratio were built. First, the effective and important dimensionless numbers were extracted. Then, the physical aspect of the question was analyzed. All the experiments have been done in atmospheric conditions with water fluid, air as the liquid jet, and gas crossflow respectively.To create crossflow, a square cross section channel and one blower which are set in the beginning of the channel were made use of. Water is injected into air crossflow by the injector installed under the channel. The system used in revealing the crossflow is shadowgraphy system.... 

With increasing demand for more efficient combustion processes, alternative processes like application of magnetic and electric fields for modification of combustion have offered possible solutions. In the recent years, liquid electro-spraying has been a hot topic with growing interest due to its wide applications in such area as fuel atomization in combustion systems, which has been investigated in this project. Notice that in this method the liquid fuel in the high voltage electric field between electrodes will be broken into small droplets. In this thesis diffusion combustion of two alcoholic liquid fuels of ethanol and butanol at the exit of a small-scale combustor of vertical Pyrex...