Journal of Advanced Materials in Engineering (Esteghlal)

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Refrigerants that are used in refrigeration systems, cause environmental difficulties such as depletion of ozone layer and greenhouse effect. Thus, we must find substitutes for such refrigerants, in this paper, the effect of refrigerant on the thermodynamics performance of refrigeration cycles are studied. To do this, a refrigerator that has designed for R12, is simulated. Results show that pure refrigerant R134a and mixture R32/R132b are convenient substitutes for R12.

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In this study, turbulent flow around a tube bundle in non-orthogonal grid is simulated using the Large Eddy Simulation (LES) technique and parallelization of fully coupled Navier – Stokes (NS) equations. To model the small eddies, the Smagorinsky and a mixed model was used. This model represents the effect of dissipation and the grid-scale and subgrid-scale interactions. The fully coupled NS equations with the multiblock method was parallelized. Parallelization of the computer code was accomplished by splitting the calculation domain into several subdomains and using different processors in such a way that the computational work was equally distributed among processors. The discretized governing equations are second order in time and in space and the pressure is calculated by Momentum Interpolation Method (MIM) to prevent the checkerboard problem. The results are obtained for the turbulent flow over five parallel tube rows. The computational efficiency, flow patterns, and flow properties are also determined. The results showed high parallelization efficiency and high speed up for the computer code. The flow characteristics were determined and compared with experimental results which showed good agreement. Also, the results showed that the mixed model is better than the Smagorinsky model for evaluation of flow characteristics and lift and drag forces on tubes.

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In this research, a 3-D mathematical model is developed for simulating electromagnetic continuous removal of inclusions from molten metals. The model includes the computation of electromagnetic force field and fluid flow in the presence of electromagnetic forces. The results of flow field together with electromagnetic force field were further used for calculating the trajectory of inclusions in the molten metal. Parametric studies were performed to evaluate the effects of various parameters such as magnetic field intensity, inclusion size, and fluid velocity on inclusion removal efficiency in molten magnesium. In order to verify the mathematical model and visualize the trajectories of particles in the melt flow under electromagnetic force, a physical model was constructed. The predicted particle trajectories and separation in the physical model were compared with those obtained from experiments, which showed a relatively good agreement.

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In this study, Al/Steel multilayer composite was produced by accumulative roll bonding (ARB) process using Al-1100 and St-12 strips. Microstructure, mechanical properties and corrosion behavior of the composite were studied by scanning electron microscopy (SEM), tensile test, Vickers microhardness tests, cyclic polarization and electrochemical impedance spectroscopy (EIS) measurement in 3.5 wt% NaCl solution. After one ARB cycle (2 roll-bonding cycles), the multilayer composite of 4 layers of Al and 2 layers of steel was produced. The tensile strength of the Al/steel multilayer composite reached 390.57 MPa after the first ARB cycle, which was 1.29 times larger than that of the starting steel while composite density was almost half the density of the steel. Corrosion behavior of the composite revealed a considerable improvement in the main electrochemical parameters, as a result of enhancing influence of cold rolling. The results indicated that strength and corrosion resistance of Al/steel composite generally decreases and elongation increases after annealing.

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In the present study, pure Aluminum powder with 5%wt Titanium Dioxide was mechanically milled at different times. Using phase analysis through X-ray diffraction (XRD), it was found that increasing of the milling times over 10 hours causes the reduction of Titanium by Aluminum and formation of Al2O3 in the structure. Also, it was shown that if the process persists, Aluminum reacts with Titanium and causes the formation of Al3Ti in the composition. The reactions were studied through the thermodynamic relations. Furthermore, after distribution of reinforcement particles in the matrix, using X-ray diffraction peak broadening, according to Williamson-Hall equation, the mean crystallite size and lattice strain were determined, and by scanning electron microscopy (SEM), the structure and morphology of the powder particles were studied.

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In the present study, silica aerogel was evaluated by a two-step catalytic process at the ambient pressure drying, under different synthesis conditions. The effects of  the catalyst and water content in the hydrolysis step on the physical properties of silica aerogel, including density, porosity and shrinkage, were investigated. The results showed that increasing the water content in the hydrolysis step increased the shrinkage of gel network and density of obtained aerogel. Moreover, in the presence of insufficient water, NH₄OH as the condensation catalyst in the gel formation step was more effective on the physical properties of silica aerogel, as compared to HCl as  thehydrolysis catalyst; Moreover, the increase in the NH₄OH content led to lower density and higher porosity. On the other hand, NH₄OH effect on the physical properties of silica aerogel was not noticeable in the presence of enough water content. In the NH₄OH/HCl molar ratio of 6, the best silica aerogel sample was obtained with the density of 0.214 g/cm³, porosity of 90% and shrinkage of 23%

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