What Is the Most Efficient Diameter for a Mill Wheel in a Stream?

[Aweir]

Hi
Im thinking of constructing a millwheel with flat fins to run in a stream.
Is there a formula i can use to calculate the most efficient ∅ to construct it , when taking into account the velocity and area the stream and the surface area of the fins when submerged. and their relevant angles to the current.
It would be good to know the formula for converting the force generated to Watts.
Thanks!

 

[Baluncore]

The design will depend on the velocity of the stream and/or the hight difference available.
The energy available will be due to the reduction in velocity, or the change in height of the flow.

The diameter of the wheel will determine the RPM of the shaft. A small diameter, wide wheel will run faster than a large diameter narrow wheel. The diameter will therefore be chosen based on the RPM you require.

 

[sophiecentaur]

You are lucky to have running water available. Could be a good project.
I don't know much about the details but I do know that 'over', rather than 'under' flow is the thing to go for in water wheel design. If you have enough drop and the possibility of building up any head of water (i.e. a mill-pond or reservoir), you will get much more Power out of your wheel if the water flows over the top of your wheel than under it.

If you have to have the wheel just dipping in the stream, you might consider a screw turbine design, which is, again more efficient than any paddle wheel (hence the way ships have been propelled for a century or more). No one uses paddle wheels for electric power on sailing boats - they all use screw turbines, these days, dragged behind, which is popular for ocean cruising.

The formula for force and power is, very basically
Power = Force times Speed
But, with a wheel, the speed varies over the radius so that formula doesn't say it all. You may be able to get 'anything' to rotate in flowing water but, to get the best out of it, you will need to do a fair bit of design prep.

Some idea of the scale you will be working on could help with useful answers. What sort of volume / water speed / depth and width of channel are you dealing with? You may find that a propellor from an old outboard motor would do very well.