Thermomagnetic Convection Research Articles

Using magnetic fluids to model convection of planetary or stellar interiors in laboratory scale

AbstractBaroclininc flow induced via thermomagnetic convection is investigated as a potential model of convection of planetary interiors at a laboratory scale. To reproduce a central force field, a permanent magnet is used that acts on a magnetic fluid contained in a spherical shell. In addition to maintaining the inner sphere and outer shell at different temperatures, the system is rotating around its central axis. Results suggest that magnetic fluids reproduce the essential qualitative character of the baroclinic flows and may be a useful tool to investigate convective flow in planetary interiors. To quantify the effect of magnetic forcing, a parameter ΘM was developed which represents the equivalent to the thermal Rossby number.

Magnetic enhancement of photothermal heating in ferrofluids

This contribution describes a multiphase CFD model, capable of simulating convective mass and heat transfer in a ferrofluid. The model was used for a parametric analysis of thermomagnetic convection, demonstrating 1.3-fold enhancement of thermal efficiency under the influence of an external magnetic field. The model was validated against two different experimental datasets.

Aspects of modeling the magnetically controlled thermal accumulators for aerospace engineering

Approaches for creating magnetically controlled thermal accumulators for aerospace engineering based on thermomagnetic convection under the action of a magnetic field created by neodymium magnet under thermal conditions are discussed. The properties of thermal accumulating material (TM) containing magnetic particles are studied. To investigate the distribution of the temperature field it was used a contactless measurement method realized using a thermal imager. The formulation of the problem of mathematical modeling for a thermal control system with TM based on the finite integral transform method is carried out. It allows to study the distribution of temperature fields at design stage. The application of the effect of thermomagnetic convection makes it possible to intensify heat exchange in TM, which increases the efficiency of charge and discharge of TM. This approach is economically feasible and technologically adapted to aerospace technology.

Lattice Boltzmann method for thermomagnetic convection and entropy generation of paramagnetic fluid in porous enclosure under magnetic quadrupole field

In this study, thermomagnetic convection in a porous cavity filled with paramagnetic fluid under a magnetic quadrupole field is studied using lattice Boltzmann method (LBM), and the corresponding entropy generation is analyzed. The horizontal walls of the porous enclosure are adiabatic, and the temperatures of the vertical walls are different. The effects of Darcy number (Da), porosity (ε), and magnetic force number (γ) on the heat transfer, fluid flow, and entropy generation characteristics are investigated. The results show that, as the magnetic force number increases, the flow is strengthened. The local Nusselt numbers (Nuloc) increase and the average Bejan number (Beave) decreases under the non-gravitational condition. Nuloc change a little along the hot wall due to the lower value of Darcy number and higher values of Darcy number lead to higher buoyancy forces, Nuloc change significantly under the gravitational or non-gravitational condition. Moreover, the average Nusselt number (Nuave) and total entropy generation (Stotal) slightly decrease at first and thereafter increase when the magnetic force dominates the entire domain under the gravitational condition. It is also observed that, when ε0 = ε1 = 0.5, γ > 10, the average Nusselt number (Nuave) is significantly higher than in the other cases. Furthermore, the average Nusselt number (Nuave) reaches its minimum value when ε0 = ε1 = 0.5, γ = 4.

Onset of thermomagnetic convection around a vertically oriented hot-wire in ferrofluid

The onset of thermomagnetic convection in ferrofluid in a vertical transient hot-wire cell is analytically and experimentally investigated by studying the temperature rise of an electrically-heated wire. During the initial stage of heating, the temperature rise is found to correspond well to that predicted by conduction only. For high electrical current densities, the initial heating stage is followed by a sudden change in the slope of the temperature rise with respect to time as a result of the onset of thermomagnetic convection cooling. The observed onset of thermomagnetic convection was then compared to that of natural convection of deionized water. For the first time, the critical time corresponding to the onset of thermomagnetic convection around an electrically-heated wire is characterized and non-dimensionalized as a critical Fourier number (Foc). We propose an equation for Foc as a function of a magnetic Rayleigh number to predict the time for the onset of thermomagnetic convection. We observed that thermomagnetic convection in ferrofluid occurs earlier than natural convection in non-magnetic fluids for similar experimental conditions. The onset of thermomagnetic convection is dependent on the current supplied to the wire. The findings have important implications for cooling of high-power electronics using ferrofluids and for measuring thermal properties of ferrofluids.

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 oxygen in a square enclosure under non-uniform magnetic field

The effects of magnet position and magnet strength on the thermomagnetic convection of gaseous oxygen in a square enclosure under combined magnetic and gravity fields are numerically studied. The magnetic field is provided by a Neodymium-Iron-Boron permanent magnet which has a high magnetic energy product. The results show that the thermomagnetic convection is significantly affected by the relative location of the permanent magnet. Both the heat transfer and buoyancy driven flow near the lower part of the vertical hot wall are greatly enhanced when the magnet is placed near the hot wall. The largest local Nusselt number is enhanced by 152.7% for the studied largest magnet strength. The corresponding largest average Nusselt number also increases by 17.9%. The overall heat transfer increases with the increase of magnet strength when the magnet is placed near the hot wall, while the heat transfer increases first and then decreases with the increase of magnet strength when the magnet is placed near the cold wall. Optimal position of permanent magnet exists for best heat transfer enhancement.

Extend of magnetic field interference in the natural convection of diamagnetic nanofluid

Main objective of the paper was to experimentally investigate the thermo-magnetic convection of diamagnetic fluids in the Rayleigh-Benard configuration. For better understanding of the magnetic field influence on the phenomena occurring in cubical enclosure the following parameters were studied: absence or presence of nanoparticles (single and two-phase fluids), thermal conditions (temperature difference range of 5–25 K) and magnetic field strength (magnetic induction range of 0–10 T). A multi-stage approach was undertaken to achieve the aim. The multi-stage approach means that the forces system, flow structure and heat transfer were considered. Without understanding the reasons (forces) and the fluid behaviour it would be impossible to analyse the exchanged heat rates through the Nusselt number distribution. The forces were determined at the starting moment, so the inertia force was not considered. The flow structure was identified due to the FFT analysis and it proved that magnetic field application changed the diamagnetic fluid behaviour, either single or two-phase. Going further, the heat transfer analysis revealed dependence of the Nusselt number on the flow structure and at the same time on the magnetic field. It can be said that imposed magnetic field changed the energy transfer within the system. In the paper, it was shown that each of presented steps were linked together and that only a comprehensive approach could lead to better understanding of magnetic field interference in the convection phenomenon.

The transition from natural convection to thermomagnetic convection of a magnetic fluid in a non-uniform magnetic field

Magnetic fluid flow and heat transfer by natural and thermomagnetic convection was studied numerically in a square enclosure. The aim was to investigate the transition from natural convection to thermomagnetic convection by exploring situations where buoyancy and the Kelvin body force would be opposing each other such that the magnetic effects would in some cases be the dominant factor throughout the domain and in other cases only in a part of the fluid. The numerical model coupled the solution of the magnetostatic field equation with the heat and fluid flow equations to simulate the fluid flow under a realistic magnetic field generated by a permanent magnet. The results suggest that the domain of influence over the flow field is largely aligned with the domain of dominance of the respective driving force. The result is that the transition from a single buoyancy-driven convection cell to a single thermomagnetically driven cell is via a two-cell structure and that the local effect on the flow field leads to a global effect on the heat transfer with a minimum of the Nusselt number in the transition region.

Thermomagnetic Convection in Porous Media: Effect of Anisotropy and Local Thermal Nonequilibrium

The onset of thermomagnetic convection in an anisotropic layer of Darcy porous medium in the presence of a uniform vertical magnetic field is investigated using a local thermal nonequilibrium (LTNE) model for energy equation representing the solid and fluid phases separately. Anisotropies in permeability as well as in fluid and solid thermal conductivities are considered. The principle of exchange of stability is shown to be valid. Asymptotic solutions for the Rayleigh number for both small and large values of scaled interphase heat transfer coefficient are presented and the comparison of results with those computed numerically shows good agreement. The mechanical and thermal anisotropy parameters have opposing influence on the stability characteristics of the system. Besides, the influence of magnetic parameters on the instability of the system is also reported.

Effect of Internal Heat Generation on the Onset of Brinkman-Benard Thermomagnetic Convection in a Saturated Porous Layer

Effect of Internal Heat Generation on the Onset of Brinkman-Benard Thermomagnetic Convection in a Saturated Porous Layer

Thermomagnetic Convection Around a Current-Carrying Wire in Ferrofluid

Thermomagnetic convection of a ferrofluid flow induced by the internal magnetic field around a vertical current-carrying wire was theoretically analyzed and experimentally validated for the first time. The Nusselt number for a heated 50-μm diameter wire in a ferrofluid was measured for different electrical currents and fluid temperatures. The experimental results are in a good agreement with the proposed scaling analysis. We found that increasing the current will increase the Nusselt number nonlinearly and ultimately enhances the heat transfer capability of the induced ferrofluid flow. We observed that the thermomagnetic convection becomes dominant, if large enough currents are applied.

Lattice Boltzmann method for thermomagnetic convection of paramagnetic fluid in square cavity under a magnetic quadrupole field

Numerical study was performed for a better understanding on thermomagnetic convection under magnetic quadrupole field utilizing the lattice Boltzmann method. Present problem was examined under non-gravitational and gravitational conditions for a wide range of magnetic force number from 0 to 1000. Vertical walls of the square cavity were heated differentially while the horizontal walls were assumed to be adiabatic. Distributions of the flow and temperature field were clearly illustrated. Under non-gravitational condition, the flow presents a two-cellular structure with horizontal symmetry, and the average Nusselt number increases with the augment of magnetic force number. Under gravitational condition, two-cellular structure only occurs when the magnetic field is relatively strong, and the average Nusselt number decreases at first and then rises with the enhancing magnetic field. Results show that the magnetic field intensity and the Rayleigh number both have significant influence on convective heat transfer, and the gravity plays a positive role in heat transfer under weak magnetic field while a negative one for magnetic force numbers larger than 1×105.

Numerical simulation of thermomagnetic convection of cold water near its density maximum in a square enclosure under a magnetic quadrupole field

Natural convection of cold water near its density maximum in a square enclosure is studied numerically under the influence of a magnetic quadrupole field without gravity. A generalized model which includes a non-Boussinesq parabolic density–temperature relationship is established. The governing equations in primitive variables are discretized using the finite-volume method and solved using the SIMPLE algorithm. The effects of magnetic force number, Rayleigh number and density inversion parameter on flow and heat transfer characteristics are analyzed. The results show that the primary flow pattern depends mainly on the density inversion parameter. Multi-cellular flow structures are observed for certain ranges of density inversion parameter independent of the value of Rayleigh number and magnetic force number. The heat transfer changes non-monotonically under combined actions of the quadrupole magnetic field and density inversion.

Effect of a Magnetic Quadrupole Field on Entropy Generation in Thermomagnetic Convection of Paramagnetic Fluid with and without a Gravitational Field

Entropy generation for a paramagnetic fluid in a square enclosure with thermomagnetic convection is numerically investigated under the influence of a magnetic quadrupole field. The magnetic field is calculated using the scalar magnetic potential approach. The finite-volume method is applied to solve the coupled equation for flow, energy, and entropy generation. Simulations are conducted to obtain streamlines, isotherms, Nusselt numbers, entropy generation, and the Bejan number for various magnetic forces (1 ≤ γ ≤ 100) and Rayleigh numbers (104 ≤ Ra ≤ 106). In the absence of gravity, the total entropy generation increases with the increasing magnetic field number, but the average Bejan number decreases. In the gravitational field, the total entropy generation respects the insensitive trend to the change of the magnetic force for low Rayleigh numbers, while it changes significantly for high Rayleigh numbers. When the magnetic field enhances, the share of viscous dissipation in energy losses keeps growing.

Using infrared thermography to investigate thermomagnetic convection under spatial non-uniform magnetic field

Thermomagnetic convection was investigated experimentally by infrared thermography and validated numerically by a finite-element analysis. A cavity filled with a magnetic liquid was under the influence of a permanent magnet that provided an external magnetic field and was subjected to thermal forcing by maintaining the two opposite horizontal walls at different temperatures. All configurations were chosen to explore situations where the Kelvin body force and buoyancy would either oppose each other or act perpendicular to each other. As both body forces depend on temperature, the aim was to explore the degree by which heat transfer may be enhanced by only varying the temperature difference. The results demonstrate that the Kelvin body force, which can be much stronger than buoyancy, leads to convective flow where only conduction would occur without the presence of the magnet. An effective force ratio, r, is developed which suggests that an increase in temperature difference across the cavity initially leads to a dominance of thermomagnetic forcing over buoyancy but that the overall force balance becomes more moderate at higher temperature differences. The global effect of both body forces on heat transfer was characterised by the Nusselt number and a suitable modified Rayleigh number.

Thermomagnetic convection of a magnetic nanofluid influenced by a magnetic field

We present a numerical study of thermomagnetic convection in a differentially heated cavity. The magnetic nanofluid (ferrofluid) is subjected to a uniform magnetic gradient oriented at an angle, ?, with respect to the thermal gradient. The motivation for this work stems largely from a desire to extent preexisting works focused on horizontal and vertical orientations ? = 0?, 90?, 180?, and 270?. Our main goal is to get data on the flow and heat transfer for any orientation in the entire range 0-360?. The generalized problem lends itself to the investigation of orientations that give maximum heat transfer. It is found that, (1) at a given magneto- gravitational coupling number, N, orientations 0?, 90?, and 270?, for which magnetization gradient is unstable, are not the optimum ones, (2) for 0 < N ? 1, heat transfer reaches a maximum between 270? and 360?, (3) for N > 1, a second maximum occur between 0? and 90? owing to reverse flow phenomenon, (4) at strong magnetic gradients, the two heat transfer peaks take the same value, and (5) optimization parameter, ?, reflecting the strongest magnetic effect, grows with N. Unlike the gravity, magnetic gradient may supply various strengths and spatial configurations, which makes thermomagnetic convection more controllable. Also, the magnetic mechanism is a viable alternative for the gravity one in microgravity, where thermo-gravitational convection ceases to be efficient..

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.

Dissipative particle dynamics simulation of biomagnetic fluid flow and heat transfer

Dissipative particle dynamics is a Lagrangian particle-based mesoscopic technique with higher computation efficiency than microscopic numerical methods, originally conceived as a hybrid of lattice gas automata (LGA), Brownian dynamics (BD) and molecular dynamics (MD). As an improved version of this method, the energy-conservative dissipative particle dynamics (eDPD or DPD+E) allows for temperature-variation in fluid system and has great potential to solve hydrodynamics problem involving heat transfer in complex fluids such as biomagnetic fluid, ferrofluid, eletrorheological fluid and so on. Considering the both effects of ferrohydrodynamic and magnetohydrodynamic, the hydrodynamics behavior of biomagnetic fluid is rather complicated and has received considerable attention from various researchers. In this work, the fluid flow and heat transfer in a semi-annulus filled with biomagnetic fluid in presence of an external magnetic field is investigated systematically by the eDPD method for different conditions. The flow is steady, two-dimensional, laminar and incompressible. To begin with, a simple and practicable strategy is proposed to matchup the mesoscopic parameters in eDPD model with the transport coefficients of real fluid because the eDPD formlation has no direct relation to transport coefficients. In detail, the viscosity and thermal diffusivity for the eDPD fluid are calculated by numerically simulating the isothermal Poiseuille flow and two-dimensional heat conduct with uniform heat generation, respectively. To study the magnetostatic interactions regardless of the induced magnetic field, a model concerned with the magnetic field is introduced. The Kelvin force and Lorentz force on DPD particles are computed in the same way as the buoyance force; the heat source induced by the external magnetic is replaced by imposing an external heat flux on the DPD particles. Then the eDPD model is first validated against the finite volume method (FVM) results for the natural convection at a given Rayleigh number and Prandtl number to access the accuracy of the eDPD simulations and it is found that the eDPD model predict the temperature and flow fields throughout the convection domains properly. Finally the effects of Magnetic number and Hartmann number, which arise from ferrohydrodynamics and magnetohydrodynamics respectively, have been examined and presented via temperature isotherms and streamlines. The obtained results simply reveal that the thermomagnetic convection is very different from buoyance-driven natural convection. Three thermal plume and four vortexes appears over the hot wall and heat transfer drop a lot near the middle zone as Magnetic number increases. It can be also found that the local Nusselt number near the hot wall significantly decrease as Hartmann number increases.

Performance of Mn-Zn ferrite magnetic fluid in a prototype distribution transformer under varying loading conditions

The temperature distribution performance of prototype transformer has been evaluated for varying loads. A 3 KVA single phase transformer is prototyped for this purpose and thermocouples were inserted at various places for the study. A novel temperature sensitive magnetic fluid (TSMF - Mn-Zn ferrite magnetic fluid) is proposed to improve the cooling performance. Winding temperature reduces when prototype transformer is submerged in TSMF compared to the same experiment performed with transformer oil, for all loading (normal load, under load, overload). The reduction in temperature with TSMF attributes to the thermo-magnetic convection, and it is the maximum when the Curie temperature of the magnetic fluid is comparable to the hot spot temperature of the transformer. The experimental results match with numerical calculation and the existing simulation. The maximum cooling is observed with TSMF for overload condition and increases normal life expectancy nine times. In case of planned overload condition normal life expectancy doubles for all situations. Study leads to the conclusion that TSMF works as a better transformer coolant under all conditions, delivers more power than its nameplate rating without using any external accessories, and improves normal life of transformer. This will reduce the basic investment cost and maintenance of transformer.