Lorentz Force Component Research Articles

Evaluation of heat transfer transition for nanofluids within an enclosure based on magnetic field angles

The present simulation deals with Magnetohydrodynamics (MHD) with focus on heat transfer attributes with the addition of nanoparticle volume concentration (NVC). Influences of the governing parameters, including natural convection strength of Rayleigh number (Ra), magnetic field intensity of Hartmann number (Ha), and magnetic directions with a complete period of γ = 0°–180° are studied. The results disclose that MHD heat transfer patterns can be divided into three groups, i.e., Type-1 (Increase), Type-2 (Transition), and Type-3 (Optimum). With the increasing Ha, the heat convection is suppressed, resulting in Type-1. Enhancing Ra improves the thermal convection and thus leads to the occurrence of Type-3. In between, a critical Ha (Hacr) at a specific Ra exists for heat transfer transition (Type-2). The flow structure and heat transfer patterns vary with γ stemming from the Lorentz force component. The maximum stream function and best heat transfer rate are found at γ = 45°, corresponding to a higher Hacr for Type-2. For a wide range of Ra values, the combination of Ha2/Ra is adopted to describe the relationship between the Hacr and γ for the heat transfer transition with an empirical equation as Ra ≥ 5 × 104.

Numerical modeling for efficiency of solar cell module combined with TEG involving Fe3O4-water nanofluid utilizing MHD

To boost the electrical power of solar panel, water has been mixed with Fe3O4 nanoparticles in the current article. The magnetic field was employed to boost the cooling procedure. Such a method can augment the electrical performance of the system and to enhance this function, TEG (thermoelectric generator) has been integrated with conventional PV module above the absorber layer. The regime of ferrofluid flow is laminar and its properties have been derived based on a single phase model. The heat sources of momentum equations in the fluid zone are Lorentz force components. The heat sources of solid layers for conduction equations have been derived from optical analysis of received irradiation. Different ranges of inlet velocity (Vin = 0.03 to 0.1 m/s), concentration of ferrofluid (ϕ = 0.005 to 0.05) and strength of B (Ha = 0 to 100) have been simulated via finite volume method which is verified based on previous publications. Results indicated that the overall performance enhances about 5.07% with growth of Ha. With augment of Vin, overall efficiency enhances about 11.70%. Loading nanoparticles can boost the performance and its effect in the absence of the magnetic field is 1.92 times greater than that of Ha = 100.

Numerical Investigation on Mechanisms of MHD Heat Flux Mitigation in Hypersonic Flows

Numerical simulations of hypersonic magnetohydrodynamics (MHD) flow over a typical sphere–cone blunt body are carried out based on the assumption of a low magnetic Reynolds number. The effects of an external dipole magnetic field on the surface heat flux are analyzed in detail, and multiple mechanisms of the MHD heat flux mitigation are revealed systematically for the first time. The following is found: (1) The external magnetic field can effectively reduce the stagnation point heat flux, and the increase in the boundary layer thickness due to the effect of counter-flow Lorentz force, which is equivalent to adding an adverse pressure gradient, is the main reason. (2) In the head region of the blunt body, the relative surface heat flux shows a complex trend of rising and falling because there are two mechanisms which could produce the opposite effects on the surface heat flux. One is that the counter-flow Lorentz force results in an increase in the boundary layer thickness, and the other is that the Joule heating increases the static temperature behind the shock wave. (3) In the shoulder region of the blunt body, the Lorentz force component, normal to streamline, could change the flow direction of the fluid elements, causing the streamline to deviate from the wall or even separate, thus affecting the surface heat flux. (4) In the large area downstream of the blunt body, the surface heat flux could still be reduced by more than 30% due to the “upstream historical effect”.

Fast computation of the Lorentz force induced by longitudinal electromagnetic stirring

In this work, we revisit a recent transient three-dimensional (3D) model for longitudinal electromagnetic stirring in the continuous casting of rectangular steel blooms. Whereas the earlier work was able to demonstrate accurate approximations to the solutions in two asymptotic limits, both of which gave economical alternatives to time-consuming 3D computations, here we show that the original governing equations can be manipulated to a form that allows for rapid computation even outside of these asymptotic limits. The resulting formulation requires the numerical solution of two steady-state complex Helmholtz-like equations in two dimensions that are coupled via a non-standard internal interface condition that is reminiscent of that occurring in the study of Marangoni convection; these equations are then solved numerically using the finite-element software Comsol Multiphysics. With this formulation, it is possible to compute the time-averaged Lorentz force components in a way that requires around four orders of magnitude less computational time than the fully 3D approach.

The role of non-axisymmetry of magnetic flux rope in constraining solar eruptions

Whether a solar eruption is successful or failed depends on the competition between different components of the Lorentz force exerting on the flux rope that drives the eruption. The present models only consider the strapping force generated by the background magnetic field perpendicular to the flux rope and the tension force generated by the field along the flux rope. Using the observed magnetic field on the photosphere as a time-matching bottom boundary, we perform a data-driven magnetohydrodynamic simulation for the 30 January 2015 confined eruption and successfully reproduce the observed solar flare without a coronal mass ejection. Here we show a Lorentz force component, resulting from the radial magnetic field or the non-axisymmetry of the flux rope, which can essentially constrain the eruption. Our finding contributes to the solar eruption model and presents the necessity of considering the topological structure of a flux rope when studying its eruption behaviour.

Чисельне моделювання нестаціонарних потоків холодної плазми при роботі плазмового актуатора

The numerical simulation of unsteady flows of cold plasma is considered in this article. A low-temperature non-equilibrium ideal plasma is formed when the plasma actuator interacts with the air. The mathematical model has been developed to describe the behavior of low-temperature plasma. It is based on non-stationary equations describing the dynamics of charged particles and plasma electrodynamics equations. The 14 types of particles: metastable and excited nitrogen and oxygen atoms, positive and negative ions, electrons and atomic oxygen are considered. Volumetric and surface chemical reactions describing processes in a barrier discharge that occur above the dielectric surface are considered. For non-stationary equations of plasma dynamics, an implicit numerical algorithm with pseudo-time iteration has been developed, which is based on a finite-volume approach. The equation for the electrostatic potential with sources was solved using the generalized minimal residual method with incomplete LU preconditioning. In non-stationary equations for the density of plasma particles, the drift derivatives were approximated using the TVD scheme with the MinMod limiter function. The derivatives in the equation for the electric potential were calculated using finite-volume relations taking into account the upwind approximation of the concentration of charged plasma particles. The numerical results of the generation, propagation and destruction of a streamer during a dielectric barrier discharge are obtained. The unsteady plasma characteristics in the region above the dielectric surface are analyzed, including the distribution of the particles density, electric potential and the Lorentz force components. The results of numerical simulation of unsteady flows of low-temperature plasma are in good agreement with the available experimental data.

Analysis of a model for longitudinal electromagnetic stirring in the continuous casting of steel

A recent three-dimensional (3D) model that revisited earlier theoretical work for longitudinal electromagnetic stirring in the continuous casting of steel blooms is analyzed further to explore how the bloom width interacts with the pole pitch of the stirrer to affect the magnetic flux density. Whereas the first work indicated the presence of a boundary layer in the steel near the interface with the stirrer, with all three components of the magnetic flux density vector being coupled to each other, in the analysis presented here we find that the component along the direction of the travelling wave decouples from those in the other two directions and can even be determined analytically in the form of a series solution. Moreover, it is found that the remaining two components can be found via a two-dimensional computation, but that it is not possible in general to determine these components without taking into account the surrounding air. The validity of the asymptotically reduced model solution is confirmed by comparing it with the results of 3D numerical computations. Moreover, the asymptotic approach provides a way to compute the time-averaged Lorentz force components that requires two orders of magnitude less computational time than the fully 3D approach.

Design, analyses, fabrication and characterization of Nb3Sn coil in 1 W pulse tube cryocooler

A laboratory scale Nb3Sn coil is designed, analysed, fabricated and characterized in 1 W pulse tube cryocooler in solid nitrogen cooling mode and in conduction cooling mode. The magnetic field profile in axial and radial direction, Lorentz force component across the winding volume in operational condition are estimated in COMSOL. The coil is designed for 1.5 T at 100 A. It is fabricated in wind and react method. Before winding, the insulated Nb3Sn strand is wound on a copper mandrel which is thermally anchored with the 2nd stage of the cold head unit via a 10 mm thick copper ‘Z’ shaped plate The temperature distribution in 2nd cold stage, copper z plate and coil is monitored in both solid nitrogen cooling and conduction cooling mode. In solid nitrogen cooling mode, the quench of the coil occurs at 150 A for 0.01 A/s current ramp rate. The magnetic field at the centre of the coil bore is measured using transverse Hall sensor. The measured magnetic field value is compared with the analytical field value and they are found to be deviating ∼5% in magnitude. Again the coil is tested in conduction cooling mode maintaining the same current ramp rate and it is observed that the coil gets quenched at 70 A at temperature ∼ 10K.

Mechanism analysis of Magnetohydrodynamic heat shield system and optimization of externally applied magnetic field

As a novel thermal protection technique for hypersonic vehicles, Magnetohydrodynamic (MHD) heat shield system has been proved to be of great intrinsic value in the hypersonic field. In order to analyze the thermal protection mechanisms of such a system, a physical model is constructed for analyzing the effect of the Lorentz force components in the counter and normal directions. With a series of numerical simulations, the dominating Lorentz force components are analyzed for the MHD heat flux mitigation in different regions of a typical reentry vehicle. Then, a novel magnetic field with variable included angle between magnetic induction line and streamline is designed, which significantly improves the performance of MHD thermal protection in the stagnation and shoulder areas. After that, the relationships between MHD shock control and MHD thermal protection are investigated, based on which the magnetic field above is secondarily optimized obtaining better performances of both shock control and thermal protection. Results show that the MHD thermal protection is mainly determined by the Lorentz force's effect on the boundary layer. From the stagnation to the shoulder region, the flow deceleration effect of the counter-flow component is weakened while the flow deflection effect of the normal component is enhanced. Moreover, there is no obviously positive correlation between the MHD shock control and thermal protection. But once a good Lorentz force's effect on the boundary layer is guaranteed, the thermal protection performance can be further improved with an enlarged shock stand-off distance by strengthening the counter-flow Lorentz force right after shock.

A Study on the Influence of the Nozzle Lead Angle on the Performance of Liquid Metal Electromagnetic Micro-Jetting.

To improve the jetting performance of liquid metals, an electromagnetic micro-jetting (EMJ) valve that realizes drop-on-demand (DOD) jetting while not involving any valve core or moving parts was designed. The influence of the lead angle of the nozzle on the jetting of liquid metal gallium (Ga) was investigated. It was found that the Lorentz force component parallel to the nozzle that jets the electrified liquid Ga is always larger than its internal friction; thus, jet can be generated with any lead angle but with different kinetic energies. Experimental results show that the mass of the jetting liquid, the jetting distance, the initial velocity of the jet, and the resulting kinetic energy of the jet increase first and then decrease. When the lead angle is 90°, the mass of the jetting liquid and the kinetic energy are at their maximum. When the angle is 80°, the initial velocity achieves its maximum, with a calculated value of 0.042 m/s. Moreover, very close and comparatively high kinetic energies are obtained at 80° and 90°, indicating that angles in between this range can produce a preferable performance. This work provides an important theoretical basis for the design of the EMJ valve, and may promote the development and application of micro electromagnetic jetting technology.

Numerical and experimental modeling of melt flow in a directional solidification configuration under the combined influence of electrical current and magnetic field

One of the key issues in the directional solidification (DS) process of multi-crystalline silicon is the control of the melt flow in order to achieve a higher quality of the crystallized material. The combination of a static magnetic field B and an electrical current I, giving rise to an electromagnetic force has a significant melt stirring effect, even for small values of I and B. In order to understand the basic features of the melt flow in a DS-like configuration under electromagnetic stirring, an isothermal model experiment in a rectangular crucible filled with a room temperature GaInSn melt and a corresponding STHAMAS3D time-dependent numerical model, were developed. Experimental velocity profiles measured by UDV confirmed the flow structure obtained in the numerical simulations. A parametrical study for a range of I and B values was performed, in the case of a symmetrical electrode positioning along the diagonal of the free melt surface. The resulting flow structure was analyzed and described in terms of a vortex or a poloidal recirculation domination and a transition between the two. A characteristic parameter was defined to quantify the different flow structures. Through the use of scaling analysis, two dimensionless numbers corresponding to the two Lorentz force components were identified and a good correlation between their values, flow structure and maximal velocity was observed. This correlation makes possible the prediction of the flow structure for any set of the system parameters I and B and a characteristic crucible length. The same conclusions would hold for a silicon melt if the dimensionless numbers are conserved by choosing different I and B in respect with the different material constants.

On possible photometric manifestation of magnetic field in Cyg X-1

We consider the influence of the magnetic field of the O9.7Iab supergiant component in the CygX-1 X-ray binary system on its atmosphere. In the frame of the simplest model (the unipolar cylindrically symmetric circum-polar magnetic spot model in static approximation, neglecting the Lorentz force component related to the force line curvature), the magnetic pressure is found to be comparable to model-atmosphere gas and radiative pressure, exceeding them in the area around the magnetic poles. As a result, bright spots are formed on the star surface. The dipolar or quadrupolar magnetic field can create large-size bright spots which can be studied by ground-based optical photometry. In the case of magnetic field inclined to the stellar rotation axis, resulting variability may be of the order of 1%. The field of higher multipolar configuration (produced by dynamo) can form smaller spots and may be detected only by space telescopes. Besides, the spots may be revealed from spectral-line profile variability. Observations of spots can be used as an instrument of magnetic field analysis.

ON THE FORCE-FREE NATURE OF PHOTOSPHERIC SUNSPOT MAGNETIC FIELDS AS OBSERVED FROMHINODE(SOT/SP)

A magnetic field is force-free if there is no interaction between the magnetic field and plasma in surrounding atmosphere i.e., electric currents are aligned with the magnetic field, giving rise to zero Lorentz force. Computation of various magnetic parameters such as magnetic energy, gradient of twist of sunspot fields and any kind of extrapolations, heavily hinge on the force-free approximation of the photospheric sunspot magnetic fields. Thus it is important to inspect the force-freeness of sunspot fields. The force-freeness of sunspot magnetic fields has been examined earlier by some researchers ending with incoherent results. Accurate photospheric vector field measurements with high spatial resolution are required to inspect the force-free nature of sunspots. We use several such vector magnetograms obtained from the Solar Optical Telescope/Spectro-Polarimeter aboard the Hinode. Both necessary and sufficient conditions for force-freeness are examined by checking global and local nature of magnetic forces over sunspots. We find that the sunspot magnetic fields are not much away from force-free configuration, although they are not completely force-free on the photosphere. The umbral and inner penumbral fields are more force-free than the middle and the outer penumbral fields. During their evolution, sunspot magnetic fields are found to maintain their proximity to force-free behaviour. Although a dependence of net Lorentz force components is seen on the evolutionary stages of the sunspots, we don't find a systematic relationship between the nature of sunspot fields and associated flare activity. Further, we examine whether the fields at photosphere follow linear or non-linear force free conditions. After examining this in various complex and simple sunspots we conclude that,in either case,the photospheric sunspot fields are closer to satisfy non linear force-free field approximation.

JET ROTATION DRIVEN BY MAGNETOHYDRODYNAMIC SHOCKS IN HELICAL MAGNETIC FIELDS

In this paper we present a detailed numerical investigation of the hypothesis that a rotation of astrophysical jets can be caused by magnetohydrodynamic shocks in a helical magnetic field. Shock compression of the helical magnetic field results in a toroidal Lorentz force component which will accelerate the jet material in toroidal direction. This process transforms magnetic angular momentum (magnetic stress) carried along the jet into kinetic angular momentum (rotation). The mechanism proposed here only works in a helical magnetic field configuration. We demonstrate the feasibility of this mechanism by axisymmetric MHD simulations in 1.5D and 2.5D using the PLUTO code. In our setup the jet is injected into the ambient gas with zero kinetic angular momentum (no rotation). Different dynamical parameters for jet propagation are applied such as the jet internal Alfven Mach number and fast magnetosonic Mach number, the density contrast of jet to ambient medium, or the external sonic Mach number of the jet. The mechanism we suggest should work for a variety of jet applications, e.g. protostellar or extragalactic jets, and internal jet shocks (jet knots) or external shocks between the jet and ambient gas (entrainment). For typical parameter values for protostellar jets, the numerically derived rotation feature looks consistent with the observations, i.e. rotational velocities of 0.1-1 percent of the jet bulk velocity.

Vortex dynamics in thin films ofYBa2Cu3O7−xwith three-dimensional nanoscale patterns

We have machined vertical transport structures in YBCO thin films in which the current is constrained to flow along the $c$ axis. When current is forced to flow outside the $a\text{\ensuremath{-}}b$ planes in YBCO a component of the Lorentz force arises acting along them. Consequently we have been able to directly measure the magnitude of the pinning force acting on vortices moving within the $a\text{\ensuremath{-}}b$ planes, and demonstrate vortex cutting, vortex channeling, and pinning by twin boundaries. Such measurements are not otherwise possible in $c$-axis films. We show that vortex channeling can be inhibited by a Lorentz force component acting perpendicularly to the channel. This is ascribed to pinning of the vortices within the channel by the channel wall.

Magnetic circuit design by topology optimization for Lorentz force maximization in a microspeaker

Bipolar magnets are common in most magnetic circuits, but circuit efficiency can be enhanced if multipolar magnets are used instead. However, the use of multipolar magnets is rare. In this investigation, the possibility of using multipolar magnets for maximizing the Lorentz force in a microspeaker is considered. The design problem of a multipolar magnetic circuit is formulated as a topology optimization problem. The circuit design region is discretized by finite elements, and a material state among a multipolar magnet, a yoke and air for every element is so determined as to maximize a desired Lorentz force component. Numerical results show that multipolar magnetic circuits obtained by the topology optimization outperform a conventional bipolar circuit.

Polarization effect of fields on vacuum laser acceleration

Concerning laser-driven electron acceleration in vacuum, a comparison was made between using circularly polarized (CP) laser field and linearly polarized (LP) field. It has been found that the main advantage for using CP field is that its acceleration channel occupies relatively larger phase space, which can give rise to greater acceleration efficiency. This feature chiefly comes from the difference in the distribution of the longitudinal electric components of these two kinds of fields. One of the disadvantages with CP field is the “energy saturation” phenomenon as the laser intensity is sufficiently high, resulting from the enhanced Lorentz force component in CP field. Physical explanations of these characteristics are addressed as well.

Vacuum laser acceleration in circularly polarized fields

This paper presents the characteristics of vacuum laser acceleration in circularly polarized (CP) fields with the capture and acceleration scenario (CAS) scheme. Compared with linearly polarized laser fields, the main advantage for using a CP field is that its acceleration channel occupies a relatively larger phase space, stemming from the distribution of the longitudinal electric component, the dependence on the laser initial phase and on the incident polarization azimuth angle, etc. These features can give rise to greater acceleration efficiency. One of the disadvantages is the ‘energy saturation’ phenomenon if the laser intensity is sufficiently high, which comes from the additional Lorentz force component in the CP field. The output properties of the accelerated CAS electron bunch, such as the energy spectra, the angular distributions, the energy-angle correlations, the acceleration efficiency, the rms emittance and the energy divergence, are also examined. Physical explanations concerning these characteristics are presented.

Simulation of Subsonic and Supersonic Flows in Inductive Plasmatrons

Simulation of sub- and supersonic thermochemical equilibrium flows in plasmatrons is considered. A physicochemical model, numerical method, and computation results for equilibrium inductive coupled plasma flows in a plasmatron are given. An effective preconditioning technique along with an implicit total-variation-diminishing scheme is used to solve the Navier‐Stokes equations in both subsonic and supersonic regimes. The governing equations include source terms corresponding to the electromagnetic field influence: the Lorentz force components (so-called magnetic pressure) and Joule heat production. The necessary transport coefficients were calculated in advance for equilibrium air plasma as the functions of pressure and temperature. Transport properties were calculated by the precise formulas of the Chapman‐Enskog method in the temperature range 300 < ‐ T < 15,000 K. Calculations of equilibrium air plasma flows for the IPG-4 (Institute for Problems in Mechanics, Russian Academy of Science) discharge channel geometry with the channel radius Rc = 0.04 m and length Zc = 0.40 m were performed. Creation of both underexpanded and overexpanded jets exhausted from the plasmatron channel is considered. A comparison with experimental results is given.

The movement and thermal history of an aluminium particle in a rf plasma generator

In previous studies on plasma-particle interaction, as far as we know,the rf plasma flow and temperature fields are all simulated by the non-self-consistentone-dimensional electromagnetic (1-D EM) field model. In the present paper, thecomplete self-consistent two-dimensional electromagnetic (2-D EM) field model in-corporating the axial Lorentz force component, which is neglected in the 1-D model,is firstly adopted to calculate the aluminium particle trajectory and thermal historyin atmospheric rf Ar plasma with the particle evaporation effect included. The cru-cial effect of reverse flow within the coil region on the particle trajectory is discoveredand the results show that the 2-D EM field model must be adopted instead of the1-D model when the plasma-particle interaction is studied. The effect of carrier gasflux on the particle movement and heating are also studied, resulting in some usefulconclusions for both plasma theory and application.