Sharif Digital Repository

Solid fuel ignition transient in dump combustor geometry is numerically studied. The objective of this work is to gain insight into the ignition of the fuel and its controlling parameters in back-step turbulent flow. The system of governing equations is solved with a finite volume approach using a structured grid in which the AUSM+ scheme is used to calculate the gas phase convective fluxes. The Spalart-Allmaras turbulence model is used in these simulations. First, the auto-ignition of solid fuel subject to an inlet oxidizer flow at elevated temperatures is considered and distinct intervals in ignition delay time are studied. We have demonstrated that for inlet oxidizer streams with high... 

Controlled islanding, also known as system splitting, can effectively prevent blackouts of power system by separating it into two or more islands. To make each island reach steady state point by available measures, each island must be observable. This paper proposes a new modified three phase method to find proper splitting strategy, considering observability as a new constraint, to minimize effect of system splitting on network observability. Some techniques about strategy searching are introduced to increase its speediness and related algorithms are described in detail in this thesis. The simulation results presented show efficiency of method in finding proper splitting strategies. In the... 

This thesis addresses the problem of dynamic security assessment in power sys-tems with an emphasis on its application in restructured power systems and issues arising in its applications. The new electric utility environment has an immedi-ate consequence of emphasis on reliability and secure operation of power systems. This requires faster and more precise methods to assess the dynamic aspect of se-curity. On the other hand, this issue is becoming challenging when the determined dispatch might not be accommodated due to violation of the limits.This thesis addresses the problem of dynamic security assessment in power sys-tems with an emphasis on its application in restructured power systems... 

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... 

The study of evaporation of fuel droplet and determination of the rate of vaporization are important in designing combustion chambers. For achieving high performance of a combustor, the evaporation of fuel droplets takes place within a high pressure environment. At these conditions, the use of low-pressure models is not appropriate and many effects that are assumed negligible at low ambient pressures become very important. For example, the solubility of the ambient gas into the liquid phase is increased by increasing the ambient pressure. In addition, the ideal gas assumption is not valid for these conditions and one should use an appropriate equation of state (EOS) that can predict the... 

In this research, transient liquid phase (TLP) bonding of FSX-414 superalloy was investigated using MBF-80 interlayer. The bonding was performed at different temperatures (1050-1200oC) for various times (1-120min). Also, different gap sizes were studied using the interlayer with various thicknesses (25-100µm). Homogenizing treatment was performed at different temperatures (1175-1225oC) and times (1-6h) for the samples with complete isothermal solidification. The microstructure of the samples were studied using the optical, scanning electron and transmission electron microscopes. Also, XRD, SEM/EDS, SEM/WDS and TEM/EDS analyses were used to analyse the observed phases at the joints. To... 

Transient liquid phase (TLP) bonding of 316L/17-4PH bimetals using a thin layer of nanostructured nickel was studied. The nanostructured nickel interface was deposited through pulse current coating in a Watt bath. The TLP bonding was performed at different temperatures ranging from 1000 °C to 1300 °C using a nickel interface having different thicknesses (2-17µm) and crystallite size (14-67 nm). The microstructure of the joint bimetals and phase formation were studied using the optical and scanning electron microscopes and X-ray diffraction (XRD) method. The mechanical properties of the bimetals were evaluated by micro-hardness and shear tests. To evaluate the corrosion behavior of the... 

Since seepage-related problems account for the cause of more than forty percent of dam failure seepage analysis serves as one of the most significant stage in the design process of an embankment dam. Currently a steady seepage analysis in a typical or maximum cross section of the dam is formed instead of transient seepage analysis for simplicity. In this way, for the change of condition (unsaturated to saturated) in the first filling of dams and it’s effects on behavior and reaction of dams until the steady seepage condition established; the results of steady seepage analysis cannot be satisfactory. In this thesis, based on finite element method and with assumption of saturated-unsaturated... 

In the research transient liquid phase bonding of AL6061-SiC composite was performed using copper as interlayer. The joining praocess was performed in various tetemperatures (560c and 580c and 600c) And times (6 and 8 hours). Optimum Joining condition was determined by using optical and of the joint, micro hardness test was performed from the bonding zone to the bulk of the base composite. The fatigue endurance limit of Al6061-SiC composite was determined by several fatigue tests and TLP bonded samples were loaded under achieved endurance limit stress. Finally fatigue fracture surface of both base composite and TLP bonded samples were studied by using of scanning electronic microscope  

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... 

Transient stability as one of the power-angle stability parts is a phenomena which is threatening the security of power systems. Following the occurrence of a major disturbance such as a short circuit, each of the generators will be in risk of transient instability due to distance from the fault location. If a generator fails to maintain its stability with the network, it become out-of-step. Out-of-step caused rounding waste of generator rotor and zero average output power. Out-of-step on the one hand lead to severe mechanical and thermal damage to the generator and the other hand disrupt load providing in the power system. So, the instability of the generator must be detected in the... 

During the last few years researchers have drawn much attention to HSLA steels because of the application of these steels in many industries. Ferrite-martensite dual phase(DP) steels are one of the most common HSLA steels which are currently used. Combination of these two phases provides good strength along with good ductility. Joining and welding of these steels has always been an important challenge. The aim of this study is to examine applicability of TLP method for joining and dual phase processing of this steels at the same time. Microstructural and mechanical examination of the bonded specimens showed that isothermal solidification occurred during holding for 50 min at the... 

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... 

Dual Phase ferritic-martensitic steels are kind of high strength low alloy steels (HASLA) which are widely used in industry because of their strength and formability. Since welding of dual phase steels in conventional welding methods causes structural change and Subsequently alters mechanical properties of both weld zone and heat affected zone(HAZ), additional operations which are not commercially viable is needed to achieve a dual phase structure in structurally changed regions. In this study, transient liquid phase (TLP) bonding method is used during dual phasing process in order to preserve dual phase ferritic-martensitic structure and avoid extra heat treatment. Applied pressure during... 

Interlayer composition as one of the most important factors of TLP bonding could affect bonding region composition and microstructure that both of them govern the mechanical properties of the joint. Due to the risk of substrate microstructure degradation , welding of dual phase steel needs more cautions. TLP bonding ideally acquires joints having more similar microstructure to substrate. Moreover , using this method could avoid the change of base metal microstructure when heat treatment of carbon steel is postponed until after bonding. Fe-based , Ni-based and commercially pure Cu are three different composition has been studied in this work. The bonded samples were investigated by optic and... 

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... 

Stainless steels have many applications in different industries like chemistry, medicine and food production. So for using these alloys it is necessary to join them with appropriate method. In this research, microstructure-mechanical properties relationship of 304LC austenitic stainless steel transient liquid phase bonded using a Co-based interlayer was investigated. For this purpose, bonding was conducted at 1180 ℃ for different holding times. Microstructure of the joints was evaluated by optical microscopy (OM) and Scanning Electron Microscopy (SEM). The results showed that isothermal solidification (IS) was completed within 1800s and no intermetallic compound was formed in the joint area... 

In the present study the production of ferrite-martensite dual-phase (DP) Steel was investigated during the bonding process by transient liquid phase method. The effect of martensite morphologies on mechanical properties of bonding zone were also studied. To make the bonding process and DP steel production heat treatment cycle simultaneously, the step in which the isothermal solidification completed, bonding process, was done simultaneously with austenitising of the steel. Homogenizing of the bond zone, was also done with DP steel making at the intercritical temperature. The St52 steel was utilized for producing DP steel and for bonding process, iron base interlayer with melting point of... 

Fluid transient in a conduit is an unsteady flow, which is followed by the change in the flow rate condition. Pressure transients in pipeline systems are caused by fluid flow interruption from operational changes such as starting/stopping of pumps, sudden changes to valve setting, etc. Fluid transient analysis is commonly based on the assumption of no air in the liquid. However, air entrainment, trapped air pockets, free gas, and dissolved gases frequently are present in the pipeline. In fact, Air changes flow properties and transient pressure peaks in comparison with single-phase flow. Experimental study has been carried out to verify characteristics of transient pressure inside a... 

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...