میدان مغناطیسی

This paper considers numerical simulation of MHD forced convection of Graphene-water nanofluid in a channel filled with porous media. To this aim, non-dimensional form of the Darcy-Brinkman-Forchheimer equations in non-equilibrium conditions are adopted and solved through programming in FORTRAN software. Simulations are undertaken according to the thermal lattice Boltzmann method with single relaxation time, adopting three distribution functions for velocity, nanofluid temperature, and temperature of the porous medium. Then, effects of different parameters including the Darcy number, the medium porosity, the nanoparticles fraction, and the Hartmann number on the Nusselt number and the local thermal-non-equilibrium are analyzed. The results show that with increase in the Darcy number, the nanoparticles fraction, and the medium porosity or decrease in the Hartmann number, the Nusselt number increases. It is also found that the local thermal-non-equilibrium has direct relation with the Darcy number and the medium porosity and inverse relation with the Hartmann number and the nanoparticles fraction.

In the present study, the effect of the angle and pitch of the spiral pipe on the rate of heat transfer through the wall of the pipe to the fluid passing through it in quiet, transition and turbulent flows has been studied Next the changes in Nusselt number due to the addition of nanoparticles TiO2 were investigated. Finally, the effect of applying a magnetic field with an intensity of 1 Tesla on the rate of heat transfer in different situations has been investigated. To do this, the spiral tube is considered in three different pitches of 10, 12.5 and 15 cm and three angles of 0, 45 and 90 degrees. According to the results, the Nusselt number increases with decreasing pitch and in different Reynolds, the Nusselt number reaches its highest value in 10 cm pitch. This number also changes with changing angle and has the highest and lowest values at 45° and 90° angles, respectively. Then TiO2 nanoparticles with mass percentages of 0.1%, 0.3% and 0.5% were added to the fluid, which increased the Nusselt number. It was observed that increasing the concentration of nanoparticles causes a higher Nusselt number. Finally, Hartmann number was calculated for the spiral tube, the maximum value of which was obtained in pitch of 15 cm and a concentration of 0.5% TiO2 and was equal to 43.83.

Nowadays, liquid cooling channels are used for cooling of electronic equipment. These channels have to change the cross-section to pass over elements, so they undergo sudden contractions or expansions. These changes create areas that are unfavorable for heat transfer. Various methods have been proposed to improve heat transfer in these areas. In the present study, the possibility of using a non-uniform magnetic field to improve heat transfer in a millichannel including two sudden contractions has been discussed. In this study, it is assumed that a ferrofluid (EMG-805-a commercial brand) flows in the channel with a laminar regime, fully developed and steady state conditions. The walls of the channel are adiabatic, while the walls of the steps, which are in the vicinity of the electronic element, have a constant heat flux. The effects of different parameters, including locations of dipoles on the bottom and top walls, the number of dipoles, the Reynolds number, and the strength of the magnetic field on the improvement of heat transfer have been investigated. The obtained results show that the increase in Reynolds number and the strength of the magnetic field causes an increase in local Nusselt. The results show that applying a magnetic dipole on the bottom wall and just after the steps results in a significant increase in the local Nusselt number and an increase of 164.05% in the average Nusselt number compared to the case without a magnetic field.

In this study, the hydraulic-thermal performance of a semi-porous wave channel with nanofluid flow and applied magnetic field has been evaluated. The magnetic field is perpendicular to the channel. In this design, single-phase, incompressible and permanent nanofluid flow is considered. The ranges of Hartmann number and Darcy number are 0 ≤ Ha ≤ 10 and 10-5 ≤ Da ≤ 10-2, respectively. Magnesium oxide nanoparticles have been investigated in four different volume fractions (0, 2, 4 and 5%). The governing equations are solved by the finite volume method. Based on the obtained results, increasing the volume fraction of nanoparticles and channel wave improves heat transfer. At constant Reynolds number, increasing the number of wave channels from 4 to 6 resulted in a 7.8% decrease in thermal hydraulics. The increase in permeability in the porous medium has increased the Nusselt number and reduced friction. The best thermal hydraulic performance is 10.08 at Darcy number 0.01 and the lowest is 0.52 at Darcy number 0.0001. Also, the presence of magnetic field has a positive effect on thermal performance. The results of this study can be useful in the design of heat exchangers.

The most obvious feature of heat sinks is their ability to transfer heat and their cooling properties. In this research, a vertical single-channel active heat sink with Galinstan liquid metal fluid was used and the discretization of Navier Stokes equations was done using the second-order upwind finite volume method. Investigation of Mixed Convection heat transfer with Richardson numbers 0.45, 1 and 10 has been done in both directions of flow from top to bottom and flow direction from bottom to top and the effects of external magnetic field in two directions perpendicular to the flow axis have been investigated. The results showed that the flow direction from bottom to top with a Richardson number of 10 without the presence of a magnetic field improved the Nusselt number by 11.30% compared to the flow direction from top to bottom. With the Richardson number of 1 and the flow direction from bottom to top, the effect of applying the magnetic field in the Z direction (perpendicular to the current axis) with the Hartmann number of 129, 164.5, and 194, respectively, is 11.29, 13.63, and 15.88 percent of the Nusselt number has been improved. With the Richardson number of 1 and the flow direction from the bottom to the top, the effect of applying the magnetic field in the X direction (perpendicular to the flow axis) with the Hartmann number of 64.6, 129 and 194, respectively, is 7.08, 8.28 and 8.76% of the Nusselt number has improved.

Spring back is one of the common defects in metal forming processes, especially in the bending process, which occurs due to the release of elastic strains after loading. To minimize or eliminate this phenomenon, new methods such as pulsed electric current and electromagnetic forming are used. In this study, the use of magnetic field to reduce the spring back is used for the first time. The research was conducted on 1050 aluminum alloy and it was observed that magnetic field causes the reduction of spring back. Performing the tensile testing, it was determined that exposing the magnetic field, during the plastic deformation, causes the increase in the strength (i.e. the flow stress). So, the existence of a plastic region with higher strength in the neighborhood of the elastic region was distinguished as the reason of the reduction in the spring back. Moreover, the effective parameters on spring back, including sheet width, mandrel speed and rolling direction were investigated for both cases of with and without exposing the static magnetic field. Response surface method was used to express the spring back as a function of these parameters and analysis of variance was used to investigate the effectiveness of theses parameters. It was determined that the magnetic field can reduce the effectiveness of the rolling direction and spindle speed on the spring back of the studied sheet in bending; while, it has no effect on the effectiveness of the sheet. Finally, the change in the mechanical properties was described by magneto-plastic effect.

A strong magnetic field provides a new method of heat transfer with high heat flux. A numerical simulation for a heat sink with high heat flux under an external uniform magnetic field in three different directions is used to investigate the flow field and displacement heat transfer between liquid metal and hot surfaces. Due to its high density and large thermal and electrical conductivity coefficients, gallinsten liquid metal has been used as a working fluid. Discretization of the Navier-Stokes equations is performed by the upstream second-order finite volume method. The results show that the effect of applying a magnetic field in the Y and Z directions (perpendicular to the flow axis) on the heat sink with a Hartmann number of 88, improves the displacement heat transfer coefficient by 15% and 8%, respectively. The best efficiency in increasing the heat transfer was obtained by applying the magnetic field in the Y direction. By applying the magnetic field in the Y direction to the heat sink, the displacement heat transfer coefficient was increased by 11.9% for Hartman number of 44, 15% for Hartman number of 88, and 17.7% for Hartman number of 132, compared to zero Hartman number. By applying the magnetic field in Z direction to the heat sink, the displacement heat transfer coefficient was increased by 4.3% for Hartmann number of 44, 8% for Hartmann number of 88, 11.4% for Hartmann number of 132 and 22.1% for Hartmann number of 330, compared to Hartmann number of zero. Also, the results show that the effect of applying a magnetic field perpendicular to the flow axis has increased the velocity gradient. As a result, the pressure drop and friction coefficient of the heat sink have increased.

Increasing the heat transfer rate in various industries in order to improve the efficiency of equipment, prevent damage to parts and reduce costs is one of the essential discussions in the industry. One of the solutions to increase heat transfer is the use of active heat sink. In this research, an active heat sink with Galinsten liquid metal fluid was used and the discretization of Navier-stokes equations was done using the second order upstream finite volume method. The effect of applying the magnetic field in the Y direction (perpendicular to the flow axis) to the heat sink has caused the creation of a force against the flow direction called the Lorentz force, which has caused the M-shaped velocity distribution. According to the constant flux boundary condition, increasing the flow velocity in the vicinity of the walls has caused the surface temperature to decrease and the heat transfer to improve. The results showed that the effect of applying a uniform external magnetic field in both Y and X directions with a Hartmann number of 517 improved the Nusselt number by 38% and 13%, respectively, compared to a Hartmann number of zero. The effect of applying a magnetic field in the Y direction to the heat well with a Hartmann number of 517, 38%, Hartmann number of 258, 22% and Hartmann number of 129, 13% has improved the heat transfer.

The problem of tracking control is addressed for rigid bodies with interior shallow-water sloshing. The liquid motion is modeled by the Saint-Venant equations, coupled with the ODE of the rigid body, leading to a global system with an ODE-hyperbolic PDE cascade structure. The paper aims to design an innovative boundary feedback framework for a pre-specified position to deal with rigid body tracking errors. Using only one control force applied to the rigid body, the formulated strategy efficiently stabilizes both the finite- and infinite-dimensional states. The main complexity lies in the fact that no sensor can be implemented in the liquid domain. Indeed, the proposed stabilizing feedback law simply requires measurements of (i) the rigid body position error and velocity and (ii) the liquid pressure at the cavity walls (liquid boundary). The asymptotic stability of the closed-loop system is analyzed using the Lyapunov direct method and LaSalle’s invariance principle without any discretization, reduction, and linearization. Additional controller features are highlighted by simulation results, including its benefits in contrast to the corresponding PD controller and its robustness to time delay and system uncertainty.

In the present work, a special type of concentric two-pipe heat exchangers has been analyzed, in which the inner tube of the heat exchanger is considered as helical grooved. The turbulent flow of water-aluminum oxide nanofluid is used on both sides of the heat exchanger and a constant intensity magnetic field is used to enhance the effect of using the nanofluid. The effect of using this system as well as the use of nanofluid and magnetic field on the total heat transfer coefficient and the total pressure drop of the heat exchanger have been investigated. The results of studies in this field show that the use of nanofluids increases the heat transfer that occurs in the heat exchanger and also decreases the total pressure. While the most suitable volume percentage of nanofluid for the best ratio of heat transfer to pressure drop is also reported to be 15%. The numerical results of this article have also shown that the best optimal conditions in terms of heat transfer enhancement and pressure drop (according to the PEC performance evaluation criteria) are at 15% volume of nanofluid. Applying a magnetic field to the nanofluid current in the transducer also helps to increase the heat transfer in the heat exchanger while also increasing the total pressure drop. The amount of flow velocity has an obvious effect on the displacement heat transfer coefficient of the heat exchanger as well as the amount of total pressure drop. The simulation of the heat exchanger in different values ​​of the turbulent flow Reynolds number shows that the increase in the Reynolds number has a direct effect on the displacement heat transfer and the total pressure drop. The results show that in the intensity of the magnetic field with Hartmann number 40, the best ratio of heat transfer to the pressure drop of the heat exchanger occurs. The best PEC number in Hartmann number 40 is 1.05. However, with further increase of Hartmann number to 60 and 80, the value of PEC number decreases and in Hartmann number 80 decreases to 0.99, which practically makes the use of nanofluid in the presence of magnetic field affectless. The rate of flow velocity has a significant effect on the heat transfer coefficient of the heat exchanger and also the rate of total pressure drop. The value of the PEC number increases from 1.01 to 1.09 in increasing the Reynolds number from 6000 to 10000. However, it seems that with a relatively small increase in the PEC number and increasing costs in increasing the Reynolds number, using a lower Reynolds number scaler is a better choice.

In this study, the magnetic field effect on the hydrodynamics and heat transfer of the laminar flow of a nanofluid in a microchannel with one-sided sudden expansion at a low Reynolds number is investigated. The governing equations including mass, momentum, and energy conservation equations are discretized and solved on a staggered grid using the finite difference method by developing a Fortran code. The simulation results show that at a low Reynolds number, no vortex is formed after the step. In the presence of a magnetic field, the friction coefficient decreases with volume fraction. At a volume fraction of 0.04, this reduction in comparison with water at Hartmann numbers of 20 and 40 is 25 and 18 percent, respectively. By applying the magnetic field, the friction factor increases. For nanofluid with a volume fraction of 0.04, the enhancement of average friction factor in comparison with water at Hartmann numbers of 20 and 40 is 176 and 337 percent, respectively. At a low Reynolds number, the magnetic field effect on the Nusselt number is negligible. The Nusselt number decreases with volume fraction. At a volume fraction of 0.04, a reduction of about 12 percent is observed in the Nusselt number in comparison with water. Examining the performance evaluation criteria shows that applying a magnetic field improves the system's performance. At a Hartmann number of 20 and volume fraction of 0.04, an enhancement of about 11 percent in performance coefficient is observed in comparison with the case that the magnetic field is absent.

The present study numerically evaluates vortex-induced vibration (VIV) for an electrically conductive fluid around a cylinder on an elastic support. The flow is subjected to a uniform magnetic field in the z-direction to evaluate the VIV attenuation performance of this open-loop active control method. To analyze this fluid-structure interaction (FSI) problem, the finite volume method (FVM) was employed based on the SIMPLE algorithm to solve the governing continuity and momentum equations of the fluid flow and an explicit integration method to solve the motion equations of the rigid structure. Simulations were carried out at reduced velocities of 2-9 and different Stuart numbers. The simulation results demonstrated that the magnetic field was significantly effective and wholly (100%) suppressed transverse and longitudinal VIVs. A rise in the magnetic field magnitude eliminated vortex shedding for all the reduced velocities, leading to a symmetric wake. The symmetric vortex even disappeared above the critical Stuart number.

Limited access to fresh water sources has raised water shortage as a crisis. Therefore, solar desalinations have received special attention today. A stepped solar desalination device which has 28 steps with a height of 30 mm, a width of 110 mm and a length of 840 mm has been designed, built and used in this research. In order to increase the daily production rate, the device has been tested and optimized in 4 different modes: first mode) simple, second mode) with phase change material (PCM), third mode) with Hybrid Nano phase change material (NPCM), fourth mode) with Hybrid NPCM under magnetic field. The phase change material and Hybrid Nano phase change material have been used as a source of thermal energy storage to continue the desalination of water when the sun sets. After conducting the tests, it was found that the fourth mode of the test, i.e. the Hybrid Nano phase change material under the magnetic field, had a 98% increase in the production rate compared to the simple mode.

In this study, numerical method of forced convection heat transfer of laminar flow of nano fluid water and copper oxide in wavy micro channel in the presence of magnetic field is investigated. The equations of flow stream function, vorticity and energy are discretized by finite difference method and are solved according to the desired boundary conditions and the development of a FORTRAN software. The effect of various parameters such as Reynolds number 50-500, volume fraction 0-10%, Hartmann number 0-20, and wave amplitude 0 – 0.3 was evaluated in improving the performance of a wavy micro channel. According to the presented results, the sinusoidal shape of the micro channel has a direct effect on the heat transfer and the Nusselt number increases with the increase of the micro channel wave amplitude. On the other hand, the vortices created at high Reynolds have also improved micro channel efficiency and increased heat transfer. The results showed that with increasing Hartmann number the flow line near the wall became more regular and due to the created temperature gradient, the Nusselt number increased

In this research, the performance of a three-dimensional oscillating heat pipe in different inclination angles from 0 to 90 degrees with distilled water and ferrofluid as operating fluid has been studied and compared experimentally. Fe3O4 ferrofluid was prepared with a mass fraction of 0.1% and tetramethyl ammonium hydroxide surfactant was used for its stability. First Experiments were performed at different input heat input fluxes (30 to 300 watts) and in filling ratios if 50 and 60%, and the optimal filling ratio (50%) was determined. Then with this value, the thermal performance of the oscillating heat pipe was evaluated at different angles without presence and also by applying a magnetic field. The experimental results showed that Fe3O4 nanoparticles could improve the thermal performance of the device. It was also found that the optimal filling ratio for this device is 50%. As the inlet heat flux increases, the thermal resistance of the OHP decreases. In addition, the results showed that the inclination angle of the heat pipe has a significant effect on the performance of the OHP. In horizontal mode, the device did not work at all, but by applying a magnetic field, the device can continue to operate.

The present study investigates the structure of a nuclear fusion reactor and the importance of magnetic hydrodynamic fluid. In reactors, the fluid is separated from the main body of the blanket by a separating layer. The separating structure is important in two ways. This structure primarily acts as a thermal insulator. The second priority is used for the separator to adjust the pressure and reduce it. The topics selected in this study are: the effect of magnetic field strength, profiles and dimensions of a blanket, wall thickness, flow velocity and pressure drop, as well as the profile of flow velocity changes due to magnetic field strength. The results show that the maximum velocity in blanket with rectangular cross section in 1T field is 11% and in 4T field is 9% faster than blanket with square cross section. Also, increasing the magnitude of the magnetic field from 1T to 4T causes a 9-fold increase in pressure drop in the blanket with a square cross-section and an 11-fold increase in the pressure drop in the blanket with a rectangular cross-section.

Recently, magnetic microrobots have attracted much attention in biomedical applications due to their minimally invasive features. One of the challenges in this field is about in-vivo autonomous control of microrobots to reach a predefined target. In concern to the submillimeter size of the microrobots, their position and orientation are controlled by an external magnetic field which is generated by permanent magnets or electromagnets. One of the advantages of using electromagnets to produce an external magnetic field is the ability to control the magnitude and orientation of the magnetic field by manipulating the electrical current of each electromagnet. In this study, by using Maxwell’s equations and considering the microrobot as a point dipole, the exerted force and torque relations are driven as a function of electromagnets’ electrical current. Moreover, a navigation algorithm is proposed to guide the robot through unknown obstacles without planning the whole path. Furthermore, the driven equations and designed algorithms are validated by simulating the microrobot’s motion using MATLAB software, which confirms the effectiveness of using three electromagnets to control an electromagnet microrobot’s planer motion.

In recent years, linear electromagnetic actuators have gained special attention in small robot actuation and calibration of milli newton thruster stands. In this paper, a linear electromagnetic actuator with a force range of milli newtons is designed and manufactured. In this regard, first the analytical relationships of the magnetic field and Lorentz force were derived and then, based on the desired design criteria meaning high force sensitivity, low heat loss and minimum geometric dimensions and weight, the appropriate design parameters of the electromagnetic actuator is obtained. According to the results, the  obtained force constant is approximately 1 mN/A while the maximum power loss is 1 mW at available stroke of 10 mm. Finally, a prototype of the linear electromagnetic actuator is manufactured and experiments are performed to validate the electromagnetic actuator. For this purpose, a precision scale with an accuracy of 0.01 gr and a power supply with a resolution of 1 mA is utilized. The results showed that the maximum difference between the calculated and measured force was 2.5%. Therefore, there is a good correlation between the experimental data and the corresponding analytical values.

In this research, the natural convection heat transfer for a square enclosure containing two Nano-fluids, water-alumina and water-copper, which is affected by a diagonal magnetic field, is simulated numerically and the effects of some parameters such as Rayleigh number ( ), Hartmann number ( ), magnetic field angle ( ), nanoparticle volume fraction ( ), type of heat source (linear or sine), length of the heat source ( ) and the non-uniformity parameter of the source ( ) have been studied on the flow and temperature fields. The results show that with increasing Rayleigh  number and Hartmann number, the amount of heat transfer increases and decreases, respectively. Also, the thermal performance of the enclosure is improved by increasing the angle of the magnetic field (from 0 to 90 degrees) and by adding solid nanoparticles to the base fluid, it means a relative increment in enclosure heat transfer is observed. In both high and low Rayleigh numbers, the maximum heat transfer is related to the constant temperature source. After that, for high Rayleigh numbers a sinusoidal source (λ = 1) and for low Rayleigh numbers a linear source (λ =0.5), where the conduction heat transfer dominates on enclosure, have the highest average Nusselt number, respectively. The results also indicate that when the length of heat sources increases, the rate of heat transfer increases too.

In the present study, considering the magnetic tractions and heat generated by electric current and eddy current, new nonlinear equations have been proposed to investigate the vibrational behavior of ferromagnetic plates carrying an electric current under a magnetic field. After extracting the governing differential equations of the system using Newton's second law, the coupled nonlinear equations are discretized using the Galerkin method and then solved numerically. The numerical results presented in the present study are compared with the results in the technical literature and then the effect of different parameters on the vibration characteristics of soft ferromagnetic plates is investigated. The results show that the magnetic field and electric current have a significant effect on the vibration behavior of the plate and lead to an increase in the amplitude oscillations of the system. The presence of a magnetic field reduces the equivalent stiffness of the plate and increases it, resulting in static instability in the system. Also, by considering the force created by magnetic tractions, a static rise is created in the plate and affects its steady-state response. In the study of thermal effects, it was found that the assumption of thermal coupling increases the natural frequency of the plate.