Particle structuring and yield stress in magnetofluidized beds
- Conference date: 20–25 June 2010
- Location: Montecatini (Italy)
A novel experimental technique to measure the tensile yield stress of fluidized beds of magnetic powders stabilized by an externally applied cross‐flow magnetic field is shown. Basically, the tensile yield stress of the magnetically stabilized bed (MSB) is measured by means of the pressure drop of a gas flow that puts the bed under tension. A first relevant result is that the yield stress depends strongly on the field operation mode. In the H off/on operation mode, the bed was driven to bubbling by imposing a high gas velocity in the absence of magnetic field. Once the gas velocity was decreased below the bubbling onset and the bed was stabilized by the natural cohesive forces alone, the field was applied. The yield stress of the naturally stabilized bed is not essentially changed by application of the field a posteriori (H off/on), which can be attributed to the inability of the field to alter the arrangement of the particles once they were jammed in the stable fluidization state. In the H on/on mode, the field was kept during the whole process of bubbling and stabilization at reduced gas velocities. In this operation mode, the field was the main stabilizing source. In contrast with the H off/on mode results, the yield stress in the H on/on mode was observed to be appreciably increased, which is a consequence of the formation of particle chains as the gas velocity is decreased in the presence of the magnetic field. The influence of other parameters such as particle size distribution reveals also a correlation between the microstructure of the MSB and its yield stress. In analogy with structured magneto‐and electro‐rheological fluids, it is found that the yield stress increases as the average particle size is increased. Moreover, the microstructure of the MSB is relevantly affected by the natural cohesiveness of the powder due to van der Waals forces, which leads to the formation of large‐scale branched chains when the field is applied, thus enhancing the yield stress. Our work shows therefore that it is the microstructure of the MSB as affected by the presence of the magnetic field what essentially determines its yield stress.
- Yield stress
- Magnetic fields
- Fluidized beds
- Magnetic field measurements
- Magnetic fluids
- Microstructure property correlations
- Van der Waals forces
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