Showing 14 results for Rte
M. R. Soltani and A. R. Davari,
Volume 20, Issue 2 (4-2001)
Abstract
A wind tunnel investigation was performed to study the flow field over a 70° swept sharped edge delta wing model at high angles of attack. The experiments were conducted in the subsonic wind tunnel at the Department of Mechanical Engineering, Sharif University of Technology. Velocity profiles have been measured using a special pitot tube and hot wire anemometer at angles of attacks of 10 to 35 degrees and Reynolds numbers between 1.5 to 5´105 over half and a full model. From these studies the shape of the leading edge vortices as well as the turbulence intensity inside the vortices were obtained and analyzed. This study revealed a region of increased velocity highly tubulent flow at the vortex core. As a result, the lift will increase nonlinearly with angle of attack.
Keywords: Leading edge vortex, Turbulence Intensity, Delta Wing, Vortex Bursting, Vortex core
J. Soltani and F. Katiraei,
Volume 22, Issue 1 (7-2003)
Abstract
In this paper, using a personal computer (PC), the practical implementation of scalar and vector control methods on a three–phase rotor surface- type permanent magnet synchronous machine drive is discussed. Based on the machine dynamic equations and the above control strategies, two block diagrams are presented first for closed-loop speed controlling of the machine drive/system. Then, the design and implementation of hardware circuits for power, insulating, and signal matching stages are explained along with a description of the written software program for the servo drive system control. These circuits are
used to produce the drive inverter switching pulses. To supply the machine drive, the sinusoidal, uniform sampling and step-trapezoidal PWM voltage source inverters are examined. For closed loop speed control of the drive system, the stator currents and rotor speed signals (in scalar control method only the rotor speed) are sampled on-line. After filtering, buffering and matching operations, these signals are transferred to a personal computer port via a high frequency sampling and high resolution A/D converter. It is worth mensioning that both methods of controlling mathematical calculations is done by computer. Finally, the practical and computer simulation results obtained are demonstrated.
Keywords: Machine Drive, Synchronous Machine, Permanent Magnet, Rotor Surface Type, Scalar and Vector Control, Voltage – Source Inverter, Control by PC.
H. Farzanehfard and A. Pakizeh Moghadam,
Volume 22, Issue 1 (7-2003)
Abstract
Soft Switcing techniques have recently been applied in the design of dc-ac converters, in order to achive better performance, higher efficiency, and power density. One of the soft switching techniques uesd in inverters is resonant dc links. These topologies have some disadvantages such as irregular current peaks, large voltage peaks, uncotrollble pulse width, etc. Another soft switching method in inverters is using Quasi –resonant links, which have PWM modulation capability. Inverters with series or parallel Quasi-resonant dc links use several quasi-resonant current or voltage pulses, respectively, to produce PWM modualation. In this paper an inverter with a novel Quasi-resonant series dc link is introduced. This topology enables current source inverters to have characteristics such as resonant pulse peak limition and pulse width controllability. This circuit provides the inverter with two to three ranges of PWM control capability which increases the switching time control in a larger range.
Various operational modes of this novel Quasi-resonant dc link is analyesed and then the circuit losses is calculated. Finally, simulation results by PSPICE software is presented to justify the circuit operation.
Keyword: Inverter, Soft switching, Novel quasi-series resonant link, increasing control areas, Losses
A. R. Bakhshai, H. R. Saligheh Rad and M. Saeedifard, ,
Volume 23, Issue 1 (7-2004)
Abstract
Pulse Width Modulation (PWM) techniques are commonly used to control the output voltage and current of DC to AC converters. Space Vector Modulation (SVM), of all PWM methods, has attracted attention because of its simplicity and desired properties in digital control of Three-Phase inverters. The main drawback of this PWM technique is
its complex and time-consuming computations in real-time implementation. The time-consuming calculation as well as software and hardware complexities of the network grow dramatically as the number of inverter levels increases. Therefore, it is necessary to develop an exact, fast, and general computation SVM algorithm for multi-level converters. This paper introduces such an algorithm. Specifically, the SVM computation algorithm based on a vector classification technique, introduced for 2-level inverters in 1996, is developed and generalized to be applicable in determining the switching sequences and calculating the switching instants in m-level inverters. The proposed technique reduces hardware and software complexities, decreases the computation time, and increases the accuracy of the positioning of the switching instants when compared with the conventional implementation of the SVM in multi-level converters
M. A. Lotfollahi Yaghin, K. Farzad and M. Naghipour,
Volume 23, Issue 1 (7-2004)
Abstract
Similar to random sea waves, forces on the offshore structures due to waves are random. These forces can be mainly divided into two components, namely, inline forces and transverse or lift forces. The random nature of lift forces is more complicated than that of inline forces and both should be combined for design purposes. In the present paper, two different approaches have been used to determine time series of lift forces. Along these lines, the determination of lift coefficients is discussed which have then been used to obtain transverse forces and compared with experimental data. The experimental data used in this study were collected at Delft Hydraulics Laboratory on a full-scale rough vertical cylinder.
H. Farzanehfard, S. R. Motahari and M.m. Tavasoulkhamseh,
Volume 23, Issue 2 (1-2005)
Abstract
One of the difficulties with PWM switching converters is high switching loss and electromagnetic interference due to switching at non-zero voltage and current, which limits the operating frequency. In order to reduce the converter volume and weight (by increasing the frequency) and reducing switching losses, zero voltage and current switching methods are recommended. In this paper, four main zero voltage switching (ZVS) methods in full bridge converters are introduced and compared. These four methods are compared on dead times required to obtain the ZVS, load range at ZVS condition, circulating energy in the switch anti parallel diodes during freewheeling periods and voltage oscillations on rectifying diodes. Finally, the results of a 3 KW prototype full bridge ZVS converter with a clamp circuit for rectifier diodes oscillations are presented and analyzed
R. Hosseini, M. Vaziri, and M. Bidi,
Volume 24, Issue 1 (7-2005)
Abstract
In this paper, the Radiation Transfer Equation(RTE) for a non-gray gas between two large parallel planes has been
solved and the temperature distribution obtained. With the RTE, solution heat fluxes are also determined. Since and are two components of most combustion products, the problem has been solved for these two gases. The results were, whenever possible, compared with data reported elsewhere. Since the simulation of exact absorbing bands has been used, it can be claimed to be relatively close to exact solution. From the results otained, it can be maintained that treating, the above mentioned gases as a gray gas could cause considerable errors in the determination of temperature distribution and heat fluxes. The error would be more for water vapour than for carbon dioxide.
R. Taherian, A. Najafi Zadeh, M. Shamanian, R. Shateri,
Volume 25, Issue 1 (7-2006)
Abstract
In this study, two CCCT diagrams are drawn to be compared with a CCT diagram. The CCCT diagrams represent continuous cooling transformations in stress assisted state. The increased Md and Bd temperatures of CCCT diagrams were also compared with those of the CCT diagrams and the cause was investigated from both thermodynamic and metallurgical viewpoints. Thermodynamic examinations revealed that stress causes the mechanical driving force to increase but the total free energy of transformation to decrease hence, Md and Bd will rise. Metallurgical investigations showed that if deformation temperatures are selected in a manner to increase the structural strength of the original austenite grains prior to deformation, the shear force required for martensite and bainite transformations will arduously obtain hence, Md and Bd will fall. However, if recrystallization or full recovery occurs during or after deformation, interior grain structure softens and the shear force required for martensite and bainite transformations will readily obtain hence, Md and Bd will rise. It was also found that the nose in CCCT curves are shifted to the left as compared to that of CCT curves. This indicates that deformation of steel enhances bainite formation more readily than that of the martensite phase.
M. Kahrom, A. Farahbode, and D. Khodadadzadeh,
Volume 27, Issue 1 (7-2008)
Abstract
A rectangular rod is placed in a flow field flowing parallel to a flat plate. Effect of chord-thickness ratio of rectangular rod on developing vortex shedding downstream to the rod is studied. Then, for each one of the aspect ratios, the distance of the rod from the neighboring flat plate is reduced until the rod sticks to the flat plate. In each case, the effect of the flat plate boundary layer on Strouhal number and the contrary effect of the boundary layer on vortex shedding from the rectangular rod are studied. Results show that as the rectangular rod enters into the flat plate boundary layer, vortex generation from the closest side of the rod reduces, thereby reducing the Strouhal number as well. Finally, when the rectangular rod sticks to the flat plate, a stationary wake forms downstream the rod and sticks to the flat plate. Meanwhile, the boundary layer over the flat plate is disturbed effectively and heat transfer coefficient from the flat plate is enhanced by an average of 50% and up to 200% in some places, locally.
R. Jamshidi-Alashti, M. Borouni, B. Niroumand,
Volume 32, Issue 2 (12-2013)
Abstract
Application of ceramic reinforcements is one of the effective and well-known ways to refine the microstructure of brittle metals such as magnesium. In this research, the influence of nano/micro particles of zirconia on the microstructure of cast AZ91 alloy was studied. At the first stage, nano and micro ZrO2 powders were blended through mechanical alloying procedure. In five specimens, the total amount of nano and micro reinforcements in the final mixture was fixed at 5 wt%, whereas their ratio was varied. Two other composites were also produced using 5wt% of nano or micro particles of zirconia. These powder mixtures were then stirred in the molten AZ91 at 650C by vortex method and finally cast in a sand mold at 615C. For comparison, two monolithic castings including a conventionally cast specimen and a super heat-treated sample were also cast. The average grain sizes for all composites were decreased with respect to both monolithic castings. The best results in terms of grain size and microstructure improvement were obtained for AZ91/5wt% nano ZrO2 composite with remarkable improvement in comparison with monolithic castings.
A. Shirali, A. Honarbakhsh Raouf , S. Bazzaz Bonabi,
Volume 34, Issue 2 (7-2015)
Abstract
Certain amount of retained austenite can increase ductility of steels because of the TRIP phenomenon during plastic deformation. One method for achieving this is partitioning of carbon into austenite to stabilize it at room temperature. The quenching and partitioning (Q&P) heat treatment leads to a microstructure consisting of martensite and stabilized retained austenite between martensite plates, which provides a better combination of strength and ductility. In this study, the effect of parameters of Q&P process (quenching temperature, partitioning temperature and partitioning time) on the microstructure and retained austenite volume fraction of a low alloy medium carbon steel was investigated. The results showed that the high increase in partitioning time causes the disappearance of martensite blades and reduction of austenite volume fraction. However, increasing
of partitioning temperature made the retained austenite films become thicker and its volume fraction increase. On the other hand, by increasing the quenching temperature, carbon content of retained austenite increased sharply.
H. Ahmadi, S. Otroj, M. R. Nilforushan, A. Dehghani Varnamkhasti,
Volume 36, Issue 2 (9-2017)
Abstract
In this study, the composition of magnesium aluminate spinle and the converter mud were used as raw materials to in-situ formation of hercynite phase in magnesite-hercynite refractory bricks. The pressed samples were sintered at 1400 and 1500℃ and then, the phase composition of bricks was evaluated after firing at 1400℃. Besides, the effect of nano-magnesia particles addition on the properties of magnesia-hercynite refractory bricks was examined. Hence, the physical peroperties, thermal shock resistance and microstructure of refractory bricks were evaluated. The phase composition results showed that hercynite is well-formed in the refractory matrix, which leads to bonding formation and its increase between magnesia particles. The evaluation of results indicated that the addition of nano-magnesia particles can reduce the porosity of brick via increasing particles packing. In this relation, 1 wt. % nano-magnesia addition was determined as optimum content. Further addition of nano-magnesia leads to increasing of porosity via agglomeration of particles. Due to the high surface area of used nano-magnesia particles, the adequate sintering of refractory brick containing nano-magnesia take places at 1400℃. This leads to increasing of particles bonding and then, increasing mechanical strength, but it can not affect the thermal shock resistance of refractory bricks. The microstructural evaluations showed the lower porosity and further particles bonding with addition of nano-magnesia optimum content.
F. Mansouri, M. Mehran,
Volume 39, Issue 1 (5-2020)
Abstract
In this paper, synthesis of inverted nano-pyramids on a single crystal silicon surface through a simple and cost-effective wet chemical method is surveyed. These structures were synthesized by MACE process using Cu as the assisted metal in the solution of copper nitrate, hydrogen peroxide and hydrofluoric acid for different etching times. FE-SEM images of the samples show that time is an important factor in the formation of silicon inverted nano-pyramids and by extending synthesis time the structures gradually fade. After synthesis, some samples were etched in KOH 2% and IPA 6% for one minute which resulted in formation of nano-pyramids besides inverted nano-pyramids on the silicon surface. Formation of these combinational structures affected the self-cleaning behavior of the silicon surface. X-ray diffraction (XRD) was utilized for studying the crystalline characteristics of the synthesized structures. Moreover, the self-cleaning behavior of them was studied using the contact angle goniometer. The results showed that the inverted nano-pyramids improve the hydrophilic behavior of the silicon surface while post-synthesized etching improves the hydrophobic behavior of the surface. These textures improve the performance of solar cells due to their self-cleaning and light adsorption properties
M. Jafari, M. Rafiei, H. Mostaan,
Volume 39, Issue 2 (8-2020)
Abstract
In this research, the effect of temperature and time on the properties of AISI420/SAF2507 dissimilar joint produced by transient liquid phase bonding process was investigated. A BNi-2 interlayer with 25 μm thickness was inserted between two dissimilar steel samples. The bonding process was performed at 1050 oC and 1100 oC for different bonding times. The microstructures of the joints were studied using optical microscope, scanning electron microscope and energy dispersive X-ray spectroscopy. Microhardness and tensile shear strength of bonded samples were investigated. Isothermal solidification was completed for the joints bonded at 1050 oC and 1100 oC for 45 min and 30 min, respectively. ASZ and ISZ areas of the bonding zone at the bonding temperature of 1050 oC indicated the highest (520 HV) and the lowest (300 HV) microhardness values, respectively. Sample bonded at 1050 oC for 1 min indicated the lowest tensile strength (196 MPa) and sample bonded at 1100 oC for 60 min indicated the highest tensile strength (517 MPa).
