Non-uniform Transverse Magnetic Field Research Articles

Effect of viscous dissipation and thermal radiation on MHD flow and heat transfer for a power-law fluid with variable fluid properties over a permeable stretching sheet

This manuscript considers the problem of MHD boundary layer flow and heat transfer of power – law fluid with variable fluid properties over a nonlinear stretching sheet in the presence of viscous dissipation and thermal radiation subject to a non-uniform transverse magnetic field and non-uniform heat generation. The governing boundary layer equations are transformed into a system of non-linear ordinary differential equations using suitable transformation which are solved numerically using fourth order Rung-Kutta integration scheme coupled with the shooting method. The effects of various parameters on the velocity, temperature, local skin-friction and the local Nusselt number are presented and discussed. We found that is as the radiation parameter R increases the temperature of a power-law fluid and the Nusselt number increase, while the viscosity parameter increasing leads to decreasing on the velocity of the fluid and local skin-friction.

Non-linear effects and electron heating dynamics in radio-frequency capacitively coupled plasmas with a non-uniform transverse magnetic field

A non-uniform transverse magnetic field is used to increase the plasma density and create an asymmetry in radio frequency capacitively coupled plasmas for plasma sputtering and plasma vapor deposition. Based on one-dimensional particle-in-cell/Monte Carlo collision simulations, the effect of the magnetic field magnitude on the non-linear behavior and the electron heating dynamics is studied for a pure helium plasma at a pressure of 30 mTorr. The results show that increasing the magnetic field magnitude can generate a more positive DC self-bias. As a result, non-linear oscillations of the electron current density and the electric field close to the grounded electrode are enhanced. An electric field reversal is induced when the powered electrode sheath collapses to balance electron and ion fluxes toward this boundary due to the strong confinement of electrons. Anomalous energetic electron beams are observed propagating from the collapsed sheath toward the plasma bulk. It is shown that such beams are reflections of the beams originating from the opposite expanding sheath based on the analysis of single particle motions. We show that energetic electron beams can be reflected by the transverse magnetic field.

Improving the nonlocal advantage of quantum coherence in the Heisenberg model by topological boundary conditions

The control of quantum correlations in a physical system is of practical importance. Nonlocal advantage of quantum coherence (NAQC) is a type of quantum correlation defined on the basis of quantum coherence. We present a comparative investigation of NAQC in the three-spin Heisenberg model with periodic boundary conditions (PBCs) and topological boundary conditions (TBCs). The results show that while there is no NAQC in the absence of an external magnetic field, one can create a considerable NAQC by applying a nonuniform transverse magnetic field to the three spins. In particular, the NAQC is significantly improved for the model with TBCs compared to that with PBCs, and the regions of nonvanishing NAQC can also be extended. The model considered with TBC can therefore serve as a potential candidate for creating NAQC.

Numerical investigation of nonuniform transverse magnetic field effects on the flow and heat transfer of magnetic nanofluid in a sintered porous channel

AbstractIn this paper, fluid flow and convective heat transfer of a ferrofluid (water and 4 vol% Fe3O4) in sintered Aluminum porous channel, which is subjected to a nonuniform transverse magnetic field have been studied. The numerical simulations supposed an ordinary cubic and staggered arrangement organized by uniformly sized particles with a small contact area for the porous media and constant heat flux at the surface of the microchannel. A wire, in which the electric current passes creates a nonuniform magnetic field, which is perpendicular to the flow direction. To do this simulation, the control volume technique and the two‐phase mixture model have been employed. The results show that the obtained local heat transfer coefficient on the channel surface increased with increasing mass flow rate and decreased slightly along the axial direction. Moreover, exerting the above‐mentioned magnetic field increases the Nusselt number that enhances the heat transfer rate while it has no effect on the pressure drop along the channel.

Numerical investigation of subcooled boiling characteristics of magnetic nanofluid under the effect of quadrupole magnetic field

This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic nanofluid (refrigerant-113 and 4 vol% Fe3O4) in a vertical annulus, which is exposed to a nonuniform transverse magnetic field generated by the quadrupole magnet. A control volume technique and SIMPLEC algorithm have been used for discretizing the governing equations and pressure-velocity coupling, respectively. The two-fluid model has been used to simulate subcooled flow boiling of the refrigerant-113. The results indicate that subcooled flow boiling characteristics change not only by using nanofluid as the working fluid, but also by applying the nonuniform transverse magnetic field. In the presence of the aforementioned magnetic field due to the Kelvin force, the fluid attracted to the outer wall. This leads to higher bubble detachment frequency so that the heat pumping is increased and the void fraction on the heated wall is decreased. Thus, the critical heat flux as one of the most important parameters in boiling processes will be increased.

Entanglement and coherence in a spin-s XXZ system under non-uniform fields

We investigate entanglement and coherence in an XXZ spin s pair immersed in a non-uniform transverse magnetic field. The ground state and thermal entanglement phase diagrams are analyzed in detail in both the ferromagnetic and antiferromagnetic cases. It is shown that a non-uniform field can control the energy levels and the entanglement of the corresponding eigenstates, making it possible to entangle the system for any value of the exchange couplings, both at zero and finite temperatures. Moreover, the limit temperature for entanglement is shown to depend only on the difference between the fields applied at each spin, leading for to a separability stripe in the field plane such that the system becomes entangled above a threshold value of . These results are demonstrated to be rigorously valid for any spin s. On the other hand, the relative entropy of coherence in the standard basis, which coincides with the ground state entanglement entropy at T = 0 for any s, becomes non-zero for any value of the fields at , decreasing uniformly for sufficiently high T. A special critical point arising at T = 0 for non-uniform fields in the ferromagnetic case is also discussed.

Forced convection heat transfer in a channel under the influence of various non-uniform transverse magnetic field arrangements

Magnetic nanofluids (MNFs) are suspensions, comprising a non-magnetic base fluid and magnetic nanoparticles. In modern set of suspensions called smart or functional fluids, fluid flow, particle movement, and heat transfer process can be controlled by magnetic fields. The present study investigated hydro-thermal characteristics of ferrofluid (water and Fe3O4) in a rib-channel exposed to several non-uniform transverse magnetic fields generated by an electric current going through several wires located perpendicularly in the flow between ribs. Two-phase mixture model and control volume technique were used to carry out the study. The effect of transverse magnetic field on heat transfer, consequent pressure loss, and wall shear stress resulting from this kind of magnetic field along the channel was investigated as well as the effect of various magnetic field arrangements. The results showed that magnetic field would have an unexpected influence on increasing Nusselt number in the channel, and raising the strength of magnetic field would cause an increase in minimum and maximum local Nusselt numbers, especially in rib regions. Furthermore, applying external magnetic field could noticeably affect fluid cooling at low Reynolds numbers. The influence of magnetic field on pressure drop and skin friction also are investigated in this study and it is illustrated that applying magnetic field increased skin friction especially in the case which whole magnetic wire located in one side on channel.

Numerical investigation of non-uniform transverse magnetic field effects on the swirling flow boiling of magnetic nanofluid in annuli

In this paper, swirling flow boiling of a dilute nanofluid (water and 0.1 vol%Fe3O4) in an annulus with a twisted fin on the outside of the inner wall in the presence of transverse magnetic gradient has been numerically investigated, using a two fluid model and a control volume technique. The results indicate that, in the boiling of swirling flow, the rate of the heat transfer increases. This phenomenon can be attributed to the effect of centrifugal force on the liquid phase flow and also reduction of the conductive sub-layer thickness that exists on the heated wall. The effects of improved surface wettability induced by nanoparticle deposition during the boiling process are accounted. The results demonstrate that the modified liquid property due to the existence of nanoparticles in the liquid has a negligible effect on the boiling heat transfer performance with dilute nanofluids while the improved surface wettability plays an important role and leads to reduction of the void fraction and consequently, an increase of critical heat flux. Applying a transverse magnetic field causes augmentation of the centrifugal force and results in increased flow turbulence. Furthermore, in the presence of the magnetic field due to magnetic force, the bubble departure diameter is reduced and bubble detachment occurs faster. Therefore, the critical heat flux will be increased. Swirling flow boiling in the presence of magnetic field is strongly suggested in devices requiring high heat transfer rates.

Numerical analysis of magnetic field effects on hydro-thermal behavior of a magnetic nanofluid in a double pipe heat exchanger

This study attempts to numerically investigate the hydro-thermal characteristics of a ferrofluid (water and 4vol% Fe3O4) in a counter-current horizontal double pipe heat exchanger, which is exposed to a non-uniform transverse magnetic field with different intensities. The magnetic field is generated by an electric current going through a wire located parallel to the inner tube and between two pipes. The single phase model and the control volume technique have been used to study the flow. The effects of magnetic field have been added to momentum equation by applying C++ codes in Ansys Fluent 14. The results show that applying this kind of magnetic field causes kelvin force to be produced perpendicular to the ferrofluid flow, changing axial velocity profile and creating a pair of vortices which leads to an increase in Nusselt number, friction factor and pressure drop. Comparing the enhancement percentage of Nusselt number, friction factor and pressure drop demonstrates that the optimum value of magnetic number for Reff=50 is between Mn=1.33×106 and Mn=2.37×106. So applying non-uniform transverse magnetic field can control the flow of ferrofluid and improve heat transfer process of double pipe heat exchanger.

Transition to a Quasi-Fully Developed MHD Flow in an Electrically Conducting Pipe under a Transverse Non-Uniform Magnetic Field

Interaction between liquid metal flows and non-uniform magnetic fields occurs in certain regions of fusion power reactors such as the breeding blanket access pipes. Here, the resulting high MHD pressure drop leads to numerous design challenges. Therefore, in this paper we perform numerical simulations to analyze the effect of a non-uniform transverse magnetic field on a liquid metal flow in a straight electrically conducting pipe. In particular, we perform parametric analyses at different conductance ratios and magnetic interaction parameters to quantify their effect on MHD pressure drop in pipes. The results also help in establishing a range for the control parameters in which the flow transforms from a quasi-fully developed to a fully three-dimensional state.

Sakiadis flow of Maxwell fluid considering magnetic field and convective boundary conditions

In this paper we address the flow of Maxwell fluid due to constantly moving flat radiative surface with convective condition. The flow is under the influence of non-uniform transverse magnetic field. The velocity and temperature distributions have been evaluated numerically by shooting approach. The solution depends on various interesting parameters including local Deborah number De, magnetic field parameter M, Prandtl number Pr and Biot number Bi. We found that variation in velocity with an increase in local Deborah number De is non-monotonic. However temperature is a decreasing function of local Deborah number De.

Numerical Investigation of the Transient Hydrothermal Behavior of a Ferrofluid Flowing Through a Helical Duct in the Presence of Nonuniform Magnetic Field

This paper investigates numerically the time dependent hydrothermal behavior of a ferrofluid (water and 4 vol. % Fe3O4) flowing in a helical channel, which is exposed to a nonuniform transverse magnetic field and its walls are subjected to uniform heat flux. The two phase mixture model and control volume technique have been used to study the flow. The results show that applying the nonuniform transverse magnetic field considerably increases the velocity and flow rate in the vicinity of the channel walls while it significantly decreases the velocity at the center of the channel. Applying magnetic field also decreases considerably the temperature of the inner wall of the helical channel. Furthermore, the average Nusselt number is increased by applying the nonuniform transverse magnetic field and it is more enhanced by increasing the magnetic field intensity.

Experimental and numerical studies of pressure drop in PbLi flows in a circular duct under non-uniform transverse magnetic field

Experiments and three-dimensional (3D) numerical simulations are performed to investigate the magnetohydrodynamic (MHD) characteristics of liquid metal (LM) flows of molten lead-lithium (PbLi) eutectic alloy in an electrically conducting circular duct subjected to a transverse non-uniform (fringing) magnetic field. An indirect measurement approach for differential pressure in high temperature LM PbLi is first developed, and then detailed data on pressure drop in this PbLi MHD flow are measured. The obtained experimental results for the pressure distribution are in good agreement with numerical simulations. Using the numerical simulation results, the 3D effects caused by fringing magnetic field on the LM flow are illustrated via distributions for the axial pressure gradients and transverse pressure differences. It has been verified that a simple approach for estimation of pressure drop in LM MHD flow in a fringing magnetic field proposed by Miyazaki et al. [22] i.e., a simple integral of pressure gradient along the fringing field zone using a quasi-fully-developed flow assumption, is also applicable to the conditions of the present experiment providing the magnetic interaction parameter is large enough. Furthermore, for two different sections of the LM flow at the entry to and at the exit from the magnet, it is found that the pressure distributions in the duct cross sections in these two regions are different.

Behavior of Negative Ion and Secondary Particles in Multi-Aperture Accelerator

The trajectories of the H− and the secondary particle in a multi-aperture accelerator for negative ion sources were calculated with a 3D numerical code, taking into account non-uniformity in magnetic field and negative ion density, and the heat loading on acceleration grids was estimated. The heat loading on a grounded grid mainly originates from the secondary electron impact and the non-uniform transverse magnetic field enhances the possibility of the secondary electron impacts on the grounded grid. Perveance condition of beamlets strongly depends on the plasma density profile and the direct interception of the H− by an extraction grid is enhanced in the non-uniform plasma case.

Numerical study of the ferrofluid flow and heat transfer through a rectangular duct in the presence of a non-uniform transverse magnetic field

This paper investigates numerically the hydro-thermal characteristics of a ferrofluid (water and 4vol% Fe3O4) in a vertical rectangular duct which is exposed to a non-uniform transverse magnetic field generated by an electric current going through a wire located parallelly under the duct. The two phase mixture model and the control volume technique have been used to study the flow.The results show that applying the aforementioned magnetic field increases the Nusselt number and friction factor and also creates a pair of vortices that enhances heat transfer and prevents sedimentation of nano-particles. Furthermore, unlike the axial non-uniform magnetic field, the increase of the Nusselt number for the transverse magnetic field is considerable in all length along the duct and it is also concluded that with increasing the Reynolds number, the effect of the transverse non-uniform magnetic field on the Nusselt number is more than that of the axial non-uniform magnetic field.

Concept of the field-driven domain wall motion memory

In this paper, the concept of field-driven domain wall motion memory is presented. It is confirmed that a domain is shifted with a carefully designed non-uniform field by micromagnetic simulations. The shift of a domain—a bit—can be established by the motion of two domain walls to the same direction and the same distance. In order to get a better understanding of the domain wall motion under the non-uniform transverse magnetic field, we investigate the motion of the transverse Néel-type domain wall by micromagnetic simulations and the collective coordinate approach. The validity of the equation of motion for the domain wall is confirmed by the micromagnetic simulations as functions of the gradient of the non-uniform field, the saturation magnetization, and the Gilbert damping parameter α.

Equation of motion for a domain wall movement under a nonuniform transverse magnetic field

We investigate the dynamics of the domain wall in a nonuniform field driven domain wall motion memory concept. The equation of motion for a transverse Néel-type domain wall under a nonuniform transverse magnetic field is obtained with a collective coordinate approach. The validity of the equation of the motion is confirmed with micromagnetic simulations. We find that the domain wall velocity depends on the domain wall width, Gilbert damping parameter α, saturation magnetization, and the gradient of the field. The domain wall velocity of ∼100m∕s is obtained with typical material as Permalloy with a moderate field gradient (100Oe∕μm). It has promising results for memory applications.

Another method for domain wall movement by a nonuniform transverse magnetic field

We propose another method for domain wall (DW) movement in nanowire geometry. We find that a DW moves to an energy minimum position in order to minimize the Zeeman energy under a nonuniform transverse magnetic field. DW dynamics under a spatially nonuniform transverse magnetic field has been investigated by means of micromagnetic simulations. The nonuniform transverse magnetic field driven DW motion has a superior nature which includes a rigidity of the domain structure during the movement of the DWs in addition to all of the advantages of conventional field driven DW movement. The direction of the DW movement only depends on the magnetization direction inside of the DW itself, not on the one of the domains. Therefore, it is possible to achieve field driven DW motion with rigid domain structures.

Magnetically Induced Thermal Rectification

We consider far from equilibrium heat transport in chaotic billiard chains with noninteracting charged particles in the presence of nonuniform transverse magnetic field. If half of the chain is placed in a strong magnetic field, or if the strength of the magnetic field has a large gradient along the chain, heat current is shown to be asymmetric with respect to exchange of the temperatures of the heat baths. Thermal rectification factor can be arbitrarily large for sufficiently small temperature of one of the baths.

The ground state of two-dimensional electrons in a nonuniform magnetic field

An exact solution to the Schrodinger equation for the ground state of two-dimensional Pauli electrons in a nonuniform transverse magnetic field H is presented for two cases. In the first case, the field H depends on a single variable, H = H(y), while in the second case, the field is axially symmetric, H = H(ρ), ρ2=x2+y2. The electron density distributions n = n(y) and n = n(ρ) that correspond to a completely filled lower level are found. For quasiuniform fields of fixed sign, the functions n(y) and n(ρ) are locally related to the magnetic field: n(y) = H(y)/ϕ0 and n(ρ) = H(ρ)/ϕ0, where ϕ0 = hc/|e| is a magnetic flux quantum. Magnetic fields are considered that are periodic, singular, and bounded in the plane xy. Finite electron objects in a nonuniform magnetic field are analyzed.