Abstract
Transition to turbulence and changes in the fluid flow structure are subjects of continuous analysis and research, especially for unique fields of research such as the thermo-magnetic convection of weakly magnetic fluids. Therefore, an experimental and numerical research of the influence of an external magnetic field on a natural convection’s fluid flow was conducted in the presented research. The experimental part was performed for an enclosure with a 0.5 aspect ratio, which was filled with a paramagnetic fluid and placed in a superconducting magnet in a position granting the enhancement of the flow. The process was recorded as temperature signals from the thermocouples placed in the analyzed fluid. The numerical research enabled an investigation based not only on temperature, but velocities as well. Experimental and numerical data were analyzed with the application of extended fast Fourier transform and wavelet analysis. The obtained results allowed the determination of changes in the nature of the flow and visualization of the influence of an imposed strong magnetic field on a magnetic fluid. It is proved that an applied magnetic field actuates the flow in Rayleigh-Benard convection and causes the change from laminar to turbulent flow for fairly low magnetic field inductions (2T and 3T for ΔT = 5 and 11 °C respectively). Fast Fourier transform allowed the definition of characteristic frequencies for oscillatory states in the flow, as well as an observation that the high values of magnetic field elongate the inertial range of the flow on the power spectrum density. Temperature maps obtained during numerical simulations granted visualizations of thermal plume formation and behavior with increasing magnetic field.
Highlights
As natural convection is one of the most common processes occurring in the environment, the heated from below configuration has been studied intensively in the literature— both from the application point of view and to delve into the theory of the phenomenon [1,2,3,4,5,6]
In 1820 Oersted discovered that a magnetic field exists around an electric current flowing through a wire and Biot and Savart demonstrated that the magnetic induction is proportional to that electric current
The emphasis of this research was placed on the changes in the flow structure under different magnetic field strengths, but the enhancement of the heat transfer in the system was analyzed
Summary
As natural convection is one of the most common processes occurring in the environment, the heated from below configuration has been studied intensively in the literature— both from the application point of view (to suppress or enhance heat transfer and fluid motion) and to delve into the theory of the phenomenon [1,2,3,4,5,6]. Wróbel and Kenjeres [30,31,32] undertook the subject of extending the analysis of thermo-magnetic convection in the Rayleigh-Benard configuration to the transitional and turbulent regimes for a cubical enclosure They provided detailed insight into the fluid flow and heat transfer changes under the influence of an external magnetic field over a range of parameters (different Prandtl numbers and values of imposed magnetic field 0–15T) and determined three flow regimes: a transient regime (RaTM ≤ 3 × 107), a regime where a thermo-magnetic mechanism dominates (3 × 107 < RaTM ≤ 3 × 108), and an additional wall heat transfer regime for RaTM ≥ 3 × 108. To describe the evolution and behavior of thermal plumes when under the influence of imposed, strong magnetic gradients
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