Sharif Digital Repository

In this study, the nonlinear vibrations of an elastic rotor that is under the elastic support and under the follower force caused by the airflow passing through the rotating plane are investigated. The elastic shaft is modeled by the Timoshenko beam theory, which is connected to a motor through a flexible coupling and the motor increases the rotational speed of the rotor by applying torque. Gibbons’ equations have been used to model the misalignment forces. First, the kinetic energy of the rotating system is calculated nonlinearly to take into account the effect of nonlinear terms due to the rotational inertia of the system, and then the potential energy and The work of external forces on... 

During the operation of a nuclear power plant, the reactor pressure vessel (RPV) is exposed to a variety of pressure and thermal stresses and neutron radiations. This will cause the loss of the initial strength of the reactor vessel component. During the occurrence of some of accidents, an excessive cooling of the coolant inside the RPV takes place which in the term is called overcooling transients. In addition, in some of these events, a break in a section of the circuit will reduce the water level at the core of the reactor. By reducing the water level, the existing emergency makeup water systems are activated and inject water into the reactor. The temperature of the added water is much... 

With the growing development of the aviation industry, the emitted jet noise from civil and military aircraft has become a challenging issue in the development of this industry. Despite recent advances in simulation methods using for fluid phenomena, understanding the nature and physics of noise generated by jet flows has been a complex puzzle for the last decades. Although numerical methods are currently able to predict the jet noise, it is still not possible to fully understand the physics of this phenomenon and subsequently use it to control the emitted sound. In this research, with the purpose of studying the pressure fluctuation caused by large-scale coherent turbulence structures in... 

TLP diffusion bonding method refers to a method in which two metal pieces can be bonded by putting an interlayer between two pieces and heating to a temperature lower than base metal melting point. Theoretically, this process shows that the bonding line disappears completely at the end of the process and it is not possible to recognize it. However, these assumptions were not accomplished in practice. Therefore, various parameters such as material of interlayer, thickness of interlayer, temperature and time of bonding, temperature and time of homogenizing and applied pressure on metal pieces should be controlled to achieve the best shearing strength results. Aluminum based composites have... 

The repair and production of Ni-based and Co-based superalloy parts have high importance in Oil and Gas, and Aerospace industries. Due to this issue, Transient Liquid Phase bonding is one of the most reliable welding methods for repair parts and fabricating new parts.In the present study, creep properties of similar transient liquid phase bonding of FSX-414 Co-based superalloy has been investigated and compared with creep properties of the un-bonded base metal. Firstly, the optimum condition of the bonding process was resulted from microstructural observations from different bonding conditions. Then, cylindrical samples for the aim of creep test specimens were fabricated in two different... 

In general, the output power of the gas turbines decreases as the ambient temperature increases. One of the ways to deal with such a performance loss is to use a wet compression system, which reduces the air temperature rise throughout the compressor. In this system, the water droplets are injected into the compressor inlet and evaporated throughout the stages. As a result, the work of the compressor decreases, and the power and efficiency of the whole engine increase. Most of the researchers in this field have tried to develop mathematical models to investigate the effects of this system on compressor and gas turbine performance in steady-state conditions. However, it is necessary to... 

Successfully bonding nickel-base alloys to titanium-base alloys has proven to be one of the most challenging modern joints. Traditional welding methods for dissimilar titanium/nickel joints often lead to cracking in the fusion and heat-affected(HAZ)zones; due to the formation of brittle intermetallic compounds with low fracture toughnesses. Additionally, the significant difference in thermal expansion coefficients between titanium and nickel introduces significant thermal stress during bonding process, exacerbating the current issue. Creating successful dissimilar Ti/Ni joints requires overcoming the formation of continuous brittle intermetallic compounds (IMCs). Brazing stands out as a... 

Transient liquid phase bonding of AISI 304 was carried out in a vaccum furnace using a Ni-based interlayer. The effect of bonding time on microstructure and mechanical properties of joints at temperature of 1150 ⁰C was investigated. The results showed that isothermal solidification was completed within 20 min and eutectic phase disappeared across the joint region. The joints with complete isothermal solidification were homogenised for 180 min at temperature of 950 ⁰C. The results of shear test showed that the average shear strength of homogenised samples was about 83% thet of the base metal at the same heat treatment cycle. Corrosion behavior of the TLP bonded and base metal was studied at... 

The importance of rainfall pattern in assessing embankment stability and deformation is widely known. However, many studies have examined an embankment with simple rainfall patterns or used average and maximum ones. The importance of diverse rainfall patterns has also been overlooked in the study of divergent soil stability in the previous study. Therefore, the hydraulic properties of a natural soil with and without salinity potential and the stability analysis of the railway embankment located on this type of soil under various rainfall patterns are investigated. The study area is part of the Zahedan Chabahar National Railway Project, which is located on divergent soil and has high... 

The thesis is focused on the process-microstructure-properties relationships in transient liquid phase (TLP) bonded IN718 nickel based superalloy. The bonding was carried out using Ni-B-(Si)-(Cr) interlayers at temperatures of 1273-1448 K with various bonding time to ensure completion of isothermal solidification. Microstructure development mechanism, kinetics of isothermal solidification during bonding process, influence of microstructure gradient induced by bonding process on the aging behavior of the joint and the response of joint region to post-bonded heat treatment were considered. It was shown that the isothermal solidification rate depends on the (1) diffusion flux of B into the... 

This thesis is about modeling of liquid-vapor two phase flows through nozzle. Liquid-vapor two phase flow is very applicable in industries such as boilers, expansion valves, refrigeration and sudden failures in pipelines. Two models are used for modeling of fluid flow through nozzle, transient and steady state. In Both model the solution field is considered as saturated liquid and vapor and the mass, momentum and energy equations as well as equation of state is used for describing of fluid flow properties. Homogeneous equilibrium model is used and for complete modeling of fluid flow heat transfer and friction force is also considered.
Numerical solutions are used for solving of the... 

This study presents a novel perspective on pressure-transient analysis (PTA) of downhole-pressure and flow-rate data by use of system-identification (SI) techniques as widely used in advanced process engineering. Key features of the algorithm are that it considers the classic PTA process from a system-theoretical perspective; derives the causal structure of the flow dynamics; proposes a method to deal with continuously varying pressure and flow-rate signals contaminated with correlated noise, which estimates physical reservoir parameters through a systematic matching procedure in the frequency domain; and can cope with arbitrary (i.e., not necessarily piecewise constant) flow-rate signals.... 

This thesis introduces a new method for identification and forecasting of future states of nuclear power plants (NPPs) parameters. The proposed method consists of four steps. First, the type of transients is recognized by the modular identifier which has been developed using the latest advances of error back propagation (EBP) learning algorithm. In second step, for more robustness of modular identifier against noisy input data, auto-regressive integrated moving average (ARIMA) method is used. A hybrid network is then used to forecast the selected parameters of the identified transient. ARIMA model is used to estimate the linear component of the selected parameters. The neural network... 

Transient liquid phase (TLP) bonding of Al2024-T6 alloy, using gallium interlayer, has been investigated. Bonding experiments were carried out using two basic methods; Conventional and Temperature gradient TLP bonding. Optimum values of isothermal solidification temperature and time, pressure, surface roughness, heating rate and homogenizing conditions were determined in conventional TLP bonding experiments. Aluminum oxide layer was not removed before TLP bonding to prevent gallium attack.this layer is crashed during bonding. Al2O3 particles are not soluble in the liquid and are pushed into liquid by solidification front progression. These particles are accumulated in grain boundaries and in... 

In the present research, transient liquid phase bonding of dissimilar st52 low carbon manganese steel to 304 stainless steel by pure Cu foil, in induction furnace and SIC furnace, was investigated. Samples were bonded in induction and SIC furnace at 1150 0C for different bonding times. Samples were then cut transversely and cross section of samples was examined by optical and scanning electron microscope. Complete isothermal solidification time was more than 9 hours and 90 minutes for SIC and induction furnace, respectively. Micro hardness and shear strength tests were done on samples bonded in optimum condition. Shear strength of none homogenized samples which had been bonded in SIC... 

The nickel-based superalloys play a key role in the development of aircraft and power-generation turbines due to their exceptional properties of high temperature strength, toughness, crack growth resistance and resistance to degradation corrosive and oxidisation environment.In this thesis, the role of base metal composition in isothermal solidification time during diffusion brazing of Nickel-based super alloys was investigated. To achieve this aim, four different super alloys Hynes230, Hastalloy X, Nimonic 263 and Monel were used in similar joining systems. Systems of H230/H230, HX/HX, C263/C263, and Monel/Monel were joined by BNi2 interlayer at 1323K and for different times, until the... 

Eliminating poverty, which is one of the most important objectives of countries, requires a deep understanding of the trends and dynamics of poverty and the distinction between chronic and transient poverty. However, given the lack of high-quality panel data, measuring movements in and out of poverty in developing countries is difficult. In this research, we build a synthetic panel by constructing a consumption model and estimating the first-round consumption of households surveyed in the second round. The analysis of Expenditures and Income Survey (HEIS) for 1398 and 1399 shows that almost 29 percent of urban and rural households were poor in both years. In female-headed households,... 

Real-time simulators are playing a critical role in the design of power electronic systems. Using real-time simulator enables designers to reduce the time and cost of design by employing hardware-in-the-loop technique. In addition, this simulator can potentially reduce the risks which are associated with performing tests on actual systems. In this thesis, a real-time simulator is designed and implemented for power electronic applications. Concerning the fact that power switches are one of the most important elements in this simulator, an ideal switch model with conduction resistor and RC snubber circuit is used. This model is more accurate as compared to other models in the... 

In this thesis, macrostructural evolutions in dissimilar TLP bonding of as-cast nickel based supperalloy IN792 to wrought HX was investigated. In the first step of this study solidification behavior of Ni-Cr-Fe-Si-B (MBF-15) interlayer in presence of HX as base metal at 1120 °C was surveyed. In second step, bonding of HX/HX, IN792/IN792 and HX/IN792 systems with MBF-15 interlayer at 1120 °C and different times were studied and isothermal solidification time and factors that affect this stage were discussed. In third step, synchronization of dissimilar bonding with standard heat treatment cycle of IN792 was investigated and effect of this cycle on aging response of different zones, isothermal... 

A statistically-based method has been developed for automated identification of steady state. The method is computationally inexpensive when compared to conventional techniques. The R-statistic is dimensionless and independent of the measurement level. Because it is a ratio of estimated variances, it is also independent of the process variance. Simulations show that for recommended critical values are also effectively independent of the magnitude and distribution of the noise.
A fuzzy-logic-based methodology for on-line steady state and transient state identification is introduced. Although steady state identification is complicated by the presence of noise, fuzzy logic can help capture...