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| Titel |
Segregation of particles in a tapered fluidized bed with implications for pyroclastic volcanism |
| VerfasserIn |
Thomas Gernon, Mark Gilbertson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250095065
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| Publikation (Nr.) |
 EGU/EGU2014-10505.pdf |
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| Zusammenfassung |
| Volcanic vents (or diatremes) such as those formed in the eruption of kimberlites, lamproites
and alkali basalts are thought to effectively behave as tapered fluidized systems. Tapered
fluidized beds are extensively used in industrial operations to induce thorough mixing of
particles of differing sizes. In the volcanic system, fluidization occurs as a consequence of
the trapping of crystals and pyroclasts during waning eruption phases, when the
increased cross-sectional area of the vent prevents most particles from escaping. This
phase, known as the pressure-adjusted regime, is accompanied by extensive gas
release from ascending melts and hot pyroclastic deposits confined to the vent. The
ascending gas is capable of fluidizing the vent-filling deposits and as such will
inevitably lead to particle segregation, similar to that observed in gas escape pipes in
ignimbrites. It has been shown that gas flow through the dense-phase of tapered beds
is heterogeneous, generating a central fluidized core and unfluidized peripheral
regions. The structure becomes much more complex when the particles reach the
minimum fluidization velocity (Umf), leading to segregation. The aim of this study is to
investigate how the two different types of structure – that is the flow structure resulting
from the tapered shape of the bed (i.e.akin to the diverging geometry of a vent)
and the segregation structures – interact and affect each other. Experiments were
performed in a tapered planar bed (θ=15°) using bidisperse mixtures of ballotini. The
growth and extent of flow and segregation structures were measured, as well as
the fabrics observed under different conditions. The most significant impact of a
tapered geometry is seen at flow-rates just in excess of Umf when vertical columns of
particles form, completely displacing larger-scale flow structures. Our results have
important implications for understanding the gas flow regimes that occur deep inside
volcanic vents, as well as those operating in pyroclastic density current deposits. |
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