No it doesn't explain anything of the sort. Why would it? The effect exists when there is nothing to hit at the top. Adding the condition that some of the particles strike something on "top of the stroke" is a needless and confusing addition to the problem because as you intimate it reduces the effect a slight amount. It contributes nothing - so why even bring it up? Also, striking the walls doesn't contribute to the effect either as someone else suggested.
Has anyone mentioned any situation where my 2 short paragraph explaination wouldn't hold?
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It's easy to prove what phenomenon dominates:
Just increase the pan diameter while keeping everything else constant.
At a minimum, everyone here can agree a simple 20-20 hind sight obvious mechanism has been completely overlooked until now.
Already asked about that on sci.materials.
The goal was to build a really cheap "free" or moving cylinder Stirling with a particle bed for the displacer-regenerator. The HX coefficients are an order of magnitude higher than gas-surface alone. The issue was reducing "dead space" between the particles -- about 25% of the bed for same size close packed spheres. Someone suggested smaller size particles to fill the voids between the larger then the same poster raised the BN effect, something I was previously told was somthing that was really baffling to great scientists but has ignored altogether in the salt shaker.
But once the issues was raised after I had spent so much time on the assymetry of the cycle I ignored all the misleading explanations.
Just conduct some tests.
Start with the limiting case of a 5mm lead weight in some microspheres. The lead ball will sink through a packed bed of glass bubbles. Then work your way to the line from there.
For the salt shaker engine, it wouldn't be an issue if the larger denser particles always were nearest the top and bottom with the smaller ones near the center, just as long as there were smaller particles filling the voids between the larger.
I can assure you that it is possible to shake a can of nuts absent a lid without the nuts going "all over the place". And the effect will occur without the lid. Likewise the phenomena is going to occur even if the nuts aren't in the can. Dump them into a shallow bowl and the effect doesn't go away or even diminish. You are focusing on things that have nothing to do with the cause. Umbrellas don't cause rain even if you do see a lot of umbrellas on a rainy day. Even primitive man understood that if you dropped a rock on a sandy beach that the rock would land and stay on top of the sand and that conversely, if you tossed a handful of sand onto a bed of rocks most of the sand would filter down thru the rocks. This is a simple and obvious concept that you are trying to turn into something complicated by focusing on things that are irrelevant. The more interesting engineering problem would be how do they keep the different sized nuts from separating before they go into the can, so that each can gets the same uniform mix of nuts.
You have to first point out that gravity is causing the bed to be deeper and longer at the bottom than the top but, like I've been saying, it's a trivial problem.
. . . .
What's really interesting is how and why such a trivial to explain effect ever got to be presented as such a curious phenomenon in the first place.
The same effect, on a long time scale is noted in fields subject to freeze-thaw cycles. Large stones and rocks rise to the surface.
As you will immediately notice - momentum effects are not required. Freezing does provide a force which effectively provides some upthrust to the entire substrate - and it is the larger objects which preferentially rise.
How does that sit with your "first real explanation invoving momentum"?
But that's not the same effect. That has nothing to do with large objects impacting packed beds of smaller objects or large objects passing through dispersed beds of smaller objects.
nuclei Brazil nuts are the same density, whereas atomic nuts are not. This is why the stony-ory planets are on the inside and the isy-wispy planets are on the outside.
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.
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