مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 11 (بهمن 1400)

In the present study, the effect of external magnetic field on the process of droplet formation with different size and frequency in a flow-focusing micro-channel is numerically studied. Moreover, the influence of non-Newtonian properties on the droplet formation characteristics is investigated using two non-Newtonian Carreau and power-law models. In order to solve the continuity and momentum equations for unsteady, two-phase and incompressible flow, finite volume method is employed. A numerical algorithm based on volume of fluid (VOF) technique is used to determine the effect of Bond number (0 to 0.2) and power-law indices (0.3, 0.6 and 1.3) on the droplet formation process along with their size and separation time. In order to validate the numerical solution, the formation of Newtonian fluid droplets at different values of magnetic field strength is compared with the results of other studies and very good agreement was observed. The results of numerical solution show that Carreau fluid droplet in the Bond number of 0.2 has the highest volume, which is equivalent to the dimensionless volume of 1.56. Also, the process of droplet formation is more affected by the magnetic field than by the non-Newtonian model. In addition, with increasing field strength, droplet separation time increases and as a result, larger droplets with lower frequency will be formed.

The salient aim of this paper is the shape optimization of an airfoil equipped with Gurney-Flap for aerodynamic performance improvement. The optimization of the slotted Gurney-Flap for improving the aerodynamic efficiency and increasing the lift force of NACA 0012 is the novelty of this research. The Genetic Algorithm, Artificial Neural Network, and Computational Fluid Dynamics are employed for shape optimization. The optimization variables include the height and thickness of the GF; also, the thickness and position of the slot. All analyses have been conducted at Re=0.45×106 and AOA=8o to simulate the take-off phase. After validating numerical results, the optimization process was conducted with two different fitness functions of lift force and aerodynamic efficiency. According to the results, the geometry representing an optimized lift coefficient compared to the geometry with optimized aerodynamic efficiency has a considerably higher height. Furthermore, the thickness of the slot in the first geometry is lower than the second geometry. The first optimized geometry leads to a 21.64 percent lift coefficient increment; there is a 293 percent increment in aerodynamic efficiency due to the second optimized geometry. As a result, it can be indicated that the help of slotted optimized Gurney-Flap can provide the required lift force for Short Take-Off Landing distance.

Lead (Pb) is one of the heavy metals having high and long-term toxicity even at very low concentrations which limits the reusability and recyclability of industrial wastewaters. In this study, the removal and recovery of Pb from synthetic wastewater by a new form of precipitation method named crystallization process in a batch system was investigated. This process has gained increasing attention in recent years because of being in access, low cost, high efficiency and no need to recover the used materials. The efficacy of removal was dependent on the factors such as pH, initial lead concentration, carbonate: lead molar ratio and amount of seed crystals. The results of the experiments performed in this study showed that when the pH is in the range between 8-9, the initial concentration of lead is 100 mg /L, the molar ratio of carbonate to lead is 3:1 and the amount of seed particles is 0.25 g dissolved in 100 ml, the lead removal efficiency is obtained as 99%. AAS, SEM and EDS analyzes were used to confirm and justify the above results.

One of the important issues in civil engineering is creating a calm place and a sense of security for the residents of high-rise structures against the force of earthquakes and wind. Therefore, the use of control systems has been considered under dynamic loads. Tuned Liquid Damper is Affordable and useful device for controlling the vibrations of structure under dynamic lateral loads. In this study, the standard high-rise structure has been modeled in ANSYS software under earthquakes (far and near-field) and wind profile with a speed of 100 meters per second. The wind tunnel has been simulated and the interaction between wind and structure has been investigated. In order to reduce the responses of high-rise structures under far-field records (Elcentro 1940 and Hachinohe 1968) and near-field records (Northridge 1994 and Kobe 1995) and wind force, a TLD with a cube tank was used. The responses of the structure such as displacement, velocity, acceleration, pressure over to the structure and wind streamline over the walls of the structure have been analyzed and also, the aerodynamic and aeroelastic behavior of the high-rise structure against wind has been investigated. Averagely, the simulation results show that the Tuned Liquid Damper could reduce the maximum displacement of the structure to 16% under far-field records, 0.5% under near-field records and 13% under wind force.

An artificial lung can help patients waiting in line for a lung transplant or for heart bypass surgery as a respiratory aid. In this study, the incompressible and pulsatile Newtonian blood flow within a complete artificial lung model was investigated including inlet manifold, porous homogeneous medium, and outlet manifold. In this scale, the effect of variation of the expansion angle (15, 45 and 90 degrees), the stroke volume, the path length of the fibers on the artificial lung impedance was studied using computational fluid dynamics. The governing equations are discretized for numerical solution by finite volume method. Also, the turbulence model was selected by measuring the system impedance. In addition to the impedence, the shear stress distribution on the housing walls was investigated. The results showed that reducing the expansion angle, reducing the stroke volume and increasing the path length of the fibers will reduce the impedance of the system. The 45-degree model has been chosen as the appropriate model. Because not only its impedance is low, but also areas with low speed flow, which can lead to clot formation, are less than the 15-degree model. In order to have lower clot formation, it is better to have the artificial lung with the natural one in series.

In the present work, the flow inside a channel with cavity is investigated as a Scramjet combustion chamber. For this aim , the parameters such as L/D (cavity length to cavity depth), H/D (channel height to cavity depth) and varied Mach numbers are studied in the supersonic flow to investigate the effect of geometric parameters on channel flow in non- reacting conditions. In this work, the vorticity is used as mixing parameter. Two-dimensional Navier-Stokes equations are used to solve the steady-state flow. The density based method and standard k-ε Model are employed to numerical simulation. The results show that vorticity of boundary layer and thus mixing in flow is increased with growing of L/D, Mach number and having sweep angle for cavity. Geometries with larger H/D performed better than other geometries in terms of generating vorticity and reducing Total pressure loss. Although the H/D = 1 ratio has a higher recirculation than others, it will not be reliable for all supersonic flow because of its considerable total pressure loss and the survival of the oblique shock in some conditions.

In this study, the flow dynamics of a high aspect ratio rectangular liquid jet issued into parallel airflow was experimentally investigated. The liquid flow was emanated from a rectangular injector with a thickness of 0.64 mm and an aspect ratio of 21. The injector was set in the center of the test section and the effects of airflow on the liquid flow were evaluated. A particular holding mechanism was designed and built to minimize the induced perturbations on the liquid flow. To identify the physics of the liquid flow shadowgraphy technique and high-speed imaging were implemented. In order to provide a comprehensive study of the problem, the experiments were performed for a wide range of flow conditions and flow visualizations were presented. Jet Weber number and Gas Weber number were varied from 3 to 120 and 0.2 to 12, respectively Also, five regimes of the liquid flow including column, column/gravity, arcade, bag and multimode were recognized. A mapping with gas Weber number and momentum ratio as the determining variables, was suggested to distinguish these regimes from each other. Breakup length of the liquid jet was also measured. It was found that with the increase of jet Weber number the breakup length was increased at constant gas speed. Moreover, it was revealed that the breakup length was elongated with the increase of gas Weber number.

Pulsating heat pipes (PHPs) are one of the devices that can transfer a lot of heat while being simple and inexpensive. in industrial applications one of the major weaknesses of PHPs is their poor performance at angles close to the horizon. Previous studies have mostly been at 90, 60, 30, and 0 degree angles and have reported the weakness of PHPs in the 30 to zero angle range, but detailed studies has not been performed in this range. Therefore, the main purpose of this study is to experimentally investigate the performance of PHP at angles of deviation close to the horizon and to provide a more accurate critical inclination angle. the performance of PHPs was evaluated for the best filling percentage (60%) at different angles 0 to 90 degrees. the results showed that by reducing the PHP angle from 90 degrees to 15 degrees, the difference in thermal resistance for different heat input powers was very small, but this difference increased from angle of 10 to 0 degrees. Further studies showed that the percentage of difference in thermal resistance between angles 15, 10 and 5 with the average value of thermal resistance is 3%, 12% and 36%, respectively. Thus, it was found that the main weakness of pulsating heat pipes is from an angle of about 10 to 0 degrees.

In this study, the natural convection heat transfer within a triangular cavity with elastic diagonal walls containing a cylindrical heat source is investigated. The assumed fluid inside the cavity is air. The flexible diagonal walls of the cavity are considered to be at a constant cold temperature of Tc and the cylindrical heat source is at the hot temperature of Th. The governing equations are discretized using Galerkin finite element method and to describe the motion of the fluid, the arbitrary Eulerian-Lagrangian approach is used. In this study, the interaction of fluid and solid fields and the effect of cylindrical heat source position on flow and temperature fields are examined. For this purpose, the effect of Rayleigh number and changing the position of the heat source along the vertical centerline on the deformation of flexible walls, flow and temperature fields, and heat transfer rate are investigated. The results show that for a fixed position of the heat source, an increase in the Rayleigh number increases the maximum of the stream function, the average Nusselt number, and the deformation of the flexible walls. Also, the results show that the position of the heat source depending on the Rayleigh number has different effects on the temperature and flow fields.

Due to the volume and mass limits of the small electronic devices, thin flat heat pipes are an ideal solution for the efficient transfer and dissipation of heat. The performance of thin heat pipes is heavily dependent on wick structure characteristics. In this research, the thermal performance of thin flat heat pipes (TFHPs) with hybrid and grooved wick for different heat inputs were studied numerically, and their heat transfer characteristics were compared. The trends of various parameters such as wall temperature, maximum axial velocity, mass transfer at the liquid-vapor interface, system pressure, and thermal resistance on the thermal performance of the TFHPs with hybrid and groove wicks were analyzed. The numerical simulation has been done using a two‐dimensional unsteady incompressible laminar flow. Results indicated that the evaporation section temperature of hybrid wick TFHP is significantly lower than the corresponding value of grooves heat pipe. It was also observed that with increasing heat input, thermal resistance of hybrid wick TFHP decreased and it has an excellent performance compared to the grooved wick. For heat fluxes of 10, 20, and 30 W, the performance of the TFHP with hybrid wick compared to grooved wick is improved by 3.59%, 20.38%, and 28.57%, respectively. Therefore, the thermal performance improvement of the TFHP with the hybrid wick was more significant. This improvement is more considerable for higher heat fluxes.

The cathode side gas diffusion layer (GDL) in polymer electrolyte membrane fuel cells discharges out the water generated as a result of the electrochemical reaction through its porous medium. This paper criticizes the generated pore network models for GDLs assuming uniform injection of liquid water from the catalyst layer into GDL. These models lead to a roughly uniform distribution of liquid water saturation in the in-plane direction making no difference between under gas channel and under-rib regions which is in contradiction with the in-situ visualizations and imaging of GDLs. It has been attempted in this paper to couple the existing two-phase flow network models to other transport phenomena in GDL and also in other layers. To achieve this, the mentioned model is coupled to network models of oxygen and electron transport at the cathode side and also to a model of electrochemical reaction at catalyst layer and a proton transport model of the membrane. As a first result of modeling, the distribution of local water generation rate and also the temporal evolution of total water generation rate at catalyst layer are presented, the latter experiencing an approximate 50% reduction from start-up to steady state. The resulting water saturation distribution in GDL is strongly non-uniform, and maximums are observed under the ribs which is a direct result of non-uniform water generation at reaction sites.

Nowadays, due to the extensive application of renewable-based co-generation systems and also the economic and the environmental necessities, their design and thermodynamic analysis has been conducted by many scientists.In this way, a novel, simple, and practical combined power and hydrogen cogeneration unit has been designed in the present study in which there are gas turbine, gasifier, trans critical Rankine cycle, and proton exchange membrane electrolyzer.This system has been analyzed from first and second laws of thermodynamics by engineering equation solver(EES).The proposed system is able to generate power and hydrogen simultaneously for users.The power and hydrogen production capacities of the system are 3.92 MW and 608.8 cubic meter per hour, respectively which consumes biomass about 1.155 kg/s. The energy utilization factor and second law efficiency ofthe system are 34.71 % and 29.44 %, respectively. It can be seen that the overall exergy destruction of the system is 11854 kW in which gasifier, gas turbine, and combustion chamber have the highest irreversibilities. In addition, it can be concluded that the exergy efficiency of condenser and heat exchanger 3 are the lowest ones among other equipments.According to the parametric studies,it was found that the increasing the inlet temperature ofthe gas turbine has a positive effect and the increasing the maximum pressure ofthe TCO2 cycle has a negative effect on the EUF and the second law efficiency ofthe system.

The use of liquid metal magnetic hydrodynamic energy units, despite reducing maintenance costs and improving reliability, requires a high-temperature source, which must be supplied by fossil fuels. The present study aims to cover this shortage by proposing a new design for liquid metal magnetic hydrodynamic power and desalination cogeneration plant by applying concentrating solar power. The results show that 73.2 kW and 21.06 m3/day power and freshwater can be produced by the proposed cogeneration plant, respectively. The energy utilization factor  and total exergy efficiency are 97.45 and 26.34%. The results also indicate that the receiver accounts for the highest exergy destruction, followed by the heliostat with 270.4 kW and 240.9 kW, respectively. Increasing the efficiency of the humidifier/dehumidifier or reducing the mass flow rate of the second magnetic hydrodynamic loop improves the energetic and exergetic performances of the system. Besides, the receiver and solar tower have the highest cost of investment and maintenance, and the total unit cost of the system is 103.4 $/GJ.

In this paper, the emission of pollutants in a cinema hall has been investigated using three air distributin systems. The simulation was performed using computational method and turbulence modeling was large vortex simulation (LES). In this study, the concentration of toxic gases of carbon monoxide and carbon dioxide as well as the heat generated by the fire in the exit doors of the cinema hall and escape routes have been investigated. Among air distribution systems, displacement ventilation and impingment jet were better in control of soot around 31%, in control of carbon monoxide 16% and in control of carbon dioxide 11% rather than stratum system. So impingment jet took carbon monoxide to 6.3 ppm and stratum ventilation system around 7.5 ppm. In the case of CO2 gas, two systems of displacement ventilation and impingment jet are recorded 330 ppm and also stratum ventilation recorded 370 ppm. In controlling the heat output from the cinema hall doors, the displacement ventilation systems and the impingment jet in door number 1 are 66.6% and in door number 2 the impingment jet system is 96% better than the stratum ventilation system.

One of the available methods for water desalination is capacitive deionization (CDI). In capacitive deionization systems where saline water passes through a channel with high contact surface electrodes. By applying voltage, the ions are absorbed under the electric field at the electrode surface, thereby reducing the salinity of water and leaving fresh water out of the system. Researchers have recently proposed various models to predict the behavior of a capacitive desalination device. The model used for the simulation is a one-dimensional transfer equation. to predict the output water concentration and identify the parameters affecting the performance of the capacitive ionization system. This study aims to investigate the efficiency of water desalination using changes in system operating parameters. The parameters studied in this study were current flow rate, applied current and inlet concentration, porosity, electrode cross-sectional area, and electrode length. The results showed that the most effective parameter in improving the "Charging current" performance by increasing 1.5 times the Charging current flow, the water desalination percentage increased by 72%, and the time required to reach maximum water desalination decreased by 76%.