Year:2007 | Volume:40 | Issue:6 (100)|Papers Count:14

In this paper a method based on reverse deformation using finite element analysis has been used to design pre-form dies for forging of H-shaped sections. When the final shape of the forging is complex and near net shape, it should be first formed in one or more pre-form dies before going through to the final die. The method presented here tries to accomplish this goal. In the method of reverse deformation the final shape of the product is the starting point for predicting the plastic deformation path. First the simulation of the forward deformation was carried out and then using the parameters such as the time of the contact of the die and the material, dimensions and coordinates of the work piece in contact with the die the path of reversed deformation was predicted. Using the path of reversed deformation the pre-form dies were designed. The work piece material for modeling and simulation of the forging process was pure aluminum taking into account strain hardening effect in terms of a empirical equation which defined the stress strain behavior. In this method parameters such as contact time, length and the distance from the center of the die were utilized to obtain the reversed plastic path. Because of the importance and impact of the geometry of the billet on the quality of the forged product the optimum dimensions of the billet were also calculated. The criteria for choosing the correct pre-form die were the complete filling of the die cavity, a faultless final product, no folding of the material and the optimum plastic strain gradient. Comparison of results with other previous methods showed that the design method presented here delivered better products.

The process of backward extrusion of polygonal shaped billets to internally circular hollow sections was simulated using finite element method. The software used in this investigation was ABAQUS (explicit) program. The objective was to study the effect of different parameters involved in the backward extrusion such as material flow, friction, reduction of area and shape complexity on the process itself. Using the finite element simulation, grid pattern deformations were obtained which would be very useful for the die and process design. The simulation was carried out for the backward extrusion of square, rectangular, hexagonal and octagonal billets into internally hollow circular sections. The results obtained from these simulations were plotted in terms of the extrusion load and pressure versus reduction of area, shape complexity factor and friction factor. Also contours for strain and stress were obtained from the finite element analysis. It was shown that as the percentage reduction of area increases the extrusion pressure and force increase with it. The effective strain near the parts of the billet which were in contact with the punch was shown to be at their maximum. Maximum deformation was observed to occur in the internal walls of the final extruded product. The results obtained in this work were compared with similar results from analytical and experimental works of others and good agreements were observed.

Condensation heat transfer of R-134a inside microfin tube was experimentally investigated. Test condenser was a double pipe counter-flow water cooled heat exchanger which refrigerant flowed inside the internal tube (microfin tube), while water flowed in the annulus. The microfin tube is provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The data are acquired for seven different tube inclinations in a range of -90 to +90 degrees and three mass velocities of 53.73, 80.59, and 107.46 kg / m2-s for the condensation of R- 134a vapor. The experimental results indicated that the tube inclination, α, significantly affected the condensation heat transfer coefficient in microfin tube. These effects increased with reduction in vapour quality and mass velocity so that in low mass velocity and vapour qualities the highest heat transfer coefficient, occurred in α= +30º, is 1.7 time greater than the minimum of heat transfer coefficient in α= -90 º. In addition, it was found that the highest heat transfer coefficient occurs when the microfin tube is horizontal at high vapor qualities and the tube with an inclination of +30 degree at low vapor qualities. The lowest heat transfer coefficient is attained in vertical tube; in high vapor qualities, upward flow has the lowest heat transfer coefficient and in low vapor qualities, the lowest heat transfer coefficient occurs in downward flow.

In some fluid dynamics problems, like flow stability and transition, accurate solution of mean flow is required. While such an accurate solution cannot be obtained by means of ordinary finite difference approximations, spectral approximations are able to surmount. Furthermore, the low inherent dissipation of spectral methods is not enough to stabilize the method in the presence of shocks. The spectral methods with shock-fitting have been shown to solve the in-viscid blunt body problem accurately and efficiently. In addition, errors decay exponentially fast with the increase in the grid points. In this study, the chebyshev spectral collocation method is used to solve Euler equations over cylinder and sphere in a supersonic flow fields. The axisymmetric and non-conservative form of Euler equations is considered as the governing equations. The sensitivity of spectral methods to boundary conditions, which can not be overstated, makes the boundary treatment the most difficult part of the solution. The Rankine-Hugoniot relations are used for shock boundary condition in accordance with a proper compatibility relation, which carry information from the flow field to the shock. The shock acceleration is derived similar to the method used by Kopriva but with different formulation to treat the shock boundary condition. Zero normal velocity along with compatibility relations in a matrix form is used as a boundary condition for solid body, which simplifies the implementation. Since the outflow is supersonic (all four characteristics are leaving the flow field), no explicit boundary conditions are necessary; therefore the governing equations are used to obtain the boundary variables. The solution is advanced in time using 4th order Runge-Kutta method.Finally, supersonic flows over cylinder and sphere are solved to show the capability of the present approach. The body surface quantities and the shock shapes compare well with previous investigators. In addition, the accuracy of the spectral collocation method is demonstrated.

Radial inflow gas turbines are widely used in diesel engines turbochargers. They increase power and efficiency and reduce SFC in engines. The flow conditions of the turbocharger’s turbine are highly varying, due to engine exhaust. The knowledge of turbine behavior in such conditions is a basic requirement for the development of turbocharger to improve engine performance. In this paper, the performance of the radial inflow gas turbine is investigated experimentally as well as advanced one dimensional modeling. The principal equation of a flow model is the dimensionless mass flow rate equation which combines the equations of continuity, energy and entropy with modeling the losses, under steady state and full admission conditions. In this study mass flow rate, pressure ratio and efficiency are unknown, with known turbine geometry, inlet and outlet total pressure and temperature of the turbine, performance characteristics can be calculated. In one-dimensional modeling the flow passage of radial flow turbine is divided into several regions such as: inlet duct, volute, incident and rotor, and the flow is modeled in each part separately. The requirement of the analytical procedure is to predict the component discharge conditions from known inlet conditions and component geometry. The computed discharge conditions then become known inlet conditions for the next component. Incident loss at the rotor inlet is the main cause of efficiency drop under off design conditions. In practice the best efficiency occurs at optimum incident angle, so any deviation from optimum incident angle causes extra losses. Incident loss is part of the rotor loss but in this study, due to modeling strategy it is modeled separately. In this study Wallace model have been adopted to calculate the incident loss, which is assumed that for the off design condition the change in deviation of the fluid entry the turbine rotor takes place at constant pressure. This model is compared with simple NASA model, which shows improving the turbine calculated performance results. The complex, three-dimensional flow pattern in the rotor gives raise the difficulties in the rotor modeling and causes the major turbine losses happening in the rotor. The following losses are used in this section: friction losses, blade loading losses, tip clearance losses and exit velocity losses. Experimental investigation of the research is carried out on special test facility under full admission conditions for a wide range of speed. The efficiency and mass parameter characteristics of the turbine are obtained from the modeling and are compared with that of experimental results over a wide rang of speeds showing good agreements. The main limitation in experimental data is due to the compressor surge. The maximum difference between experimental and theoretical results under full admission conditions is 5.1% for mass parameter, and 7.2% for efficiency.

Proper tolerancing of an assembly would presumably improve its entire lifecycle, including such aspects as function, cost, manufacturability and serviceability. The complex topological nature of most mechanical assemblies and the interrelations among their geometrical and dimensional tolerances have impeded the development of a comprehensive method for the tolerance analysis of such complex systems. Aside from the approximate, traditional tolerance analysis methods currently in use by the industry, a number of simulation-based methods have been proposed over the past few years that are potentially capable of predicting the tolerances of a target point in a mechanical assembly for a given set of component level tolerances. Most of these methods, however, have remained at the conceptual level and have not been implemented due to practical complications. It is understandably true that designers tend to use tight tolerances to ensure the proper functioning of an assembly; whereas manufacturers prefer loose tolerances which make the production easier. This is mainly because it is rather hard for the designer to have a thorough perception of the error-propagation mechanism and they would prefer tolerancing problems to be dealt with during process planning, tool selection or production phases.Over the past two decades, several attempts have been made to develop 2D and 3D tolerance analysis methods. Among these methods is the Virtual Joints (VJ) Method proposed by Laperriere in 1999 which substitutes a kinematic rotational or sliding joint for every dimensional and angular tolerance in the assembly model and in this way creates an equivalent virtual model. This model would then be able to predict tolerancing errors in any given point of an assembly using tolerance loops and dimensional and geometric tolerances. This paper presents a novel approach to the 3- D tolerance analysis of mechanical assemblies.The proposed approach is an enhancement to the original Virtual Joints method and would enable the automatic detection of assemblysurface locations and surface types at “assembly nodes” which, in turn, would lead to the immediate choice of equivalent kinematic joints to be used in the tolerance analysis algorithm. The VJ method builds on the premise that tolerance errors in a mechanical assembly could be represented by a set of small translational and/or rotational transformations, and thus generates a mathematical model of error propagation based on the accumulation of tolerance variations across tolerance loops and chains. Through replacing translational kinematic joints for longitudinal dimensional/geometric errors and rotational kinematic joints for rotational dimensional/geometric errors, an equivalent virtual assembly is created whose governing equations could predict overall tolerances at any desired point in the actual assembly. The enhanced algorithm is applied to an actual 3-D case to demonstrate the performance of the proposed approach.

In the central hole drilling method, the calibration factors relate the released strains to the residual stresses. In order to calculate the residual stresses for isotropic materials, two calibration factors from released strains are enough. However, for composite materials nine calibration factors are needed. These factors are presented in a matrix format and determining them for orthotropic materials is a tedious task. In this article, a new method for calculating the calibration factors for measuring the residual stresses in different material systems is presented. The simulated hole drilling method can be used instead of experimental techniques. In this method the process of hole drilling, using a finite element method, is simulated. The drilling location is simulated by a finite element technique and after applying the initial load in the form of residual stresses, the elements in the hole area are deleted from the model. Then, the strain around the hole area under the strain gages are calculated. The two and three dimensional simulations of the hole drilling method for isotropic materials are presented. The calibration factors are calculated and compared with those available in the standards. The results show a difference about 0.3% between the two methods. The simulation of the hole drilling process for the orthotropic materials, by different Poisson’s ratio, different shear stiffness and different module of elasticity is performed. For orthotropic materials, using the method presented in this study, the calibration coefficient matrix is obtained. The results are compared with the available analytical results. The consistency of the results show the reliability of the modeling process presented in this research. Also for laminated composite materials the presented method is utilized and the calibration coefficient matrix is obtained. Different laminated composites with various lay ups and materials are considered. Using the simulated hole drilling method and calculating the residual strains around the hole area and calculating the calibration coefficient matrix, the residual stresses due to curing process are calculated. In this study different laminated composites are simulated and by finding the calibration coefficient matrix, the residual stresses are obtained. The results are compared with the available experimental data and a very good correlation is obtained. The main advantage of the presented method is the capability to simulate the residual stresses in orthotropic materials with any degree of orthotropy. This method is able to simulate the behavior of different strain gages with different sizes and hole diameters. Simulation of the residual stresses for components and parts with complicated geometry is an application of this method. For very small components where measurement by with experimental methods is not possible, the SCHD method can calculate the calibration factors and therefore the residual stresses can be obtained. The calculation of the residual stresses in complicated specimens is another capability of the method presented in this study. Also, if the gradient of the variation of the residual stresses is too high, the SCHD University College of Engineering, University of Tehran 5 method is a suitable method to measure the residual stresses. Moreover, this method can be used to measure the calibration factors for calculation of non-uniform stresses through the thickness of the thick components.

In general, vibrations have been found to induce tube wear and failure in metal tubes. Because of this, vibrations in tube bundles are minimized in well designed heat exchangers. Recently tube bundles made of flexible material such as Teflon have been used in heat exchangers operating in corrosive environments like strong acids in high temperature and pressure conditions. However, another consequence of tube vibrations is higher heat transfer rate caused by increased fluid mixing.Thus, the use of heat exchangers made of Teflon and other plastics is becoming common practice for many industrial processes.The pressure drop and heat transfer coefficient in tube bundle of shell and tube heat exchangers are investigated considering viscous dissipation effects. The governing equations are solved numerically. Because of time dependent viscosity the equations should be solved simultaneously. The flexible tubes vibration is modelled in a quasi static method by taking the first tube of the row to be in 30 asymmetric positions with respect to the rest of the tubes which are assumed to be fixed and time averaging the steady state solutions corresponding to each one of these positions. The numerical method used for this problem is a finite deference method, upwind method, and applied for a group of five tubes in a row and can be generalized to bundles of more than five tubes in a square or triangular arrangement. It is observed that the eccentricity of the first tube significantly affects the velocity and temperature fields. The results show that the eccentricity of the first tube increases pressure drop and heat transfer coefficients significantly comparing to the case of rigid tube bundles, symmetrically placed. In addition, it has been shown that in spite of unfavorable effect of viscous dissipation on heat transfer coefficient, the overall heat transfer rate increases because of favourite effect of flexible tube vibrations. The results obtained from our numerical method have a good agreement with the available experimental data of constant viscosity for flexible tube heat exchangers and it shows that this numerical method is reliable despite its relative simplicity. Finally, because of higher heat transfer coefficient and lighter material and being strong enough to withstand moderately high temperature and pressure particularly in corrosive environment heat exchangers with this kind of material can be a good alternative for solid tube heat exchangers.

Plate fin-and-tube heat exchangers are employed in a wide variety of engineering applications like air-conditioning apparatus, process gas heaters, and coolers. In plate –fin and tube heat exchangers the fins forms a series of channels that create narrow passages for the external flow. Also the fins act as external surfaces to increase external heat transfer from the tubes in these heat exchangers. The corresponding heat transfer performance of the fin-and-tube heat Exchanger is highly dependent on the pattern of fins for its dominant thermal resistance for typical applications of air-cooled Heat exchangers; the air-side resistance generally comprises over 90% of the total thermal resistance. The fins are extended surfaces which enhance convective heat transfer to the over-tube fluid but at the same time they increase tube to tube conduction. This phenomenon is undesirable because it degrades the performance of heat exchanger. In this paper the effect of conduction heat transfer between the tubes of a plate-fin and tube heat exchanger by conduction through the fins has been studied. A two dimensional continuum model is developed to evaluate the fin conduction and conditions under which it may or may not be neglected are determined. The effect of parametric variation such as number of tubes and variation of convective heat transfer has been considered detail. Tube-to-tube conduction is found to degrade the performance of the heat exchanger. It is found that the same heat transfer area distributed in a different number of tubes affects its performance.

Springback is one of the key factors to influence the quality of drawn sheet metal parts in deep drawing process. Previous investigations about the springback have shown that, the amount of work-piece deformation can be affected by this phenomenon in the deep drawing process.Therefore, to optimise this manufacturing process and to increase the dimensional accuracy of the parts, it is essential to find out the effective parameters and bring the deep drawing process under producer control. In this paper, the finite element analysis of springback was applied and the effects of many physical parameters on the springback such as, type of materials, blankholder force, coefficient of friction and tool clearance were studied. Then, it was proposed how to decrease the effect of springback for different materials. For simplicity, two dimensional analyses (plane-strain simulation) were performed and the results were compared with experimental data. A good agreement was found between numerical and experimental results.

The performance and desirability of a passenger car is no longer assessed based on its in-gear accelerations, gradability and maximum speed alone, but also, and more importantly, based on its emissions and fuel economy. Of these various attributes, the first three may, fairly straightforwardly, be determined through analyzing the power flow from the prime mover to the driver tire(s) through driveline components; whereas the last two are quite difficult to calculate and predict. This is because they are the products of a complex mechano-chemical process involving an array of interconnected parameters. This paper reports on the optimal design of a passenger car’s transmission based on fuel consumption and vehicle performance metrics. Decision parameters affecting the choice of a transmission for a given engine are first introduced and modeled. To minimize an objective function that would encompass fuel economy and performance parameters and take into account the effects of suspension nonlinearities on vehicle directional response, an Elitist Genetic Algorithm is employed which can readily handle such discrete parameters as the preferred gear ratios. Evaluation of this objective function in each iteration of the optimization algorithm involves the solution of a set of non-linear differential equations to determine the performance metrics. To validate the proposed model, a 5-speed manual transmission for a 1.6 liter gasoline engine is designed and resulting values of the decision parameters including acceleration, top speed, gear-shift points, deviation from optimal power and fuel consumption are compared with experimental results. Using the proposed model, and defining an objective function to include performance- and fuel consumption metrics, ratios for the 2nd, 3rd and 4th gears are then determined using the elitist genetic algorithm.

Tube hydroforming as a new metal forming process, has attracted the attention of many industries especially automotive industry. In this study finite element simulation of tube hydroforming of a tee joint was performed with use of a commercial code ABAQUS/EXPLICIT 6.3-1 and the effects of the key parameters such as friction coefficient, strain hardening exponent and the fillet radius on parameters, protrusion height, thickness distribution and clamping force have been analyzed. Also the relation between axial feeding and axial force which is very applicable for the designer has been presented. By the use of this curve and the clamping force curve one can determine the capacity of the side jacks and the press. Then it was shown that there is a good agreement between the results of the experiment and finite element simulation. So one can use the FEM to design the process for complicated pieces and reduce the number of costly trials and errors. At the end, some important experimental facts about the velocity of the side pistons, surface finishing and counter punch position and pressure were given. Also it is shown that the coefficient of friction affects axial force more than claming force and has most dominant effect on the protrusion height and thickness distribution. As results by using the parametric studies presented in this paper a designer can gain an understanding of how each parameter affects the product.

The accurate computation of helicopter rotor flow leads to an accurate calculation of rotor blade aerodynamic loads. The flowfield around a rotor is difficult to model due to the presence of vortical wake. The complexity of the rotor flowfield results from the effects of strong vortical wake and primary vortical structures that are convected away from the rotor disk at relatively low speeds. In the present work, a free wake vortex lattice method is used to accurately predict the vortical wake and blade loading of an isolated rotor in hover. In the vortex lattice method, the blades are modeled as flat plates with zero thickness and ring vortices are distributed on the surface of each blade. When the blades rotate, the vortices are shed into the wake and freely moved with a velocity induced by the effects of the vortices located on the blades and in the wake. The wake can freely deform over time to take its natural shape. The distribution of lift can be computed from the strength of each vortex on the blade. The aerodynamic analysis of a wing in oscillating motion for different reduced frequencies is performed to show the accuracy of the free wake vortex lattice method for unsteady calculations. Then, the aerodynamic calculations are performed for a hovering helicopter rotor for two operating conditions of subsonic and transonic tip Mach numbers (Mtip = 0.44, 0.88). To accurately compute the compressible flow case, the Prandtl- Glauert correction is applied to the Biot-Savart Law. The present calculations based on the free wake vortex lattice method are compared with available theoretical and experimental results. The effects of number of panels, vortex core radius and also computational time step on the aerodynamic results and wake structure are also studied. Finally, the capability of the free wake vortex lattice method for aerodynamic analysis of the rotor blades with twist is demonstrated.

Recently the metal-bonded diamond tools have been found dramatic applications for the processing of natural stone in civil engineering. Although, significant improvement in tool performance have been made, however diamond tools are still the main factor in order to achieve the economic and optimum sawing conditions.The influence of machining parameters on diamond tool wear during sawing a kind of granite stone belongs to Kezab region of Yazd city in Iran was investigated. A machine tool was designed and made for this purpose. A series of tests were scheduled. The radial wear of the saw-blade was measured by using a micrometer at the end of experiments. The cutting forces during sawing were measured with using of mechanical dynamometer. The results show that with the increase of feed rate the tool wear reduces. This may be due to the reduction of cutting forces and/or the reduction of cutting temperature. On the other hand, with the increase of cutting speed the tool wear also increases. This may be because of rising the temperature and mechanical impacts during machining. These factors affect on the diamond grains and the cobalt binder of the segments.