JOURNAL OF MECHANICAL ENGINEERING
Year:2019 | Volume:49 | Issue:3 (88) |Papers Count:40

In this paper, the hybrid front tracing method is used to simulate the rising of a single bubble in a quiescent liquid. This method is a new approach to capture of fluid interfaces in the constant grid and using a one field concept. A one field approximation is used, where one set of governing equations are only solved in the entire domain and different phases are treated as one fluid with variable physical properties. The interfacial effects are accounted for by adding appropriate source terms to the governing equations. The related unsteady incompressible full Naiver-Stokes equations were solved using a conventional finite volume method with a structured staggered grid. The interface was tracked explicitly in to connected marker points via hybrid front capturing and tracking method. A computer program was developed for numerical simulation of bubble movement in a viscous liquid in different gravity conditions, especially zero gravity conditions have been used. In addition to general studies of microgravity effects, the initiation of hydrodynamic Marangoni convection solely due to the variations of interface curvature (surface tension force) and thus generation of shearing forces at the interfaces was also studied. Recognizing this movement in the study of bubble dynamics problems, especially in the nucleate pool boiling in zero gravity is important. In this regard, the simulation results were obtained for different conditions and the role of the hydrodynamic Marangoni convection on bubble movement in the absence of buoyancy force was investigated in the obtained results. The simulation results on the inclined plate show that bubble behavior in according to a critical angle from sliding on the wall to departure from wall and finally fluctuate departure have a good agreement with experimental results.

Nowadays, with the advancement of technology, high-speed electronic instruments with small sizes are required. Regarding the low density of metal foams, they are recommended for many applications in order to energy absorption and heat transfer purposes. In this paper, the heat sinks made of metal-foams are simulated and the results are compared with simple finned heat sinks. In the heat sinks that made from aluminum foams with porosity value of 95. 6%, the amount of heat transfer coefficient of displacement increases up to 31. 127%. In another part of the dissertation, a finned heat sink, which metal-foam is placed between its fins is modeled. Obtained results show that, the Nusselt number in the heat sinks that made of aluminum foam with porosity of 95. 6%, in the finned mode could increase up to two times. Finally, the effects of parameters such as the amount of mass discharge, the heat sink height and the amount of porosity of heat sink foam on the amount of heat transfer through heat sinks is determined at the end part.

Abstract In this study, fluid-structure interaction is studied by using of Lattice-Boltzmann-Lattice Spring Model (IB-LB-LSM). In this hybrid method, second-order accuracy is achieved by adopting split-forcing technique. The immersed-boundary method which is known as a fixed-mesh method is implemented to calculate boundary force. To apply deformation of elastic solid due to vicinal fluid, Lattice Spring Model (LSM) is utilized in which solid is substituted by collection of springs. In numerical method section of this paper, a implicit algorithm based on lattice spring model is represented which it can easily combine with fluid solvers and promote previous explicit method. Validation of structure and fluid solver are done by simulation of cantilever beam and motion of single particle in shear flow, respectively. In the results section, nearby motions of circular particles in shear flow are compared for two different types of rigid and deformable condition which it is shown how the deflection of solid body changes particles path.

The research carried out is a study of spray behavior and a better understanding of the Pressure-Swirl Duplex Injector to improve its future design. Therefore, the effect of a number of functional parameters is expressed as a function of pressure, Reynolds and Weber numbers. Water was the experiment fluid and all of the experiments performed at atmospheric standard condition. Spray area shooting were carried out based on fast shooting method. The results show that increasing Reynolds number would tend to change trickle mode regime to atomization then fully developed regime would be dominated as Reynolds numbers further increases. Moreover, Increasing Reynolds numbers would increase spraying angel of both paths followed by maintaining at constant values. Due to Injector's design pressure is estimated equaling to 10 bars because lowest beak-up angel was achieved around this condition. Furthermore, spatial distribution and spraying area measurements showed that the injector has the significant performance to creating a symmetric corn hollow spraying pattern. Additionally, it was realized that discharge coefficient is not only dependent on the geometry of the injector and would raise while Reynolds changes.

Surface hardening is one of the most popular ways for improving surface properties. In this research, surface hardening operation on low carbon steel was investigated via formation of vanadium nitride. To do so, three weld layers with 4, 3 and 2 passes on the surface of St37 steel were cladded by using flux cored arc welding via flux cored electrode containing 17% vanadium element. For investigating surface hardening and identifying the formation of vanadium nitride, hardness measurement test, x-ray diffraction and optical microscope were used. Obtained result from x-ray diffraction showed the formation of vanadium nitride and iron nitride precipitants. It was also found that these nitrides are main factor in surface hardening of low carbon steel. The result of hardness measurement showed that with increasing distance from base metal surface, the hardness has increased from 163 HV for base metal to 365 HV on surface. Microstructure investigation demonstrated that vanadium nitride precipitation was prevented to grow initial austenite and was caused the formation of finer ferrite in the microstructure of weld layers and consequently, the improvement of properties.

In the present study, the tribological response of graphene (Gr)-epoxy nanocomposites was investigated. At the first step, the surface of Gr nanoplatelets was modified with 3-aminopropyltrimethoxy silane (3-APTMS), then the influence of silane modified Gr (SGr) loading (0, 0. 05, 0. 1, 0. 3 and 0. 5 wt. %) and surface functionalization on the tribological response of specimens was assessed. Approximately 40% and 68% decreases in coefficient of friction and wear rate of epoxy matrix, respectively, were observed at 0. 3 wt. % SGr loading. Also, the results revealed remarkable decreases of 28% and 48% in the coefficient of friction and wear rate of 0. 3 wt. % SGr loaded specimen compared to that specimen containing 0. 3 wt. % unmodified Gr. The possible mechanisms behind the reinforcing effects of Gr in the epoxy matrix were proposed.

The Cement industry is one of the highest energy consuming industriesin the world. The cement industry consumeda large part of total energy. A large portion of this energy iswasted byexhaust gases fromthe chimney and ventedto the atmosphere. With using the time value of money in payback period, in this study, thefeasibility of using hot chimneygases from BojnourdCement Company chimneys to cogenerate heat and power system has been investigated. Theminimum temperatures of the hotgases from the chimney NO. I and II were 130oC and 250oC, respectively. The thermal energy of the chimney gases wasused to convert inlet water stream to superheated vapor. The results revealed that using an energy recovery system at this plantand generating electricity, it could be possible to return the investment in 2. 26 years. Finally, thetemperature of theexhaust gases from combined heat and power generation system reached to 112oC and 104oC.

In this research, the deformation of a core-shell droplet under the influence of high voltage electric field is studied. In order to produce this kind of droplets two coaxial nozzles are utilized. Here, distilled water and transformer oil are fluids used as the core and shell, respectively. Two parallel copper plates connected to a high voltage supplier are considered so as to produce an electric field. By increasing the electrical potential difference, the intensity of electric field and consequently the drop deviation angle increase. At particular electrical potential difference of 8 kV, the core breaks due to the intense electric field, therefore it has a different deformation. After separation of the inner droplet from the outer one, the outer droplet on the grounds of surface tension and inertia forces begins to return to the spherical form. Due to its initial velocity toward positive electrode, the core-shell droplet tends to move in this direction. However, the remaining piece inside the outer droplet tends to move in opposite direction. Eventually, outer droplet deforms and stretches again.

In the present research, effects of flow acceleration magnitude on velocity profiles, near-wake and far-wake turbulences, and drag coefficients of a model vehicle are studied. Moreover, rate of changes in the mentioned parameters in an accelerated flow at various accelerations was considered. An open-circuit blowing wind tunnel was used for fluid simulation, whose maximum nominal turbulence and velocity were 0. 1% and 30 m/s, respectively. Velocity was increased continuously at different accelerations using an inverter device. The results indicate that, geometry of the developed wake is consistent and independent of inlet flow acceleration. Furthermore, with the passage of time and increasing velocity, near-wake and far-wake velocity defects decreased largely and slightly, respectively. With increasing the inlet flow acceleration, momentum and turbulence components of drag coefficient increased. By enhancing the flow acceleration, drag coefficient increased. The lowest value of the drag coefficient often occurs in the current with minimal acceleration, which is approximately equal to the corresponding values in the flow at constant speed.

In this research, recrystallization behavior and coarsening kinetics of 7075 alloy were investigated during conventional and newly modified SIMA processes. The conventional SIMA process consisted of applying 10-55% uniaxial compression strain at ambient temperature and subsequent semi-solid treatment at various temperatures and times. A new modified SIMA process was developed by introducing repetitive upsetting-extrusion (RUE) for the first time for semi-solid processing of 7075 Al alloy. Specimens of 7075 alloy were subjected to different RUE cycles at 250 ° C, and then the semi-solid treatment was carried out. Microstructural studies revealed that the recrystallization rate is accelerated, the average grain size is reduced considerably and the sphericity of solid grains is improved by the newly modified SIMA process. Lifshitz-Slyozov-Wagner (LSW) theory was used to describe the coarsening process of the semi-solid slurries. Coarsening rate of the solid grains in the newly modified SIMA process was slower than the conventional SIMA process.

The use of solar energy in the refrigeration cycle is one of the newest ways to increase the coefficient of performance and use of clean energy. The purpose of this paper is to present a parametric simulation and the performance analysis of a water-lithium bromide absorption refrigeration cycle equipped with a solar receiver in terms of thermodynamics and exergy. The effect of different parameters on system efficiency, heat transfer rate in generator, exergy efficiency and exergy changes of the whole system is investigated. The results of the study show that there is an optimal radiation intensity at a given temperature for the evaporator and condenser, in which the total exergy changes of the system are minimized and the system coefficient of performance and its exergy efficiency reach its maximum. The results show that when the evaporator temperature is in the range of 4 to 10 ° C and the condenser temperature is in the range of 33 to 39 ° C, the maximum yield coefficient for the single-cycle cycle is in the range of 0. 75 0 to 80/0. Also, surveys show that the maximum exergy efficiency for a single-cycle refrigeration cycle is in the range of 13 to 23. 7 percent.

Dynamic behavior study of elastic structures under moving loads is one of the most famous problems in different engineering fields. In this study, dynamic stability of a simply supported rectangular plate under periodic passage of moving masses is investigated using Floquet theory and strained parameters method. Considering effects of all inertia components of the moving masses, the governing partial differential equation of motion is derived. The Galerkin’ s method is applied to reduce the governing partial differential equation to a set of ordinary differential equations. The periodic loading across the plate results in a time-varying equation. Considering a straight path for passage of moving masses and applying the Floquet theory and strained parameters method, the dynamic instability of the plate and coexistence phenomenon are analyzed. In addition, defining a diagonal transition path of moving masses over the plate’ s surface, the significance of the coexistence phenomenon and also sensitivity of plate instability to mass transition paths are investigated. Numerical simulations validate the accuracy of both methods results.

The process of interference fit and bolt clamping force are two effective techniques for improving the fatigue life of the joints and are used in various industries, especially in aerospace and automotive. Most of the previous studies have investigated separate effects of the interference fit and bolt clamping force on the fatigue life of the specimens. In the present research fatigue behavior of interference fitted specimens combined with bolt clamping force has been investigated yet. On the other hand, the pervious researches on the interference fit and bolt clamping force have been studied at laboratory temperature and the effect of temperature changes has not been investigated. To investigate the effect of temperature on the fatigue life of specimens, two different temperatures (i. e. 60° C and 120° C), aside from room temperature, were selected and fatigue life of specimens are studid at these tempratures. The fatigue tests were performed on the specimens of Al-alloy 7075-T6 alloy to obtain S-N data.

Rotary kilns have some applications in industrial due to high production capacity, uniform and extensive combustion. Rotary kiln performance not easy and despite of technology development, the problems exist in the filed from combustion them. Reasons of some problems are separate design of burner and kiln and inattention to coordination and harmony between them. According to in the present study, flame behavior has been studied under governed condition on the kiln and secondary air flow. In the first step, suitable methods and models are verified on the basis of benchmark problem due to lack of experimental data. Finally, rotary kiln simulation done with realizable k-ε turbulence model, partially stirred reacting flow combustion model and P1 radiation model. The present simulation done in OpenFOAM open source software by using reactingFOAM solver. Also in the present work, ability of applying radiation model has been created with addition source term to energy equation and ability of applying rotation boundary condition has been created with addition source term to momentum equation. According to obtained results, applying of gravity acceleration leading to deviation of flame to the upper wall. The results show that radiation heat transfer is important in the governing condition on the problem. Finally, effect of excess air studied on the temperature distribution that obtained results show temperature of kiln and wall temperature kiln reduced with increasing of excess air percent in the investigation limit.

One of the important methods for seawater desalination is Multi Effect Desalination (MED). The economic and environmental seawater desalination in Persian Gulf countries is very important. The main purpose of this paper, using accurate thermodynamic modeling for exergetic, exergoecomic and exergoenvironemtnal modeling and simulation of MED desalination. In this paper, an accurate improved thermodynamic model has been applied for simulation of MED desalination. Next, exergetic, exergoeconomic modeling and analysis have been performed based on accurate simulation data. In this regard, economical and exergoeconomical parameters have been calculated using an accurate data. Furthermore, the cost of water production and environmental impacts of the final product have been evaluated. Finally, exergetic, exergoeconomic and exergoenvironemtnal parameters in different conditions have been evaluated.

One of the very important cooling processes in industries is cooling via droplet spraying. In this research, the impact of Water-Silica nanofluid drop on a hot surface in the film boiling regime is studied. Governing equations are incompressible continuity, momentum and energy equations. The level set method is used for interface tracking and the ghost fluid method is used to impose discontinuities at the interface. The effect of adding nanoparticles and also impact velocity on the droplet contact time, droplet spreading and heat transfer from surface is investigated. Droplet spreading and heat transfer rate increase by an increase in impact velocity. But, the effect of impact velocity on the droplet contact time is negligible. Droplet contact time and spreading increase by an increase in nanoparticle’ s volume fraction. Nanoparticles addition has more effect at higher impact velocities. Volume fraction enhancement, increases heat transfer rate and total heat dissipated from the surface.

In recently years, Stiffened cylindrical shells are used in the major components of aviation, missile and marine structures. In this study has been analyzed buckling of composite lattice cylinder with inner and outer shell under external pressure. This was accomplished by developing an analytical model for determination of the equivalent stiffness parameters of a grid stiffened composite cylindrical shell. This stiffness contribution of the stiffeners was superimposed with the stiffness contribution of the shell to obtain the equivalent stiffness parameters of the whole panel. Equations governing the cylindrical lattice structures are solved based on the displacement and stress-strain relations in the form of a matrix using classical shell theory under boundary conditions clamp supported and simply supported. The results show increase in grid cell length, angle and cross-section parameters caused increase in stiffness and critical buckling load but increase further them cause local and global buckling. Finally for all of them can be considered optimal value and for prevent local and global buckling and the effectiveness of the lattice structure to a limited number of stiffener with optimal angle, longitudinal and cross sections required.

In this paper, the effect of helical nozzles morphology on the vortex tube performance is analyzed using simulations and computational fluid dynamics method. The k-epsilon turbulence model is used to solve the flow field equations, also, the model geometry is considered as a fixed structure. The main goal is to achieve the minimum cold outlet temperature and maximum rotational speed in the vortex tube. In this paper, the machine is equipped with three sets of nozzles including 3 straight nozzles, 6 straight nozzles and 3 helical nozzles. The results indicate a lower cold temperature for a vortex tube with 3 helical nozzles than the other two models. Also, this type of nozzle generates a higher rotational speed inside the vortex tube, especially in the vortex chamber.

Oscillation or rotation rates change flow behavior and wake patterns over and around cylinders. In this study, laminar fluid flow over a rotational cylinder with triangular cross section is studied numerically. Because of rotating geometry, dynamic mesh is used for the numerical solution and simulations are done for 50, 100, 150, 200 Reynolds numbers and 1, 2 and 3 rotation rates and effects of rotation rate and Reynolds numbers on wake and flow patterns are investigated. Investigating average Lift, drag and moment coefficients express that increasing in Reynolds number and rotation rate lead to lower lift and drag coefficients, while moment coefficient has a gradual growth similar to the circular cylinder case. However, the dependency on the Reynolds number is higher in a way that by increasing the rotation rate from 1 to 3 the change in the coefficients is at least 40 percent more than changes of coefficients due to tripling the Reynolds number.

In this paper, the principles of constructal theory is utilized for modeling and designing of three stream tubular heat exchangers. The target is improvement of thermal performance of this kind of heat exchangers using constructal theory especially dendritic systems. In this regard, the differential equations of a threes stream heat exchanger are extracted and its solution is achieved. A consecutive division ratio is obtained using constructal theory and Lagrange method for three stream tubular heat exchangers. this ratio reduce remarkably the design and optimization procedures of a tree-shaped three stream heat exchanger which is actually a complicated and cumbersome task. Finally, the design algorithm of a three stream dendritic heat exchanger is extracted using geometric and thermal modeling. In order to evaluate the presented algorithm, a case study is used and the results will be shown. The results shows remarkable reduction of pressure drop and heat transfer area in tree-shaped heat exchanger rather than the typical tubular heat exchangers.

In this paper, flow control ability with suction over a circular cylinder has been numerically investigated in order to drag reduction and elimination of unsteadiness and the resultant vibrations. The efficiency of this flow control method is dependent to different parameters. Some of these parameters are dependent to geometry and the rest are dependent to flow characteristics. For this purpose, the geometric parameters of width and position of suction slat and also the suction flow velocity are studied in this investigation. 2D flow physics around the cylinder have been simulated in Re=3. 3*104 with K-ε (RNG) turbulence model. The obtained results show the elimination of Von Karman vortices under sufficient suction. Suction causes vacuum in viscous sublayer and elimination of low momentum regions via momentum transfer from outer layers, and consequently postpone the separation. The results indicate that pressure drag is decreased, skin friction drag is increased and total drag is decreased about 55%.

There are many two-layerd components where the outer layer is a metal of high acoustic impedance and the inner (insulating) layer is a non-metal with low acoustic impedance. In most cases, it is impossible to directly access the inner layer for inspection or thickness gaging. The key problem in thickness gaging of the inner layer from the outer surface is that the backwall echo from the insulation cannot be identified in the very many backwall echoes coming back from the metal layer. In this paper, we use the signal processing techniques of wavelet transform and EMD algorithm to overcome this problem. To demonstrate the repeatability of these methods, three two-layered samples called A, B and C are prepared. The backwall echo of the insulation layer cannot be identified in any of these samples without further processing. The results indicate that both methods can facilitate the measurement of the insulation thickness, however, due to higher sensivity of the wavelet method, it is more suitable than the EMD algorithm.

Magnetic levitation systems are recently used for contactless transportation of objects. In magnetic levitation, the object can be levitated by actively controlling the magnetic field based on the measured air gap between the object and the levitator. These systems are affected from the velocity of moving objects, the distance of suspended object from magnetic core and intensity of magnetic field as well as dynamic loads. Environmental noises cause instability in these systems. In this paper, a contactless transportation system has been proposed for vertical transportation of a metal sphere. PID controller was used to stabilize the transportation. The performance of this transportation system at different vertical speeds was analyzed. Results show that the suggested method could precisely monitor sudden moves. The proposed magnetic suspension system can maintain the stability of suspended object at the maximum velocity of 4. 53m/s in upward movement.

Abstract In this paper, multi-objective optimization (MOO) of heat transfer and flow field in interrupted micro channel heat sinks (MCHS) with rectangular fins using in computer chips is performed using Computational Fluid Dynamics (CFD) techniques and Nondominated Sorting Genetic Algorithms (NSGA II). At first, fluid flow is solved numerically in 100 various interrupted MCHS with rectangular fins using CFD techniques. Finally, the CFD data will be used for Pareto based multi-objective optimization of fluid flow in MCHS with rectangular fins using NSGA II algorithm. In the MOO process there are three geometrical parameters and the conflicting objective functions are to simultaneously maximize the amount of heat transfer and minimize the pressure drop. It is shown that the achieved Pareto solution includes important design information on fluid flow in MCHS with rectangular fins.

Turbulent flow and heat transfer of water-copper nanofluid at the leading edge of a surface with two unequal obstacles issimulated numerically. The importance of this study is to simultaneously consider the effect of flow obstacles and nanoparticles on flow and heat transfer of turbulent boundary layer, which can not be found in previous literatures. Turbulent modeling is performed by a modified k- model with two-layer zonal Wolfstein model for near wall treatment. The effects of Reynolds number, distance between the obstacles, the volume fraction and diameter of nanoparticles on the flow parameters are investigated. The results indicates that by changing both the blocks distance and solid volume fraction, the size of recirculation zone between the blocks changes and consequently, the Nusselt number is affected. At large distance between the obstacles, the recirculation zone behind the first block can not reach the second block and hence, the rate of heat transfer decreases. At low Reynolds numbers, convective heat transfer is less than thermal diffusion and therefore, the heat transfer rate is less influenced by the solid volume fraction. Moreover, due to the role of nanoparticles motion on the rate of convective heat transfer, smaller nanoparticles in diameter are more effective in heat transfer enhancement.

In radial turbines, two types of the radial and tangential volute are usually used in terms of their applications. In the application of the pump as turbine, the utilized type of radial or tangential volute is very important in terms of their operation areas. In this paper, the effect of the volute type and its diameter reduction as one of the effective parameters in turbine state was investigated. In this regard, PAT was designed with CFTurbo software, and mesh generation process was performed with ANSYS Meshing software, and by CFX software Analyzed with finite volume method approach and turbulent k-ω SST model, and the results of numerical simulation with the experimental results were compared and validation. Investigating the effect of volute type in turbine mode on hydraulic parameters and efficiency showed that when the flow rates is less than BEP point (QQBEP) use of tangential volute causes an increase in the efficiency up to 1. 03% compared to the radial volute. The results of the reduction in the diameter of the volute indicate an increased of 4. 05% of the efficiency at BEP point, of course in lower points of BEP point is more effective than higher points (Q

Plasma actuator is one of the noteworthy devices in flow control techniques which can delay separation by inducing external momentum to the boundary layer of the flow. In this paper, Numerical analysis of NACA0012 airfoil in plunging motion and some effective agents on it (reduced frequency, amplitude, Reynolds number), has been studied in with and without plasma actuator. The results indicates that, increasing in reduced frequency, increase the values of lift coefficient. By increasing reduced frequency, power of vortices shed into downstream and separate from the airfoil surface increases and the velocity variation in downstream decreases. Reducing friction in flow lead to increase velocity; as a result, the amount and range of thrust increases. Increasing amplitude and Reynolds number, respectively lead to increasing and reducing aerodynamic performance. When plasma actuator is on, lift coefficient and thrust would increase and aerodynamic coefficients would improve. Increasing in Reynolds number decreases the effects of the plasma actuator on the improvement of the aerodynamic coefficients.

In this study, firstly, a three-bladed vertical axis wind turbine with straight blades was three-dimensionally modeled using computational fluid dynamics (CFD) and the obtained results were compared to the experimental data presented by other researchers. Then, a two-bladed vertical axis wind turbine with straight blades was simulated under the same conditions, and its aerodynamic characteristics and performance were compared to those of the three-bladed turbine. The comparison on aerodynamic characteristics between the two turbines was made at different tip velocities considering the peak power produced by each turbine at each tip velocity. Results showed that, despite its lower rigidity compared to the three-bladed turbine, the two-bladed turbine was highly efficient, with the largest difference between power factors of the two turbines occurringat a tip velocity ratio of 2. 55. Furthermore, in this research, the produced power by the two-bladed turbine exceeded that of the three-bladed turbine by 54%, further confirming cost-effectiveness and efficiency of the considered two-bladed turbine.

Homogeneous charge compression ignition (HCCI) is an advantageous combustion concept in terms of efficiency and pollutant emissions. However, its limited operating range suppressed its successful utilization. One applicable method to use this concept is to combine it with other combustion strategies. Direct fuel injection into the combustion chamber is an applicable strategy aiming at this target. To make this concept an applicable one for engines, there should be validated models both for performance prediction and control applications. Single zone models are good and rapid performance predictors and can be used in control applications. Due to the intrinsic characteristics of HCCI being a kinetically controlled combustion, chemical kinetics should be incorporated in modeling it. Therefore, in this study a stand-alone single-zone thermo-kinetic model is developed and validated against experimental measures. Since the main assumption of the developed model is the immediate vaporization of the injected liquid fuel, the validity of the model confirms the idea which states that in the case of early direct fuel injection, in-cylinder combustion can be regarded as HCCI. Furthermore, the results show good agreement with the measurements. The model predicts combustion phasing within 0. 5 crank angle degrees while engine power is estimated with an acceptable accuracy of 90 percent.

The impinging jet resulted from corona discharge has been used in different applications such as cooling, refining and enhancing heat transfer. In this research, the characteristics of this flow including velocity and the force applied by the impinging jet have been numerically studied. Hence, the governing equations of plasma flow under the influence of electrostatic field has been solved by using a finite element method. The final results show that in atmospheric condition, by applying a voltage of 500 V between two electrodes with a 200 μ m gap, the maximum velocity after impingement reaches to 2. 3 m/s. Also the maximum volume force applied on the cathode surface is almost equal to 1. 23×. The conducted simulation has successfully shown the process of jet formation in four steps including ion production, ion flow stretching, ion flow impingement to the cathode surface and finally jet expansion.

In this paper, the atomistic finite element model is used to study the compressive behavior and mechanical properties of singlewalled carbon nanotubes-graphene nanosheets junctions with different boundary conditions. Considering nanotubes as space frame structure, classical structural mechanics approaches can be used to study their mechanical behaviors. The beam and mass elements are used to model the bonds and atoms, respectively. In order to determine the properties of the beam elements, the energy tems in molecular mechanics are equated to those in the structural mechanics. The present analysis provides useful data about the applicability of finite element model to predict the elastic modulus and critical compressive forces of carbon nanotube-graphene junctions. The results show that in a constant aspect ratio (the ratio of the length to the radius of nanotubes), increasing length of the nanotube leads to decreasing the critical compressive force. While the elastic modulusis constant. Moreover, increasing aspect ratio results in decreasing the critical compressive force negligible increasing in elastic modulus. The amount of critical compressive force of the nanotubes under clamped-clamped boundary condition is more than the other boundary conditions. Finally, the first ten mode shapes of the armchair and zigzag nanotubes are drawn for one-side and two-sided junctions, and it is shown that the mode shapes of the armchair and zigzag nanotubes are almost similar

In this study, corrosion and mechanical properties of AISI 420 martensitic stainless steel after welding and heat treatment were studied. Heat treatment cycle in order to optimize the mechanical and corrosion properties of the steel after welding was investigated. After TIG welding and heat treatment of tempering, hardness and tensile, and impact mechanical tests and corrosion polarization tests were performed on the samples. Also to study the microstructure and identify the type of the happened corrosion, by optical microscopy and transmission electron microscopy (SEM), the samples were examined. By The results of mechanical tests and corrosion, heat treatment conditions to achieve optimum mechanical properties and good corrosion properties was determined after welding. In conclusion, the cycle of heat treatment of tempering at a temperature of 300˚ C and 1 hour, is an appropriate cycle in order to achieve Suitable corrosion and mechanical properties for welded 420 stainless steel.

Nowadays, the centrifugal pumps are one of the essential components of various industries. So improving their efficiency is so important. In this paper, the effective parameters in flow losses and increasing the efficiency of a centrifugal pump impeller have been investigated by numerical solution of three-dimensional turbulent flow of water fluid in a steady-state, incompressible and noslip condition on the walls and the surfaces of the vanes using ANSYS 16. 0 software. Optimizing these parameters has led to increasing the pump efficiency. For this purpose, the parameters such as the number of the impeller vanes, inlet and outlet angles and leading and trailing edge thicknesses, have been studied. To validate the numerical solution, the provided results are compared with the pump characteristic curves of Manufacturer Company. With the change in the pump impeller design, the efficiency of the pump has been increased to 48 percent.

In this paper, fluid flow in Francis turbine draft tube is simulated with and without water injection condition in unsteady state using actual and straight diffuser geometry of draft tube. Also a new method for selecting nozzle diameter for injection is proposed. Simulation is carried out using Fluent software and k-ε and SST k-ω turbulence models in the straight and actual geometries of the draft tube, respectively. In two considered geometries, results of proposed method for selecting the nozzle diameter, which is based on the ratio of the total loss to the pressure recovery factor, have been compared with the method used in the previous researches, which was based on the total loss calculation. Inlet boundary conditions and validation are based on the experimental data. Results show that selecting nozzle diameter is depended on the geometry of the draft tube, and using the ratio of total loss to pressure recovery factor approach for selecting nozzle diameter will improve the size of selected nozzle diameter up to 33%, velocity fluctuations up to 16. 3% and pressure fluctuations amplitude up to 19% in draft tube.

In this study equal channel multi angular pressing (ECMAP) process as one of the most effective severe plastic deformation (SPD) techniques in producing ultrafine grained AL5754 alloy has investigated using combined Taguchi-Grey method. Response parameters of Taguchi were obtained using Deform2D software. Due to the number of investigated factors, Taguchi L27 orthogonal array were selected. Process parameters and die geometrical parameters considered as input parameters and the plastic strain of final strip, the strain inhomogeneity of the final strip and the process force were selected as objective parameters. Then, input parameters values of optimum condition were extracted which the objective of the optimization was minimizing both process force and strain inhomogeneity and maximizing the equivalent plastic strain. Moreover, using the regression analysis, first order relation among input parameters and objectives were obtained. Then, with the aim of specifying the effectiveness of each input variable on objective parameters, analysis of variance (ANOVA) was employed. The results revealed that, three parameters namely friction coefficient (Fc), die channel angle (Φ 1) and die corner curvature angle (Ψ 2) are the most effective parameters, respectively.

The IN792 casting superalloy is used in the manufacture of movable blades for gas turbines. Due to exposure to hard conditions, these parts are subjected to a variety of microstructural changes. The control of various stages of blade manufacturing includes casting, forging and heat treatment. In the meantime, controlling the heat treatment variables during the processing of nickel base superalloys is easier due to the ease of switching than other variables and the great influence on the microstructure and mechanical properties. Therefore, . The present research work is directed towards determining the effect of heat treatment`s parameters (time and temperatures of the final stage of aging treatment) on the hardness and microstructure properties of IN792 nickel-based superalloy. For this purpose, the 9 samples were annealed and then were solution heat treated, followed by a two-stage ageing process. In final stage of ageing, 3 different temperatures (870, 845, 800° C) in different ageing time (16, 24, 30 hours) have been chosen. The result revealed that in a constant time by increasing temperature, size of 𝜸 ́ phases increased while the amount of them decreased. The results was the same in a constant temperature and increasing time. The strength of these alloys is basically depends on a factors like volume fraction, size, growth rate, composition and well distribution of 𝜸 ́ precipitates. Homogeneous and uniform distribution of 𝜸 ́ precipitates was observed in a sample that was age treated at 845° C for 16 hours in which, the maximum hardness had been occurred. Also by increasing time and temperature, eutectic phases dissolved in matrix and carbides precipitate in the matrix and near boundaries which will improve mechanical properties.

Graphene is one of the youngest member of the nano-carbon material with two-dimensional structure which has extraordinary mechanical and electrical properties. Graphene sheets can also be used as mass sensors. In this paper, the special applications of this substance as a mass sensor are considered. The vibrational behavior of a graphene mass sensor is studied by molecular dynamics simulation method. Also, the effects of increase the dimensions of the rectangular sheet in two zigzag and armchair directions has been studied on natural frequencies. For this purpose, a square graphene sheet has been created in the molecular dynamic environment and the particle displacement is obtained from the molecular dynamics method. The displacements are analyzed using the frequency domain decomposition method and the first natural frequency of the sheet is calculated. Then, the dimensions of graphene sheet have been changed in the zigzag and armchair directions, and the natural frequency of the sheet is also estimated. In order to validate the method, the estimated natural frequency is compared with the results of one of the improved theories of continuous mechanics. In the following, the ability of graphene sheet for detection of attached mass has been examined using the presence of concentrated gold particles. Finally, the sensitivity of graphene sheet to the attached masses is presented.

The purpose of this study is to investigate the effect of injection timing on various exergy terms of reactivity control combustion ignition with natural gas fuels and n-heptane, which are considered low and high fuel reactivity, respectively. The engine is simulated using a three-dimensional model and the numerical results are compared with the experimental data and the accuracy of model is validated. Thermomechanical exergy, chemical exergy, work, exergy of heat transfer and the amount of irreversibility are calculated in each times step. The results show that fuel burns more perfectly by advanced start of injection due to more fuel and air mixing time. This causes the in cylinder temperature charge to be high enough to increase the heat transfer exergy due to increased heat transfer. In addition, the high temperature in the case when the start of injection is too advanced leads to an increase in irreversibility due to an increase in the number of chemical reactions.

Fuel cells have attracted considerable interest during the recent years. Among the different types, high temperature fuel cells are very promising because of their high efficiency, fuel flexibility and high temperature of the exhaust heat which can be used for cogeneration. Among the high temperature fuel cells, Molten Carbonate Fuel Cell (MCFC) has several advantages but is studied less than others. Co2 capturing and internal reforming are its benefits. Internal reforming eliminates the need for external reformers and increases the overall efficiency of the system. In order to better understand the performance of MCFC, in this study, the model of MCFC with indirect internal reforming is developed step by step. The model is developed in a way that has a proper response speed and accuracy. Given that MCFC is a developing technology particularly in our country, evaluating the performance of this technology under different condition could be useful in further progress of MCFC. In order to evaluate the parameters of molten carbonate fuel cell, the impact of parameters such as pressure, fuel cell inlet temperature, steam to carbon ratio, utilization factor of fuel and utilization factor of air and carbon dioxide on the efficiency of the fuel cell is examined. Given that MCFC is a developing technology, particularly in our country, evaluating the performance of this technology under different conditions could be useful in further progress of MCFC. According to the sensitivity analysis conducted, parameters such as pressure, steam to carbon ration, fuel cell inlet temperature and utilization factor of fuel have the most impact on the efficiency of MCFC. With increasing pressure, efficiency is reduced from 56% to 42%. Also, increasing steam to carbon ratio, fuel cell inlet temperature and utilization factor of fuel increases the efficiency by 6. 2%, 7. 5% and 14. 3%, respectively. On the other hand, the utilization factor of air and carbon dioxide have the least impact on the efficiency of MCFC.

In the present paper, using the non-equilibrium molecular dynamic (NEMD) method, thermal conductivity coefficient of water based nanofluids was calculated and the influence of the metal nanoparticles type with four particle types including copper, silver, platinum, and gold inside a cooper nanochannel was investigated. Also, Pcff force field was used for modeling of the bonded and unbonded interactions among the molecules of water, nanoparticles, and the walls. The results, show that the silver nanoparticle has the maximum effect on the increasing of the nanofluid thermal conductivity coefficient. The interaction and tendency between water and nanoparticles were investigated by using of the radial distribution function (RDF) analysis and the diffusion coefficient of the nanofluid inside the nanochannel was then evaluated. Moreover, the thermodynamic properties of nanofluids including Cv and Cp were studied by using equilibrium molecular dynamic (EMD) method. The studies indicate that by adding nanoparticles to water, the specific heat value is reduced in constant pressure and volume, of which the minimum and maximum specific heat reduction is related to copper and platinum nanoparticles. By investigating the diagram of atoms’ density distribution, it was found that the highest density is related to the silver nanoparticle and the maximum thickness of water layer is formed alongside with the gold nanoparticle. The viscosity of the nanofluids was also calculated by two methods of equilibrium and non-equilibrium dynamics and it was specified that the viscosities obtained for the nanofluids were increased by adding the nanoparticles to water fluid.