نشریه مهندسی مکانیک مدرس
سال دوازدهم شماره 4 (آبان 1391)

This paper is concerned with the conduction heat transfer between two parallel plates filled with a multi-layer porous medium under a Local Thermal non - equilibrium condition. Analytical solution is obtained for both fluid and solid temperature fields in the porous channel incorporating the effects of thermal conductivity ratio, porosity, and a non-dimensional heat transfer coefficient at the pore level. The effects of the variable porosity on the temperature distribution are completely shown and compared with the constant porosity model. The presented method for analysis of heat transfer in a multi-layer porous medium is a generalized solution which is valid for arbitrary number of internal porous layers. The local temperature difference between fluid and solid phases is also calculated for a wide range of parameters. The results confirmed that the conductivity ratio and the porosity of the internal layers have significant role in the thermal modeling of the porous channel.

Effects of geometrical changes (due to twist, lean and bow) for the second stage blade of Avon gas turbine on the turbine performance have been studied. At first, a 3-D approach for gas turbine simulation has been validated by using experimental results for one stage of Hanover turbine. Then, second stage of Avon turbine was modeled with this numerical method and the results were validated by performance data of this turbine. In the next step, geometrical changes for second stage rotor of Avon turbine were applied on turbine geometry and flow and temperature fields due to these changes were simulated with numerical softwares. Finally, effects of these changes on performance of the second stage (efficiency, pressure ratio etc.) were evaluated. In this study, the geometry changes including lean, bow and stagger angle of the blades were investigated. Finally it was observed that changing blades stagger angle caused the mass flow rate to change greatly. This factor will lead to changes in the turbine power. Lean and bow effects on the performance of turbine are Negligible.

Low frequencies vibration is known as a method for stress relieving of metals. This paper shows that the high frequency vibration (ultrasonic) is also a promising method for stress relieving. In this paper the effect of parameters including stress relieving time, vibration amplitude, grain size and pre-load on samples are investigated. In order to evaluate the residual stress before and after ultrasonic relieving process Almen test was used. The samples were made according to Almen standard and after heat treatment, were shot peened so that, the compressive and tensional residual stress induced into the samples, then the stress relieved by means of ultrasonic vibration. Results show that the residual stress has decreased about 46 percent for coarse grain samples and 27 percent for fine grain samples. Also it was found that the most effective factor on the stress relieving rate is the grain size and the pre-load has the least effect.

This paper presents a new and accurate methodology for transient and dynamic behavior of slug hydrodynamic instability modeling in horizontal and inclined channels. The base of this methodology is on numerical solution of hyperbolic two fluid model equations by means of a class of high resolution shock capturing methods. The privilege of this method is that it can model and predict initiation and growth of slug in stratified flow automatically and directly by solving the flow field differential equations. The well-defined test case considered for the verification and validation of the results is the Ransom Water Faucet case. The results obtained for slug flow modeling in horizontal duct were compared with two sets of experimental results. The good agreement of the present modeling results with the experimental results of own and other investigators and also grid independency study show the model is capable of slug tracking and slug capturing and the numerical method which is used here can predict with high enough accuracy.

In certain applications, it is improtant to measure the mechanical properties of steel with high accuracy. These properties are usually measured by destructive methods. However, the small changes made during heat treatment processes are not usually detectable by destructive methods. Ultrasonic non destructive testing is an alternative method that can be used for measuring the mechanical properties of steels. In this paper, the ultrasonic method has been used for measurement of meachanical properties of AISI D6 steel samples processed by various heat treatment processes. Each sample has a differnet micro stucture and hardness due to its specific heat treatment. To find the meachanical properties, the velocity of longitudinal and shear ultrasonic waves were measured for each sample. Comparison of the results obtained from ultrasonic measurements with those available in reference tables shows that the ultrasonic non destructive method can measure the elastic properties of AISI D6 samples with high accuracy.

The aim of this paper is to show a new geometrical specimen in fatigue to define the behavior of semi elliptical crack growth in thick-walled pressure vessels. This research emphasize on the importance of the behavior of fatigue crack in test specimen and real conditions in vessels. In other word, there must be more adaptations between stress intensity factor in new specimen and real vessel that includes crack along the wall, so, under Fatigue loadings, as a result, more adaptation will be obtained between growth speed and relative crack lifetime according to loading cycles. By introducing this new specimen we compare the results of fatigue loading on propositional new specimen with results of vessel fatigue loading and standard specimen. We estimate the behavior of fatigue crack growth in specimen and pressure vessel with (FEM) and experimental method. In order to comparing of limit component with experimented results, some piece of with new specimen made of steel (ck45) can be used and all of these must be under fatigue testing and results are obtained in these methods.

Numerical simulation of turbulent flow and heat transfer in film cooling from a cylindrical hole in three-dimensional case is considered. For this purpose, turbulent heat flux term of energy equation is usually modeled by simple eddy diffusivity model with constant turbulent Prandtl number, while experimental and numerical researches show that the prescribed constant value of turbulent Prandtl number is far from reality. In the present study, second moment closure (SMC) models with wall-reflection term is applied for modeling the turbulent flow and heat transfer in film cooling flow. Comparison between the numerical and experimental results show that the explicit algebraic second moment closure models have more ability to better prediction of temperature field in film cooling. In addition, turbulent Prandtl number distribution for film cooling has been investigated. The range of this parameter for the desired geometry has been identified from 0.1 to 0.95 in the near of injection hole to far from it respectively. Finally, in order to enhance the capability of simple eddy diffusivity model in simulating film cooling heat transfer, the value of 0.7 has been proposed for turbulent Prandtl number instead of the common value of 0.85.

In most of methods and among Savitsky's semi experimental method, deadrise angle is assumed constant or used its amount in cross section of gravity center. In fact the deadrise angle of high speed craft is variable in longitudinal direction of hull. According to pervious assumptions for the deadrise angle, real physic of Problem didn’t model.In this paper, hydrodynamic resistance per weight is calculated by development of Savitsky’s semi experimental method and has been shown that results of present method have more accurate than results of Savitsky’s semi experimental method at comparison with empirical results. Also results of present method are good agreement with the empirical results over a wide range of volumetric Froude numbers. Then, optimum effective parameters such as position of gravity center, beam and rate of deadrise angle variation is determined by using genetic algorithm method. Objective function is considered resistance per weight in optimization that it is calculated in first part of the paper. Dynamic and static stability are as constraints of optimization that dynamic stability includes transverse dynamic stability and porpoising. Finally, Optimum answers group is presented for use of naval architect in concept design of monohull high speed craft.

Energy consumption is very much to achieve comfort condition in buildings. Therefore, using and finding methods are necessary to reduce building energy consumption. First, the building energy consumption must be calculated. Computer Simulation is a method to calculate building energy consumption. In this paper energy consumption of an education building has been calculated using Energyplus software in Isalmic Azad University Tabriz branch. This building has more energy consumption according to geographical type of Tabriz city. Several parameters of this building have been changed to reduce building energy consumption. The result shows that, in optimized building, energy consumption reduces about 35.5 percent in winter, about 42.22 percent in summer and about 40 percent (3.82E+12 joule) in year. In addition, the building parameters had been changed according to minimum building cost. The result shows that, in optimized building, energy consumption reduces about 35.5 percent in winter, about 42.22 percent in summer and about 40 percent (3.82E+12 joule) in year. In addition, the building parameters had been changed according to minimum building cost.

In this paper, the forward kinematic of the special cases of 4- R" R' R' R" R", 4- R" R" R" R' R' and 4- R" R" R' R' R" parallel mechanisms that respectively lead to two 4- R R U R with different geometric structures and one 4- R U U spatial 4-DOF parallel robots has been studied. They are originated from the type synthesis of 4-DOF parallel mechanisms with motion patterns of 3 T1R. Each of them is composed of four kinematic chains and each chain consists of five revolute joints. The directions of revolute joints have been different with each others that create three different geometrical structures. The forward kinematic problem is done in three dimensional Euclidean space and finally, a univariate mathematical expression of degree 344, 462 and 8 is indicated the forward kinematic problem of each parallel robot. Also, the results are compared with simulations.

In all of power plants, which are made only for electricity generating, more than half of fuel energy is wasted. This waste energy could be recovered by cogeneration or trigeneration systems. In this study a cogeneration combined cycle with triple pressure HRSG has been simulated. Second product was considered heat that can be generated as steam or hot water depends of intended consumption. At first, exergy and energy analysis has been carried out and potential of waste heat in power plant was determined and evaluated. By using TRR method Thermoeconomic analysis was done and cost of products calculated. At the end of this research an objective function defined and to optimize with algorithm Genetic method, some decision variables were selected. After various parameter optimizations at different situation of power plant, optimum structure specified. Results are shown at which status of power plant, producing heat and power has maximum efficiency and profitability.

In this paper the performance of an ejector refrigeration cycle was investigated theoretically and experimentally. Making use of the conservation of mass and energy as well as the exergy balance equations, a two-dimensional thermodynamic model was developed. The influence of flow viscosity is taken into account through considering a two-dimensional flow near the ejector inner wall. The results indicate a decrease of COP with increasing generator temperature and an increase of second law efficiency with increasing evaporator temperature and/or decreasing generator temperature. It is found that at any generator temperature, there exist a particular evaporator temperature above which the exergy destruction in the condenser is higher than that in the ejector. The maximum relative and the root mean square errors in calculating the entrainment ratio at three generator temperatures of 77, 83 90 o C are obtained as 7.76% and 5.13% respectively. Also the exergy destruction in the evaporator at an evaporator temperature of 13.5 oC, was found to be the highest among those occur in the other components of the cycle.

In this research, a novel method to decrease macro-residual stresses of laminated composites by adding carbon nano-fiber (CNF) is proposed. To this end, using micromechanical and modified micromechanical approaches, coefficient of thermal expansion (CTE) and Young’s modulus of carbon nano-fiber reinforced polymer are calculated. Then using this enhanced matrix, longitudinal and transverse of thermal expansion and Young’s modulus of a CNF/glass/epoxy unidirectional lamina in presence of CNF are obtained. Finally, using the classical lamination theory (CLT), macro-residual stresses for CNF/glass/epoxy laminated composites are obtained. The results explain how CNF can result in reducing macro-residual stresses in nano particle filled laminated composites. The results for both cross ply and angle ply glass/epoxy laminated composites are presented and show that adding CNF through the matrix can decrease macro-residual stresses in each plies up to about 30%. The main reason for the decrease in residual stresses are due to the sever decrease of coefficient of thermal expansion of the unidirectional ply.

In this paper a neural network with a feed forward topology and a back propagation algorithm was used to investigate the effect of chemical composition on hardness and impact energy in API X65 microalloyed steel. Experimental data was obtained by cutting 100 specimens from pipes manufactured in industrial scale (with 1219 mm diameter, 14.3 mm wall thickness, with similar heats and manufacturing processes). The chemical analysis, Vickers hardness and Charpy impact tests were conducted then according to requirements specified by API 5L standard. The weight percent of C, Si, Mn, P, S, Ni, Cr, Mo, Al, Cu, V, Ti, Nb and Ca were considered as input parameters of the network; while Vickers hardness and Charpy impact energy were considered as output. Scatter diagrams and two statistical criteria: correlation coefficient and mean squared relative error were used to evaluate the prediction performance of developed ANN model. With regard to the exact performance of the developed neural network, it was used then to investigate the effect of chrome and vanadium on Vickers hardness and Charpy impact energy of tested steel.