Convection Of Magnetic Fluid Research Articles

Effect of Particle Thermophoresis on Convection of Magnetic Fluids in Narrow Channels Heated from Below

Effect of Particle Thermophoresis on Convection of Magnetic Fluids in Narrow Channels Heated from Below

Effect of Particle Thermophoresis on Convection of Magnetic

The effect of positive thermodiffusion of colloidal particles under convection of magneticfluids in connected vertical channels of 3.2 × 3.2 mm2 square cross-section and height 50 mm heated from below is analyzed. Below the critical Rayleigh number, particle thermophoresis in vertical generates unstable density stratification in fluid at rest. This leads to rapid bursts (~1 min) of concentration convection arising periodically (~4 h). Under developed convection, above the critical Rayleigh number particle thermophoresis in horizontal direction generates concentration inhomogeneities in the neighborhood of the channel walls and provokes convective flow instability that leads to the periodicchange (~1 h) in the direction of convective stream. The reasons of the oscillatory instability of mechanical equilibrium observed experimentally at positive sign of the Soret coefficient are discussed.

Penetrative Internally Heated Convection in Magnetic Fluids

Convection in an internally heated magnetic fluid is analyzed for rigid – rigid and rigid – free boundary surfaces with zero temperature at top and no heat flux at the lower surface. Linear stability analysis is performed for a variety of internal heat source models and the corresponding Eigen value problem is solved by Chebyshev Pseudospectral method.

Infrared thermography of wall temperature distribution caused by convection of magnetic fluid

Convection of a magnetic fluid within a perspex container was investigated experimentally and complemented by a computational Finite-Element model built according to the same physical specification. The enclosure was heated at two opposite side walls and exposed to a magnetic field provided by a Neodymium-Iron-Boron permanent magnet placed either above or below the container. The spatial temperature distribution on the front side wall of the container was recorded via infrared thermography (IR) and compared to computational results that reproduced the spatial temperature fields. The results show a significant effect on heat transfer by the location of the permanent magnet and gave evidence that the Kelvin body force can be much stronger than buoyancy. As both body forces are temperature sensitive an increase in temperature difference increased both, buoyancy and Kelvin body force, albeit with a different intensity that was explained via Curie's Law and expressed as a temperature dependent magnetisation through the pyromagnetic coefficient, K. The heat transfer was characterised by the Nusselt number and a suitable modified Rayleigh number that took the orientation of both buoyancy and Kelvin body force in account. The degree of heat transfer enhancement reported varied between a 23% reduction to a 20% enhancement.

Thermomagnetic convection in a cylindrical enclosure. Part 1: Zero gravity

The effects of a uniform external magnetic field on the thermomagnetic convection of magnetic fluid in a cylindrical enclosure are numerically studied. It was found that uniform magnetic field can be cause the convection in the non-uniformly heated magnetic fluid in a cylindrical enclosure even in the case of zero gravity. There are two threshold values of fluid magnetization and temperature gradient. Above the first threshold the convective flow is realized as circular flow, above the second one there are two cells in the enclosure. The angle between a magnetic field strength and a temperature gradient is found to be significant factor influencing structure of a convective flow and heat transfer control.

Thermomagnetic Convection of Magnetic Fluid in an Annular Space under a Non-Uniform Magnetic and Thermal Field

Two-dimensional thermomagnetic convection of magnetic fluid possessing internal spin and relaxation of magnetization with high thermal sensitivity is numerically analyzed under a non-uniform magnetic and thermal field, using a spectral difference scheme, magnetic fluid is assumed to be placed between concentric cylinders, an azimuthal magnetic field being produced by an electric current though the inner cylinder, which is an adiabatic wall, whereas a half of the outer cylinder is kept at a high constant temperature and the rest half is at a lower constant temperature. As a result, in case of ▽T × ▽|H| ≠ 0, thermomagnetic convection is found to be produced, the convection pattern depends on electric current distribution, depending on which multiple or single circulation is produced, thus, it shows that thermomagnetic convection pattern is controlled by changing relative direction of temperature gradient to that of a magnetic field.

Natural solutal convection in magnetic fluids: First-order phase transition aspect

Concentration stratification of magnetic fluids under the action of external magnetic field can disturb mechanical equilibrium in the system and cause intensive solutal convection. The current paper is devoted to the study of free solutal convection in magnetic fluids undergoing first-order phase transition. Simulation of solutal convection in OpenFOAM package makes it possible to compare numeric results with physical experiment observations. The numeric simulation of convective hydrodynamic flows was carried out in the framework of several theories of first-order phase transition in ferrocolloids. The numerical results are compared with experimental observations in order to choose the theory which predicts most accurately the concentration stratification in magnetic fluids undergoing magneto-controllable first-order phase transition.

On natural solutal convection in magnetic fluids

An experiment was carried out to investigate natural solutal convection in a magnetic fluid caused by non-homogeneous initial distribution of colloidal particles in a vertical Hele-Shaw cell. For experiment, we used a dilute magnetic fluid of the “magnetite–kerosene–oleic acid” type. The initial distribution of particles was formed by magnetophoresis of the drop-like aggregates and their sedimentation on the surface of the diamagnetic disk located in the center of the cell. Application of the magnetic field on the system led to the onset of the Rayleigh-Taylor instability and formation of descending convective jets. The velocity of the flow at the front of descending jets was measured for different values of cell thickness (up to 0.18 mm) and strength of the magnetic field generating the drop-like aggregates (up to 21 kA/m). The solutal Rayleigh numbers varied in the range Ra = 50–105. It was shown that the intensity of the convective flow characterized by the Reynolds number Re, increases with the Rayleigh number according to the power law: Re = 1.16 × 10−5Ra0.86.

The influence of an external uniform magnetic field on convection in magnetic fluid filling a spherical cavity

The influence of an external uniform magnetic field on convection in magnetic fluid filling a spherical cavity

Effect of Gravity Modulation on the Onset of Ferroconvection in a Densely Packed Porous Layer

The stability of a horizontal porous layer of a ferromagnetic fluid heated from below is studied when the fluid layer is subject to a time-periodic body force. Modified Darcy law is used to describe the fluid motion. The effect of gravity modulation is treated by a perturbation expansion in powers of the amplitude of modulation. The stability of the system, characterized by a correction Rayleigh number, is determined as a function of the frequency of modulation, magnetic parameters, and Vadasz number. It is found that subcritical instability is possible for low frequency g-jitter and that the magnetic and g-jitter mechanisms work against each other for small and moderate values of frequency of modulation. The effect of Vadasz number is shown to be reinforcing the influence of gravity modulation for small and moderate values of frequency. The magnetic, porous and modulation effects disappear altogether for sufficiently large values of the frequency of modulation. Keywords-Ferromagnetic fluid, Gravity modulation, Perturbation method, Porous layer, Stability. Gupta and Gupta (3) investigated thermal instability in a layer of ferromagnetic fluid subject to coriolis force and permeated by a vertical magnetic field. It is substantiated that overstability cannot occur if the Prandtl number is greater than unity. Gotoh and Yamada (4) investigated the linear convective instability of a ferromagnetic fluid layer heated from below and confined between two horizontal ferromagnetic boundaries. The Galerkin technique is used and the Legendre polynomials are taken as the trial functions. It is shown that the magnetization of the boundaries and the nonlinearity of fluid magnetization reduce the critical Rayleigh number and the effects of magnetization and buoyancy forces are shown to compensate each other. Blums (5) examined the possibility of having convection in ferromagnetic fluids as a result of magneto- diffusion of colloidal particles which give rise to non-uniform magnetization. Stiles and Kagan (6) examined the thermoconvective instability of a horizontal layer of ferrofluid in a strong vertical magnetic field. Their work also questioned the satisfactory agreement claimed to exist between the experiments and the theoretical model of Finlayson which has been generalized by them. Odenbach (7) investigated the convective flow generated by the interaction of a magnetic field gradient with a gradient in magnetization in a magnetic fluid. This gradient was caused by the diffusion of the magnetic particles in the field gradient. Aniss et al. (8) investigated the effect of a time-sinusoidal magnetic field on the onset of convection in a horizontal magnetic fluid layer heated from above. The Floquet theory is used to determine the convective threshold for free-free and rigid-rigid cases. The possibility to produce a competition between the harmonic and subharmonic modes at the onset of convection is discussed. Abraham (9) investigated the Rayleigh-Benard problem in a micropolar ferromagnetic fluid layer in the presence of a vertical uniform magnetic field analytically. It is shown that the micropolar ferromagnetic fluid layer heated from below is more stable as compared with the classical Newtonian ferromagnetic fluid. The effect of radiative heat transfer on ferroconvection has been studied by Maruthamanikandan (10) using the linear stability analysis. Consideration is given to two asymptotic cases, viz., transparent and opaque layers of fluid. The critical values marking the onset of convection are obtained using the Galerkin technique. Bajaj (11) considered thermosolutal convection in magnetic fluids in the presence of a vertical magnetic field and bifrequency vertical vibrations. The regions of parametric instability have been obtained using the Floquet theory. Ramanathan and Muchikel (12) investigated the effect of temperature-dependent viscosity on ferroconvective instability in a porous medium. It is found that the stationary mode of instability is

Influence of thermodiffusive particle transport on thermomagnetic convection in magnetic fluids

Influence of thermodiffusive particle transport on thermomagnetic convection in magnetic fluids

Thermal magnetic nanosuspension convection in narrow channels

The results of comparative experiments on the convection of magnetic fluids and molecular binary fluid mixtures in connected vertical channels heated from below are discussed. In both media, near the equilibrium stability threshold, flows in the form of specific swing oscillations are observed. The results of the experiment form a basis for a three-component model for magnetic fluid convection which takes into account the thermodiffusion separation of the dispersion medium components and the weak sedimentation of magnetic particles. The results of numerical simulation and experiment are compared.

Patterns of thermomagnetic convection in magnetic fluids subjected to spatially modulated magnetic fields

An analysis of thermomagnetic convection in a thin horizontal layer of magnetic fluid constrained horizontally by impermeable layers and subjected to a spatially and symmetrically modulated magnetic field is presented. The magnetic field as well as the temperature gradient are oriented vertically. For any nonzero magnetic field the base state is a convective one formed by a double vortex which reflects the symmetrical modulation. For long wave modulations the linear stability analysis reveals that the critical Rayleigh number Ra(c) for the stability of the base state against small disturbances increases with increasing magnetic driving force. An analytical expression for Ra(c) is derived which shows the characteristic feature of sudden jumps of Ra(c) at particular values of the magnetic driving force.

Convection of magnetic fluids in connected channels heated from below

The convective flows of a binary mixture in connected channels heated from below are studied experimentally. In contrast to homogeneous fluids, in magnetic colloids “hard” convection excitation, specific transient flows, and oscillatory convection regimes can be observed. The temperature fields and concentration inhomogeneities are measured.

Numerical study of Natural Convection of Magnetic Fluids in a Cubic Cavity with a Heat Generating Object Inside by the Lattice Boltzmann Method

Numerical study of Natural Convection of Magnetic Fluids in a Cubic Cavity with a Heat Generating Object Inside by the Lattice Boltzmann Method

Onset of convection in magnetic fluids

Regimes of irregular convection in a spherical cavity filled with magnetic fluid and heated from below are detected near the convection threshold. The temperature oscillations during long experimental runs (ranging from one to several weeks) are due to the concentration heterogeneity caused by the barometric and thermal diffusion effects.

Thermomagnetic convection in magnetic fluids subjected to spatially modulated magnetic fields

A horizontal layer of magnetic fluid subjected to a vertical temperature gradient and a spatially modulated magnetic field is considered. For the case of symmetric modulation the initial state characterised by a nonzero flow field is identified. By conducting a linear stability analysis the area of stability of the initial state against small perturbations is determined.

Thermomagnetic convection in magnetic fluids influenced by a time‐modulated magnetic field

AbstractMaterial– and flow properties of magnetic fluids can be influenced by applying an external magnetic field. In this work we will particularly consider the onset of convection in magnetic fluids which is influenced by a magnetic force. In a horizontal magnetic fluid layer the force arises if a temperature gradient and an external magnetic field is applied. The behaviour of the onset of convection is investigated for a static and a time–modulated magnetic field. For the case of a static magnetic field the onset of convection depends on the strength of the field and for a time–modulated magnetic field an additional dependence on the frequency of magnetic field variation is found. The experimental results presented here confirm in principle the theoretical predictions about the influence of static and time–modulated magnetic forces on the onset of convection. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermomagnetic Convection as a Tool for Heat and Mass Transfer Control in Nanosize Materials Under Microgravity Conditions

Magnetic colloids are relatively new man-made nanomaterials whose magnetic susceptibility is several orders of magnitude larger than that of natural substances. Experiments conducted in magnetic fluids show that strengthening or weakening of thermal convection in colloids is dictated by a competition between the gravitational and magnetic mechanisms as well as by the effect of the fluid density stratification due to gravitational sedimentation of magnetic particles and their aggregates. Therefore experiments in microgravity conditions are required to eliminate gravitational sedimentation. This will enable an accurate investigation of convection in magnetic fluids and unambiguous study of the interaction of a magnetic field with a magnetopolarized medium. Such experiments are also needed to perform an accurate measurement of fluid’s transport coefficients.

Parametric modulation of thermomagnetic convection in magnetic fluids

Previous theoretical investigations on thermal flow in a horizontal fluid layer have shownthat the critical temperature difference, where heat transfer changes from diffusion toconvective flow, depends on the frequency of a time-modulated driving force. The drivingforce of thermal convection is the buoyancy force resulting from the interaction ofgravity and the density gradient provided by a temperature difference in thevertical direction of a horizontal fluid layer. An experimental investigation ofsuch phenomena fails because of technical problems arising if buoyancy is tobe changed by altering the temperature difference or gravitational acceleration.The possibility of influencing convective flow in a horizontal magnetic fluid layer bymagnetic forces might provide us with a means to solve the problem of a time-modulatedmagnetic driving force. An experimental setup to investigate the dependence of the criticaltemperature difference on the frequency of the driving force has been designed andimplemented. First results show that the time modulation of the driving forcehas significant influence on the strength of the convective flow. In particular apronounced minimum in the strength of convection has been found for a particularfrequency.