MODARES TECHNICAL AND ENGINEERING
Year:2010 | Volume:- | Issue:37 (SPECIAL ISSUE ON MECHANICAL ENGINEERING)|Papers Count:13

Radiative heat transfer must be considered in retail refrigerators with glass doors. Some methods have been proposed for reducing radiative losses, like usage of double glased windows filled with argon or other types of transparent material with low emissivity in infrared band. For evaluation of thermal radiation in commercial refrigerator compared to infiltration loss, thermal conduction and facilities loss, a three dimensional model has been developed. In this model all wall surfaces are isothermal and evaporator with specific temperature located in roof. The radiative properties of glass are considered as actual.The results show that decreasing internal temperature of cabinet incentive radiation losses. These losses are almost independent of window surface temperature. Increasing of emissivity factor of evaporator, causes increasing of thermal radiation flux of evaporator which can be used in design improvement. Radiation flux of each surface have been compared with convection flux. This comparison show the importance of covering windows in unutilized times.

In this paper a new approach for forming sheet metals by explosion of gas mixtures is presented. As the sheet metal shapes by the impact and pressure resulted from the explosion, it undergoes through a plastic deformation phase. Testing apparatus which is built for the first time in Iran, consists of a thick-walled cylinder (expolsion chamber), various dies for shaping, and measuring instruments.Unlike techniques used in conventional systems, in this method, the wave impact acts as the male part of the die to form and produce different engineering components.Experimental results presented show the effect of various parameters such as thickness, boundary conditions, and the material type of the work-piece, also the percentage of gas mixture on the distribution of thickness/circumferential strain in the work-piece. Furthermore, an analytical model based on the plastic work calculations for the sheet metal deformation is presented.

Considering frictional, inertial and machining forces, the authors have presented an enhanced analysis of a hexapod table as used in milling machines. The Newton-Euler analysis of hexapod’s components has been implemented by a simulation program developed by the authors in MATLAB environment and the results have been verified by those of others. The impact of various loads involved in machining operation carried out on a milling machine equipped with hexapod table has been presented in the paper. This provides a potential machine tool designer with guidelines on the importance of these loads and helps him give appropriate weights to them.

In cold season, draught or undesired local cooling sensation in ankle and neck region is one of the most frequent cases of complaint of the occupants. A person who are subjected to draughts in winter, tend to elevate the room temperature to counteract the cooling sensation, thereby increasing the energy consumption. In naturally ventilated buildings, draught is due to windows and other cold surfaces in the room. Draught is dependent on the air speed and on the magnitude of turbulence intensity. Serious draught complaints can often occur at mean speeds lower than those recommended by standards when turbulence intensity is high. So investigation of undesired local cooling in floor heating systems is very important, although in these systems the mean air speed is not significant. In this paper, the effects of size of window on draught are investigated in floor heating systems. Results demonstrated that, undesired thermal discomfort caused by local cooling phenomenon in floor heating systems is negligible. At the end, the probability of occurrence of local cooling phenomenon in floor heating systems is compared to vertical heating panels. Thereby the floor heating systems are more effective than the vertical heating panels in aspect of thermal comfort and energy consumption.

This paper reports the growth analysis of a preexisting axial external surface crack in a thin cylindrical tube under gas detonation loading using the finite element method. The loading consists of a shock wave with pressure peak of 6 MPa that travels at 2.4 km/s. First, numerical modeling of the structural response of the tube to detonation loading was performed using WARP3D, which is a 3D nonlinear-dynamic fracture simulation code using FEM. Modeling of crack growth in the thin-walled tube was based on CTOA as the fracture parameter. The final part of the paper is devoted to comparisions between the numerical crack growth simulations and the experimental results reported in the literature. The differences between the full dynamic and the quasi static crack growth simulations are highlighted.

A honeycomb panel consists of an array of open hexagonal cells which their walls are perpendicular to face sheets although other panel sandwiches don’t have these perpendicular walls. Their design is often performed based on minimum weight. This research is aimed at minimizations of weight by means of computing honeycomb core girth. Weight optimization is done by means of Naive and numerical procedures. Numerical optimization is done by the sequential quadratic programming (SQP) method. Geometric parameters and optimized weight are calculated for hexagonal and square cells. Optimized weights for these two cross-sections are compared.

In this study, in addition to forming a steel tube of D6AC by direct flowforming, in order to examine the effects of different parameters of heat treatment process on the a for mentioned part properties, different cycles of heat treatment process are designed and after doing so, mechanical properties and microstructural changes of flowformed tube are investigated. For this purpose, at 850, 900 and 950oC austenitizing temperatures for 15 and 30 minutes and AUS-BAY quenching at 510oC for 10 minutes, and then quenching at 60oC oil are used. Initial tempering for 2 hours at 250oC and final tempering for 2 hours at 250, 300, 350 and 450oC was performed. Investigations show that the optimum heat treatment cycle is austenitizing at 850oC for 30 minutes, and final tempering at 300oC, because of creating high strength and suitability for fracture toughness. Due to small grains of initial austenite, the ductility properties are also adequate.

In this paper linear aeroelastic analysis of a swept wing with two degrees of freedom in an incompressible flow is investigated in time - domain. The equations of the motion of an elastic wing are derived from Lagrange’s equations in time - domain. The wing is modeled as a cantilever beam rigidly connected to the root. Considering assumed modes of cantilever beam, aerodynamic forces and moments acting on the wing are derived using strip - theory in an unsteady incompressible potential fluid flow. The governing aeroelastic equations of the system have been introduced in dimensionless form. These equations are solved via a numerical method. Comparisons between obtained results and both available experimental data and the results of some cited references indicate a close agreement.

There are numerous applications for gas turbine discs in the aerospace industries such as in turbojet engines. These discs normally work under high temperature while subjected to high angular velocities. Minimizing the weight of such items in aerospace applications results in benefits such as low dead weights and lower costs.Optimization of rotating discs is historically, an area of research due to their vast utilization in industry. The gas turbine disc is one of examples to name. Gas turbine discs work mostly at high temperature gradients and are subjected to high angular velocities. High speed results in large centrifugal forces in disc and simultaneous high temperatures reduces the disc material strength, thus the later increases stress in disc automatically.In order to obtain a reliable disc analysis and arrive at the corresponding correct stress distribution, solution should consider changes in material properties due to the temperature field throughout the disc. To this end, an inhomogeneous disc model with variable thickness is considered. Using the variable material properties method, stresses are obtained for the disc under rotation and a steady temperature field. In this paper this is done by modeling the rotating disc as a series of different rings with constant properties. The optimum disc profile is arrived at by sequentially proportioning the thickness of each ring to satisfy stress requirements.In this paper these are done using the simplex method. Simplex algorithm is applied in Ansys software and the results are presented.

In this study, pulsating hydroforming of tube in a box die is simulated using the three dimensional finite element method to investigate the mechanism of improvement of the corner filling. In addition, the results obtained from the simulation are compared with experimental observations, and the validity of the simulation results is verified. Based on a better understanding of the mechanism of improvement of the corner filling, a new pulsating pressure path is proposed to increase the corner filling. It has been shown that the proposed pulsating pressure path is more effective in increasing the corner filling of the box shape tube hydroforming process.

This paper presents an analytical solution for steady state conductive heat transfer in a cylindrical composite laminate. The results of this solution can be pretty useful in investigating heat transfer in pipes and reservoirs. In this research, tensor of thermal conductivity coefficients for composite materials is presented and the procedure of determination of the coefficients is described based on the properties of fibers and matrix material. Then, the equation of heat transfer of composite materials has been determined in cylindrical coordinates. The research has been done for conditions that fibers are wound around the cylinder and the heat transfer equation has been solved via separation of variables method.

In recent years many researchers have been focused on active magnetic bearing (AMB) systems. AMB offers advantages such as, contact less and friction free operation, excellent performance over a wide range of temperature, no need for lubricant and longer life. Technology of magnetic bearings can be considered as a new field of research in Iran. This paper presents design, manufacturing and control of a magnetically levitation system with successful operation. This research concluded with documentation of the AMB technology which is prerequisite for earning the technology of active magnetic systems (AMS) and paves the way to develop it.