مجله مهندسی مکانیک
سال چهل و چهارم شماره 1 (پیاپی 67، بهار و تابستان1393)

In this paper after the formulation and manufacture of thick 8YSZ and 1. 5Ni-8YSZ layers by screen-printing method on Alumina substrate، the amount of YSZ and Ni-YSZ band gaps and the sensitivity of these sensors to ethanol vapor have been studied and compared. The objectives of this study are to examine and compare the amount of band gap، activation energy، sensitivity changes، response and recovery time. Regarding to the sensing results، YSZ causes a lower band gap and better sensitivity than Ni-YSZ، other advantages of the sample without dopant are the lower working temperature and sensitivity at lower temperature.

Continuum damage mechanics is a new tool in mechanical engineering. This new tool is known as the complementarity for theory of plasticity and fracture mechanics. ABAQUS commercial software owns different damage models for ductile, brittle, and composite materials. In this study, first, ductile damage models exist in the ABAQUS/Explicit software are introduced. Then, performing the numerical simulations, the prediction results of the above damage criteria in a number of benchmark problems, and also sheet metal forming processes such as tube hydroforming and hydrodynamic deep drawing are achieved. In the continue, the numerical simulation results are compared with the experimental results and the proper models for predicting the damage evolution in sheet metal forming processes are specified. Comparison of the results reveals that Hooputra's ductile damage criterion is a suitable model for predicting the ductile damage evolution in sheet metal forming processes.

Condensing flows in nozzles and stationary blades of steam turbine have always been the subject of many studies in recent years. The slip between two phases of droplet and vapor has been ignored in calculations but, in the present study, the governing equations by considering slip between two phases and also incorporating droplet momentum equation have been developed to the solution. The governing equations have been solved using the Eulerian–Lagrangian method and fourth order Runge- kutta scheme. The numerical results of slip and no slip cases have been compared with the experimental data. The innovation of this study is developing an analytical code by adding the droplet momentum equation to governing equations and modeling the slip between phases in supersonic condensing flow. All the solutions are based on a one dimensional analytical code in convergingdiverging nozzles.Considering slip in these three nozzles shows that the radius of droplets has a good agreement with experimental data and its value is increased. Also, wetness fraction is reduced compared to no slip case.

Two-phase flows are of great importance in the oil, gas and nuclear power plant industries. In this paper drift-flux model was used for mathematical modeling of two-phase flows and then its hyperbolic analysis was performed. For numerical solution of hyperbolic equations, primitive (PRI) centered (CE) or PRICE-C scheme, which is a conservative scheme along the central path was used. The scheme does not need calculation of whole eigenstucture for solving the system of governing equations; therefore, the speed of solution is high. Shock-tube, pure rarefaction and transonic rarefaction problems were solved using this method. The results indicate that the PRICE-C scheme has high ability in capturing the discontinuities in the flow field and can predict the flow behavior successfully.

In this paper, the discharge coefficient and the flow linearized coefficients for hydraulic valves with spool are examined. The discharge coefficient was determined for orifice created by spool in turbulent flow and the linear part slope of C Re d - curve, 0.6229 and 0.222, respectively. Furthermore, the variation range of pressure-flow coefficient for valves with zero overlap is less than valves with negative overlap. On the other hand, the flow gain for valves with zero overlap has more changes than valves with negative overlap.

In this study, an experimental and numerical analysis based on computational fluid dynamics is done to study the flow characteristics of a turbulent compressible jet impingement on a flat plate. The purpose is to investigate the behavior of cutting gas jets. In numerical model a finite volume approach with realizable k-ε model is used. Pressure and shear stress distribution over the flat plate are determined for various distances between nozzle and plate. Comparisons between numerical results and experimental measurements validate the accuracy of numerical results. Obtained results show that if the horizontal distance between the nozzle outlet and flat plate is 7.84 times of characteristic length of jet, the shear stress on the flat plate has maximum value. Therefore, in this case jet has a maximum performance in cutting procedure.

In order to perform experiments in wind tunnels it is needed to arrive in the “run” stage, and knowing that the “run” stage is arrived after passing the triggering and “start” stage, it is necessary to study the tunnel flow in the start stage. In the start stage, which is occurring in a very short time interval in an unsteady and transient mode, some strong shock waves are formed who should rapidly pass the test section to be swallowed by the second throat of the supersonic diffuser. Otherwise, the normal shock waves formed in the test section will remain and in addition to causing severe pressure drop, will hinder the flow in the tunnel. Such a phenomenon will practically make the tunnel unusable. In this paper, the “start” stage of a blow-down supersonic wind tunnel with a Mach number of 2.8 and a nuzzle geometry designed with “method of characteristics”, has been studied under unsteady and transient conditions. Mach counters and their diagrams were plotted to assess the propagation of shock waves and achieve goal Mach number. y+ diagram was used to cross examine the modeling of the flow turbulence done in Spalart – Allmaras method. The results confirmed the accuracy of the model.

In this study, accelerating motion of a sphere in incompressible fluid has been nvestigated by the Navier-Stokes equations. The motion is set in a small time interval, having the constant velocities out of it.The laminar and unsteady flow has been solved in various Reynolds numbers. In a short moment the Reynolds number is rised which cause large variations in the drag force. This method is applicable to other accelerating motions such as oscillation and also spherical submarines.