نشریه مهندسی مکانیک مدرس
سال بیست و سوم شماره 12 (آذر 1402)

Among solar collectors, solar parabolic dishes have attracted the attention of researchers due to their high working temperature and high thermal performance. The purpose of this study is to compare two absorber receivers (normal-black color) for the solar parabolic dish. The analysis of energy and exergy was investigated for different conditions at different hours. Two-axis tracking system was applied to the outlet temperature in the solar parabolic dish system. The results show that the maximum temperature of the normal and black absorber is 101.52 and 115.53 ℃ , respectively, and the maximum energy and exergy efficiency of the black spiral absorber plate is 0.7 and 0.21. In addition, the designed parabolic solar container raised the temperature of 80 liters of water to 60 ℃ in 5 hours after sunlight, which is suitable for bathing and washing clothes in winter in backward and rural areas without electricity and fuel. Therefore, it is enough for four to five people.

At the end of the forming process, when the part is removed from the mandrel and the matrix and the part is loaded, a deformation occurs in the part, and this deformation after forming is called spring back. This research was carried out in order to experimentally determine the spring return of stainless steel 316 and carbon steel ST37 with different thicknesses in C-die forming and compare it with finite element simulation. The parts with three thicknesses of 1, 1.5, and 2 mm, forming and the geometrical dimensions of the spring back of the sheets have been verified. The results showed that the experimental spring back in the finite element simulation is consistent with the test, and also by reducing the thickness, spring back increases, which is affected by perfect elastic zone and surface plastic strain and membrane and bending stress.

Applying ultrasonic vibrations (UV) in manufacturing processes has resulted in various significant improvements. This study investigates longitudinal UV effect on the performance of electric hot incremental forming (EHIF) process of Ti–6Al–4 V sheets. In this technique, UV with a high frequency and low axial amplitude were combined with EHIF. Required devices were designed, manufactured, and used for performing the EHIF process. In this mechanism, UV were transmitted to a rotary forming tool attached to a CNC machine spindle. Hyperbolic geometry was fabricated to find out the most effective parameters values. Design of experiment and analyses of variance (ANOVA) were employed to identify optimum conditions for effective parameters. These parameters were optimized by response surface methodology (RSM).  Alternating among various input values of EHIF parameters including feed rate, pitch, and current has affected output parameter values such as surface quality and maximum forming angle were investigated in both conventional-EHIF and ultrasonic-assisted EHIF processes. Experimental results have demonstrated that combination of EHIF with UV has resulted in improving surface quality. Also, XRD (X-ray diffraction) phase analysis showed that the β phase (BCC structure) was increased due to ultrasonic stimulation. Intergranular heating has been done in Ti-6Al-4V alloy. The results have proved that the transformation from α phase to β phase has taken place in a faster and simpler manner. This occurrence has changed the phase composition from a dominant percentage of alpha phase (α) and HCP structure to a combination with a higher percentage of beta phase (β) and BCC structure. This phenomenon has improved the formability while it has increased the maximum forming angle about 25% ~ 30%

In this research, the deformation of circular metal sandwich panels with vertical tube cores under blast load has been investigated numerically and experimentally. The relationship of energy balance in different components of the structure has been considered. The core tubes are installed in a cross arrangement and vertically with the same height between the upper and lower sheets of the sandwich structure. The amount of energy absorbed by the cores is determined according to their location in the structure and the effect of their number and diameter. The grouping of the desired tests for this research has been done according to the thickness of the sheet 1.2 and 2 mm and with aluminum cores with diameters of 12 and 16 (mm). Numerical simulation has been done in the form of free explosion and by defining the pressure function using the Conwep method in Abaqus software. To validate the numerical results, experimental tests have been carried out with the construction of sandwich structure. In both methods, the maximum lateral displacement of the structure at its center and the displacement in terms of distance from the center of the structure, at cores location have been measured. Increased number of tubes in the core of the structure decreased the maximum rise in the upper layer and decreased the transverse displacement of the lower sheet. Structures with fewer cores and less sheet thickness showed more energy absorption. The average difference between the results of numerical and experimental methods was approximately 11%.

To investigate the effect of relative humidity percentage on heat transfer and distribution of droplets in the condensation phenomenon, a test device with the ability to provide and control different environmental conditions was made, and therefore, the hydrophilic (copper) and hydrophobic (Teflon coating on copper) surfaces were measured under controlled environmental conditions. In all the tests, the inlet air flow rate, inlet air temperature, air temperature reaching the test surface, water temperature, water surface height, and test surface temperature were kept constant at specific values using PID control. Each test's relative humidity values of 80, 88, and 96% have been determined and controlled. The results of the transient investigation of heat transfer show that it takes time for the condensation phenomenon to occur, and the higher the surface hydrophilicity and relative humidity, the shorter this time will be. Also, the average heat transfer for 60 minutes was calculated. It showed that the average heat transfer coefficient increases with increasing humidity. Under the same environmental conditions, the heat transfer coefficient on hydrophilic surfaces is higher than on hydrophobic ones. In the graphical analysis of the droplet size, it has been observed that the most oversized droplets on hydrophilic surfaces at relative humidities of 88 and 96% are in the hydraulic diameter range of 0.35 to 0.4, and on hydrophobic surfaces are at relative humidities of 80 and 88% in the hydraulic diameter range of 0.2 to 0.25 mm.

Waspaloy is a type of nickel-based superalloy that is mainly used in aircraft turbine parts, compressor disks, shafts, and turbine parts. Waspaloy, like many nickel base superalloys, is difficult to machine at room temperature (conventional turning). In this paper, the cutting and feeding forces and cutting temperature have been evaluated by changing rotational speed, feed rate and constant depth of cut, in the dry oblique turning process of Waspaloy. The workpiece's diameter and hardness were 25 mm and 385±10 Vickers, respectively. In order to investigate of the cutting force, temperature and the surface roughness, a full factorial design experiment was used, and a regression model of the influencing factors was presented. Moreover, the surfaces roughness and micro hardness of the machined parts were evaluated. The results showed that the hardness on the surface of the workpiece increases with the increase of the cutting depth and the feed rate.