مجله مهندسی مکانیک
سال چهل و ششم شماره 2 (پیاپی 75، تابستان 1395)

In this paper, the accuracy of AL7075 electrical discharge machining was investigated. The experimentation based on Statistical Design of Experiment methodology was conducted by selecting five process inputs including Peak current, Off-time period, Voltage, Discharge time and Polarity with the aim of studying four outputs which are Surface roughness, Material removal rate, Over cut and also, Wear of the tool. The purpose of this research is access to a final machining part with a desirable surface quality along with the minimum possible deviation from nominal tolerances under different parametric settings. The results indicate that the machining quality is functionalized directly through the inputs. Interestingly, the figures related to tool wear is considerable lower than initial expectation of Aluminum EDM. Selection of machining polarity did not have priority on electrode wear here however; other responses were dramatically affected by this factor. Between all of the existed parameters in the process, the most influential one was peak current in so far to have a machining with higher accuracy and quality, its relevant level must be kept at least as much as possible whereas, in this case, this selection will lead to a condition with a smaller material removal rate. Accordance to Grey Relational Analysis, a simultaneous calculation performed on the four responses: wear rate , material removing rate, overcut and Surface roughness and the parametric treatment for an acceptable machining in terms of quality and time, suggested as I = 8A, Ton = 50µs, Toff = 5 µs, V = 40v and P = 0. It should be noted that on the basis of relative degrees of gray as the primary and secondary quantitative index optimization, by selecting this parameter, the overall performance is improved by about 20% .

In this paper, an experimental research was carried out for investigating the characteristics of the flow pattern and separation region behind a backward-facing step (BFS) with low expansion ration (aerodynamic step), in the Reynolds number range of 18000

An experimental investigation on the cylinder having a length of 390 mm and a diameter of 20 mm at Reynolds number 18750 has been done. At the first, mean streamwise velocity profiles, then streamwise turbulence intensity profiles fixed and rotating circular cylinder (for spans 1000, 2000 and 3000 rpm) at three stations (x=30, 60,120 mm) behind the cylinder are discussed and compared. In order to measure the mean velocity and turbulence intensity, the hot-wire anemometry has been used. Also, the Strouhal number of fixed and rotating cylinders is compared. The results show that by increasing the rotational speed at a constant Reynolds number, if the direction of the cylinder is the agree of the fluid movement, the width of wake becomes smaller, and the Strouhal number of the wake associated with the Karman vortex increases. At the stations of 1 and 3 turbulences the rotating cylinder is more than the fixed cylinder, and the second station turbulences fixed cylinder is more than the rotating cylinder. It is found that by increasing the rotational speed, the parameters of drag coefficient and velocity defect reduce. By increasing the rotational speed, the drag coefficient reduced by about 10%.

In this research, free vibration analysis of single and double walled carbon nanotubes (DWCNT) are considered, Analytical and numerically. In FEM simulation, two node beam elements are used and spring elements added as Van Der Waals (VDW) interaction forces between various walls of nanotube. For the first time in this research, the effect of nonlocal forces are prescribed as inclined nonlinear spring elements between nanotube walls. For better understanding of nonlocal force effects, free vibration analysis of DWCNT is performed analytically and numerically without nonlocal effect, too. In the analytical section, parabolic shear deformation theory is used. The nonlocal mode of FEM shows higher non-dimensional natural frequencies as compared to the analytical and linear FEM solutions. The later result is because of the increment in the flexural stiffness of simulated nanotube, by addition of nonlocal springs. However in the higher mode frequencies, the effects of nonlocal forces are reduced. Thus, nonlocal effect is higher in low frequency mode numbers. By comparing the results of analytics and FEM results, there shown more agreement between them by raising up the frequency mode number.

The objectives of this paper are to examine the effect of nanoparticles on flat-plate collector efficiency. This study employes direct synthesis method to prepare Cu–water nanofluid to serve as a working experiment fluid in a solar collector. ASHRAE 93 was used to test the solar collector. According to the experimental results, the collector efficiency washigher when the concentration of nanoparticles was raised and the results show that the efficiency of collector at 0.05wt%was approximately 24%more than that of the pure base fluids for given conditions. This study confirms thatnanofluids will have considerable potential for use in solar collectors.

Rotating discs have been used in numerous industrial machineries. In most of these applications discs are worked at high temperatures and high rotational speeds. Such a working conditions lead to the more sever thermo-mechanical stresses in rotating discs. In this situation of high temperature and stress the creep phenomenon is the most important and it can impose the irreversible damages to structure. Therefore it is important to investigate the creep behavior in rotating discs. Studies have shown that the creep is entirely dependent on the material structure. In recent years using of a new type of material called functionally graded materials (FGM) has been expanded continuously. Nowadays the functionally graded materials are used in construction of various component of machinery including rotating discs because of their spatial benefits. As a result the creep analysis in FGM rotating discs is a valuable problem. In this paper the rotating discs steady state creep equations are extracted and the effect of particle distribution and threshold volume fraction as well as prevailing temperature on the creep rates and stress fields of FGM rotating disc are investigated.

In this paper, a new 1-dimensional model is developed to predict the ejector performance. This model is based on the gas dynamics with the principles of mass, momentum and energy conservation. The model is able to calculate the ejector entrainment and area ratios for different thermodynamic conditions with high accuracy. Due to the frictional losses in different parts of the ejector, isentropic efficiencies are considered for the ejector. The results of the validation show that the maximum relative difference, compared with the experimental data, in calculating the area and entrainment ratios are 13.27% and 5.8% respectively, when in the one dimensional model reported in the literature these values are 22.04 % and 9.78 % respectively. The results also indicate that with decreasing the ejector back pressure or increasing the evaporator pressure, area and entrainment ratios of the ejector increase.

In the present study, Repetitive extinction-ignition dynamics for premixed hydrogen-air mixture is investigated numerically in a heated micro channel. Low Mach number approach is considered for governing equations in numerical simulation. Also detailed chemistry and different mass diffusivity of species is utilized. The effect of inlet velocity, equivalence ratio and channel height are studied on repetitive extinction-ignition dynamics using two parameters, amplitude and frequency. The results show that the amplitude of repetitive extinction-ignition dynamics increases with increasing the inlet velocity, while the frequency has a descendingascending behavior. The rising of equivalence ratio from 0.5 to 1 and channel height from 0.4 mm to 1mm lead to growth of amplitude and frequency reduction. Bifurcation of propagating flame is also investigated by hydrodynamic and kinetic approach. Details of flow field indicate that bifurcation of flame is due to creation of recirculation zones formed close to the walls at the beginning of ignition phase. The presence of water in the zone between two propagating flame can be the main reason of bifurcation flame which is obtained from kinetic analysis.

Elliptical subsurface cracks, under rolling contact fatigue, are special due to the coupling of the fracture modes and the crack closure. In this paper, the ability of the matrix of two dimensional weight function, to analyze the completely closed subsurface crack is investigated. Weight function method has a super position base and so, the ability is evaluated for the frictionless contact between crack surfaces. To do this, mixed mode stress intensity factors of a subsurface crack front under semi-ellipsoid of contact pressure are calculated using the matrix of two dimensional weight function and subsurface stress field. The results are compared to finite element stress intensity factors with high accuracy. It is concluded that the matrix of weight function is able to quick analyze the completely closed subsurface crack with high accuracy if the compression normal stress is vanished. Also, the effects of the aspect ratio and depth of the crack on the stress intensity factors are investigated.

Experimental study of convective heat transfer behavior and the AC dielectric breakdown voltage of nano oil for use in transformers is the main scope of this work. The nano oil containing colloidal magnetic Fe3O4 nanoparticles at different volume concentration and the Nytro Libra oil that is the conventional oil in power transformers has been considered as the base fluid. The convective heat transfer behavior was studied by using an experimental setup in laminar flow regime and under constant heat flux. The AC dielectric breakdown voltage by usingBA100 portable dielectric oil tester according to IEC 60156 was carried out. Results show that nano oil with 23 nanometer size and volume concentration of 0.1% has convective heat transfer coefficient to 4.5% higher than the base oil. The AC dielectric breakdown voltage improved 23.8% for nano oil with volume concentration of 0.1% compared to the base oil. The results of the convective heat transfer coefficient behavior of nano oil were compared with othe researchers.

In this paper, an analytical study has been carried out in homogeneous porous media and in the fully developed pipe flow, as constant heat flux tube wall from outside influences. Using a mathematical model, the equations of continuity, momentum, and energy dissipation due to viscous friction intended heat of the semester in which it is, Analytical solution for the profile of velocity, temperature and Nusselt number (Nu) obtained versus dimensionless Darcy numbers (Da) (represents the relative effect of the permeability of the medium versus its cross-sectional area commonly the diameter squared) and Brinkman (Br *) which is a dimensionless number related to heat conduction from a wall to a flowing viscous fluid. The profile of velocity, for smaller Darcy numbers uniformly in the central region of the tube, whereas in the area near the wall has sharp slope. The effect of these parameters was observed on the Nusselt number in certain Brinkman (as shown by Brc *) and also Darcy can be changed with the Nusselt number. Using curve fitting techniques, function of Brc * was obtained Darcy number with good accuracy.

In this paper, energy harvesting from base vibration of cantilever beam is considered.With taking into account the high electromechanical coupling of piezoelectric materials in comparison to other vibration energy harvesting methods, the considered structure of energy harvester is a three layers beam that consists of one metal substrate layer and two piezoelectric layers at up and down of it and a proof mass at free end.The considered harvester is modeled in ANSYS finite element software using CPC-FEM technique and effects of geometrical parameters of energy harvester on mechanical and electrical behavior of it are investigated. The optimized dimensions of harvester by using genetic algorithm (GA) are presented and the achieved results from CPC-FEM technique are compared with analytical method counterparts. Results indicate that finite element and analytical methods have an acceptable adaptation. But, because of considering the stress strain relation at whole of beam length with electrode, finite element method presents the accurate results. Also, results show that piezoelectric energy harvester can harvest 4.4 mW power and 9.3 Vvoltage at optimum electrical resistance of 10kohm that is sufficient to power sensor nodes and other low consumer electronic equipments.

In this paper, the fracture analysis and fatigue crack growth life assessment for a gas turbine blade of the hot gas path is studied. At first, applying FEM analysis using ANSYS software, the stress analysis for the turbine balde with no crack in the case of of steady state full load condition has been performed. In this way, thermomechanical loadings, i.e., rotational speed, temperature and pressure distributions are all included in the analysis. From the outcome results the critical regions on the blade airfoil surfaces having high stresses are recognized. Then, after modeling the crack in these regions the fatigue crack growth life assement analysis has be done. To do this, using ANSYS software the crack stress intensity factors (KI) for the cracks with different dimensions has been calculated. Having on hand the obtained values for KI in conjucture with the specified graph for the crack growth rate, i.e., (da/dN) vs. the.crack intensity factor for the blade material, the cyclic crack growth rate for the cracks with different dimension ratios (a/c) can be obtained. Finally, using numerical integration method, the number of cyles Nf to reach the critical crack depth value (af) for the turbine balde has been calculated.

Most structures experience large deformations under applied loads. Due to the deforming of geometry of structures, geometrical nonlinear analysis is needed. The co-rotational method is a simple and practical technique for nonlinear geometrical analysis. In this technique, unlike other nonlinear methods, is supposed the strain of the element to be small. The aim of this research is to develop a new co-rotational method for geometrical nonlinear analysis of two-dimensional truss structures based on small strains. A co-rotational method makes the separation of rigid body motion and small strains possible for a truss element. Therefore, global and local coordinate systems are utilized. Linear stiffness matrix of two-node element is used for small strains in local coordinate systems. Then, by writing co-rotational equations, transformation matrix is introduced for this element. The transformation matrix is used to calculate tangential stiffness matrix and element node force vector in global coordinate systems. At the end, the accuracy of the proposed method is evaluated by numerical tests. These tests confirm high accuracy of the proposed element for geometrical nonlinear analysis of twodimensional truss structures.

This study aims at investigation the effects of material distribution and thermal boundary condition on stress field of FGM rotating hollow disk. For this purpose, three different problems with four distribution functions for material properties and three different thermal boundary conditions have been applied. Young’s modulus, density, and coefficient of thermal expansion distributions are defined as functions of the radius, and Poisson's ratio is assumed uniformly because it has equal value in two considered materials. The first, with assumption of elastic perfectly plastic behavior for material, disk plastic area rate with linear, power, exponential, and logarithmic material property distributions at three different temperature boundary conditions is studied and the best distribution for minimizing plastic area is detected. In the following with the assumption of constant rotational speed, thermal boundary conditions and external and internal radius of the disc, by utilizing criterion Von-Mises stress and the Taguchi method, the distribution of material properties for minimizing the maximum Von-Mises stress in each of the thermal boundary conditions is introduced. Finally, the sensitivity of Von Mises stress subject to the rotational speed, thermal boundary conditions, and material properties of the inner and outer radii is investigated by utilizing the Taguchi method. It’s concluded that the sensitivity of maximum Von-Mises stress to temperature distribution is more than to material distribution and rotating speed.

In this article, free vibration of a composite cylindrical shell reinforced with circumferential rib have been analyzed with analytical and numerical approach. The equilibrium equations based on the classical theory are presented. The stiffness and the mass matrix, and the structural frequencyequationsarederived. Then, using the numericalGalerkin solutionthe equations were solved. The results of Analysis of reinforced composite cylinders under S-C and C-C boundary conditions are compared with each other. The results show that the circumferential ribs increase the transverse stiffness and the flextural frequencies of the structure. In the present study,the results wereverified with existing resources.