Sharif Digital Repository

Fuel cells are a reliable technology for electrical power generation, widely regarded as a promising alternative for stationary and mobile applications. In order to supply vehicle required power, fuel cell must possess suitable transient response. In this work, a mathematical zero dimensional control-oriented model for a PEM fuel cell stack is developed. Afterwards, to increase the efficiency, four parameters including cathode air inlet flow rate, return manifold cross section area, anode hydrogen inlet flow rate and hydrogen humidity are optimized in different vehicle current demand. Also, in order to decrease water production inside the fuel cell membrane, considered parameters are... 

Restructuring energy systems for increasing the competence and improving the efficiency has been regarded by energy policy makers. Also, the emerging of new technologies and also the district generation has increased the competence in energy industry. Planning and exploiting the energy system in a restructured environment, has basic differences with integrated systems. Models that can assess the behavior of independent players are needed to assess energy systems in a restructured space.The goal of the present study is to present a model to optimally develop the energy system in a restructured environment. The concept of energy hub was applied to model multicarrier systems. Three steps were... 

Condition monitoring and fault detection is an important way to provide economy in both time and costs. Many methods are developed for this purpose. Vibration analysis is one of the most important ways in this field. In this thesis, fault detection of mechanical rotary systems with nonlinear and non-stationary nature has been developed by use of empirical mode decomposition (EMD) and Hilbert transform. Firstly the measured signal is transformed into some intrinsic mode functions (IMF’s) by EMD, and then the Hilbert transform is implemented on each IMF. With Hilbert transform, the instantaneous frequency of system and then the mean frequency are obtained. Mean frequency is a key definition... 

So far, investigations have shown that open dielectric resonator antennas (DRAs) offer several advantages at microwave and millimeter wave frequencies. These advantages include small size, large impedance bandwidth, high radiation efficiency, ease of manufacturing and simple feed-coupling mechanisms (such as probe coupling, aperture coupling or proximity coupling to micro-strip lines). Most microwave ferrites have dielectric constants in the desirable range and they can be used in place of dielectric materials. Such ferrite resonator antennas (FRAs) offer a potential advantage in that their permeability tensor can be controlled by applying an external magnetic field. Up to now and by the... 

Internet video networks have already attracted millions of Internet users, and Internet video is one of the fast growing Internet services. The peer-to-peer (P2P) network is an efficient solution to supporting large scale video streaming on the Internet. P2P networks can deliver live multimedia content to millions of users with low cost at any time, and at anywhere they want. In this thesis, we investigate solutions to enhancing the performance of the unstructured mesh-based. Locality awareness or network proximity is one of the essential characteristics of P2P networks. In live P2P video streaming we face to some challenges like bandwidth heterogeneity of peers, and peer churn. In the P2P... 

The failure in rolling element bearings as an important part of rotary machine can lead to machine breakdown.Consequently, bearings can reduce reliability of these components and therefore a maintenance strategy is essential to keep reliability in a desired level. The first and primitive strategy is called breakdown or unplanned maintenance. The next is preventive or planned maintenance. In this scheme, maintenance is done after a specified time period irrespective of machine health status. Out of place maintenance is the disadvantage of preventive maintenance. The most effective generation of maintenance strategy is condition based maintenance (CBM) which suggests maintenance decision based... 

Rotor contact with a stationary element of a rotating machine and the subsequent rubbing at the contact area is a serious malfunction that may lead to the machine’s catastrophic failure. In the past decades, many studies have been done on the rubbing phenomena, its different regimes and its various consequences on the machine performance and safety. However, most of them have used a simple model to investigate the rubbing interaction between the rotor and the stator. This simple model consists of the Jeffcott rotor, as a model for the real rotors, and a rigid annular block surrounding the rotor, as a model for the real stators. The objective of this thesis is to build an algorithm to... 

The effect of Vibration on the human body is considered as one of the limitations in design and use of HSCs at high speeds. One way to reduce the effect of vibration on the human body is to isolate deck vibrations from the crew by means of seat suspension system. This thesis has studied the effect of using an MR damper on the performance of seat suspension in vibration reduction. In fact, employing MR dampers in seat suspension systems enables us to change their damping. Using an MR damper makes it possible to control seat suspension by semi active control strategies. In this work, a number of proposed mathematical models of an MR damper were studied for numerical analysis and the most... 

Today as various electronic and telecommunication devices like cell phones and PDAs are being popularized, MMIC technology is gaining a significant importance. One of the priorities of this technology is trying to miniaturize these popular devices. But reducing size of passive microwave components used in these devices requires microwave wavelength to decrease.Components fabricated by conventional deposition techniques aren’t thick enough to reduce the wavelength, so permittivities much higher than those of conventional dielectrics could be a possible solution. Artificial dielectric layers (ADL) can provide such high permittivities.So utilizing these layers can possibly reduce the size of... 

So far, investigations have shown that open dielectricresonator antennas(DRAs) offer several advantages at microwaveand millimeter wave frequencies and make them appropriate in low directional-gain applications . These advantages include small size, large impedance bandwidth, high radiation efficiency, ease of manufacturing and simple feed-coupling mechanisms (such as probe coupling, aperture coupling or proximity coupling to micro-strip lines). Most microwave ferrites have dielectric constants in the desirable range and they can be used in place of dielectric materials. Such ferrite resonator antennas (FRAs) offer a potential advantage inthat their permeability tensor can be controlled by... 

Torsional vibration is undeniable in rotary systems, and it is more important in gearbox because it is the main cause of failure and noise. In this research The stiffness and damping between teeth are considered and mean stiffness between teeths are calculated by using material, contact ratio,and geometrical parameters. Transfer matrix method is used for obtaining natural frequencies and mode shapes.
In this thesis, effects of geometrical parameters of gears, backlash value, harmonic and constant torque on rattle noise show that increase in backlash, amplitude and frequency of excitation, magnify the power of impacts, also the time of rattle phenomenon and number of impacts decrease by... 

In wireless communication systems increasing the gain of antennas is seminal to increase in coverage and data transmission rate, and reducing the possible errors. One of the main factors for improving the antenna gain is matching the polarizations of the transmitter and the receiver antennas. Towards this end, both the transmitter and the receiver antennas should have the same axial ratio, specific direction and polarizations. As far as the flexibility of the direction angle between the transmitter and the receiver is concerned, the so-called circular polarization is preferred do the linear polarization. The antennas with circular polarizations are prominently utilized in satellites,... 

Operational modal analysis (OMA) techniques are proper methods to identify the modal parameters of the structures such as natural frequencies, damping ratios and mode shapes based on the system responses. First research in this area are stared from civil engineering. Difference between civil structure and industrial machines are in the type of excitation.in most industries such as aerospace or mechanical, colored or harmonic forces due to rotary machines, excited the system. Under this condition, traditional methods in OMA are not capable any more. Many works has been done to modify traditional methods. Most of these work limited to situations that forces are harmonic and harmonic...