Natural Convection in an Enclosure with a Partially Active Magnetic Field

Chaotic natural convection flow of a molten gallium in a square enclosure with the upper and lower surfaces being insulated was studied by two-dimensional numerical simulation. Constant temperatures are imposed along the left and right walls of the enclosure with a volumetrically heated enclosure. In addition, a nonuniform partially active magnetic field is applied in a vertical direction. The flux lines spread out into a fringing field so the effective cross-sectional area of the gap is larger than that of the pole face. A chaotic regime is considered under steady state boundary condition. This study was done for an internal Rayleigh number of 10⁷, external Rayleigh number of 10⁵, and Prandtl number of 0.024. The study covers various magnet pole effect widths of 1/4, 1/2, and 3/4 from enclosure width and the magnetic field strength ranges 0.0 ≤ B _o  ≤ 10 Tesla. The transport equations for continuity, momentum, and energy are solved. The numerical results are reported for the effect of the partially active magnetic field on the velocity vectors, counters of temperature, streamline, and heat transfer coefficient. The numerical study shows that a magnetic field is damping chaotic oscillation behavior and decreases the amplitude of oscillation. Also, at a certain magnetic field strength the chaotic flow tend to becomes periodic flow at certain amplitude and frequency, and at high magnetic field strength the flow in the square enclosure flow tends to become steady laminar flow with stable average Nusselt number values; so, the random oscillation behavior disappeared. The effect of a nonuniform magnetic field tends to push the fluid to flow away from magnetic field region.     

Numerical Investigation of Transient Magnetohydrodynamic Mixed Convection in a Ventilated Cavity Containing Two Heated Circular Cylinders

A two-dimensional finite volume computation is performed to analyze the transient magnetoconvective transport in a ventilated cavity containing two inner heated circular cylinders with identical shape. An electrically conducting fluid (Prandtl number 0.01) enters the cavity through an opening at the middle of the left wall and is taken away by a similar opening at the middle of the right wall. A uniform magnetic field is applied along the horizontal direction normal to the vertical wall. Simulations are performed for the parameters, Richardson number (0, 0.25, 0.5, and 1), Reynolds number (380–550), Hartmann number (0, 10, 20, and 50) and dimensionless gap between the cylinders 0.1, 0.2, and 0.3. The analysis indicates that the transport process is a complex function of the magnetic field strength, mixed convective strength and the cylinder distance. Some typical combinations of these controlling parameters may produce three different transport characteristics such as the steady state, periodic oscillatory, and chaotic. With a lower cylinder distance and higher mixed convective strength, the flow instability increases causing periodic and even chaotic oscillations, whereas the magnetic field due to its damping nature imparts stability to the flow resulting in a steady state flow condition.     

A Novel Magnetic Separation Oxygen-enriched Method and the Influence of Temperature and Magnetic Field on Enrichment

A novel oxygen-enriched method is presented.Using two opposite magnetic poles of two magnets with certaindistance forms a magnetic space having a field intensity gradient near its borders.When air injected into themagnetic space outflows from the magnetic space via its borders,oxygen molecules in air will experience the in-terception effect of the gradient magnetic field,but nitrogen molecules will outflow without hindrance.Therebythe continuous oxygen enrichment is realized.The results show that the maximum increment of oxygen concen-tration reaches 0.49% at 298 K when the maximum product of magnetic flux density and field intensity gradientis 563T~2/m.The enrichment level is significantly influenced by the gas temperature and the magnetic field.Themaximum increment of oxygen concentration drops to 0.16% when the gas temperature rises to 343 K,and dropsto 0.09% when the maximum product of magnetic flux density and gradient is reduced to 101 T~2/m from 563T~2/m.     

Quasi-3-D analytical modeling of the magnetic field of an axial flux permanent-magnet synchronous machine

A quasi-three-dimensional (3-D) analytical model of the magnetic field in an axial flux permanent-magnet synchronous machine is presented. This model is derived from an exact two-dimensional analytical solution of the magnetic field extended to the 3-D case by a simple and effective radial dependence modeling of the magnetic field. The obtained quasi-3-D solution allows rapid parametric studies of the air-gap magnetic field. Then, analytical modeling of the cogging torque is presented. It is based on the obtained quasi-3-D analytical solution. Results issued from the proposed model in the air gap are compared with those stemming from a 3-D finite-element method simulation as well as with prototype measured values.     

On a Contact Problem of Two-Layer Beams Coupled by Boundary Conditions in a Temperature Field

We consider a mathematical model of two-layer beams coupled by boundary conditions in a stationary temperature field taking into account geometric nonlinearity. The stationary temperature field is defined by a 2D heat transfer equation with boundary conditions of the first kind. The geometric nonlinearity is introduced via von Kármán's relations for both beams. Equations of beam deflection are derived due to the Euler–Bernoulli hypothesis. The contact interaction is described using Winkler's model. Scenarios of a transition from regular to chaotic regimes are studied. Phase synchronization of beam vibrations versus both character and intensity of the applied temperature field is investigated.     

Effect of magnetic field on the thermal performance of a ferrofluid-based solar flat plate collector

In this paper, the effectiveness of ferrofluid as a working fluid for solar flat plate collectors (FPCs) is studied. A mini-ferrofluid-based solar FPC is designed and tested under laboratory conditions. The thermal performance of the designed solar FPC is evaluated under different conditions and it is observed that it provides higher efficiency for the case of ferrofluid in the presence of a magnetic field. The thermal efficiency is observed to increase by 54% for ferrofluid in presence of a magnetic field as compared to no magnetic field. The thermal efficiency is observed to increase further with the increase in the magnetic field. The increase in thermal efficiency is attributed to the combined effects of higher thermal conductivity and magnetohydrodynamics of ferrofluid, which result in higher convective heat transfer from the riser tube walls into ferrofluid. The higher heat transfer for ferrofluid with a magnetic field is established by calculating the Nusselt number numerically using COMSOL. Simulation results show an increase in Nusselt number for ferrofluid with magnetic field and hence higher thermal efficiency for the solar FPC. The designed FPC provides simple modifications to conventional FPCs to use ferrofluid with magnetic field for higher thermal efficiencies.     

燃煤飞灰中可吸入颗粒物在磁场中聚并收尘试验研究

针对燃煤电站锅炉飞灰这种弱磁性可吸入颗粒物,将由专门研制的气溶胶发生器产生的气溶胶(PM10)颗粒按照粒径大小分为12等级,利用低压电称冲击器ELPI测试在不同磁感应强度下各级颗粒浓度的变化,考察磁场对颗粒磁聚并的影响.结果表明,燃煤飞灰这种弱磁性粉尘在磁场中有一定的聚并作用,对其中PM2.5的颗粒聚并的效果更明显.     

Laminar Free Convection in a Square Enclosure Driven by the Lorentz Force

A numerical study is presented of laminar free convection flow driven by magnetic forces. An external magnetic field with one spatially varying component is applied to an electrically conducting fluid in a square enclosure. This magnetically-driven flow is controlled by the intensity and the wave number of the applied magnetic forcing. In addition, when the enclosure is heated laterally in a non-zero gravity environment, the resulting buoyant forces may contribute or resist the magnetically-driven fluid motion. The present results show that a strong magnetic field can even reverse the buoyant flow. The circulation intensity of the flow and the heat transfer from the sidewalls is increased with increasing magnetic field or with decreasing magnetic Reynolds number. The wave number of the magnetic forcing is also an important parameter that determines the vortex patterns and, consequently, the convection heat transfer.     

Global simulation of a silicon Czochralski furnace in an axial magnetic field

Control of melt flow in crystal growth process by application of the magnetic field is a practical technique for silicon single crystals. In order to understand the influence of axial magnetic field on the silicon melt flow and oxygen transport in a silicon Czochralski (Cz) furnace, a set of global numerical simulations was conducted using the finite-element method for the magnetic field strength from 0 to 0.3 T, the crystal rotation rates from 0 to 30 rpm and the crucible counter-rotation rates from 0 to −15 rpm. It was assumed that the flow was axisymmetric laminar in both the melt and the gas, the melt was incompressible and a constant temperature was imposed on the outer wall of the Cz furnace. The results indicate significantly different flow patterns, thermal and oxygen concentration fields in the melt pool when a uniform axial magnetic field is applied.     

Determination of magnetic field constant of DC permanent magnet motor powered by photovoltaic for maximum mechanical energy output

The DC permanent magnet motor coupled with centrifugal pump has the better matching when directly powered by photovoltaic (PV) array. The important parameter of DC permanent magnet (DC PM) motor is magnetic field constant. The method for the determination of optimum magnetic field constant of DC PM motor, when powered by PV, has been obtained, and its analysis has been carried out for different magnetic field constants. It has been found that the maximum output is available at the output-energy-weighted average value of magnetic field constant. The parameter, magnetic field constant, should be properly selected during the design of DC PM motor in order to extract the maximum power from PV array.     

Numerical investigation of natural convection in an inclined enclosure filled with porous medium under magnetic field

In the present study, natural convection of fluid in an inclined enclosure filled with porous medium is numerically investigated in a strong magnetic field. The physical model is heated from left-hand side vertical wall and cooled from opposing wall. Above this enclosure an electric coil is set to generate a magnetic field. The Brinkman–Forchheimer extended Darcy model is used to solve the momentum equations, and the energy equations for fluid and solid are solved with the local thermal non-equilibrium (LTNE) models. Computations are performed for a range of the Darcy number from 10⁻⁵ to 10⁻¹, the inclination angle from 0 to π/2, and magnetic force parameter γ from 0 to 100. The results show that both the magnetic force and the inclination angle have significant effect on the flow field and heat transfer in porous medium.     

On the control of solidification using magnetic fields and magnetic field gradients

Solidification from the melt to near net shape is a commonly used manufacturing technique. The fluid flow patterns in the melt affect the quality of the final product. By controlling the flow behavior, the final solidified material can be suitably affected. Most of the magnetic field approaches to melt flow control rely on the application of a constant magnetic field. A constant magnetic field results in the Lorentz force which is used to damp and control the flow. However, simultaneous application of a magnetic gradient results in the Kelvin force along with the Lorentz force. This can be used for better control of the melt flow resulting in higher crystal quality. In the present work, a computational method for the design of solidification of a conducting material is addressed. The control parameter in the design problem is the time history of the imposed magnetic field. A steady, constant magnetic gradient is also maintained during the process. The design problem is posed as an unconstrained optimization problem. The adjoint method for the inverse design of continuum processes is adopted. Examples of designing the time history of the imposed magnetic field for the directional growth of various materials are presented to demonstrate the developed formulation.     

New magnetic field analyzer device dedicated for polymer electrolyte fuel cells noninvasive diagnostic

Inside the Fuel Cell, the magnetic field distribution can indicate normal or abnormal operation and therefore provides an effective diagnosis approach. The magnetotomography is the only noninvasive current density mapping method and is based on the measurement of the external magnetic field of the fuel cell stack. The present work addresses the development of a new magnetic field analyzer device devoted to assess the current density distribution inside the Fuel Cell, within the surrounding external magnetic field. The proposed magnetic field analyzer associates magnetic sensors with a ferromagnetic circuit, which is essentially different in comparison with other methodologies proposed until now. Providing a higher magnetic field variation at the level of magnetic sensors, this new approach enables a more accurate analysis of the current distribution inside the Fuel Cell. This study considers the Proton Exchange Membrane Fuel Cell case.     

Effect of a magnetic field on heat and mass transfer by natural convection from vertical surfaces in porous media-an integral approach

The study deals with natural convection heat and mass transfer from vertical plates embedded in electrically conducting fluid saturated porous media with constant surface temperature and concentration. The flow is exposed to a transverse magnetic field. The integral method is applied to obtain the analytic solution. Results are illustrated graphically for various features of the solutions. The application of a transverse magnetic field normal to the flow direction decreases the Nusselt number and Sherwood number. Moreover, the thickness ratio of the thermal boundary layer to the concentration boundary layer is found to be independent of the intensity of the magnetic field.     

Simulation study on magnetic field distribution of PEMFC

As a promising next-generation energy source, proton exchange membrane fuel cell (PEMFC) still suffers from durability and reliability issues, where PEMFC performance will decay during its operation. In this study, a three-dimensional, multi-component and multi-physics PEMFC model is developed to investigate the effect of PEMFC performance degradation on its external magnetic field. By comparing simulation results and experimental data, the capability of the developed model in simulating magnetic field due to PEMFC current is demonstrated. With developed model, different PEMFC degradation mechanisms, including flooding, dehydration, PEMFC aging are simulated, and the amplitudes and distributions of magnetic field under different mechanisms are investigated. Moreover, considering local defects may happen in practical PEMFC systems, its influence on magnetic field distribution is also studied. From the results, the correlation between PEMFC performance degradation and its magnetic field distribution is clarified, which will be beneficial for researches utilizing PEMFC magnetic field for analyzing PEMFC performance variation.     

Effect of magnetic field on the performance of new refrigerant mixtures

Performance test results of new alternative refrigerant mixtures such as R-410A, R-507, R-407C, and R-404A under various conditions of magnetic field are discussed, analysed and presented. The test results were obtained using an air-source heat pump set-up with enhanced surface tubing under various magnetic field conditions. Performance tests were conducted according to the ARI/ASHRAE Standards. The test results demonstrated that as magnetic field force increases, compressor head pressure and discharge temperature slightly increase as well as less liquid refrigerant is boiling in the compressor shell. This has a positive effect in protecting the compressor. The effect of magnetic field on mixture behaviour varies from one mixture to another depending upon the mixture's composition and its boiling point. Furthermore, the use of magnetic field appears to have a positive influence on the system COP as well as thermal capacities of condenser and evaporator. Copyright © 2002 John Wiley & Sons, Ltd.     

The effect of magnetic and optic field in water electrolysis

Hydrogen production via water electrolysis was studied under the effect of magnetic and optical field. A diode solid state laser at blue, green and red were utilized as optical field source. Magnetic bar was employed as external magnetic field. The green laser has shown a greatest effect in hydrogen production due to its non-absorbance properties in the water. Thus its intensity of electrical field is high enough to dissociation of hydronium and hydroxide ions during orientation toward polarization of water. The potential to break the autoprotolysis and generate the auto-ionization is the mechanism of optical field to reveal the hydrogen production in water electrolysis. The magnetic field effect is more dominant to enhance the hydrogen production. The diamagnetic property of water has repelled the present of magnetic in water. Consequently the water splitting occurs due to the repulsive force induced by the external magnetic field. The magnetic distributed more homogenous in the water to involve more density of water molecule. As a result hydrogen production due to magnetic field is higher in comparison to optical field. However the combination both fields have generated superior effect whereby the hydrogen yields nine times higher in comparison to conventional water electrolysis.     

Multi-field coupling thermo-acoustic radiation using free-standing nano-thin films in a static magnetic field

Abstract

Thermo-acoustic radiation from nano-thin film has been widely reported these years. In this paper, a static magnetic field is introduced to enhance the acoustic power in the multi-field thermo-acoustic generation. The coupled thermo-acoustic system in a static magnetic field shares the same acoustical unit with very few additional components. In the presence of a static magnetic field, thin-film vibration is excited instead of being still in the classical thermo-acoustic system. Not surprisingly, a steady magnetic field has very little impact on the thermo-acoustic generation system when a constant amplitude sinusoidal current is introduced. The oscillation perpendicular to the nano-thin film is driven by the electromagnetic force. The system response of thermo-acoustics and magneto-acoustics can be generally matched when suitable parameters are introduced. The acoustical pressure output can be significantly improved in the presence of a static magnetic field although more electrical input power should be provided in the multi-field thermo-acoustic system. Evidently, the coupled multi-field acoustical system is able to handle more electrical power input and the power input can be more easily dissipated relatively. The results show that the acoustic response of this new multi-filed coupled system can be significantly improved as compared to the classical system without magnetic effects.     

Suppressing Rayleigh–Benard convection in a cube using a strong magnetic field — Experimental heat transfer rate measurements and flow visualization

The average heat transfer rate of joint gravitational and magnetic convection of a paramagnetic fluid (an 80% mass aqueous solution of glycerol with 0.8 mol/kg concentration of gadolinium nitrate hexahydrate) in a cubic enclosure heated from below and cooled from above is experimentally measured for five different strengths of the magnetic field. The cubic enclosure of 0.032-m sides is placed in the bore of a 5-Tesla super-conducting magnet below the solenoid centre at a position where the magnetic force distribution is relatively uniform. The magnetic field imposed in the vertical direction acts against gravity and the heat transfer rate decreases with the increase of the magnetic induction. Furthermore, steady and transient states flow visualizations are carried out with a 10-Tesla super-conducting magnet to show a stable stratification obtained in the form of magneto-thermal.     

磁场旋转方向对引发火焰旋转临界值的影响

通过对旋转火焰涡运动微分方程式的分析，研究旋转磁场引发火焰旋转时的磁场旋转临界速度与磁场参数之间变化的关系，以及旋转磁场方向的变化对旋转磁场引发旋转火焰时磁场旋转速度临界值的影响规律．实验研究结果表明，随着磁场强度的增加，引发火焰旋转的磁场上临界转速增加，下临界转速则下降；磁场顺时针方向旋转时，引发火焰旋转的磁场上临界转速较逆时针旋转时要小，而下临界转速较逆时针旋转时要大．