Negative Magnetic Field Gradient Research Articles

Effects of annular-cylindrical combined channel Hall thruster length on the discharge characteristics

In order to increase the degree of ionization and improve the efficiency, the effects of different cylindrical outer wall lengths on the discharge characteristics of the annular-cylindrical combined channel Hall thruster are studied. The cylindrical outer wall is set to the following positions: 95% (with positive magnetic field gradient), and 100%, 95%, 90%, 75%, and 50% (with negative magnetic field gradient) of the maximum magnetic field. The ionization gain and wall energy loss under different positions are analyzed through numerical simulations. The simulation result shows that when the cylindrical outer wall is set to the position where it is 100%–90% of the maximum magnetic field (negative magnetic field gradient zone), the power deposition on the channel wall is small, the propellant utilization can be largely improved, and the comprehensive performance is optimal for the annular-cylindrical combined channel Hall thruster.

Laminar ferrofluid heat transfer in presence of non-uniform magnetic field in a channel with sinusoidal wall: A numerical study

This research is allocated to numerical study of two dimensional laminar forced convective heat transfer of ferrofluid (water and Fe3O4) in the presence of a non-uniform magnetic field in a channel with a sinusoidal corrugated wall. The hydrodynamic and thermal behavior of the ferrofluid is examined using the mixture model and the finite volume method. The effects of an increase in wavy channel amplitude, volume fraction, Reynolds number, and negative magnetic field gradient on the hydrodynamic and thermal behaviors are investigated and discussed. Nusselt number is found to increase as wave amplitude, volume fraction of nanoparticles, Reynolds number, or negative magnetic field gradient increase. The negative magnetic field gradient increases the pump work with a greater effect at lower Reynolds numbers. The results of this research can be utilized to enhance heat transfer in heat exchangers and to increase their efficiency.

A two-phase simulation for ferrofluid flow between two parallel plates under localized magnetic field by applying Lagrangian approach for nanoparticles

Abstract Hydrothermal characteristics of water–MnZnFe2O4 magnetic nanofluid between two parallel plates are evaluated under effect of localized magnetic field. The Eulerian–Lagrangian approach is employed considering the effects of Brownian, thermophoretic, magnetic, lift and drag forces. Convective heat transfer enhances by increasing each of the parameters of nanoparticle size, concentration and magnetic field strength. The rate of enhancement for convective heat transfer is lower at higher magnetic field strengths due to the magnetic saturation phenomenon. Concentration is non-uniform in transverse direction, and nanoparticles migrate to central regions. In the sections with positive magnetic field gradient, the magnetic force is exerted on the particles in the flow direction and consequently, velocity increases in the central regions while decreases near the walls. Conversely, in the sections where the magnetic field gradient is negative, the velocity decreases in the central regions while increases near the walls. This causes local changes in convective heat transfer. Moreover, the pressure increases along the channel in the sections with positive magnetic field gradient, whereas in the sections with negative magnetic field gradient, the pressure drops with a greater slope compared with the case without magnetic field. Additionally, the wall temperature decreases with increasing the magnetic field gradient.

Extraction of Dysprosium Ions with DTPA Functionalized Superparamagnetic Nanoparticles Probed by Energy Dispersive X-ray Fluorescence and TEM/High-Angle Annular Dark Field Imaging.

The extraction of dysprosium (Dy3+) ions from aqueous solution was carried out successfully, using magnetite (Fe3O4) nanoparticles functionalized with diethylenetriaminepentaacetic acid (MagNP@DTPA). The process was monitored by energy dispersive X-ray fluorescence spectroscopy, as a function of concentration, proceeding according to a Langmuir isotherm with an equilibrium constant of 2.57 × 10-3 g(MagNP) L-1 and a saturation limit of 63.2 mgDy/gMagNP. The presence of paramagnetic Dy3+ ions attached to the superparamagnetic nanoparticles led to an overall decrease of magnetization. By imaging the nanoparticles surface using scanning transmission electron microscopy equipped with high resolution elemental analysis, it was possible to probe the binding of the Dy3+ ions to DTPA, and to show their distribution in a region of negative magnetic field gradients. This finding is coherent with the observed decrease of magnetization, associated with the antiferromagnetic coupling between the lanthanide ions and the Fe3O4 core.

Experimental analysis of magnetic field effect on the pool boiling heat transfer of a ferrofluid

In this research, an experimental study was conducted to investigate the pool boiling heat transfer of Fe3O4/water nanofluid (ferrofluid) in the atmospheric pressure. This study also investigated the influence of the magnetic field on the rate of boiling heat transfer of nanofluid. Deionized (DI) water was used to examine the repeatability, integrity and precision of the experimental apparatus where a well agreement with the existing correlations was observed. The investigation of various volume concentrations of nanofluid revealed that boiling heat transfer in high concentrations decreases with an increase of concentration while it rises with the increase of concentration in low concentrations. The boiling heat transfer coefficient at 0.1% volume concentration nanofluid was evaluated as optimal (increasing up to 43%). In addition, experimental studies showed that the presence of positive and negative magnetic field gradients decrease and increase the boiling heat transfer, respectively. The findings of this study showed that at higher concentrations of nanofluid, the effect of the magnetic field on nanoparticles is boosted. The results of the experiments indicated that adding nanoparticles would not necessarily increase the boiling heat transfer coefficient. In fact, the surface roughness and the magnetic field gradient on the boiling surface were the main factors that could affect the boiling heat transfer coefficient, significantly.

Observation of high-frequency ion instabilities in a cross-field plasma

This paper reports on the examination of the high-frequency ion dynamics in a low-pressure cross-field plasma. Measurements of the time-varying ion velocity distribution function were carried out by photon-counting laser-induced fluorescence spectroscopy. A temporal correlation was found between the low-frequency ionization process and the high frequency ion instability. The high-frequency spectral properties of the ion mean velocity are compared with a perturbed fluid model of the plasma flow on the source axis. The wavelength of the high-frequency mode is similar to the electric field extent outside the cavity. Surprisingly the mode appears only in the area of negative magnetic field gradient.

Lengths of Lifted Laminar Flames Under Vertical Magnetic Field Gradient

The effect of a vertical magnetic field gradient on stoichiometric lengths of methane-air diffusion lifted flames was numerically investigated. The study was developed for understanding the evolution of these lengths when coaxial jet air velocity was varied keeping constant fuel velocity. As the air jet velocity is increased, gradient in mixture fraction and liftoff height increase and flame length decreases. Calculated reacting flow results of liftoff and flame length are presented for two cases, with and without magnetic field. For the same initial flow conditions, the application of the negative magnetic field gradient results in an increase of the stoichiometric flame lengths and a reduction of liftoff heights. For the lifted flame under magnetic field gradient, the Froude number was modified to take into account the magnetic buoyancy effect and it is shown that the correlation proposed by Altenkirch et al. (1976) can still be applied to represent lengths of lifted flames.

Numerical study of non-uniform magnetic fields effects on subcooled nanofluid flow boiling

The main goal of the present work is to investigating the effect of non-uniform axial magnetic field with positive and negative gradients on subcooled nanofluid flow boiling. Also, for better understanding the existing phenomena, single-phase convection has been studied in the first part. Control volume technique for discretizing the governing equations, SIMPLEC algorithm for pressure-velocity coupling, and SST k-ω model for turbulent flow have been used in this research, respectively. In the second part, a three dimensional two-fluid model is used to subcooled flow boiling and the available R-113 boiling experimental data are used to validate the results. The obtained results indicate that single-phase convection as well as subcooled flow boiling characteristics change not only by using nanofluid as a working fluid, but also by applying the non-uniform axial magnetic field. For the case of magnetic field with negative gradient, single-phase convection heat transfer rate increases. Also for the subcooled boiling flow, negative magnetic field gradient will lead to increase critical heat flux (CHF), due to the decrease in evaporation rate in the wall surface which is eventually responsible for wall dryout. As a result, a more safe operation condition of the industrial equipments can be achieved.

Euler‐Lagrangian Simulation of Magnetic Field Effects on the Mixed Convection of Ferrofluid

In this article, the hydro‐thermal behavior of a ferrofluid flow (kerosene and 4 vol% Fe3O4) in a vertical tube in the presence of different magnetic field gradients using a two‐phase Euler–Lagrange model and control volume technique has been reported. Two cases for the magnetic field gradients have been considered to affect the mixed convection of the ferrofluid: non‐uniform axial negative and positive field gradients. Unlike our previous studies instead of uniform nanoparticle distribution within the flow, it has been considered as non‐uniform (maximum concentration profile is at the flow centerline). Based on the obtained results in this case, the velocity profiles in the negative magnetic field gradients are more affected than our previous studies. Also the time variations of the Nusselt number and skin friction factor have been presented. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res 43(2): 148‐166, 2014; Published online 12 September 2013 in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21069

A 3D numerical simulation of mixed convection of a magnetic nanofluid in the presence of non-uniform magnetic field in a vertical tube using two phase mixture model

In this paper, results of applying a non-uniform magnetic field on a ferrofluid (kerosene and 4 vol% Fe 3O 4 ) flow in a vertical tube have been reported. The hydrodynamics and thermal behavior of the flow are investigated numerically using the two phase mixture model and the control volume technique. Two positive and negative magnetic field gradients have been examined. Based on the obtained results the Nusselt number can be controlled externally using the magnetic field with different intensity and gradients. It is concluded that the magnetic field with negative gradient acts similar to Buoyancy force and augments the Nusselt number, while the magnetic field with positive gradient decreases it. Also with the negative gradient of the magnetic field, pumping power increases and vice versa for the positive gradient case.

Thermally activated mineralogical transformations in archaeological hearths: inversion from maghemite γFe2O4 phase to haematite αFe2O4 form

AbstractA series of laboratory experiments were conducted in an effort to understand why magnetic field gradient survey techniques failed to detect hearths at a prehistoric archaeological site in southern California. The study used various methods of environmental magnetism to examine the effects of exposing soil samples to a temperature of 650°C over a period 26 h. Results of the study indicate that the failure was associated with a reduction in soil magnetic susceptibility to below background levels within hearth soils. This reduction was due to high‐temperature transformation of iron oxides from a highly magnetic form to a relatively non‐magnetic form. The reduction in susceptibility is thought to have proceeded via the oxidation of primary (lithogenic) magnetite, Fe3O4 to maghemite, γ Fe2O4 followed by the inversion of maghemite to haematite, αFe2O4. The study suggests that in some instances high temperature inversion can reduce the proportion of ferrimagnetic minerals within a hearth to below initial concentrations (resulting in a negative magnetic field gradient anomaly, the opposite of what is normally expected). A field experiment was also conducted to determine what soil temperatures might be achieved 2 cm beneath a hearth. The experiment recorded temperatures ranging from approximately 400°C to 650°C, with an average temperature of about 470°C. Soil colour changes and magnetic susceptibility enhancement observed at the conclusion of the field experiment indicate that these temperatures were sufficient to activate some mineralogical changes, possibly including inversion to haematite. The implications of high temperature inversion to archaeological prospection are discussed, as is a potential archaeological application. Copyright © 2006 John Wiley & Sons, Ltd.

Bubble Behavior in Magnetic Fluid under a Nonuniform Magnetic Field.

Experimental study is made to clarify the effects of nonuniform magnetic field on bubble behavior in a magnetic fluid. The behavior of vapor bubbles is visualized with ultrasonic wave echo under the nonuniform magnetic field. It is found that the void fraction in two-phase flow decreases with increase in the magnetic field due to the effect of magnetic body force. Furthermore, the effects of nonuniform magnetic field on translational motion and deformation of a single gas bubble in a magnetic fluid are experimentally examined using a transparent thin duct, called a Hele-Shaw cell. The experimental results indicate that a single bubble is decelerated in the region of positive magnetic field gradient and accelerated in the region of negative magnetic field gradient, and that it is elongated in the direction of the magnetic field.

Fast electron cooling with a magnetically expanded electron beam

A method of obtaining an electron beam in an electron cooler where the transverse temperature is lower than the cathode temperature is presented. The method is based on the fact that the kinetic energy transverse to the magnetic field, mv ⊥ 2 2 , of an electron that passes through a negative magnetic-field gradient is reduced by a factor equal to the ratio between the initial and final field strengths, provided that the field change is adiabatic with respect to the cyclotron motion of the electrons. The requirements on the magnetic field and the effects on cooling forces are discussed. It is concluded that a reduction in transverse electron temperature by a factor of 10 to 120 K is realistic for an electron cooler such as the one in CRYRING.