Sedimentation of gas-fluidized particles with random shape and size

Abstract

This work deals with the fluidization and sedimentation of fine solid particles, of random shape and size, similarly to those commonly involved in geophysical mass flows, such as pyroclastic flows.While heated to avoid the effect of moisture and the formation of clusters, particles were first uniformly fluidized by a hot gas flow, up to a high expansion rate, then let sedimented after stopping the gas supply. Three different materials are explored, involving contrasted geometries, each characterized by a specific particle volume fraction at packing Φ pack . Within the range of values of the solid volume fraction Φ s /Φ pack studied here, from 0.65 to 0.95, the dense suspension forms a fully fluidized homogeneous mixture, with no segregation, for which the fluidization and sedimentation velocities are equal. Despite a significant discrepancy between the intrinsic properties of the different materials used, all measured velocities are observed to collapse into a single master curve f (Φ s /Φ pack ) provided that they are normalized by the relevant scaling. Regarding to the sedimentation velocity, Φ pack turns out to be sufficient to characterize the material made with a random distribution in particle shape and size. Furthermore, these new findings suggest that such fluidized gas-solid mixtures behave as a homogeneous equivalent fluid with a bulk apparent viscosity that only depends on Φ s /Φ pack .

Physical modeling of the dam-break flow of sedimenting suspensions

“…This conclusion has been obtained by taking advantage that the front velocity U F of the dam-break flow in the investigated configuration remains constant during almost the entire process (hypothesis H1). However, in contrast with the sedimentation velocity which can be determined from the sole knowledge of the initial properties of the suspension [27], the dam-break flow does depend on the geometry of the channel. In particular, U F is not expected to be constant in general and, Shallow-water equations are commonly used to describe the flow of a heavy fluid into a lighter one [7], as well as that of a fluid laden by solid particles into the same fluid [31].…”

“…The Reynolds number of the dam-break flow can be estimated by considering that the mixture can be described as an equivalent fluid of density ρ m and viscosity µ m . According to [27], the kinematic viscosity µ m /ρ m at large concentrations (Φ s /Φ pack ≈ 0.95) hardly reaches that of water. The Reynolds number of the flow mixture is thus larger than 10 5…”

“…Thus, combining eqs 14-15 with a model for the sedimentation velocity [27], should constitute the first-order approximation of a dense layer of pyroclastic flows along the major part of its course, excluding the initial formation which is fully three-dimensional and the very last stage, when the thickness of the boundary layer becomes comparable with that of the moving layer. Solving these equations is beyond the scope of this paper.…”

“…The sedimentation velocity in the non-flowing case (fig 1c) has been recently analyzed [27] for fine heated particles including volcanic ash of random shape and almost spherical synthetic particles (FCC). In any case, it is well described by the following semi-empirical expression,…”

“…When the gate is opened, the particle sedimentation is initiated simultaneously to the dam-break flow (figs 1d and 2). The fluidization technique, which enables to significantly vary Φ s within the mixture [27], allows to distinguish the dynamics of dry granular materials governed by frictional interactions [9,28] from that of significantly expanded suspensions [8,12]. Let us focus on the second case, which is the subject of the present study.…”