مجله مهندسی مکانیک
پیاپی 149 (خرداد و تیر 1402)

The purpose of this research is economic thermodynamic optimization of flat plate solar collector and investigation of transient heat transfer. In this way, modeling and optimization are done for the worst conditions in terms of radiation. The research method in this article is optimization with the help of multi-objective genetic algorithm (NSGA-2) which is done with MATLAB software. The main components are the objective functions, constraints and design variables, which are the objective functions of collector efficiency and annual total price. As the results, the effect of increasing the length of the collector on the thermodynamic efficiency and the total annual price, which shows that with the increase in length, the efficiency decreases and the total annual price increases. In addition, with the increase in the width of the collector, the thermodynamic efficiency first increases and then decreases. and the total annual price increases. Also, with the increase in the number of pipes, the total annual price decreases and the thermodynamic efficiency first increases and then decreases. It is done on four different days.

Structural Health Monitoring (SHM) is a method in which the structural health is monitored in order to detect damages before catastrophic failure. In composite laminates, the layer-by-layer fabrication process makes it possible to embed strain sensors between the layers to make the part intelligent with respect to the incoming loads and to obtain information about the stress state inside the structure. The innovation of the present article is to use the health monitoring system for sandwich structures with lattice core in such a way that it would be economical. For this purpose, during the construction of the sample, a strain gauge is embedded between the core and the shell to monitor the common failure mode in this type of structure, i.e. the delamination of the shell from the core. The results of this research show that it is possible to make the sandwich panel smart and monitor its health using the present method in the elastic range and in quasi-static loading with good accuracy. Outside of the elastic range, the data obtained from the strain gauge is mainly affected by the damage and failure near the strain gauge.

Investigating the flow around a circular cylinder is one of the classical problems of fluid mechanics. Unsteady forces formed behind the cylinder are caused by vortices separated from its surface. Controlling the intensity of vortex shedding leads to controlling the intensity of forces caused by vortices. One of equipment that is used to reduce fluctuations caused by vortices is trip wires. Therefore, in the present study, the fluctuating velocities, Reynolds shear stress and drag coefficient around the circular cylinder with the trip wire have been investigated at different Reynolds numbers. For this purpose, the diameter of the circular cylinder is 20 mm and the diameter of the installed trip wires is 0.25, 0.5, 0.75, 1, 1.5 mm. Trip wires are installed at θ=±40 and ±140 positions. The results show that the use of trip wires with a diameter of 0.25mm causes a decrease of 68%, 50% and 55% of fluctuating velocities (u', v', w') and a decrease of 85% Reynolds shear stress compared to the smooth cylinder. Also, the presence of a peak in the fluctuating velocity indicates the presence of large vortices behind the cylinder. Reynolds shear stress has a direct relationship with the diameter of the trip wire.

Optimization of Ferritic-pearlitic steel properties is one of the most important aims of the researchers in this study branch due to increasing use of these steels in industry applications.Therefore, improving their final microstructure is necessary to enhance the level of the mechanical properties. On the other hand, employing some manufacturing methods such as rolling can improve the microstructure and the mechanical properties in turn. In this research, the hot rolling operation and quenching in two different environments was carried out on CK45 steel. The samples were rolled in different temperatures and different rotational speeds of work-rolls under the same reduction percentage. They were then quickly cooled down in room atmosphere and cold water and the final microstructure was examined using of optical microscope. The results show that in hot rolling process, over 850○C temperatures, the microstructure is always in the form of martensite phase after rolling and cooling in cold water and the microstructure gets smaller even with increasing the temperatures. About the samples which are cooled in room atmosphere and with increasing the temperature, there is enough time for growth and forming continuous background of ferrite phase’s in the sample, so the samples return to their before rolling state. However, in low temperatures and high speeds, because of high strain rate, material has not enough time for retrieving and forming ferrite phase and the ferrite phase is formed only in rolling direction.

In the industry, MF-10-60-GRZ welding wire is used according to DIN8555 standard for surface hardening of the guides with the welding method. Cracks called stress release cracks or check cracks can be observed in the welds that have been created by this welding wire. The existence of a longitudinal crack more than the standard leads to reconstructions or rejection of the part. The purpose of this article is to review the various solutions used to remove longitudinal cracks created during surface hardening. In this research, the methods of Submerged Arc Welding (SAW) and Flux Cored Arc Welding (FCAW) were used. The results of surface hardening with different methods showed that there is no practical way to reach a hardened layer of 15 mm, and the only reliable way is to create the low-carbon steel layers (welded metal deposited using ER70s-6) between the hard layers (high-alloy cast iron).

In this research, by reviewing more than 50 authentic and up-to-date international articles, the advantages and challenges of fuel cell cars have been stated. Also, while presenting the latest statistics of the fuel cell car market in the world, the solutions of different countries to solve the challenges and development of these cars, as well as predicting their future, have been mentioned. Many countries have announced the years 2030 to 2050 as the beginning of banning the sale of internal combustion vehicles. One of the important challenges of the fuel cell car is hydrogen production. According to the European Union report, the average cost of producing hydrogen from fossil fuel is 1.8 dollars per kilogram, while the cost of producing hydrogen from renewable energy is between 3 and 6.6 dollars per kilogram. By reducing the cost of hydrogen production through the use of renewable energies, the ideal goal of low cost and low emissions can be achieved, so that according to the roadmap of the International Energy Agency (IEA) in 2050, the cost of fuel cell vehicles will be very close to gasoline cars and the market share of fuel cell cars will be around 17%. Currently, the allocation of government subsidies is very colorful. For example, we can mention a 65% discount in California for the sale of Toyota Mirai and a 50% discount in South Korea for the sale of Hyundai NEXO.