مجله مهندسی مکانیک
سال چهلم شماره 1 (پیاپی 59، بهار و تابستان 1389)

In this study, flow and heat transfer characteristics of Al2O3/water nanofluid, flowing through a 90° curved pipe for the both boundary conditions of constant wall heat flux and temperature have been numerically investigated. The incompressible and laminar Navier-Stokes and energy equations are solved in a body fitted coordinates system using a control volume technique. The effects of nanoparticles concentrations and centrifugal forces on the flow pattern, Nusselt number and wall shear stress have been studied. The results indicated that the curvature and nanofluids effects improve the heat transfer characteristics, dramatically. Furthermore, the results showed that in order to obtain the optimum operating conditions of nanofluids, different parameters such as heat transfer enhancement, pressure drop and erosion of pipe wall must be considered.

In this study, validity and accuracy of the particle tracking method for estimating the distribution of recirculated gases between intake manifold runners have been investigated. For this purpose, several experiments have been performed on EF7-TC engine on various working conditions and with different rates of recirculated gas injection. The experimental method is designed based on the LPG gas injection and its effect on the amount of λ of each cylinder exhaust gases. Contribution of each cylinder of the injected gas is determined by performing a simple calculation on the measured values obtained from experiments. In addition the numerical results are calculated by using the boundary conditions derived from 1-D model which is calibrated with the EF7-TC engine. In order to better compare and evaluate the results, based on experiences of previous researches, four different injection points on the intake manifold was considered. Results comparison showed the particle tracking method can qualitatively predict the distribution of recirculated gases between the intake manifold runners with a high confidence level. Only one qualitative difference between experimental and numerical results is observed. Besides, it is revealed that with decreasing the recirculated gas rate the accuracy of particle tracking method is enhanced and differences between experimental and numerical results are decreased. In addition, by comparing the results of four injection points it was showed that the particle tracking method is more accurate for injection points which distribute recirculated gases more uniformly.

In this research, a fixed-grid finite volume numerical approach is developed and used to simulate unsteady natural convection flow during freezing of water in a circular enclosure. This approach is based on the enthalpy-porosity method which is used to track the motion of the liquid-solid front and to obtain the freezing length and time of the solidification. The Navier-Stokes equations are solved on a staggered mesh by pressure-based implicit procedure. Results of the solidification are then validated against published results. Findings show a remarkable quality of the simulation of solidification problems.

In this paper, mathematical modeling and simulation of the counter flow cooling tower were performed by using heat and mass transfer equations without using simplified assumptions. Governing differential equations for air and water in the cooling tower were solved by numerical method and the results are validated by experimental results available in the literature. Also, experimental tests were performed on the cooling tower of a power plant to obtain its characteristic curve. Using the performance curve of the tower and the governing differential equations, the behavior of the tower was predicted at different weather conditions and compared with actual data on the plant which shows good agreement. The results indicate that ambient wet bulb temperature has important effect on the tower performance. By prediction the tower performance and specially by proper prediction of water outlet temperature at hot weather condition it is possible to make necessary arrangement like increasing the air flow rate to prevent disturbance in the tower performance.

This paper addresses the implementation of the element-free Galerkin method for calculation of stress intensity factors in cracked functionally graded materials. The physical interpretation of Lagrange coefficients is used in the element-free Galerkin method which has more advantages than the finite element method and the common element-free Galerkin method. The obtained results for problems with different geometries and loadings are in good agreement with reported analytical and numerical works.

Dual phase steels are modern steels with structural uses. They have better strength and formability than those of mild steels. In this research the effect of intercritical annealing temperature between 740 and 810 oC on microstructure and tensile properties of dual phase steel, prepared of primary ferritic pearlitic microstructure, was studied. Primary steel was annealed at four temperatures, and was subjected to tensile test, hardness test and SEM metallography test. The results showed that morphology and percentages of final phases were dependent on the annealing temperature, and had effects on the final properties of steel. The martensite vol.% was increased with increasing annealing temperature, hence the tensile strength was increased and the elongation was decreased. The good combination of strength and elongation in the steel can be achieved using the selection of suitable intercritical annealing temperature.

Gas Metal Arc Welding (GMAW) using inert gas is an important welding process for making permanent joints in metal parts. The quality of joint in welding is usually expressed by weld bead geometry which includes bead height, width and penetration. Weld bead geometry and joint quality, in turn, depend on the welding parameter settings. In this paper, using regression modeling, the exact mathematical relationships between welding input parameters and weld bead geometry specifications have been established. Then, based on statistical analyses, the best and most fitted model has been selected. The proposed model in the present research has better fit and is simpler than those in related literature. In the second part of this paper, Simulated Annealing algorithm has been employed in order to determine the optimum set of welding parameters values. The comparison between optimization results and actual values demonstrates that the proposed procedure is quite capable in modeling and optimization of GMAW process.