Ferrofluid Flow Research Articles

Dynamics of magnetic modulation of ferrofluid droplets for digital microfluidic applications

Active control of droplet generation in a microfluidic platform attracts interest for development of digital microfluidic devices ranging from biosensors to micro-reactors to point-of-care diagnostic devices. The present paper characterizes, through an unsteady three-dimensional Volume of Fluid (VOF) simulation, the active control of ferrofluid droplet generation in a microfluidic T-junction in presence of a non-uniform magnetic field created by an external magnetic dipole. Two distinctly different positions of the dipole were considered – one upstream of the junction and one downstream. While keeping the ferrofluid flow rate fixed, a parametric variation of the continuous phase capillary number, dipole strength, and dipole position was carried out. Differences in the flow behaviour in terms of dripping or jetting and the droplet characteristics in terms of droplet formation time period and droplet size were studied. The existence of a threshold dipole strength, below which the magnetic force was not able to influence the flow behaviour, was identified. It was also observed that, for dipoles placed upstream of the junction, droplet formation was suppressed at some higher dipole strengths, and this value was found to increase with increasing capillary number. Droplet time period was also found to increase with increasing dipole strength, along with droplet size, i.e. an increase in droplet volume.

Flow restrictive and shear reducing effect of magnetization relaxation in ferrofluid cavity flow

In this study, we report the effects of a uniform stationary magnetic field on the flow of ferrofluid (FF) inside a boundary driven cavity. A coupled set of conservation equations for the flow field, the Maxwell’s magnetostatic equations, and the constitutive magnetization equation are solved numerically. The non-dimensional groups primarily influencing the phenomenon are first systematically identified through the normalization of the complete set of equations. We find the magnetization relaxation effects, under the stationary uniform field, to be flow restrictive in nature. The misalignment between the local magnetic field and the magnetization suppresses the vorticity field in the cavity, shifts the primary central vortex, and reduces the average shear stress at the boundaries. As a consequence, it becomes apparent that at a given Reynolds number, the application of uniform magnetic field can reduce the shear drag at the boundaries of the cavity, of course at an expense of reduced flow rate in their vicinity. Our study uniquely reveals that the relaxation time effects are dominant in the regions of ferrofluid flow where the change in the magnitude of the vorticity takes place over a length scale which is much smaller than the characteristic length scale of the flow geometry. Depending on the magnitudes of influencing parameters, the solution exhibits anomalous characteristics, such as creeping and saturating behavior.

The Influence of magnetic field on heat transfer of magnetic nanofluid in a sinusoidal double pipe heat exchanger

In this article, the effect of magnetic field on the Nanofluid flow inside a sinusoidal two-tube heat exchanger is investigated numerically. This study focuses on the influence of variable magnetic field in the heat transfer of heat exchanger while mixture is single phase. In this heat exchanger, the inner tube is sinusoidal and the outer tube is considered smooth. The magnetic field is established orthogonal to sinusoidal tube. The basis fluid is water with 4vol.% Nano particles (Fe3O4). In our study, Ferrofluid flows in the internal tube (sinusoidal tube) as hot fluid and air flows counter currently as cold fluid in external tube. The finite volume method with the SIMPLEC algorithm is used for handling the pressure⿿velocity coupling. The numerical results present validated data with experimentally measured data and show good agreement with measurement. The influence of the variation of different parameters like geometric shape, intensity of magnetic field non-dimensional number and Reynolds number, on heat transfer is investigated. According to obtained results, sinusoidal formation of the internal tube significantly increases the Nusselt number inside a two-tube heat exchanger. Also, magnetic field enhances diffusion of the cold boundary layer to the central parts of the inner tube for various geometric shape coefficients. Our findings show that the diffusion also elevates as the intensity of the magnetic field is increased. So, Nusselt number and heat transfer increase and this augmentation intensifies in high Reynolds number.

Thermofluid Analysis in Magnetic Hyperthermia Using Low Curie Temperature Particles

Magnetic hyperthermia uses the targeted therapeutic heat from magnetic particles (MPs) in an alternating magnetic field to kill cancer cells. MPs with low Curie temperature (Tc) (in the range 42 °C–45 °C), high magnetization, and magnetic permeability/susceptibility are good candidates for their use in hyperthermia therapy. This paper analyzes the hyperthermic effects determined by the MPs with low Tc within a tumoral configuration from healthy tissue when an alternating magnetic field is applied. The temperature field was determined as a solution of the Pennes bioheat transfer equation. The spatial distribution of the particles after their injection within the tissues was computed by solving the convection–diffusion equation in porous tissues. Results show that the MP injection rate significantly influences the spatial distribution of the particles and the temperature field of the tumor. Higher values of the ferrofluid flow rate push more particles to the tumor center, thus also elevate its central temperature. The temperature field becomes uniform in a percentage of 90% of tumor volume if a blood vessel is localized at the tumor center.

A diffuse interface model for two-phase ferrofluid flows

We develop a model describing the behavior of two-phase ferrofluid flows using phase field-techniques and present an energy-stable numerical scheme for it. For a simplified, yet physically realistic, version of this model and the corresponding numerical scheme we prove, in addition to stability, convergence and as by-product existence of solutions. With a series of numerical experiments we illustrate the potential of these simple models and their ability to capture basic phenomenological features of ferrofluids such as the Rosensweig instability.

Magnetic Trapping of Bacteria at Low Magnetic Fields.

A suspension of non-magnetic entities in a ferrofluid is referred to as an inverse ferrofluid. Current research to trap non-magnetic entities in an inverse ferrofluid focuses on using large permanent magnets to generate high magnetic field gradients, which seriously limits Lab-on-a-Chip applications. On the other hand, in this work, trapping of non-magnetic entities, e.g., bacteria in a uniform external magnetic field was studied with a novel chip design. An inverse ferrofluid flows in a channel and a non-magnetic island is placed in the middle of this channel. The magnetic field was distorted by this island due to the magnetic susceptibility difference between this island and the surrounding ferrofluid, resulting in magnetic forces applied on the non-magnetic entities. Both the ferromagnetic particles and the non-magnetic entities, e.g., bacteria were attracted towards the island, and subsequently accumulate in different regions. The alignment of the ferrimagnetic particles and optical transparency of the ferrofluid was greatly enhanced by the bacteria at low applied magnetic fields. This work is applicable to lab-on-a-chip based detection and trapping of non-magnetic entities bacteria and cells.

Viscoelastoplastic bodies under cyclic loading in thermal-radiation fields

Deformation of viscoelastoplastic bodies in the neutron and thermal fluxes under a one-time loading has been studied previously in [1–3]. A mathematical model has been set forth in [4] for a cyclic deformation of elastoplastic bodies in a neutron flux. In the paper [5,6] ferrofluid flow and heat transfer in a semi annulus enclosure is investigated considering thermal radiation. This article [7] explores the effect of thermal radiation on Al2O3–water nanofluid flow and heat transfer in an enclosure with a constant flux heating element. In the article [8], effect of thermal radiation on magnetohydrodynamics nanofluid flow between two horizontal rotating plates is studied. In the study [8], heat and mass transfer characteristic of unsteady nanofluid flow between two parallel plates is investigated considering thermal radiation.This study considers the process of a combined effect of the heat, radiation and force cyclic loads on a viscoelastoplastic laminated body within the frames of the theory of small elastoplastic deformations.

Numerical study of homogeneous–heterogeneous reactions on stagnation point flow of ferrofluid with non-linear slip condition

This study deals with the stagnation point flow of ferrofluid over a flat plate with non-linear slip boundary condition in the presence of homogeneous–heterogeneous reactions. Three kinds of ferroparticles, namely, magnetite (Fe3O4), cobalt ferrite (CoFe2O4) and manganese zinc ferrite (Mn–ZnFe2O4) are taken into account with water and kerosene as conventional base fluids. The developed model of homogeneous–heterogeneous reactions in boundary layer flow with equal and unequal diffusivities for reactant and autocatalysis is considered. The governing partial differential equations are converted into system of non-linear ordinary differential equations by mean of similarity transformations. These ordinary differential equations are integrated numerically using shooting method. The effects of pertinent parameters on velocity and concentration profiles are presented graphically and discussed. We found that in the presence of Fe3O4–kerosene and CoFe2O4–kerosene, velocity profiles increase for large values of α and β whereas there is a decrement in concentration profiles with increasing values of K and Ks. Furthermore, the comparison between non-magnetic (Al2O3) and magnetic Fe3O4 nanoparticles is given in tabular form.

Ferrofluid flow due to a rotating disk in the presence of a non-uniform magnetic field

The flow of a ferrofluid due to a rotating disk in the presence of a non-uniform magnetic field in the axial direction is studied through mathematical modeling of the problem. Contour and surface plots in the presence of <i>10</i> kilo-ampere/meter, <i>100</i> kilo-ampere/meter magnetization force are presented here for radial, tangential and axial velocity profiles, and results are also drawn for the magnetic field intensity. These results are compared with the ordinary case where magnetization force is absent.

Multiphase ferrofluid flows for micro-particle focusing and separation.

Ferrofluids have demonstrated great potential for a variety of manipulations of diamagnetic (or non-magnetic) micro-particles/cells in microfluidics, including sorting, focusing, and enriching. By utilizing size dependent magnetophoresis velocity, most of the existing techniques employ single phase ferrofluids to push the particles towards the channel walls. In this work, we demonstrate a novel strategy for focusing and separating diamagnetic micro-particles by using the laminar fluid interface of two co-flowing fluids-a ferrofluid and a non-magnetic fluid. Next to the microfluidic channel, microscale magnets are fabricated to generate strong localized magnetic field gradients and forces. Due to the magnetic force, diamagnetic particles suspended in the ferrofluid phase migrate across the ferrofluid stream at the size-dependent velocities. Because of the low Reynolds number and high Péclet number associated with the flow, the fluid interface is sharp and stable. When the micro-particles migrate to the interface, they are accumulated near the interface, resulting in effective focusing and separation of particles. We investigated several factors that affect the focusing and separation efficiency, including susceptibility of the ferrofluid, distance between the microfluidic channel and microscale magnet, and width of the microfluidic channel. This concept can be extended to multiple fluid interfaces. For example, a complete separation of micro-particles was demonstrated by using a three-stream multiphase flow configuration.

Heat transfer in MHD stagnation point flow of a ferrofluid over a stretchable rotating disk

In this article, heat transfer analysis in three dimensional boundary layer stagnation point flow of incompressible ferrofluid over a stretchable rotating circular disk in the presence of uniform external magnetic field has been investigated. For this purpose, three different types of ferroparticles namely, magnetite (Fe3O4), cobalt ferrite (CoFe2O4) and Mn-Zn ferrite (Mn-ZnFe2O4) are considered with water as a base fluid. The governing partial differential equations of the considered problem are transformed into non-dimensional ordinary differential equations using similarity transformation (Turkyilmazoglu, 2012). The solution of obtained equations is computed numerically using an efficient implicit finite difference scheme. The effects of pertinent parameters namely, volume fraction parameter of magnetic nanoparticle, rotating parameter, magnetic parameter, velocity ratio parameter and Prandtl number on both radial and azimuthal velocity profiles, temperature profile, skin friction coefficients and local Nusselt number are computed and shown through graphs and tables. For magnetite ferroparticle (Fe3O4) radial velocity decreases and azimuthal velocity and temperature increase with increasing value of volume fraction parameter ϕ. The values of skin friction coefficient in radial direction and local Nusselt number of magnetite ferroparticle (Fe3O4) become higher than that of other ferroparticles due to the maximum density and thermal conductivity.

Continuous-flow sheathless diamagnetic particle separation in ferrofluids

Separating particles from a complex mixture is often necessary in many chemical and biomedical applications. This work presents a continuous-flow sheathless diamagnetic particle separation in ferrofluids through U-shaped microchannels. Due to the action of a size-dependent magnetic force, diamagnetic particles are focused into a single stream in the inlet branch of the U-turn and then continuously separated into two streams in its outlet branch. A 3D numerical model is developed to predict and understand the diamagnetic particle transport during this separation process. The numerical predictions are found to agree well with the experimental observations in a systematic study of the effects of multiple parameters including ferrofluid flow rate, concentration and magnet-channel distance. Additional numerical studies of the geometric effects of the U-turn reveal that increasing the outlet-branch width of the U-turn can significantly enhance the diamagnetic particle separation in ferrofluids.

Effect of magnetic field on laminar convective heat transfer characteristics of ferrofluid flowing through a circular stainless steel tube

Ferrofluids have promising potential for heat transfer applications, as they can be easily controlled and manipulated by applied magnetic field produced by either permanent magnets or electromagnets. The present study reports the effect of magnetic field on the convective heat transfer characteristics of ferrofluid flowing through a circular stainless steel (SS) tube (2mm ID×2.6mm OD) under constant heat flux conditions. The convective heat transfer coefficient for various ferrofluid flow rates and applied magnetic field gradients is reported using infrared thermography (IRT) technique. COMSOL simulation has been carried out to calculate the magnetic field and magnetic force distribution inside the SS tube. Bright field visualization of clusters of nanoparticles and aggregation of nanoparticles at the inner wall of the SS tube in the presence of magnetic field have also been carried out in order to explain the possible mechanism for heat transfer enhancement for ferrofluid in the presence of magnetic field. The convective heat transfer coefficient for ferrofluid flow in the presence of magnetic field may increase or decrease depending upon several factors such as, ratio of magnetic force to inertia force acting on the ferrofluid, interaction of ferrofluid flow with the aggregate of nanoparticles formed at the wall of the tube near the vicinity of the magnets, and enhancement in local thermal conductivity of ferrofluid flow resulting from the chain like clusters of nanoparticles within the ferrofluid in the presence of magnetic field.

Effects of homogeneous–heterogeneous reactions in flow of magnetite-Fe3O4 nanoparticles by a rotating disk

This article addresses the flow of ferrofluid due to a rotating disk in the presence of homogeneous–heterogeneous reactions. The disk rotates with constant angular velocity about the z-axis. Water is used as base fluid while magnetite-Fe3 O4 as nanoparticle. Fluid is electrically conducting in the presence of an applied magnetic field. Effects of viscous dissipation are also considered. The disk is kept at uniform temperature while there is ambient temperature away from the disk. In view of the rotational symmetry, the derivatives in the azimuthal direction are neglected. Appropriate transformations reduce the system of nonlinear partial differential equations to system of ordinary differential equations. Convergent series solutions are computed using homotopy analysis method (HAM) for the resulting nonlinear problems. Effects of different parameters on the velocity, temperature and concentration profiles are shown and analyzed. Computations for skin friction coefficient and Nusselt number are presented and examined for the influences of pertinent parameters. It is noted that concentration distribution decreases for larger values of strength of homogeneous reaction parameter while it increases for strength of heterogeneous reaction parameter. Skin friction coefficient and rate of heat transfer are enhanced when the strength of magnetic field is increased.

Rotating Flow of Magnetite-Water Nanofluid over a Stretching Surface Inspired by Non-Linear Thermal Radiation.

Present study explores the MHD three-dimensional rotating flow and heat transfer of ferrofluid induced by a radiative surface. The base fluid is considered as water with magnetite-Fe3O4 nanoparticles. Novel concept of non-linear radiative heat flux is considered which produces a non-linear energy equation in temperature field. Conventional transformations are employed to obtain the self-similar form of the governing differential system. The arising system involves an interesting temperature ratio parameter which is an indicator of small/large temperature differences in the flow. Numerical simulations with high precision are determined by well-known shooting approach. Both uniform stretching and rotation have significant impact on the solutions. The variation in velocity components with the nanoparticle volume fraction is non-monotonic. Local Nusselt number in Fe3O4–water ferrofluid is larger in comparison to the pure fluid even at low particle concentration.

Analysis of Porous Layered Journal Bearing Lubricated with Ferrofluid

This paper presents an analysis of porous layered long journal bearing lubricated with ferrofluid using displaced infinitely long wire magnetic field model. The ferrofluid flow in the porous region is analyzed using modified Brinkman model. Nondimensional pressure and shear stress expressions are derived using Reynolds boundary conditions. Nondimensional load capacity and coefficient of friction are evaluated under the influence of permeability of porous media, porous layer thickness, lubricant layer thickness, magnetic field intensity and distance ratio parameter. A porous layered journal bearing lubricated with ferrofluid increases the load carrying capacity and reduces the coefficient of friction.

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.

A multi-phase ferrofluid flow model with equation of state for thermomagnetic pumping and heat transfer

A one-dimensional multi-phase flow model for thermomagnetically pumped ferrofluid with heat transfer is proposed. The thermodynamic model is a combination of a simplified particle model and thermodynamic equations of state for the base fluid. The magnetization model is based on statistical mechanics, taking into account non-uniform particle size distributions. An implementation of the proposed model is validated against experiments from the literature, and found to give good predictions for the thermomagnetic pumping performance. However, the results reveal a very large sensitivity to uncertainties in heat transfer coefficient predictions.

Heat Transfer in MHD Mixed Convection Flow of a Ferrofluid along a Vertical Channel.

This study investigated heat transfer in magnetohydrodynamic (MHD) mixed convection flow of ferrofluid along a vertical channel. The channel with non-uniform wall temperatures was taken in a vertical direction with transverse magnetic field. Water with nanoparticles of magnetite (Fe 3 O 4) was selected as a conventional base fluid. In addition, non-magnetic (Al 2 O 3) aluminium oxide nanoparticles were also used. Comparison between magnetic and magnetite nanoparticles were also conducted. Fluid motion was originated due to buoyancy force together with applied pressure gradient. The problem was modelled in terms of partial differential equations with physical boundary conditions. Analytical solutions were obtained for velocity and temperature. Graphical results were plotted and discussed. It was found that temperature and velocity of ferrofluids depend strongly on viscosity and thermal conductivity together with magnetic field. The results of the present study when compared concurred with published work.

Thermomagnetic Convective Flow Characteristics of Oil-Based Magnetic Nanofluids in Rectangular Enclosure.

The characteristics of thermomagnetic convective flow in a rectangular enclosure heated from below and filled with oil-based nanofluid (EFH-1, Ferrotec.), so called ferrofluid, were numerically investigated. The enclosure contained obstacles with rectangular or triangular configurations mounted on the top and bottom walls. To generate homogeneous magnetic fields, a permanent magnet with a uniform magnetic field strength of 600 kA/m was located in the lower part of the rectangular enclosure, and specified the horizontal or vertical direction. Coupling calculations between thermal-flow field and magnetic field in the analysis model were performed using the commercial code, COMSOL Multiphysics. Results showed that the ferrofluid flow fields were affected by the applied external magnetic field directions and that the eddy flow phenomena in the rectangular enclosure were generated in the vicinity of the section of high magnetic flux density fields such as the edge of the permanent magnet. The effect of parameters like temperature distributions and local Nusselt number (Nu) profiles on the thermomagnetic convective flow was graphically depicted with various flow conditions.