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             BCCs - especially see "Superb example" link below:

The Archnimedes screw pump (or hydraulic motor) does NOT usually
operate in the manner which may be intuitively assumed.

ie IF the screw had zero centre diameter then it would act as a pipe
with a spiral guide in it and water would flow down it when stopped.
BUT as the centre diameter is made an increased  fraction of the outer
diameter you form "buckets" at the bottom of each spiral loop. The
size of the buckets as a proportion of the total pipe internal volume
depends on both the inner/outer ration AND the tilt angle.

Look at the decorative screw pumps which consist of a spiral tube with
a large air space down the middle. It is very easy to visualise the
"buckets of water trapped in each loop. Attached GIF gives a good
visualisation and an excellent real world example is here.

SUPERB EXAMPLE :

        http://www.swansea.ac.uk/grst/images/Archimedes'%20screw.jpg

This allows the crucialrole of the OD/ID ratio to be seen.
Note that here the screw is formed on the outer wall so there is NO
leakage when static.
A screw with given ID/OD ratio can also be inserted in a tube, when
there will be leakage between the tube and screw.
For this design the outer wall height of the buckets becomes relevant
- for a screw in tube design the outer wall is continuous.

I guess that an eg Teflon coated version of the above shown screw
should be able to provide an extremely high efficiency.

As implemented here water is spilling from successive turns due to the
buckets being brim full, but an inlet scheme could be arranged to
slightly underfill each bucket initially. As seen here tilt angle and
ID/OD ratio are not optimised - as the buckets are spilling on the
outer edges but water level is not up to the ID edge. IE a slightly
steeper tilt would lose less water for this % of fill.

Note that this pump can operate across a wide speed range - and stop
without losing water. If manually operated this pump would allow
delivery rate to vary with user's input energy.

____________________

A screw with zero or "too small" ID/OD ratio can operate as a pressure
turbine that pumps dynamically BUT in most cases the system is
effectively operated so as to effectively be a series of buckets. ie
it is a positive displacement pump with gravity providing the cap to
the buckets.

A horizontal screw will still transport liquid with the maximum outlet
head being the height of the inner edge of the lower part  of the
spiral, and the input head minimum being level with the lower outer
edge.

Note that the various heat engine solutions being discussed in
response to Olin's challenge do NOT use the screw in the above
manner-they use it as a an inertial or momentum driven system with the
fluid flow reacting against the rotor. This would work conceptually
but is liable to have much of the flow bypass the screw - depending a
lot on design.



                    Russell McMahon




> >> > What backflow? =A0 :)

> >> I'm not sure if that's a joke or not. I realize that the water which
> >> has fully reached the top will not backflow since it is lower than the
> >> end of the screw. However, any water inside the screw will flow back
> >> down into the lower pool if the screw speed is insufficient to keep up
> >> with the pull of gravity on the water. Am I missing something?

> Isn't the screw, when stopped, a continuous inclined plane so that
> water poured in at one end would just run "downhill" around the turns
> of the screw and out the other end? I did look at the photos and
> that's what it appeared to me.

> > It's not a joke, as for most of the screw the water does not cover the =
screw, so the water cannot backflow. If the screw is made as a channel with=
 an open top (see the pictures in the links I posted previously about power=
 generation using a screw) then the only backflow is when the water overflo=
ws a turn of the screw.
> >
> > There will be a trickle backflow due to an inevitable gap between the s=
crew and the channel, but careful design would make this reasonably minimal=
.

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