Experimental studies on heat transfer in thermo-magnetic onvection for para- and diamagnetic fluids

Experimental studies on heat transfer in thermo-magnetic onvection for para- and diamagnetic fluids

Mulder, M.

Kenjeres, S. (mentor)

Applied Sciences

Chemical Engineering

Transport Phenomena

2014-04-23

In industrial heat transfer processes, natural convection enters in various forms. One form of natural convection is thermo-magnetic convection. Besides gravitational force, magnetic force causes warmer fluid to rise or fall dependent on the fluids magnetic susceptibility and direction of magnetic field gradient. Magnetic susceptibility is a material property which indicates a degree of magnetization in a material. For paramagnetic fluids magnetic susceptibility depends on temperature, is positive and therefore attracted by magnetic field. Magnetic susceptibility of diamagnetic materials is independent on temperature, is negative and hence repelled by magnetic field. Magnetic force can be used to enhance or suppress gravity. This phenomena is widely investigated for many materials, magnetic field strengths and set-up geometries. In this research thermo-magnetic convection and the effect it has on internal heat transfer is experimentally investigated for para- diamagnetic fluids. Making use of a 10 Tesla superconducting magnet, which can generate field gradients up to 870 T2/m, steady, oscillating and turbulent flow regimes can be observed. I performed the experiments at the AGH University of Science and Technology in Krakow, Poland. A small cubical enclosure filled with para- or diamagnetic fluid is placed at different positions in the magnet to get enhancement or suppression of internal heat transfer. Enclosure is heated from below and top is kept at constant temperature. Temperature of the fluid is measured with thermocouples at six different positions inside the enclosure. From these temperature-time measurements a power spectrumis obtained to determine the characteristic flow regime. Internal heat transfer is investigated by measuring different variables and calculate thermo-magnetic Rayleigh and Nusselt numbers. As paramagnetic fluid a 40% water-glycerol solution is used and gadoliniumnitrate is added to create a higher magnetic susceptibility. Enclosure was placed above the magnet centre which should give a magnetic force that enhances gravitational buoyancy. Temperature difference between the hot and cold plate of the enclosure is 5 and 11±C, respectively case G5A and G11A. Case G5A shows transition in the flow regime from steady to oscillating to turbulent with increasing of magnetic field. Case G11A shows turbulent regime for each measurement. Nusselt number calculations for glycerol solution measurements show an increase, up to 2.5 times, in internal heat transfer. Turbulence causes better mixing and hence better heat transfer. Relation between RaTM and Nu are compared with previous (experimental) relations and show good agreement. Pure water is used as diamagnetic fluid. First enclosure is placed below magnet centre and temperature difference is 5 and 1 ±C, respectively case W5B and W1B. Here magnetic force should enhance internal heat transfer. Case W5A and W3A are measured above the magnet centre and have respectively a temperature difference of 5 and 3 ±C. Case W5B and W5A both show turbulent flow regime for all measurements. Internal heat transfer is about the same for both cases but show a slight increase for W5B and decrease for W5A. There can be concluded that for turbulent flow regimes magnetic force direction has no significant influence on internal heat transfer. For smaller temperature differences, case W1B and W3A, magnetic force does influence measurements. CaseW1B shows steady flow regime first and for higher magnetic field strengths fluid plumes start to rise and sink due to magnetic force. Case W3A shows a very clear transition from turbulence to oscillating flow. Small temperature differences cause large measurement errors and internal heat transfer is assumed to be constant. Recommendations for further research is to get a better impression of fluid structures and temperatures in the enclosure. Fluid behaviour can be visualized with liquid crystals and velocity fields can be determined by using PIV on these visualizations. The velocities can be compared to simulations. To get realistic simulations, fluid properties need to be measured for different temperatures and if necessary differentmagnetic field strengths.

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Student theses

master thesis

(c) 2014 Mulder, M.