نشریه مهندسی مکانیک مدرس
سال یازدهم شماره 1 (بهار 1390)

Repowering means addition of gas turbine unit(s) to a steam power plant in order to make use of the exhaust gas heat and to increase efficiency of the new combined cycle. There are two groups of repowering partial repowering and full repowering. Full repowering is more common and is used in power plants with nearly ended useful lifetime. In this case the capital investment is considerably reduced compared with the case of making a similar combined cycle. Objective functions are per kWh electricity cost and exergy efficiency. These functions are based on important independent variables of heat recovery boiler, steam turbines, gas turbine. Finally, considering the introduced objective functions, it is tried to achieve the most optimized techno-economic characteristics for Besat power plant repowering cycle, using genetic algorithm optimization method with two scenarios of single and multiobjective.

Residual stress measurement is one of the most interesting research areas in experimental mechanics. Residual stress is introduced to material due to plastic deformation of parts and can be one of the most effective parameters on design and operation of parts. ASTM E837-01 standard studies residual stress determination in parts by hole drilling method and represent calibration coefficients for flat sheets with constant stress profile. However, there is no certain standard on the residual stress measurement by Incremental Hole Drilling Method (IHDM) which is the subject of this study. IHDM can obtain stress profile by using two modified stress calibration coefficients. In this article, the stress calibration coefficients have been extracted for incremental hole drilling by using finite element analysis (FEA). FEA contains both biaxial tension test and pure shear test which a hole has been drilled step by step in the parts by removing elements and the strains changes were determined at three strain gauge positions on the surface. At last, the calibration coefficients are determined for each step and the accuracy of coefficients have been verified by a set of experimental test and a FE analysis. The experimental test contains four-point bending of an AA5056 flat aluminum sheet. The numerical analysis contains four-point bending of a flat sheet. In both cases, the stress profile can be determined easily by using analytical equations. Average analytical stress in each increment has been calculated and compared with the result of numerical incremental hole drilling method. The comparisons show that numerical and experimental results have no significant differences in first six steps but in the last four steps show an increasing errors due to the change in stress profile and hole geometry. Results presents that the calibration coefficients have suitable accuracy in stress profile determination.

Passing manoeuvres and crosswind can have significant effects on the stability and fuel consumption of road vehicles. When two vehicles overtake or cross, they mutually influence the flow field around each other, and under certain conditions, can generate sever gust loads that acts as an additional forces on both vehicles. The forces acting on them are a function of the longitudinal and transverse spacings and of the relative velocity between the tow vehicles. In this paper, the models were designed to study the effects of various parameters such as the longitudinal and transverse spacing, the relative velocity and the crosswind on the aerodynamic forces and moments generated on the overtaken and overtaking vehicles using Ansys CFX. The aerodynamic forces have been predicted by a SST model solution of the Navier-Stokes equations for turbulent flow. The numerical predictions for the evaluation of aerodynamic coefficients agree well with the scaled-down air tunnel experimental work.

The inertial forces and moments, due to the motion of robotic arms installed on a mobile base, lead to reaction forces on the moving base which may cause its unexpected motion. In this article, a method of designing a path of motion in the Cartesian space between the initial and final positions is presented which guarantees no reaction on the moving base. To this end, developing the system dynamics model, the moment equations are derived. Based on the conservation of momentum in the absence of any external force and moments, the angular motion due to the motion of robotic arms is solved. Then, based on the definition of reaction null-space map for dynamic coupling matrix, the joint speeds are projected to the reaction null-space, to obtain the joint speeds in this space. Next, using numerical integration of the obtained joint rates, the motion in the joint space with no reaction on the base is obtained. Therefore, motion of robotic arms according to these joint specifications, the total momentum of the system remains zero, and due to no reaction forces applied on the moving base, its position and attitude remains unchanged.

In this paper a new approach about relation of Acoustic Emission(AE) method and mechanical properties of ferrite-martensite dual phase steels(DPS) has presented. The AE signals from a tensile test using a range of DPS with different volume fractions of martensite (VM)s, in the range of 12–65% VM, were obtained and their AE signals were investigated. In order to better study DPS internal behaviour, a function named “sentry function” was used. The amount of this function depends on the strain energy and acoustic emission energy. the Results show that AE monitoring and sentry function are efficient tools for detection of micromechanisms, consisting of Ferrite-Martensite interface decohesion and/or martensite phase fracture, identifying the correlation of failure mechanisms to microstructure in DPS.

In this paper the idea of energy regeneration of active suspension system in hybrid electric vehicle is presented and its influence on the fuel consumption and emissions of vehicle is investigated through computer simulations. Active suspension systems employ active actuators to apply force and control the vibrations of vehicle body. The active actuators either insert energy to the system or extract the energy of vibrations when required. Using an energy regeneration system, the extracted energy of vibrations can be recovered and stored in the energy storage system. In hybrid electric vehicles, the active suspension supplies its required energy from the electric energy storage system of vehicle. In this work, a hybrid battery/supercapasitor energy storage system is employed to supply the required energy of active suspension and other electric components of vehicle. The simulation results show that with application of the energy regeneration system, the fuel consumption and exhaust emissions of vehicle is reduced.

The window is an external envelope of the building that has more effect on the building energy consumption and human thermal comfort. So, calculating of window heat transfer is an important task. Since, calculating of the window energy transfer is difficult and must be calculated with the computer simulation, simple equations are necessary to estimate the window energy transfer and to compare the different window types. In this study, using computer simulation, a new equation has been presented to calculate double pane glazing window energy transfer. Using this equation, the window parameters can be designed based on the minimum window energy transfer. Also to compare the different window types (with or without overhang) a coefficient as "Efficiency Coefficient" has been defined. The result show that the window energy transfer decreases with the Efficiency Coefficient decreasing. Therefore, this coefficient can be used as a criterion to select the optimum window based on yearly minimum energy consumption.

In this paper a continuous-time state-space aerodynamic model has been developed based on the boundary element method. First, boundary integral equations for unsteady potential subsonic flow around lifting bodies are presented with emphasis on a modified formulation for thin wings. Next, the BEM discretized problem of unsteady flow around an arbitrary wing is recast in the form of a state-space model using some auxiliary assumptions. To validate the proposed model, its predictions for unsteady aerodynamic coefficients due to various unsteady flows about different wing geometries were compared to the verified results of the direct boundary element solution and good agreement was observed. Because of the resulting aerodynamic model has been constructed in the continuous-time domain, it is particularly useful for optimization and nonlinear analysis purposes. Moreover, its state-space representation is the appropriate form for an aerodynamic model in control applications.

The aim of present study is to investigate the kinematics of tool-workpiece’s relative movement in conventional and ultrasonic-vibration assisted turning (UAT). The kinematic analysis of UAT shows that the movement of cutting tool edge relative to the workpiece resulted from the cutting speed, feed speed and tool’s vibration affects the lateral machined surface of workepiece and leaves a repeating pattern of crushed and toothed regions on it. This results in an increase in the surface hardness of the lateral machined surface in comparison with conventional turning (CT). A model of the tool-workpiece’s relative movement has first been developed in the present study. This model predicts a surface hardening effect for the lateral surface in UAT in comparison with CT. Several experiments were subsequently carried out employing a surface micro-hardness testing machine and an optical microscope to verify the predicted results.