Experimental Measurement of the Solid Particle Concentration in Geophysical Turbulent Gas-Particle Mixtures
Résumé
Dilute gas-particle mixtures in which the particles are carried by the turbulent fluid are found in
various geophysical contexts, from cold snow avalanches to hot pyroclastic density currents. Though
previous studies suggest that such mixtures have maximum particle concentrations of a few volume percent,
the dependence of this maximum concentration on the Reynolds number is unclear. We addressed this issue
through laboratory experiments in a vertical pipe, where dilute gas-particle mixtures were created by
injecting a turbulent air flow from below. Nearly monodisperse mixtures of glass beads of different grain sizes
(77 to 1,550 μm) were used with varying bulk concentrations from 0.025 to 8 vol. %. To create quasi-static
mixtures, the mean air velocity matched the terminal settling velocity for the grain sizes investigated. The
maximum Reynolds numbers of the mixtures were ~104–106. The air pressure indicated full support of the
particle weight at concentrations down to 0.025 vol. %. Above a critical particle concentration, at which all
the particles were suspended, subsequent additional particles were not maintained in the mixture and led to
the formation of clusters that settled downward in the pipe to form a dense fluidized bed. Maximum
mean particle concentrations of the dilute mixtures increased from ~1 to ~2.8 vol. % and reached a plateau at
increasing mixture Reynolds number. These results give insights into the maximum particle concentrations of
geophysical turbulent gas-particle mixtures and may serve to constrain observations as well as the input
and output data of models.
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