نشریه مهندسی مکانیک مدرس
سال بیست و یکم شماره 5 (اردیبهشت 1400)

Supplementary firing system is one of the common methods of increasing the power generated of combined cycle units. Low cost of investment to the rate of increase of the generative power has encouraged the designers to use this method in the above power plants. In this article, field study of the performance changes of a real combined cycle unit with and without supplementary firing has been performed from energy and exergy viewpoints. Studies show that in all operation modes subject to research, using supplementary firing causes an increase in power generation up to 26.3MW, energy efficiency of steam cycle about 2.43% and decreases the exergy destruction of steam flow control valves. But this system has a negative impact on energy and exergy efficiencies of the whole combined cycle, which at the worth case about 1.17% decreases energy efficiency. In addition, it was specified that the power plants operation in the partial loads causes high exergy destruction in the cycle which steam cycle increased energy and exergy efficiencies due to use   supplementary firing system cannot fully compensate it.

In the current research study, the large plastic deformation and failure mechanism of sandwich structures with aluminum facesheets and pumice core under low-velocity impact loading have been investigated. The drop hammer testing machine was used to apply the impact load on the specimen at seven different energy levels 34.3, 68.6, 102.9, 137.2, 171.5, 205.8, and 223 J. To achieve the mentioned energy levels, the weight of the impactor was considered constant and equal to 3.5 kg and the standoff distance of the hammer was changed from 1 to 6.5 m. 16 experimental samples were considered in two types of layering with and without pumice core. Also, in this series of experiments, the thickness of facesheets was fixed and two different thicknesses of 16 and 32 mm were considered for the core. Experimental results showed that at all energy levels, the sandwich panel with 16 mm pumice core shows a smaller deformed area due to the shorter porous space between two facesheets. Also, at low energy levels, the thickness of the pumice core does not play a key role in improving the impact resistance of the structure. Compared to the coreless sandwich structure, the use of a very low-mass pumice core can prevent the plastic deformation of the rear facesheet, and the 16 mm thickness of the pumice core can even prevent the front surface from petalling.

Nowadays, torsion springs are used widely in aerospace equipments. In most of gravity gradient booms and some of solar panel extruders, torsion springs plays an important role in expanding these mechanisms.  Since these mechanisms are constantly exposed to severe thermal gradients during orbital circulation and because of sensitivity of these mechanisms, it is important to identify the behavior of their elements under temperature variations. Therefore, in this study, the effect of temperature and its variation on these springs and their performance are investigated analytically and experimentally. To investigate the torsional behavior of the spring, an specific test setup is designed and constructed. This test train includes power transmission, temperature control chamber, temperature controller and torque meter. Torsion springs are selected from hard drawn spring steel. At the temperature range of  to  ,the effect of temperature variation on the produced torque due to different forced torsion angles are investigated. As well as the spring constant variation with temperature is obtained and compared with analytically obtained data. The obtained results validate the consistency and accuracy of the analytical modeling and the applied experimental test train and procedure.

Increasing heat transfer and preventing sedimentation in equipment have always attracted the attention of engineers. In this work, the variations of salt concentration were effective on bubble diameter, departure frequency and generation points and its sediments acted as a heat transfer resistance. Therefore, first, the effect of ultrasonic waves on salt sedimentations in pool boiling was investigated. The results revealed that the ultrasonic waves had positive effect by suspending the soluble particles in the fluid and preventing them from precipitating on the surface of heat transfer. Increasing turbulences and perturbations due to changes in bubble dynamic and cavitation phenomenium, led to improve the heat transfer coefficient, significantly. The role of roughness on the surface heat transfer in bubble production was other investigation of the work. Bubble production by increasing the roughness with ultrasonic wave’s irradiation had direct and important effects on enhancing the heat transfer. Finally, salt and nanofluid sediments were compared. The nanoparticles precipitate faster and more easily under the bubble layer, but less in the salt solution if its dissolution is maintained. The ultrasonic waves were employed at three powers of 30%, 60% and 90%. Finally, the heat transfer coefficient and bubble departure diameter increased as 8.43% and 7.54%, respectively. In addition, the sedimentation decreased by 37.19%. As a result, the waves reduced their deposition by preserving salt dissolution.

In forming conical parts by traditional deep drawing techniques, due to the stress concentration at the contact area between the punch and the workpiece, thinning and rupture occurs on the sheet. There is also a high possibility of wrinkling in the free area of ​​the sheet; where there is no contact between the punch and the sheet. Therefore, new methods have been examined in forming this group of parts. Electromagnetic forming is one of the relatively old methods of high-speed forming that has attracted more attention in recent years. In the present study, the process of pre-forming of aluminum conical parts using electromagnetic force has been discussed numerically and experimentally. First, experiments were carried out by a simple spiral coil and after confirming the validity of the numerical simulations, the effect of electromagnetic force density in radial and axial directions was investigated in different areas of the sheet. Using the obtained results, a new coil was designed and built that has the ability to provide suitable distribution of the force in the radial and axial directions. Reduction in power consumption by up to a quarter, an increase in the amount of radial inward force and the height of the preform formed cone up to 2 times, minimizing the friction force, reduction of the workpiece center thinning by 3% (while increasing the height by 2 times) and elimination of wrinkles in the flange area of the sheet are the advantages of using the new coil compared to the primary coil.

In this research, vinyl ester matrix composite coatings reinforced by E-glass fibers, Nano TiO2, and Carbon Nanofiber were prepared by hand lay-up method and their mechanical properties were investigated. The mechanical properties of fiber-reinforced composites were investigated by the tensile, impact, hardness, shear test, and wear tests. Scanning electron microscopy was employed in order to study the fracture surface of the prepared samples. The results of the tensile test showed that the presence of the E-glass fibers in the vinyl ester matrix increases the strength about 4 times and the elongation about 8 times. There was no change in the fiber-reinforced composite strength by reinforcing the composite with nanoparticles of TiO2 and carbon Nanofiber, but the elongation of the fiber-reinforced composite increased by 1.6 times. Impact resistance of fiber-reinforced composite and fiber-reinforced nanocomposite relative to vinyl ester resin increased about 20 and 29 times. The presence of glass fiber and Nanoparticles in the vinyl ester matrix increases the hardness of the samples about 1.5 to 2 times. The results of the adhesion test demonstrated that the presence of nanoparticles in fiber-reinforced nanocomposite improves adhesion to concrete surfaces. Also, the results of the wear test showed that the presence of glass fiber in the matrix of vinyl ester reduces wear resistance and the presence of Nanoparticles in fiber-reinforced nanocomposite improves wear resistance of the fiber-reinforced composite.