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Aerated granular flow over a horizontal rigid surface

Eames, I; Gilbertson, MA; (2000) Aerated granular flow over a horizontal rigid surface. J FLUID MECH , 424 169 - 195. 10.1017/S0022112000001920. Green open access

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Abstract

The effect of a vertical gas flow on the dynamics of a coulombic granular material moving over a horizontal rigid porous surface has been studied experimentally and theoretically. The presence of a fluidizing gas significantly alters the granular flow dynamics. When the gas velocity, u(g), is below the minimum fluidization velocity, u(mf), the effect of the gas is to reduce the angle of repose theta from the value measured in the absence of a gas flow. When material is poured from a point source onto a horizontal surface it forms a pile, which adjusts through episodic avalanching to a self-similar conical shape. Under these conditions, the development of the pile is determined by the local force balance on individual particles and its extent may be expressed in terms of the volume of particles added and the angle of repose. A volume of material is poured continuously from a point source onto a surface according to Qt(alpha). Below the minimum fluidization velocity, a quasi-static description gives the encroachment distance of the granular pile as l = (2Q/(2 pi /3)(n-1) tan theta)(1/n+1)t(alpha /n+1) where n = 1 for a planar release and n = 2 for an axisymmetric release.A continuum description of fluidized granular flow has been developed by vertically averaging the mass and momentum conservation equations and including the momentum exchange between the gas and granular flow. The bulk movement is driven along the ground by horizontal gradients of particle pressure and is resisted by a viscous drag force due to the particles moving horizontally through a vertical gas flow. Above the minimum fluidization velocity the character of the granular how is significantly altered and takes on fluid-like properties. The model predicts the shape of the fluidized granular pile and that the encroachment distance grows as l = lambda (n alpha) (Q (u(g) + u(mf)) /<(<epsilon>)over bar>)(1/n+2)t(alpha +1/n+2), where <(<epsilon>)over bar> is the void fraction in the bed and lambda (n alpha) is a constant. Below the conditions for fluidization (u(g) < u(mf)), the pile of granular material grows quasi-statically when t > t*, where t* similar to (<(<epsilon>)over bar>(n+1)Q(u(g) + u(mf))/mu (2+n)(u(mf) - u(g))(2+n))(1/1+n-alpha) corresponds to the critical time when frictional forces are comparable to gradients of particle pressure and the drag force. Numerical solutions describing the granular how are presented.Experimental observations of the shape and extent of planar and axisymmetric granular flows when a = 1 compare well with theoretical predictions for Various values of particle volume flux Q, time t, and gas flow rate u(g). The mathematical description of fluidized granular flows along rigid surfaces indicates a strong analogy with buoyancy-driven flows in a porous medium. This analogy permits extension of our description to include flows down slopes and the effect of internal stratification.

Type: Article
Title: Aerated granular flow over a horizontal rigid surface
Open access status: An open access version is available from UCL Discovery
DOI: 10.1017/S0022112000001920
Publisher version: http://dx.doi.org/10.1017/S0022112000001920
Language: English
Additional information: © 2000 Cambridge University Press
Keywords: FRICTIONAL COLLISIONAL EQUATIONS, FLUIDIZED-BEDS, MOTION, PARTICLES, CHUTES
URI: https://discovery.ucl.ac.uk/id/eprint/129948
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