مجله مهندسی مکانیک امیرکبیر
سال پنجاه و یکم شماره 2 (خرداد و تیر 1398)

Water management in a proton exchange membrane fuel cell is numerically modeled by considering the 2D, non-isothermal steady flow assumptions. Governing equations are solved in all cell layers including cathode and anode electrodes by finite volume method using a single-region approach. The effect of gas cross-over through the membrane is studied on cell performance. This consideration, not only improves the general accuracy of modeling, but also makes it possible to model energy losses due to direct reaction of reactant gases. The effect of some key variables such as liquid water diffusivity, current density, membrane thickness, etc. on PEMFC conditions such as the amount of saturated liquid water, power density, cell temperature, cross-over efficiency and so on are examined. It was observed that the amount of saturated liquid water on the anode side is considerably important. This observation addresses needs for further investigation of liquid water behavior in the anode electrode. The amount of liquid water saturation in both the cathode and anode electrodes is increased with increasing the current density. The results showed that at the current density of 0.2 A/cm2, cross-over effect causes about 10% reduction in cell efficiency and by decreasing the current density this effect is enhanced.

In the present study turbulent wind flow field around inline surface-mounted and supported buildings has been investigated numerically. In order to model turbulence, re-normalization group k‑ε and realizable k‑ε turbulence models are employed. According to the numerical simulations, stream-wise velocity profiles around single surface-mounted and supported buildings are compared with the experimental data. Consequently, the validated model with realizable k‑ε turbulence models is used to simulate flow field around two inline surface-mounted and supported buildings. Results have been reported for two Reynolds numbers (17000, 170000). Approximately, same velocity field was observed for non-supported buildings at two flow Reynolds numbers. Although, for supported buildings small difference is observed in the velocity profile under and above the building. Comparison of results for non-supported and supported buildings shows that behind the supported buildings the near ground reversed flow region was removed and lead to the lower drag force on such building. Moreover, supports increase the reattachment length on the upstream building.

In the present work the flow around a horizontal axis wind turbine has been studiedusing large Eddy simulation at different rotational speeds. The results show increasing rotational speedscauses a higher velocity deficit in the downstream direction. For example, in 1D after the wind turbinethe minimum velocity is 54% of the initial velocity and reach to the 67% of the initial velocity after waketravel 6D. At the rotational speed of λ3= 10 the minimum velocity is 26% of the initial velocity and reachto the 68% of the initial velocity after wake travel 6D. The frequency of vortex shedding is increasedby increasing the rotational speeds. Shed vortices tend to be extended in the y direction and its intensityaugmented by increasing the rotational speeds. The strengthen of vortices at higher rotational directionin far wake region not only due to the increased of swirling strength, but it is also due to the collision ofvortices and the formation of new vortices. This issue has not been reported in previous works. Also, theincrease of turbulence intensity and Reynolds shear stress in the flow direction is due to the severe windshear and high mechanical production of turbulent kinetic energy.

In this paper, drying of the moist object is numerically investigated in the forced convection with and without the electric field. Finite volume method is used to solve governing equations of electric, flow, temperature, and the concentration fields in flow phase, as well as the temperature and the moisture fields in the moist object. In this study, the effect of applied voltage and the arrangement of the emitting electrode are evaluated. The results indicated that in presence of electric field, the increment of the applied voltage for 18 kV to 24 kV, the mass transfer from porous object 3.78 times and power consumption 7.96 times are increased. It is also found that the drying rate is increased by decreasing the distance between the emitting and collecting electrodes. According to numerical results, the mass transfer enhancement is usually accompanied by penalty of electric energy consumption. Therefore, the specific energy consumption has been evaluated as final criterion. It is shown that the specific energy consumption of the electrohydrodynamic drying process has been remarkably affected by the changing of the emitter arrangements. Finally, an optimum arrangement has been introduced as the affordable arrangement.

In this research , the effect of an electric field on the deformation and phase change ofa perfect conductive drop suspended in a perfect dielectric fluid is studied . Basic equations are theincompressible flow and energy equations . Electric field effect appears as normal stresses at interfacewhich are taken into account in solving flow equations . The level-set method is used for interfacetracking. Discontinuities at interface are imposed using the ghost fluid method. In the first step , the effectof an electric field on the hydrodynamic of a drop is studied . A good agreement between the simulationand experimental results is observed. Due to electric stresses , drop deforms in the direction of electricfield . The drop deformation increases with the electric capillary number. If the electric capillary numberexceeds the critical value, deformation will be unsteady. Novelty of this research is related to the studyof electric field effect on the drop evaporation. Based on the results, drop evaporation rate is enhancedin the presence of an electric field . If the electric capillary number exceeds a specific value (evaporationcritical electric capillary number) , drop evaporation will increase considerably . This critical value isintroduced in this research, for the first time.

In this study, Immersed Boundary-Lattice Boltzmann Method (IB-LBM) as a fluid solver is combined with Discrete Element Method (DEM) as a collision model. The consequences of this arrangement go to a numerical great model (IB-LB-DEM) which is capable to simulate particulate flows with second-order accuracy. To apply non-slip boundary condition, Eulerian velocities are interpolated in Lagrangian nodes using diffuse delta function. In DEM, two particles can penetrate to each other which this approach generates more realistic model rather other collision rules. Generally, in this model, the most important parameter is overlap distance between two particles which is directly related to amount of particles rigidity. The mentioned hybrid method is validated by simulation of dry-contact of two particles and sedimentation of single particle in vertical channel, individually. Finally, sedimentation of two circular particles in vertical channel is studied and effects of physical parameters such as rigidity, restitution coefficient and friction coefficient in particles behavior has been investigated. Finally, it is shown that increasing friction coefficient leads to increasing in kissing time that causes a change in particles path. For this particular model, It is also dedicated that restitution coefficient does not have significant effect in particles behavior.

In the present study, noise emission from a circular cylinder model with 22 mm diameterand 500 mm length has been experimentally investigated. For this purpose, the surface pressurefluctuations have been measured both in spanwise and azimuthal directions by employing miniaturecondenser microphones, Pa-WM-61A. All the experiments are carried out in a subsonic wind tunnel withthe turbulence intensity of 0.3% and maximum upstream velocity of 25 m/s. The results show that tonalnoise for velocities of 10, 15 and 20 m/s takes place at vortex shedding frequencies of 98, 142 and 186Hz respectively which correspond to typical Strouhal number of 0.2. Moreover, frequency of the firstand second harmonic occurs at two and three times of the vortex shedding frequency respectively. In thisstudy, the best collapses of the surface pressure spectra at low and middle frequencies can be obtainedusing the upstream flow scales whereas at high frequencies data are collapsed by employing downstreamscales at vortex formation location. Furthermore, the longitudinal and lateral coherences can provideadequate information about the lifespan (or, inversely, the decay) of eddies and their physical size.

Resistance and wave pattern due to the motion of an underwater vehicle model are obtained by experimental and numerical methods. The tests on the model are carried out in Shohada-e- Khalij-e-Fars National Marine Laboratory. The model is towed with the carriage at various speeds and two depths of submergence in the basin. The model is made from polyethylene and is attached to the carriage by a strut at the end. The strut at the end allows to measure the wave pattern of the body alone but the measured resistance is for the body and the strut. The computational fluid dynamics is used to study the interaction of body and strut and the resistance of the body is obtained by eliminating the effect of the strut. The wave profile is measured by four fix sensors at the transverse section of the basin. The wave profile is also obtained by computational fluid dynamics computations and compare with the experimental measurements. The numerical and experimental results are comply with each other. These experimental results can be used to validate and calibrate the numerical solutions.

In the present work to evaluate the cross wind flow effects due to the subway train motion on efficiency of condenser fans in air conditioning system, the behavior of fluid have been studied. Velocity profiles in output fan, temperature variations at near fan were studied at different velocity of train. In numerical analysis, for the turbulent and incompressible flow, Navier-Stokes and energy equations and k‑ε turbulence model has been used for modeling of turbulent flow. Variations of temperature and velocity of outflow of the fan at horizontal and vertical directions and the effective length as outflow guidance of the fan in opposite direction of train at difference velocity of train have been reported. At high velocity of train, negative output velocity of the fan and high effective length have been observed. Dimensionless effective length in high velocity of train at height of 10 and 20 cm were obtained 0.528 and 0.951 respectively. Finally, a parameter that is heat transfer rate to maximum heat transfer rate at height of 10 cm is defined which maximum amount is 5.88 percent. Due to the prevailing crosswind flow on the outflow of the fan, this parameter reduces.

In this study three dimensional fluid flow and heat transfer of Al2O3-water nanofluid in a triangular microchannel heat sink, consisting from seven isosceles triangular microchannels, have been investigated numerically by considering conduction in solid parts. The governing equations have been solved using finite volume method based on finite element and utilizing coupled algorithm. The objective has been investigating the effects of four inlet/outlet flow arrangements on flow field and heat transfer of Al2O3-water nanofluid. These arrangements consist of: inlet from the center of the north wall and outlet from the center of the south wall (I-type), inlet from the right side of the north wall and outlet from the left side of the south wall (N-type), inlet and outlet from the top and bottom parts of the west wall (D-type) and inlet from the upper part of the east wall and outlet from the bottom of the west wall (S-type). Also the effects of the Brownian motion of nanoparticles and temperature-dependent properties of the nanofluid have been considered. The results showed that increasing the nanoparticles volume fraction from 0 to 4% increases the average Nusselt number between 4.72% and 5.47, decreases thermal resistance between 1.81% and 2.34% and decreases the ratio of maximum temperature difference of heat sink substrate to heat flux between 1.28% and 1.56%. Also the results indicated that the I-type arrangement has a better heat transfer performance, lesser thermal resistance and provides more uniform temperature distribution. In this case, the I-type arrangement has higher Nusselt number between 1.69% and 18.33%, lower thermal resistance between 3.55% and 29.29%, and a smaller ratio of maximum temperature difference of heat sink substrate to heat flux between 5.23% and 36.25%, when compared with those of other arrangements. The heat sink performance characteristics have improved between 0.1% and 0.75% by considering the Brownian motion and between 1.9% and 3.9%, by considering temperature dependent properties.

Minimizing the number of axial flow compressor stages for a specific work output, and thereby lowering the engine size and weight has always been the designer’s goal. A major limitation on the pressure rise in a subsonic axial-flow compressor stage is boundary layer separation on the blade suction surface. One method of mitigating the suction surface separation is to employ tandem airfoil blades. Tandem blading is a method of increasing the flow deflection by delaying the separation in diffusing cascade arrangements. The basic concept is that a new boundary layer forms on the second (aft) airfoil, allowing for high overall loading without the large flow separations that would be seen with a single airfoil. The unsteady 3D flow fields in a single-stage compressor with tandem blades under designed conditions are simulated numerically to investigate the stage performance and the aerodynamic interaction between the blade rows. In this work, the Time Transformation method (TT) to stage modeling has been employed to predicting stage compressor performance. In the compressor, three main aerodynamic structures are responsible for the unsteadiness of the flow: the wakes, the corner stalls and the tip-clearance flows. The study of the aerodynamic structures is the subject of this paper.

In this study the flow patterns in downward air-water two-phase flow was studied experimentally and numerically. An experimental setup was designed and fabricated to allow visual observation and camera recording. The setup includes a transparent vertical pipe with a diameter of 50 mm and height of 4 m. Water and air were used in the experiments and flow map was prepared by data obtained from a total of 391 test cases by changing in air and water superficial velocities. Using flow pattern map, obtained from experimental results, simulation of two-phase flow in downward pipe has been performed. Multi-fluid model with Eulerian-Eulerian approach was used in Ansys-Fluent software for numerical simulation. Comparison of numerical with experimental results shows acceptable agreement for all expected regimes from flow map and it can be concluded that for downward two-phase flow patterns prediction numerical methods can be used. At the end, the numerical flow pattern map was plotted and compared with experimental results which also show good agreement. Experimental and numerical values of superficial velocities in transition boundaries were also compared.

Waves influence on breakwaters is vital for Prediction of harbors design. Floating breakwaters can be installed, displaced and can be used again in different conditions, even deep waters. But floating breakwaters are useful for special periods due to their complicated reaction to dynamic response of wave transmission. In this study, by an incompressible smoothed particle hydrodynamics method in three steps, coastal waves effect has been investigated on a pair of floating breakwaters and combination of floating-submerged breakwater. The floating breakwater behavior is assumed as a massspring system and the influences of inhibitor system tension and wind on the breakwater are neglected. For the validation, the oscillation amplitude variations are compared between the incompressible smoothed particle hydrodynamics results and experimental model which yields to a good adaptability. Breakwater hydrodynamic behavior is investigated versus the sinus-shaped wave different periods, less than 3 seconds. Based on results, using floating breakwater is optimized in wave period of lower than 2 seconds and The presence of a floating breakwaters in the vicinity of the immersion breakwaters helps to stabilize the pressure and reduce the fluctuations. It is also concluded that Floating breakwaters with heave displacement are better than floating breakwaters that are in sway motion.

In the present study, for the first time, the locally power-law preconditioning method for analyzing steady and unsteady incompressible turbulent flows around airfoils in high Reynolds numbers is utilized. In this method, the governing equations are modified by altering the time derivatives terms. The governing equations are discretized by the numerical method derived from the cell-centered Jameson’s finite volume algorithm. In addition, for solving the unsteady flows, an implicit dual-time procedure and for simulating the turbulent flows, Baldwin and Lomax algebraic model have been employed. The computations are presented for steady and unsteady turbulent flows around NACA0012 and ONERA-A airfoils at various angles of attack and Reynolds number. Results presented in the paper focus on the velocity, pressure and eddy viscosity profiles, distribution of pressure coefficient, lift and drag coefficients and the effect of the power-law preconditioning method on the convergence rate. The numerical solution indicates an acceptable accuracy with the aid of the power-law preconditioning method in both steady and unsteady turbulent flows for high Reynolds numbers. Moreover, using the power-law preconditioning method improves the convergence speed significantly and reduces the iteration number of solution steps and central processing unit time simultaneously in both steady and unsteady flows.

In the vortex-induced vibration the structure interacts with the fluid flow that results in the vibration of the structure and sometimes leads to the destruction of it. In this paper, the effect of the vortex-induced vibration on one and two cylinder(s) in the rotating and non-rotating states are studied, numerically. For the case of single non-rotating cylinder, the effect of the reduced velocity and damping ratio on the displacement and velocity of circular cylinder and also on the lift coefficient and its components are investigated. The cylinder displacement decreases by increasing the damping ratio. The vortex shedding pattern for all examined reduced velocity and damping ratio is in the 2S mode. In all cases, two patterns of vibration, harmonic and beating phenomena, can be observed. The maximum value of the vibration amplitude is related to the reduced velocity of 4 for ζ= 0 and the minimum is belonged to the reduced velocity of 3. For the case of rotating cylinder, it is observed that the rotation of cylinder(s) affects the vortex shedding pattern and their strength, significantly. Therefore, the fluctuation of lift coefficient and vibration of cylinder(s) are reduced considerably.

In this paper tip leakage flow structure of a transonic axial compressor rotor in different performance conditions will be simulated. Results have been presented according to a 3D numerical simulation of the viscous flow and solving Navier-Stokes, Continuity and energy equations using Ansys- CFX software. Initially, performance curves have been derived and compared with experimental results and have shown good agreement. Then, results have been obtained from three mass flow rates including design, choke and near stall conditions. The results have indicated that reduction of mass flow rate from chock to stall condition leads to increase in the tip leakage flow strength. This phenomena causes to more loss, especially in the blade tip region. In addition, position of shock line moves to upstream when the mass flow rate decreases. The tip leakage flow, shock and main flow contact cause a complicated flow structure near stall condition which leads to increase in entropy, vortex flow and blockage.

In this paper, the inertial and non-isothermal flow of viscoelastic fluid inside thesymmetric planar sudden expansion channel with an expansion ratio of 1:3 has been numericallyinvestigated in the range of Brinkman numbers (0.01≤Br≤20). The rheological and nonlinear model ofPhan Thien-Tanner (PTT) is used for modeling viscoelastic fluid behavior. The finite volume method(FVM) is employed to discretize the governing equations and the PISO algorithm is used to solve theseequations simultaneously. Due to the significant effect of temperature changes on the viscoelastic fluidproperties, these properties are considered as temperature-dependent and the viscous dissipations termis considered in the energy equation. The main purpose of this study is to investigate the effects ofBrinkman numbers on the heat generation by viscous dissipations term used in the energy equation.Therefore, the streamlines, the length of vortices, the isothermal lines, the distributions of velocity andtemperature and local Nusselt numbers have been examined in the channel expanded part. The resultsshow that for the hydrodynamic and thermally developing zone, the maximum value of the local Nusseltnumbers on the walls of the channel expanded part is located at the end of the first and second vortices.

Numerical simulation of blood flow in Configurations is the aim of this study. In order to predict the best configuration in a patient with double stenosis 65 and 50 percent is examined at rest. The computational domain was created from CT. In this study, blood is assumed homogeneous, non- Newtonian and pulsatile. To consider non-Newtonian effects the Carreau model is used and, a Seven– element lumped model is used in outlet coronary artery and a Three-element Windkessel model is used in outlet aorta. Results of the Comparison between configurations indicate that is TAWSS minimum is less than the critical value 0.4 in the junction in parallel with the host vessel, In bed and the surrounding junction, furthermore OSI maximum is more than critical value 0.1, Because of sharp curvature of graft branch on the vascular graft. The parallel sequential is more prone than cross with respect to fat accumulation and diseases such as intima hyperplasia. The results of this study indicate by doing bypass surgery, sequential parallel and cross can be improve critical values of shear stress in the area of stenosis and it reduced the risk of rupture of plaque fat and moves the heap toward downstream vessels.

Nowadays, according to the pollution and reduction of available water resources, accessing to fresh water for agricultural and drinking purposes are restricted. In this paper, the solar subsurface irrigation system, which is one of the most efficient ways of fresh water production has been simulated. This system consists of a pool as a humidification unit and pipes that are buried in the soil as a condenser unit. In this simulation eight parameters including relative humidity, water pool temperature, pressure, pipe temperature, inlet air cross section, air temperature, air velocity and pipe diameter have been considered in three levels and 27 tests to calculate the amount of fresh water were presented using Taguchi method. In order to maximum use of sunlight, the mirror was used. The results showed that the lower humidity, pipe temperature, velocity and inlet air cross section increase the amount of water production. In addition, the higher pipe diameter, water temperature and air temperature is directly proportional to the amount of produced water. The optimal pressure is 95 kPa. The optimal amount of fresh water obtained from Taguchi method is 5.15 kg/m.day which shows an error of 5.23% compared to simulation result.