Abstract
The occurrence of coherent structures in multiphase flows is widespread: quantitative experimental data with respect to their size, their velocities, and their frequency in both bubble columns and fluidized beds are presented; these data have been collected by such different techniques as Laser-Doppler Velocimetry, Particle Image Velocimetry, Optical Bubble Probes, and Gamma- Ray Densitometry. In addition, results are discussed from Computational Fluid Dynamics simulations as to particle swarms in the freeboard beyond bubbling fluidized beds. The question is raised whether there is a common explanation for the appearance of coherent structures in such different systems. The similarity of multiphase flows and natural convection phenomena is discussed, while some features of turbulent multiphase flows resemble those in shear- induced turbulence in single-phase flows. Phase locking, on the basis of interplaying time scales, and an analogy with the vortices in a Karman vortex street are suggested as possible mechanisms behind the occurrence of coherent structures.
Original language | English |
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Pages (from-to) | 123-136 |
Number of pages | 14 |
Journal | Powder Technology |
Volume | 100 |
Issue number | 2-3 |
DOIs | |
Publication status | Published - 1 Dec 1998 |
Externally published | Yes |
Keywords
- Bubble columns
- Coherent motions
- Flow structures
- Fluidized beds
- Phase locking
- Vorticity