Nonuniform Magnetic Field Research Articles

FHD flow in an irregular cavity subjected to a non-uniform magnetic field

In this paper FHD flow in a rectangular pipe constricted by two analogous semi-cylinders attached to the left and the bottom walls is investigated. The laminar, axial flow is produced by a constant pressure gradient, and the flow is affected by a spatially varying non-uniform magnetic field caused by two electric wires. The current-carrying wires are placed along the axes of the semi-cylinders. The fully developed flow is studied on the 2D cross-section of the pipe, a cavity, where the wires act as point magnetic sources. The pressure equation is added to the mathematical model, and the velocity-pressure form governing equations are numerically solved by the dual reciprocity boundary element method (DRBEM). The Dirichlet type pressure boundary conditions are approximated through a process using the radial basis functions and a finite difference. The flow, velocity, and pressure variations are investigated for different magnetic field strengths and current ratios. The grid independence study is also carried out. The proposed iterative scheme is capable of generating numerical results by performing a non-uniform discretization for the boundary. Dense discretizations are applied at the places where the flow shows a sudden fluctuation. It is shown by the numerical results that the flow and the pressure variations are dominated by the strong magnetic source. With an increment in the magnetic number, the planar flow is accelerated, the axial flow is decelerated, and the pressure increases, especially around the strong point magnetic source.

Effects of external magnetic field arrangements on thermomagnetic convection in ferrofluid around heated Micro-wire

Thermomagnetic convective cooling of a heated micro-wire in quiescent ferrofluid is experimentally investigated under various non-uniform external magnetic fields produced by permanent magnets. In the past studies, in most of the cases for this geometry the external magnetic field is applied under forced convection conditions. This work provides the first published data set of heat transfer from a microwire in quiescent ferrofluid (free convection conditions) under various arrangements of external magnetic fields produced by permanent magnets. For the different arrangements, magnetic flux density and Kelvin body force density are calculated to aid discussions of the observed effects. The experiment tracked the transient temperature rise of a heated micro-wire, highlighting effects of applied magnetic field orientation, symmetry, and strength. The results showed that greater non-uniformity in radial magnetic field could enhance the cooling effects with up to 7% reduction in the heated wire temperature rise. A magnetic field aligned with the heated wire axis produced only a minor enhancement of 3% in heat transfer in contrast to application of radial magnetic fields. Surprisingly, a stronger external magnetic field did not always result in better heat transfer. Magnetophoresis of ferro-particles induced by the external magnetic field was found to be an important consideration when using permanent magnets for ferrofluid applications.

Effect of Particle–Particle and RBC–Particle Interactions on Capture Efficiency of Magnetic Nanocarriers Under the Influence of a Nonuniform Magnetic Field

In this study, the capture efficiency (CE) of magnetic carrier particles at the region of interest (ROI) in a micro-vessel was investigated. A nonuniform magnetic field produced by a Nd-Fe-B magnet was applied to capture the carrier particles at the ROI. The particle–particle and red blood cell (RBC)–carrier particle interactions were taken into account. Other parameters including carrier particle diameter, magnetic force, and non-Newtonian behavior of blood were also considered. The finite element method was used to solve the governing equations. Four cases were considered: (1) Newtonian without interaction forces, (2) Newtonian with interaction forces, (3) non-Newtonian without interaction forces, and (4) non-Newtonian with interaction forces. The results showed that for a particle diameter of 2000 nm, the CE values for the case without interaction force and with interaction force were 70% and 53%, respectively. It was found that by considering the particle–particle and RBC–carrier particle interactions for all magnet–vessel distances, the CE of carrier particles decreased, on average, by nearly 19%. At a magnet–vessel distance of 2.5 cm (d=2.5cm),\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$d=2.5\\,\\mathrm{cm}),$$\\end{document} it was seen that the CE values for case 2 and case 3 were nearly 70% and 45%, representing the maximum and minimum CE values, respectively.

Magnetic-Field Mediated Active Propulsion of Ferrofluid Droplets on a Wire.

Droplet actuation on wires or fibers is highly relevant to digital microfluidics applications. Several different passive or active actuation mechanisms have been studied, including capillary pressure, thermal gradient, tilting, acoustics, and electrowetting, to name a few. However, the lack of precise control and accurate droplet positioning during its actuation on a wire limits the use of earlier methods. On the other hand, using a nonuniform magnetic field to actuate a droplet on a micrometer-size wire helps overcome the above limitations. In this context, we elucidate the motion of microliter-size ferrofluid droplets, smaller than the capillary length, in the presence of a permanent magnet using a simple yet robust experimental setup, wherein the clamshell shape droplets hang on a wire at a predetermined distance from the magnet. Beyond a critical volume, the droplet starts moving toward the magnet while exhibiting shape evolution continuously. Using a high-speed videography technique, we uncover the intricate relationship between the magnetowetting, favoring continuous shape evolution of droplets of different sizes, and their actuation dynamics (velocity) on a wire, which has scarcely been explored. The most significant contribution of this study is the detailed evaluation of the shape factor (k), relating the shape evolution of the droplet to its velocity through the contact angle hysteresis force. A theoretical framework has also been proposed, comprising different forces in action, which is found to set forth a good match with the experimental results. The results from this study are expected to open up new avenues in digital microfluidics, specifically in drug delivery, ferrobotics, and droplet logic gates, to name a few.

Unidirectional operation criterion in monolithic nonplanar ring oscillators.

Monolithic nonplanar ring oscillators (NPROs) under an applied magnetic field can operate unidirectional single-frequency lasing due to the loss differences among its four eigenpolarizations, where the minimum was empirically estimated to be 0.01%. However, this value has never been verified because the applied magnetic field is not uniformly distributed, making it hard to resolve both theoretically and experimentally. Here, we propose a method to resolve the applied magnetic field through an NPRO by combining finite-element analysis and experimental verification. By introducing the non-uniform magnetic field information to the eigenpolarization theory, the loss differences can be calculated by path integration along the optical path in the NPRO. The critical point, where the bidirectional lasing is emerging, is identified by the relative amplitude noise (RAN) of the laser and by the beating signal between the clockwise (CW) and counterclockwise (CCW) lasing. With this method, we determine that unidirectional operation is possible with loss differences as low as 0.0001% and 0.0003%, corresponding to two different NPRO designs with out-of-plane angles of 90° and 45°, respectively, which increases the precision of the loss differences for unidirectional single-frequency lasing by more than one order of magnitude. Our findings will greatly facilitate NPRO laser design with lowered magnetic field intensity requirements.

The application of non-uniform magnetic field for thermal enhancement of the nanofluid flow inside the U-turn pipe at solar collectors

The improvement of heat transfer inside the solar heat exchangers is important for the development of solar energy in an urban area. In this study, the usage of a non-uniform magnetic field on the thermal efficiency of the nanofluid (Fe3O4) streaming inside the U-turn pipe of solar heat exchangers is examined. Computational fluid dynamic is applied to visualize the nanofluid flow inside the solar heat exchanger. The role of magnetic intensity and Reynolds number on thermal efficiency are fully investigated. The effect of single and triple sources of the magnetic field is also studied in our research. Obtained results indicate that the usage of the magnetic field results in the production of vortex in the base fluid and heat transfer improves inside the domain. Our finding indicates that the usage of the magnetic field with Mn = 25 K would improve the average heat transfer by about 21% along the U-turn pipe of solar heat exchangers.

Effect of non-uniform magnetic field on combustion products of aluminum nanopowder in mixture with titanium and zirconium dioxides

The morphology and phase composition of the combustion products in air of Al nanopowder mixed with diamagnetic TiO2 and ZrO2 was studied. The influence of non-uniform magnetic field on the combustion process of the oxide-metal powdery mixtures led to the stabilization of TiN and ZrN in the final products. The two-level whiskers of AlN formed at the combustion of Al-TiO2 mixture, while the agglomerated structures covering the large particles of ZrN formed at the combustion of Al-ZrO2 mixture. During combustion of oxide-metal mixtures in non-uniform magnetic field, diamagnetic metal dioxides are reduced to metals or their suboxides and then interact with atmospheric nitrogen.

Controlling Selection Rules for Magnon Scattering in Nanomagnets by Spatial Symmetry Breaking

Nanomagnets are the building blocks of many existing and emergent spintronic technologies. The magnetization dynamics of nanomagnets is often dominated by nonlinear processes, which have been recently shown to have many surprising features and far-reaching implications for applications. Here we develop a theoretical framework uncovering the selection rules for multimagnon processes and discuss their underlying mechanisms. For its technological relevance, we focus on the degenerate three-magnon process in thin elliptical nanodisks to illustrate our findings. We parameterize the selection rules through a set of magnon interaction coefficients which we calculate using micromagnetic simulations. We postulate the selection rules and investigate how they are altered by perturbations that break the symmetry of static magnetization configuration and spatial spin-wave profiles and that can be realized by applying off-symmetry-axis or nonuniform magnetic fields. Our work provides the phenomenological understanding of the mechanics of magnon interaction as well as the formalism for determining the interaction coefficients from simulations and experimental data. Our results serve as a guide to analyze the magnon processes inherently present in spin-torque devices in order to boost their performance or to engineer a specific nonlinear response in a nanomagnet used in a neuromorphic or quantum magnonic application.

Influence of streamwise and spanwise wall magnet arrays on near-wall MHD turbulence

ABSTRACT The effect of a permanent, localised and non-uniform magnetic field on the near-wall turbulence in a fully developed channel flow is analysed through direct numerical simulations. The magnetic field distribution is obtained by an arrangement of magnets placed on the upper and lower channel walls, producing preferentially either a streamwise (i.e. in the main flow direction) or a spanwise magnetic field component. The wall shear stress is drastically reduced under the effect of the streamwise arrangement of the wall magnets wherein both streamwise and wall-normal magnetic fields are involved. The magnetic braking effect leads to an important increase of the body force. Paradoxically enough, the small-scale turbulent activity is significantly increased above the low buffer layer in this case. On the opposite, imposing a magnetic field with a predominant spanwise component reduces tremendously the population of the turbulent shear stress producing eddies by directly affecting the regeneration of the buffer layer quasi-streamwise vortices. The flow is quasi-relaminarised between the magnets in the low-buffer and viscous sublayers. The wall shear increases in a predictable and deterministic way over the magnets, wherein the wall normal magnetic field component induces electric current loops.

The Effects of Uniform and Nonuniform Magnetic Fields in Plant Growth: A Meta-Analysis Approach.

Magnetic field (MF) effects have been reported in plants' growth, seed germination, gene expression, and water consumption. Accordingly, magnetic treatments have been proposed as a sustainable alternative to improve yields. Nevertheless, a comprehensive quantitative assessment is needed to understand whether their effects are general, species-specific, or dependent on the experimental setting. We conducted a multilevel meta-analysis of 45 articles that studied 29 different plant species. A positive and neutral effect of a nonuniform MF was found on fresh weight and germination rate, respectively. A significant association was found between a uniform MF and germination. These results suggest that MFs improve plant growth. However, the effects are highly dependent on the experimental setting. This opens exciting questions about the biophysical mechanisms underlying the perception and transduction of this environmental cue and about the possible translation to agricultural practices. © 2023 Bioelectromagnetics Society.

Extended investigation of an abrupt crystal diameter change during Czochralski silicon growth with transverse magnetic field

An investigation about the phenomenon of an abrupt diameter deviation during Czochralski crystal growth, a characteristic when using a transverse magnet, has been carried out both experimentally and computationally. 300-mm silicon crystals have been pulled under certain conditions and the diameter deviations have been recorded in these processes. Three-dimensional calculations of the melt flow have been performed using CGSim Flow Module 3D, accounting for distribution of the actual non-uniform magnetic field, to accurately simulate the heat transfer and melt flow for selected growth stages along the entire process. The sudden change of melt temperature near the triple point that leads to the diameter deviation, is attributed to the evolution of melt flow pattern, which can be altered by positioning the magnet at a different height. The discovery of such melt flow behavior proposes an effective solution to eventually eliminate the abrupt diameter deviation during body growth. The visualization of the melt flow transformation by 3-D simulation offers another adjustability to the process of modern industrial crystal growth with transverse magnet. The mechanism of how the magnet position impacts the melt flow is also discussed.

Scrutiny of melting rate of phase change material in a four petals cavity with internal branch fins under magnetic field

This paper attempts to conduct the numerical simulation of the behavior of a phase change material (PCM) in a four petals enclosure with internal fins. The enthalpy-porosity method was used to simulate the thawing process. Two new enclosures with the different arrangements of the fins have been considered. The simulation results for the new finned geometries have been compared with those of the simple double-tube geometry. The effects of fin length (FL), angle between the fins (ABF), non-uniform magnetic field, Stefan number on the PCM melting process (PCM-MP) have been inspected. Results display that the average liquid fraction for new no-fin geometries has increased by about 14% compared to the simple double-tube. In both storage enclosures, the PCM-MP has increased by enhancing FL and falling ABF. ABF = 40° is about 18.7% higher melting performance. Also, the use of non-uniform magnetic field and boosting its power improves the PCM-MP about 10%. Increasing Stefan number augments the PCM-MP by about 15%. The material of the fins has a momentous result on the PCM-MP.

Computational study of heat transfers and hydrodynamic of nanofluid flow inside double pipe heat exchanger in presence of non-uniform magnetic field

The improvement of heat exchangers is important for energy savings and environmental protection. In this work, the application of the non-uniform magnetic source for the enhancement of the hydrodynamic in double-pipe heat exchangers is comprehensively studied. The hydrodynamics of nanofluid flow is also investigated in our research by performing computational modeling. In addition, the thermal efficiency of a selected system in different conditions of magnetic field and nanofluid velocity are studied. In addition, role of wall profile of inner tube on hydrodynamic is also disclosed in this work. The simulation of the nanofluid flow is done by computational fluid dynamics and solving Navier-stokes equations. The usage of non-uniform magnetic field would raise average skin friction about 230 %, 400 % and 500 % on sinusoidal, triangular and rectangular profiles, respectively.

SITAR: A Code for ICRH Antenna–Plasma Coupling

We have developed an ICRF (ion cyclotron range of frequency) heating antenna code, SITAR (Simulation of ICRH anTennA Resistance), for stripline antenna to find out its impedance in the presence of plasma. The antenna is of finite poloidal and toroidal dimensions and screened from the plasma by the Faraday shield that shorts out the toroidal components of the radio frequency field. Antenna lead current as well as toroidal and poloidal current of antenna have been taken into account. A slab geometry model for coupling of antenna with tokamak plasma and a linear model of the perpendicular refractive index for the propagation of ICRF wave in the evanescent region have been considered. The plasma in the coupling region is described by a cold plasma dielectric tensor with nonuniform density and toroidal magnetic field along the radial direction.

Electron dynamics in planar radio frequency magnetron plasmas: I. The mechanism of Hall heating and the µ-mode

The electron dynamics and the mechanisms of power absorption in radio-frequency (RF) driven, magnetically enhanced capacitively coupled plasmas at low pressure are investigated. The device in focus is a geometrically asymmetric cylindrical magnetron with a radially nonuniform magnetic field in axial direction and an electric field in radial direction. The dynamics is studied analytically using the cold plasma model and a single-particle formalism, and numerically with the inhouse energy and charge conserving particle-in-cell/Monte Carlo collisions code ECCOPIC1S-M. It is found that the dynamics differs significantly from that of an unmagnetized reference discharge. In the magnetized region in front of the powered electrode, an enhanced electric field arises during sheath expansion and a reversed electric field during sheath collapse. Both fields are needed to ensure discharge sustaining electron transport against the confining effect of the magnetic field. The corresponding azimuthal -drift can accelerate electrons into the inelastic energy range which gives rise to a new mechanism of RF power dissipation. It is related to the Hall current and is different in nature from Ohmic heating, as which it has been classified in previous literature. The new heating is expected to be dominant in many magnetized capacitively coupled discharges. It is proposed to term it the ‘µ-mode’ to separate it from other heating modes.

Ferrofluid double emulsion generation and manipulation under magnetic fields

Magnetically controlled double emulsions offer a promising tool for targeted drug delivery. Here, the effect of a non-uniform magnetic field induced by a permanent magnet on ferrofluid double emulsion generation and morphology is studied. In the numerical part, the double emulsion (and its ferrofluid core) migration was analyzed for different transverse distances of the magnet from the channel centerline (Ly) and different core and shell diameters. In the experimental part, water-based ferrofluid is encapsulated in a UV-curable polymer phase in water, forming a W/O/W double emulsion. As the magnet’s downstream distance from the ferrofluid junction reduces, the core aspect ratio increases while its diameter decreases. Finally, the core eccentricity was calibrated with Ly, and by shining a UV light, this core eccentricity was affixed inside the UV-curable medium. These double emulsions can then rearrange and orient under a uniform magnetic field for more efficient targeted drug delivery.

Design of anti-misalignment PCB coil for wireless power transfer

This paper focuses on the problem of the non-uniform magnetic field distribution in short-distance magnetic coupled wireless power transfer (MC-WPT) systems, which causes the transfer power and power transfer efficiency to decrease when the transmitter and receiver coils are offset. At first, the distribution law of the magnetic field generated by the printed circuit board (PCB) coil is studied. A new method is proposed to design the transmitter coil, which combines a genetic algorithm (GA) with a partial element equivalent circuit (PEEC). The appropriate turn radii are first obtained by the GA, and the quality factor is further evaluated by the PEEC to select the optimal coil turns. Finally, the PCB coils with uneven pitches are fabricated, and experiments are carried out on an MC-WPT system. The effective power transfer area is expanded noticeably and the high efficiency of the system remains stable, which is very useful to provide a wider working space and steady wireless energy supply for mobile objects.

Electron dynamics in planar radio frequency magnetron plasmas: II. Heating and energization mechanisms studied via a 2d3v particle-in-cell/Monte Carlo code

The present work investigates electron transport and heating mechanisms using an (r, z) particle-in-cell simulation of a typical rf-driven axisymmetric magnetron discharge with a conducting target. Due to a strong geometric asymmetry and a blocking capacitor, the discharge features a large negative self-bias conducive to sputtering applications. Employing decomposition of the electron transport parallel and perpendicular to the magnetic field lines, it is shown that for the considered magnetic field topology the electron current flows through different channels in the (r, z) plane: a ‘transverse’ one, which involves current flow through the electrons’ magnetic confinement region (EMCR) above the racetrack, and two ‘longitudinal’ ones, where electrons’ guiding centers move along the magnetic field lines. Electrons gain energy from the electric field along these channels following various mechanisms, which are rather distinct from those sustaining dc-powered magnetrons. The longitudinal power absorption involves mirror-effect heating (MEH), nonlinear electron resonance heating, magnetized bounce heating (MBH), and the heating by the ambipolar field at the sheath-presheath interface. The MEH and MBH represent two new mechanisms missing from the previous literature. The MEH is caused by a reversed electric field needed to overcome the mirror force generated in a nonuniform magnetic field to ensure sufficient flux of electrons to the powered electrode, and the MBH is related to a possibility for an electron to undergo multiple reflections from the expanding sheath in the longitudinal channels connected by the arc-like magnetic field. The electron heating in the transverse channel is caused mostly by the essentially collisionless Hall heating in the EMCR above the racetrack, generating a strong azimuthal drift velocity. The latter mechanism results in an efficient electron energization, i.e. energy transfer from the electric field to electrons in the inelastic range. Since the main electron population energized by this mechanism remains confined within the discharge for a long time, its contribution to the ionization processes is dominant.

Dynamic tuning of phase change interfacial thermal energy transport in hybrid nano-enhanced phase change materials by a non-uniform magnetic field

The present study aims to explore the synergistic enhanced phase change heat transfer behavior between magnetic field and hybrid nanoparticles under the consideration of non-Newtonian effects. The visualized experiments are conducted using thermochromic liquid crystal technology to extract the interconnection between dynamic phase change interface evolution and temperature change of hybrid nano-enhanced phase change material (HNEPCM), and thermocouples to measure temperature data inside the latent heat thermal energy storage unit. The magnetic field and hybrid nanoparticles synergistic mechanism is then further revealed based on the analysis of liquid phase fraction, Nusselt number, energy storage and energy storage efficiency. The results show that under magnetic field, the magnetic nanoparticles form a “thermal conductivity layer” at the solid-liquid interface, while the non-magnetic nanoparticles in the liquid phase region drive the phase change heat transfer to proceed. The non-Newtonian effect induced by high concentration of HNEPCM can weaken the convection intensity. The competition between natural convection and magnetic viscous effect under magnetic field is induced at different hybrid ratios of magnetic and non-magnetic nanoparticles. High hybrid ratios (R = 1.0, 1.5 and 2.0) will be more favorable to satisfy the high thermal conductivity requirement in the liquid phase region and the dynamic adjustment condition at the phase change interface. Under a non-uniform magnetic field, 1 wt.% HNEPCM with R = 1.0 is considered as the optimum strategy with better energy storage performance and higher magnetic field effect.

Influence of non-uniform magnetic field on spatial orientation of Heisenberg dimers with spin [formula omitted

The paper presents exact results for the Heisenberg dimer with spin S=1 placed in a non-uniform magnetic field. It was assumed that the dimer has a degree of freedom of movement enabling the spatial arrangement of the dimer axis parallel or perpendicular to the field gradient. Based on the calculations of the Gibbs magnetic energy, the occurrence of magneto-spatial phase transitions of the 1st kind was found, in which the dimer changing its magnetization state simultaneously changes the spatial orientation relative to the field gradient. Ground state phase diagrams and temperature diagrams are presented, as well as calculations of the Gibbs energy, magnetization, entropy and dimer quadrupolar moment. These calculations form an internally consistent thermodynamic description and are presented in figures. The discussed model illustrates the possibility of a magnetic contribution to the self-assembly of dimers on the surface.