Sharif Digital Repository

In this study, transient 2D/axisymmetric simulations of cavitating flows are performed using a modified 'Volume-of-Fluid' (VOF) technique. Simulation of the cavitation is based on a homogenous equilibrium flow model. To predict the shape of the cavity, the Navier-Stokes equations in addition to an advection equation for the liquid volume fraction are solved. Mass transfer between the phases is treated as a sink term in the VOF equation. The numerical method is used for different geometries in a wide range of cavitation numbers. Computed shapes of cavities were found to be in good agreement with those of the reported experiments. The simulation results also compared well with those obtained... 

This paper has investigated the performance of a linear, 6H-SiC high power photoconductive semiconductor switch. A three-dimensional device modeling with SILVACO ATLAS tools was used to model the optically initiated 6H-SiC switch. The 6H-SiC PCSS device is designed in a rear-illuminated, radial switch structure. The material properties of vanadium compensated 6H-SiC PSCC have been analyzed for breakdown, photocurrent profile such as rise and fall time in terms of their applications as a photoconductive switch at high bias conditions. This structure and also new type of illumination extends the blocking voltage by reducing the peak electric field near electrodes. In this presentation the... 

This article aims to simulate the sudden and fast pressure drop of VVER-1000 reactor core coolant, regarding acoustic phenomenon. It is used to acquire a more accurate method in order to simulate the various accidents of reactor core. Neutronic equations should be solved concurrently by means of DRAGON 4 and DONJON 4 coupling codes. The results of the developed package are compared with WIMS/CITATION and final safety analysis report of Bushehr VVER-1000 reactor (FSAR). Afterwards, time dependent thermal-hydraulic equations are answered by employing Single Heated Channel by Sectionalized Compressible Fluid method. Then, the obtained results were validated by the same transient simulation in a... 

A highly reliable tool for transient simulation is vital in the safety analysis of a nuclear reactor. Despite this fact most tools still use diffusion theory and point-kinetics that involve many approximation such as discretization in space, energy, angle and time. However, Monte Carlo method inherently overcomes these restrictions and provides an appropriate foundation to accurately calculate the parameters of a reactor. In this paper fundamental parameters like multiplication factor (K eff ) and mean generation time (t G ) are calculated using Monte Carlo method and then employed in transient analysis for computing the neutron population, proportional to K eff , during a generation time... 

Here, using the probabilistic evaluation based on the Monte Carlo method, back-flashover rate and shielding failure flashover rate of 230 kV overhead transmission lines in the western regions of Iran are evaluated. To such an aim, first, the number of thunderstorm days per year is collected from the reported weather information in order to determine the ground flash density. Then, using MRU-200 equipment, the tower-footing resistance of several towers is measured. Matlab® software is used in order to produce lightning surges considering its probabilistic nature and randomly distribution on the ground to evaluate striking distance based on the geometric model. Then, calculated parameters are... 

This paper presents a novel approach for fault location of overhead transmission lines by voltage and current measurements in a remote substation. The method is applicable when not all of the transmission lines in an area are equipped with fault-locators, although there may be a critical substation (CS) in the area equipped with a digital fault recorder (DFR). In the proposed method, the circuit equations of the network are used to find the transfer function between the fault location and each voltage and current measurement in the CS. Next, two auxiliary variables are defined to transform the nonlinear fault location estimation problem into a linear least squares problem. A closed-form... 

This study presents a novel method for fault location of transmission lines by multiple fault current measurements. In contrast to conventional methods, it is proposed to utilise several current measurements, which may be far from the faulted line. The circuit equations of the network are used to express each fault current as a function of fault location. Fault location is then estimated using a least-squares estimation technique. To achieve a robust estimation of fault location, statistical hypotheses-testing is employed for identifying erroneous measurements. The method is applicable to both synchronised and unsynchronised measurements. Moreover, fault location can be estimated regardless... 

In this paper a novel supplementary subsynchronous damping controller (SSDC) is proposed for the STATic synchronous COMpensator (STATCOM) which is capable of damping out subsynchronous oscillations in power system with series compensated transmission lines. An auxiliary subsynchronous damping controller (SSDC) for a STATCOM using the generator rotor speed deviation signal as the stabilizing signal has been designed to damp subsynchronous oscillations. Eigenvalue analysis and transient simulations of detailed nonlinear system are considered to investigate the performance of the controller. Robustness of the controller has been analyzed by facing the system with disturbances leading to... 

Development of cities as well as population growth causes to development of public transportation especially subway lines. The high capacity besides the high speed in transportation makes them the popular transportation system. Fire is the one of the most important issues that may occur in subways. The difference in flame size, emissionheat, smoke and pollutants generation of subway fires attracts an especial attention of fire investigators. The emergency ventilation of subways in the case of fire should have the ability of discharging heat, smoke and pollutants from passenger escape route and preparing a safe place for a specific duration. The optimal performance of emergency ventilation... 

Smart transmission grids are intended to utilize various measurements for enhanced operation. In particular, synchronized phasor measurements have found many applications in this context. This paper presents a linear weighted least-squares (WLS) method for fault location estimation of transmission lines by synchronized voltage measurements. The measurements may be taken from the faulted line terminals or buses far from the faulted line. The circuit equations of the network are used to find the transfer function between the fault location and each voltage measurement. Next, two auxiliary variables are defined to transform the nonlinear fault location estimation problem into a linear WLS... 

In this paper, a transient model is developed to simulate the start-up operation of an industrial twin-shaft gas turbine based on real geometry and control logic. The components’ characteristic curves of the studied gas turbine were generated with the aid of CFD simulation tools and according to the real geometry of each component. The control logic of the studied gas turbine was simplified and coupled with the developed engine performance model. Also, the gas turbine actuators including variable inlet guide vanes, compressor bleed valves, and fuel valves were simulated so that their concurrent effects on the gas turbine transient behavior can be captured precisely. Moreover, the air-cooled... 

A vapor-compression cooling system utilizing PCM is studied whereby the electricity consumption peak load is shifted. More specifically, the dynamic performance of the cooling system with and without PCM is evaluated and is presented with details on the hottest day of the year in Tehran, Iran. The proposed system uses the cooling energy to freeze or “discharge” the PCM during nighttime when the cooling load is minimally needed and uses the stored cooling energy during the peak load hours by melting or “charging” the PCM. This leads to better performance during the peak load hours when higher cooling loads are required. Oleic acid was chosen as PCM. The simulation was performed in EES... 

In this paper, a new configuration of a solar combined cooling, heating, and power (CCHP) system is proposed to recover the waste thermal energy of a steam power plant, which provides the cooling and heating needs of an apartment complex located in Tehran. The required energy of the system is supplied by the parabolic trough solar collectors (PTCs) and, if necessary, an auxiliary heater is also used. An ejector refrigeration cycle (ERC) and a steam Rankine cycle are used for cooling and power generation, respectively. The cycle is dynamically modeled over a year using a TRNSYS-EES co-simulator. It is found that the highest Rankine cycle efficiency is obtained in the cold months (January)... 

This paper proposes a novel FPGA-based matrix-inversion technique that is specifically tailored and optimized for real-time electromagnetic transients simulation of power electronic converters with high switching frequency. This is the first reported solution that is capable of solving the real-time equations related to using ideal switch model and the associated circuitry in very small time-steps (e.g., an average of 36 ns in a three-phase back-to-back converter case study), without requiring large amount of memory, being limited to small number of switches, adding parasitic elements, or depending on a priori knowledge of the circuit operation or switching strategy. The accuracy of the...