Variety of the patterns in the case of a magnetic convection (original) (raw)
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Flow Visualization of Magnetic Convection of Paramagnetic Fluid in a Cylinder
2004
Convection of a paramagnetic fluid inside a vertical cylinder placed in the bore of a superconducting magnet was studied. The bore axis and cylinder axis were placed coaxially. The upper side wall of the cylinder was electrically heated and lower side wall was cooled isothermally. The upsidedown case was also studied. The mixture of water and glycerol was used as a working fluid. The magnetic susceptibility of the mixture was increased by adding the particular amount of Gd(NO3)3 ⋅ 6H2O. A horizontal mid cross section of the cylinder was illuminated to see the isotherms with dispersed liquid crystal slurry. The color image showed that the magnetic field affected the fluid flow structure it enhanced the convective flow and also induced flow from a quasi conduction state.
International Journal of Thermal Sciences, 2008
A cube was filled with an aqueous solution of glycerol with the addition of gadolinium nitrate hexahydrate to make the working fluid paramagnetic. A very small amount of liquid crystal slurry was then added in order to visualize the local temperature inside the enclosure. One vertical wall of the cube was uniformly heated by nichrome wire from a DC power supply while the opposite one was cooled by cold water flowing from a thermostatic circulator. The system was placed close to the solenoid of a superconducting magnet which was horizontally oriented. Two cases were considered in the experiment: the first with the cooled wall close to the solenoid and the second with the heated wall close to the magnet's electric multi-wires. Natural convection was investigated for both cases: first without a magnetic field and second with various strengths of magnetic force acting on the system. The experimental results showed clearly suppression and enhancement of natural convection. Corresponding numerical computations were carried out for comparison with the experimental data. For this purpose, isotherms of the experimental data were extracted from the color images using the Particle Image Thermometry method and were compared with the numerical results.
Thermal science and engineering, 2006
The magnetic convection of paramagnetic fluid is studied in a strong magnetic field. The fluid in a cubic enclosure is heated from one vertical wall and cooled from the opposite one. The fluid is the 80% mass aqueous solution of glycerol with 0.8 mol/kg concentration of gadolinium nitrate hexahydrate to make the working fluid paramagnetic. The small amount of liquid crystal slurry is added to the fluid in order to visualize the temperature profiles in a vertical cross-section. This system is placed directly below the solenoid of the superconducting magnet which is oriented vertically. The temperature of cold wall is constantly controlled by the water flowing from a thermostating bath. On the other hand, the hot wall is heated by a nichrome wire from a DC power supply. In the numerical computations, the configuration of the system is modeled to be as close as possible to the real system. The physical properties of the working fluid are used to compute dimensionless parameters in the ...
Physical Review E, 2010
Thermal convection experiments in a liquid gallium layer were carried out with various intensities of uniform horizontal magnetic fields. The gallium layer was in a rectangular vessel with a 4:1:1 length ratio ͑1 is the height͒, where the magnetic field is applied in the direction normal to the longest vertical wall. An ultrasonic velocity profiling method was used to visualize the spatiotemporal variations in the flow pattern, and the temperature fluctuations in the gallium layer were also monitored. The observed flow pattern without a magnetic field shows oscillating rolls with axes normal to the longest vertical wall of the vessel. The oscillatory motion of the flow pattern was suppressed when increasing the applied magnetic field. The flow behavior was characterized by the fluctuation amplitude of the oscillation and the frequency in the range of Rayleigh numbers from 9.3ϫ 10 3 to 3.5ϫ 10 5 and Chandrasekhar numbers 0-1900. The effect of the horizontal magnetic field on the flow pattern may be summarized into three regimes with increases in the magnetic intensity: ͑1͒ no effect of the magnetic field, ͑2͒ a decrease in the oscillation of the roll structure, and ͑3͒ a steady two-dimensional roll structure with no oscillation. These regimes may be explained as a result of an increase in the dominance of Lorentz forces over inertial forces. The power spectrum from the temperature time series showed the presence of a convective-inertial subrange above Rayleigh numbers of 7 ϫ 10 4 , which suggests that turbulence has developed, and such a subrange was commonly observed above this Rayleigh number even with applied magnetic fields when the rolls oscillate.
Onset of convection in magnetic fluids
Physics Procedia, 2010
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.
Investigation of Magneto Hydrodynamic Natural Convection Flows in a 3-D Rectangular Enclosure
Journal of Applied Fluid Mechanics, 2016
The article deals with magnetic field of free convective flows in cavities similar to those used in artificial growth of single crystals from melts (horizontal Bridgman configurations) and having aspect ratios an equal to "4". The combined effect of wall electrical conductivity and vertical direction of the magnetic field on the buoyancy induced flow of mercury was investigated numerically. The validation of the numerical method was achieved by comparison with both experimental and analytical data found in the literature. The plotted results for variation of velocity, temperature and Nusselt number in terms of the Hartmann number Ha and Rayleigh number "Ra" showed a considerable decrease in convection intensity as the magnetic field is increased, especially for values of "Gr" situated around 10 7. The calculations also showed that the vertically directed magnetic field (perpendicular to the x-z plane) is the most effective in controlling the flow and hence the speed of growth of the crystal. Also, wall electrical conductivity enhances damping by changing the distribution of the induced electric current to one which augments the magnitude of the Lorentz force.
Effect of an external magnetic field on the Rayleigh-Bénard convection of liquid metal within a rectangular enclosure was numerically studied. The liquid metal filled in the enclosure is heated from a horizontal bottom wall and cooled from an opposing top wall both isothermally, whereas vertical walls are adiabatic. The direction of the magnetic field is either in vertical or horizontal. The governing parameters of such Rayleigh-Bénard convection in the presence of a magnetic field are the Rayleigh number, Ra, the Prandtl number, Pr, the Hartmann number, Ha, and the electric conductance of walls, c. The numerical computations have been carried out for Pr = 0.025 (approximately mercury or gallium) and c = 0 (insulating walls) by using the HSMAC algorithm for correction of both pressure and electric potential, and also by using a third-order upwind scheme for inertial terms in the Navier-Stokes equation. The numerical results exhibit significant differences depending on the direction of the applied magnetic field. When the vertical magnetic field is applied, the magnetic damping effect is quite strong and the convection structure is different from an ordinary Rayleigh-Bénard convection. On the other hand, when the horizontal magnetic field is applied, a quasi-two-dimensional roll cell structure is organized along the direction of the magnetic field. Once, such a convection structure is organized, the magnetic damping effect is much weaker than that of the vertical magnetic field. For the case of the horizontal magnetic field with the high Hartmann number, the modeling of the Hartmann layer is used in order to make computations more accurate and to save the computational meshes and time. The numerical results are plotted in a graph of Nu versus Ra for the various Hartmann numbers, and finally compared with some data previously reported.
Energies, 2020
Natural convection of liquid metal in an annular enclosure under the influence of azimuthal static magnetic field was numerically studied. The liquid metal in the enclosure whose cross-sectional area is square was heated from an inner vertical wall and cooled from an outer vertical wall both isothermally whereas the other two horizontal walls were assumed to be adiabatic. The static azimuthal magnetic field was imposed by a long straight electric coil that was located at the central axis of the annular enclosure. The computations were carried out for the Prandtl number 0.025, the Rayleigh number 104, 5 × 105 and 107, and the Hartmann number 0–100,000 by using an in-house code. It was found that the contour map of the electric potential was similar to that of the Stokes stream function of the velocity regardless of the Hartmann number. Likewise, the contour map of the pressure was similar to the Stokes stream function of the electric current density in the case of the high Hartmann n...
The present study is devoted to the problem of onset of oscillatory instability in convective flow of an electrically conducting fluid under an externally imposed time-independent uniform magnetic field. Convection of a low-Prandtl-number fluid in a laterally heated two-dimensional horizontal cavity is considered. Fixed values of the aspect ratio ͑length/heightϭ4͒ and Prandtl number ͑Prϭ0.015͒, which are associated with the horizontal Bridgman crystal growth process and are commonly used for benchmarking purposes, are considered. The effect of a uniform magnetic field with different magnitudes and orientations on the stability of the two distinct branches ͑with a single-cell or a two-cell pattern͒ of the steady state flows is investigated. Stability diagrams showing the dependence of the critical Grashof number on the Hartmann number are presented. It is shown that a vertical magnetic field provides the strongest stabilization effect, and also that multiplicity of steady states is suppressed by the electromagnetic effect, so that at a certain field level only the single-cell flows remain stable. An analysis of the most dangerous flow perturbations shows that starting with a certain value of the Hartmann number, single-cell flows are destabilized inside thin Hartmann boundary layers. This can lead to destabilization of the flow with an increase of the field magnitude, as is seen from the stability diagrams obtained. Contrary to the expected monotonicity of the stabilization process with an increase of the field strength, the marginal stability curves show nonmonotonic behavior and may contain hysteresis loops.