Brazil Nut Effect Easily Explained With Fluidization

Bret Cahill · 2007-05-22T02:48:43+00:00

This is the first real explanation of why larger particles rise to thetop when you shake a container filled with different sized particlesall of the same density.Actually it's almost trivial. They made an easy problem morecomplicated than what it is.When the container rises, it flings all the particles up to the topand when it falls it flings all the particles down to the bottom.The accelleration on the nuts at the bottom is the accelleration ofthe can plus the accelleration of gravity. At the top, however, it'sthe accelleration of the can minus gravity so the nuts spend more timepacked on the bottom than packed on the top.During the flight up and down the larger particles will have a highermass/drag ratio than the smaller particles. Mass ~ D^3 and drag ~D^2. The drag is from the air-particle fluid but a "drag" may alsocome from the particles scattered in a vacuum.The larger will blast right past the smaller ones when moving from oneend to the other.Basically the smaller particles act as a fluid when in transit withthe larger ones passing through that "fluid."This happens in both directions.The difference is on the way down the smaller particles pack earlierand longer than at the top and cease to be like a fluid. The largerparticles cannot make much headway in a packed bed.The small particles may pack at the top as well but not as long as atthe bottom so the big particles will rise relative to the small oneach shake.Bret Cahill

May 22, 2007 21 Replies

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Autymn D. C. 19 years ago

like your mind and reality

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Bret Cahill 19 years ago

I tried three phase fluidization -- liquid, particles, gas -- which has heat transfer coefficients that approach those of condensation but I could never figure out a way to keep the two heavier phases from sloshing to one side of the cylinder.

Bret Cahill

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