We study the transport of a slurry of heavier than liquid circular particles in a plane pressure driven flow in a direct simulation. The flow is calculated in a periodic domain containing 300 circular particles. The study leads to the concept of fluidization by lift in which all the particles are suspended by lift forces against gravity perpendicular to the flow. The study is framed as an initial value problem in which a closely packed cubic array of particles resting on the bottom of the channel are lifted into suspension. All the details of the flow are resolved numerically without model assumptions. The fluidization of circular particles first involves bed inflation in which liquid is driven into the bed by high pressure at the front and low pressure at the back of each circle in the top row. This kind of bed inflation occurs even at very low Reynolds numbers but it takes more time for the bed to inflate as the Reynolds number is reduced. It appears that the bed will not inflate if the shear Reynolds number is below the critical value for single particle lift-off. The flows with a single particle are completely determined by a shear Reynolds number and a gravity parameter when the density ratio and aspect ratio parameters are specified. In the multi particle case the volume fraction and distribution also matters. The transition to a fully fluidized slurry by waves is discussed.
An analytical model of the steady motion of a single particle dragged forward in a Poiseuille flow is derived and compared with a simulation. The undisturbed fluid velocity is always larger than the particle velocity producing a fluid hold-up. The effect of the hold-up in the many particle case is to greatly reduce the velocity of the mixture which may be described by a two-fluid model in which the solid laden mixture is regarded as a second fluid with effective properties.
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February 2000. Contact H.G. Choi for questions about this article.
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