مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 3 (خرداد 1400)

Effects of flapping on lift and thrust forces in a 3D flapping wing has been investigated at low Reynolds numbers and several reduced frequencies, using experimental tests in a subsonic wind tunnel. Tests has been performed at Reynolds numbers 42000 to 170000 and reduced frequencies 0 to 0.45 that most birds flight at this ranges. Also the ranges of angle of attacks are between 0°-24°. Results has shown that increase of reduced frequency can enhance the lift force by up to 100 percent and in some cases reduce drag force to zero. Furthermore increment of reduced frequency has caused delay at stall of wing. Also by increasing the Reynolds number from 42000 to 86000, major region of the boundary layer of wing surface becomes turbulent, so maximum lift force increases by 40 percent. Wind tunnel test results show that the effect reduced frequency on the lift force was dependent on the angle of attack, so at the lower attack angles, the increase of reduced frequency did not affect the lift coefficient, but, with increment in angle of attack, the positive effect of the reduced frequency on the coefficient of the lift force increased.

In the present study, effects of various parameters on the simulation of flow around an H-type vertical axis wind turbine with NACA0018 airfoils is studied. All computations are carried out by computational fluid dynamics method using finite volume approach. Free-wind velocities of 5, 10 and 15 m/s and tip speed ratios of 3 and 5 are considered. All obtained results are compared with experimental data and show good agreement. Examination of obtained results reveal that by increasing free-wind velocity, maximum momentum coefficient occurs at higher azimuth angles. Also, by reducing tip speed ratio, fluctuations in the rotor due to the penetration of more volume of air into the rotor increase, thus lifecycle and performance of wind turbine reduce. Furthermore, the effect of tip speed ratio on the performance of wind turbine is more significant than free-wind velocity so that by increasing tip speed ratio from 3 to 5 at a constant velocity of 10 m/s, power coefficient increases by 81/87% and by increasing free-wind velocity from 5 to 10 at a constant tip speed ratio of 3 power coefficient increases by 58.2%.

Improvement of an axial compressor performance through suppression of flow disturbances due to inlet flow blockage, utilizing air injection at the blade tip region, is the subject of the present study. Method of investigation is based on experimental measurements conducted in a low speed isolated axial compressor rotor blade row. Four different blockage screens of different blockage ratios ranged between 5 and 20 percent of the inlet area are located at the compressor entrance. Instantaneous and time-averaged static pressure are recorded at different locations of compressor casing. Frequency analyses of pressure signals, show that the flow disturbances are being created in the presence of the blockage screens. These disturbances cause to appearance of rotating stall in the flow field when the compressor operates under distorted inflow with low blockage ratios (5 and 10%). To reduce the destructive effects of the inlet distortion, air is injected at the tip region of the rotor through 12 injectors which are located evenly spaced around the compressor circumference. Air injection in small quantities, 1.5% of main flow, has considerable effects on the compressor performance under inlet distortion. The rotor performance and compressor delivery pressure is improved up to 35% than to the no injection case. By implementation of Short-Time Fourier Transform (STFT) technique effects of air injection on elimination or reduction of flow disturbances are demonstrated, too.

In this research, a transparent PEM fuel cell is designed and manufactured to visualize the anode and cathode side flow channels and study flooding phenomenon in them. Manufacturing challenges are found, and cell design is simulated prior to manufacturing. Restrictions are expressed in the first place and then practical solutions are presented to cope with them; materials and dimensions are then determined accordingly. As the second step, the cell’s operation is simulated and the polarization curve is extracted using a steady-state two-phase computation fluid dynamics model. After the manufacturing of all components, due to the high influence of the torque of the bolts on the leakage, lifetime and operation of the cell, this parameter is optimized using polarization curves. A torque of 1N.m for the bolts is found to be optimum. In order to validate the numerical model, the polarization curve of the model is compared with that of the experiment. An error of 6.76% demonstrates the suitable accuracy of the numerical model. Finally, the accumulated water on the cathode side is detected using direct visualization and image processing technique.

Effects of a controllable axial flow on the stability of rotational flow of pseudoplastic fluids in the gap between concentric cylinders are studied. It is assumed that the outer cylinder is fixed while the inner one has simultaneous and independent rotational and translational motions. The fluid follows the Carreau–Bird model and it imposes mixed boundary conditions. The four-dimensional low-order dynamical system resulting from Galerkin projection of the conservation of mass and momentum equations includes highly non-linear terms in velocity components originating from the shear-dependent viscosity. In the absence of axial flow as the pseudo plasticity effect increases, the critical Taylor number decreases, at which the rotational low loses its stability to the vortex. The appearance of the vortices corresponds to the onset of a supercritical bifurcation, which is also seen in the rotational flow of a Newtonian fluid. The existence of an axial flow induced by the translational movement of the inner cylinder appears to make the vortex flow more evident. Complete flow analysis together with viscosity maps are given for the entire flow field.

Cellular metabolism is strongly dependent on the sufficient and continuous supply of oxygen. Cellular oxygenation is performed by blood flow. Blood flow and oxygen delivery to cells can be influenced under several conditions such as arterial stenosis. In the present study, the oxygen delivery to the arterial wall of a precise model of the Main Left Coronary artery (LM) and its two main branches for normal artery and stenosis degrees of 75% and 84% is numerically investigated. For all simulations it is assumed that the flow is steady, blood is Newtonian, and the arterial wall is rigid. Transported oxygen by hemoglobin as well as oxygen consumption in the vessel wall are considered for determining the oxygen content in the vascular tissue. The results of oxygen concentration in the lumen and vascular tissue is validated with a benchmark study. The results indicate that, centrifugal forces and secondary flow are formed due to curvature and results in a significant reduction in the mass transfer of oxygen to the myocardial wall relative to the epicardial one. Arterial stenosis results in areas of low oxygen concentration with 3-4 mmHg less than normal artery, that increase the likelihood of hypoxia in these areas. Finally, the results show that 12.8% reduction in P50 does not have a significant effect on the oxygen concentration in the lumen and vascular tissue.

In this paper, the effect of blowing in a rod on the flow structure and its noise in a rod-airfoil is investigated. To this aim, the simulation of the flow around the rod-airfoil was performed using URANS equations and employing k-ω-SST turbulence model. . The prediction of the flow-induced noise is performed using F-WH analogy. Since Vortex's periodic producing is the main cause of the noise mechanism, by reducing its effect on the airfoil leading edge, the acoustic propagation reduces as well. In the present study, in order to control flow and reduce noise, the blowing active control in the rod has been used. The intensity of the blowing (I) is chosen between 0.1 and 0.5 where I is the ratio of blowing velocity to the inlet freestream flow). The results showed that increasing the blowing intensity to 0.5 reduces the noise emitted from the rod by 90% and the airfoil and rod-airfoil by 64%. In addition, by applying blowing, the lift force is increased and the drag force of the rod is reduced, which is aerodynamic favorable. In addition, the vortex shedding frequency is decreases while blowing applied.

Selection the appropriate anastomosis angle for the creation of a fistula for surgery is very important. Therefore, in this study, three anastomosis angles of 45, 90 and 135 degrees, representing acute and obtuse angles, are designed and simulated in a complete pulsation cycle. Carreau non-Newtonian blood model is used and the flow is considered as incompressible flow. Finally, after modeling, important parameters such as TAWSS, OSI, RRT and maximum pressure drop are extracted and compared at different angles. After comparing the results, it is observed that the time average wall shear stress and the range of the high shear stress at anastomosis angle of 135 degree is lower than the two other angles and the probability of thrombosis disease in this angle is reduced. 80% of fistula failure is caused by thrombosis disease, therefor this angle is chosen as the most appropriate angle for fistula creation. Based on the results of the RRT in which are shows areas of sediment, it is found that at all three angles of anastomosis, the right branch of the fistula and flow separation sites have probability of sedimentation and it decreases at angle of 135 degree. This angle also has the lowest pressure drop between the main inlet and the fistula outlet.

In this study a two-dimensional (2D) numerical simulation using two-phase level set method (LSM) have been carried out to investigate the influence of continuous phase entrance flow rate on the microdroplets generation process. The analysis of the breakup process of microdroplets in immiscible liquid/liquid two-phase flow in T-junction microchannel has been predicted. Governing equations on the flow field have been discretized and solved by using finite element method. Obtained numerical results have been validated by comparing the experimental data reported in literature which show acceptable agreement. The simulation results show that the continuous phase entrance flow rate has a major effect on the size of generated droplets. Studies have shown that pressure diagram of the junction point can reflect the number of formed droplets and the triple stages of droplet formation. Also, Examinations of the pressure and velocity gradient inside the main channel show that the pressure difference of the droplet’s tip and rear and shear force caused by viscosity dominates the droplet formation which the pressure difference between two side of droplet is more effective.

The Multizone model is one of the most popular models for simulating indoor energy and airflow; however, the basic problem of this model is that it cannot provide detailed and accurate airflow characteristics in the computational domain. To obtain the detailed airflow information, CFD model can be used but its high computational cost restricts the applicability of this method. Therefore, it is necessary to develop a model which can provide detailed airflow information with a reasonable accuracy and computational time. In this study, Fast Fluid Dynamics (FFD) method which has an unconditionally stable algorithm is proposed. To investigate the functionality of FFD model, four case studies of flow in a lid-driven cavity, flow in a plane channel, natural flow convention and forced flow convection are analyzed and the results are compared and validated with the results of CFD model, experimental data and analytical solution. The main focus of this study is increasing the simulation speed of FFD algorithm. For this purpose the sequence of equations has been modified and an efficient numerical method has been applied to solve the equations. Using the proposed FFD solver the simulation time of the case studies has decreased between 52 to 94 percent compared to the CFD and a faster than real time simulation has been achieved on a regular computer system.

Two-phase flow instabilities are occurred in many systems such as turbo machinery, refrigeration systems, boiling water reactors, two-phase heat exchangers and nuclear reactors. Fluid flow parameters such as pressure drop, stability range in boiling process are the determining factors in design and operation condition of two-phase flow equipment. In this paper, analysis of two-phase flow instability in boiling process is investigated and a simple and comprehensive model is modified to express pressure drop. The defined model and nondimensional numbers give a comprehensive sight of different parameters effect on the oscillations. By using Lyapunov stability analysis, conditions in which instability occurs are identified. The effect of parameters on diagram of pressure drop versus mass flow rate are investigated and the existence of extremum is discussed. The oscillation form varies according to the value of the basic oscillation damping parameter from an elliptical orbit to a quadrilateral, corresponding to the pressure drop curve. In addition, by nonlinear analysis, variation of the oscillation period and amplitude is examined and its relation to the parameter of systems is investigated.

At the present study, nucleation pool boiling of the saturated refrigerant R-245fa on the horizontal tube at pressure of 123.8 kPa and temperature of 20°C under different heat fluxes (18-27 kW/m2) has been simulated numerically and the details of the flow are investigated. Numerical simulation is carried out based on the volume of fluid model with piecewise linear interface construction method, without pre-determined bubbles by Lee phase change model and surface tension model of continuum surface force. The importance of this study and the numerical model, in addition to its industrial applications at nuclear reactors and flooded evaporators, is the model validation at predicting boiling portion of forced falling or climbing flows on horizontal tubes or bundle of tubes. The heat transfer coefficient of the present model, in comparison with the experimental data at two heat fluxes of 18 and 24 kW/m2 and the results of pool boiling Cooper correlation at heat fluxes of 18-27 kW/m2 has the maximum error of 6.67% and 8.64%, respectively. In addition, at this study, bubble nucleation and its departure from the tube wall and superheated liquid beside the wall, liquid and bubble temperature and modality of fluid movement due to the generated bubbles around the tube have been studied.

In this paper, the heat transfer of the forced convection of nanofluid in a annular porous channel on internal and external walls is numerically investigated. The nanofluid is simulated using the dual phase and flow model in the porous region by the Darcy-Brinkman-Forcheimer model. The fluid flow is considered as two-dimensional, laminar, steady, axi-symmetric axial and incompressible. The porous medium is uniform and homogeneous, and the physical properties of the nanofluid and the porous medium are assumed constant. The governing equations have been solved using the finite volume method and SIMPLE algorithm. The effect of parameters such as Darcy number, porosity barrier height and thickness, thermal conductivity of porous region to fluid and volume fraction of nanoparticles on flow field, heat transfer and pressure drop have been investigated. The results show that the use of porous barriers in the flow path leads to significant changes in the characteristics of flow and heat transfer. Reducing the Darcy and Reynolds numbers leads to the formation of a vortex behind the barriers that have a significant impact on heat transfer. By reducing the Darcy number, the heat transfer increases significantly. It will also cause a severe pressure drop in the flow. Increasing the thermal conductivity ratio of the solid-liquid matrix will increase the local Nasslet number of the wall in the area of the barrier, which will be more significant in the high permeability. Increasing the height of the porous barriers reducing the thickness of the boundary layer and increasing the heat transfer.

In the present study, the new ADI-CEIDD approach is proposed by combining the ADI method with the explicit-implicit domain decomposition method. The method is used for solving the two-dimensional conduction heat transfer equation on GPU .In this method, an explicit numerical scheme is used to predict values at inner boundaries and an implicit scheme based on the ADI method is used to solve the sub-domains. Then, an implicit scheme is used to correct the values on the inner boundary. The present method increases the number of independent sets of equations and enables more threads to occupy the device. Numerical experiments are done to investigate the accuracy and speed of the method. The results show that the ADI-CEIDD can achieve a speedup of 1.3 to 2.6 times compared to the ADI method. By increasing the number of subdomains from 2 to 32, the speed of the proposed method is increased up to 1.6 times and the accuracy decreases. Although the error of the presented method is higher than the ADI method, numerical experiments show high stability of the ADI-CEIDD. Furthermore, the results show that the ADI-CEIDD method is more advantageous to problems with coarse grid. By increasing the grid size from 256 * 256 to 512 * 512, the value of the Sp decreases from 2.4 to 1.7.

In the present paper, simulation of laminar natural convection heat transfer of 〖Fe〗_3 O_4 -water nanofluid in a circular enclosure has been carried out numerically using Buongiorno two phase model. A porous layer is inserted into the hot wall of the enclosure and applied constant external magnetic field caused to MHD effect in the cavity. The simulations are performed utilizing two phase model and nanoparticle concentration distribution is presented. All of the equations are solved in dimensionless form. Influence of the porous layer existence on heat transfer enhancement nanofluid flow is studied. The control parameters in this study are Darcy number (1e-6 ≤Da≤1e-1), angle of applied magnetic field (0≤γ≤90), Hartmann number (0≤Ha≤200), effective conductive heat transfer coefficient of porous layer ( 10≤k_eff≤100), Rayleigh number ( 1e3≤Ra≤5e5), geometrical parameters like porous layer thickness (0.01 ≤t_Layer≤0.09) and central angle of cavity (0≤θ≤90). The gained results which are derived in form of plots, contours and streamlines show the dependency of Nusselt number to control parameters. According to the results, any changes in Darcy number causes to Nusselt number variations and also there is a specified Darcy number that heat transfer reduces by increase of Darcy number. Moreover, by increment of Hartmann number, leading to higher Lorentz force, the average Nusselt number will reduce because the momentum of fluid flow and consequently convective heat transfer decrease inside the enclosure.

In this paper, the heat transfer coefficient and pressure drop are investigated by creating a hole between the vortex generators and their proper positioning. For this purpose, a numerical modeling of water flow in a rectangular channel in two laminar and turbulent flow regimes and for 5 models with different positions of vortex generator and hole including vortex generator in left and right, hole in top and bottom and Finally, both the vortex generator and the hole in the middle of the fin are provided for constant-size geometric parameters. In order to create a better comparison between the these positions, two relative ratios j/f and j/f1/3 are introduced and used. The results showed that in both flow regimes, by placing the hole in top and bottom of the trapezoidal fin, the pressure drop increased and the hydrothermal performance decreased, and the trapezoidal fin has the best hydrothermal performance when the vortex generator is in the left and right and the hole in the middle. So that the highest values of 0.0539 and 0.01504 are obtained, respectively, at the maximum and minimum Reynolds numbers for j/f and j/f1/3 in the turbulent flow.

The heat transfer and pressure drop of the air flow inside the square channel with perforated ribs are investigated experimentally. The effect of the perforation on the reduction of the low pressure region behind the ribs, and thus reduction in overall pressure drop, is evaluated. The Reynolds number is between 15000 and 50,000. Two values of 0.1 and 0.14 are selected for the ratio of the ribs height to the channel hydraulic diameter and three values of 20, 25 and 30 are considered for the ratio of the ribs pitch to the ribs height. The ratio of the hole diameter to the rib height are 0.3 and 0.5 respectively. It is found that the perforated ribs result in less pressure drop as well as reduced Nusselt number. Moreover, the effect of perforation is found to be unfeasible in the smaller pitch ratios. On the other hand, in long ribs, the perforation improves the performance coefficient up to 17%, the improvement is more evident in low Reynolds. In particular, for applications such as gas turbine or heaters, where the prime concern is to lower the wall temperature or increase the heat transfer and the blower energy consumption is less important, the current results show that using holes in the ribs can effectively enhance the flow thermal performance.

In present paper, an experimental study using pine wood in solar steam generation is carried out. The unique properties of wood such as high porosity, hydrophilicity, lightness and low thermal conductivity have led to consider for localizing light on water surface and generating steam in this paper. At the first step, steam generated by water and the pine wood which can float on the water surface was compared. The results showed that the wood as a surface membrane can improve the evaporated mass, so that its evaporation rate increased to 26.3% of water. To enhance the light absorption via wood and evaporation rate, wood’s surface was carbonized with a metal plate in temperature 400°C. In addition, the optimum thickness and the effect of the duration of the carbonization process were tested. According to the results, the optimum thickness of carbonized wood and carbonized time were 10 mm and 150 s respectively. However, the using carbonated wood enhanced the evaporation rate about 1.86 times larger than water and allocated evaporation efficiency of 64.2 % to itself.

In this paper improvement of efficiency of a household gas water heater has been investigated experimentally. For this purpose, six baffles have been used for deceleration of the hot exhaust gases in the middle tube of the water heater. Three of the baffles have almost the same geometry and dimension and the other three are different from geometry and dimension points of view. Also the same distances between the baffles have been considered. The geometry and dimension of the baffles have been chosen by experimental tests. Experimental results show that by employing the optimum geometries and dimension of the baffles and by selecting the optimum arrangement of the baffles, the thermal efficiency of the household gas water heater has been improved and increased up to 82.5 percent. Experimental results also show that by improvement of the thermal efficiency of the household gas water heater, pollutant emissions which have been measured by the gas analyzers at the laboratory remained almost at the lowest level. Finally by taking into account the effective parameters on the uncertainty for the thermal efficiency, in terms of the level of almost 95 percent for insurance against the reported number for the efficiency, the total uncertainty of the final optimized model for the thermal efficiency is ± 0,17.