A reverse Prop Question

[boatmon1955]

I am not sure if I can post the picture, but will try and describe the wheel.
a propeller with outer ring and a shaft thru middle.... (prop).

12' dia outer ring that is about 3 feet long, 4 blades going to center shaft (hollow , 12" diam) blades are at 45 degree angle and close 75% of the circle (25 % open from front elevation).

We have patent and are building some models for test, but I need some ideas of working parameters.
The wheel will have two struts to lower into water and will generate power by a shaft coming up one strut (kind of like the reverse of an outboard motor drive train/ lower unit). At the top of the shaft will be electric generator that can vary load. This is where it gets way out of my expertise... If I have the wheel freewheeling in a 5 mph current, how much power can I load to get best efficiencies for generation. Then can I chart for 2 mph current up to say 10 mph, and adjust efficiencies to guess at power generated?

[daiquiri]

Sounds like an axial-flow hydro generator with a shrouded prop.
I didn't quite understand whether the prop blades are attached to the shaft or to the outer ring and whether the outer ring's inflow area is constant or variable.
That said, everything is possible, and you can make a mathematical model of your propeller. The complexity of the math depends on the geometry of the prop/shroud. If prop blades have a sufficiently high aspect ratio, a simple strip-theory model could be used for simulations.

[baeckmo]

Look for texts describing wind turbines; they are operating from an open flow like yours, the only difference is the density of the medium. There is an optimum operating point where the incoming velocity (Vo) is retarded to 1/3 of its undisturbed value (ie the velocity in the turbine disc area is 2/3 of V0).

The outer ring that you have proposed will not be of any help in increasing power or efficiency; it is a waste of effort. In this kind of power extraction from a "diluted source", the prime subject is to enclose the maximum flow. So the focus must be on maximizing turbine swept area. All the "material expenses" should be invested there, not in idling structures, that most of all increase friction and generate disturbing turbulence!

Sorry to be the "messenger for bad news", but there is an abundance of basic knowledge on turbomachine technology available, that might lead you on the track forwards!

[boatmon1955]

No bad news at all. Thank you both very much for the info. We have a few other reasons for the outer ring, but that info gives us some better design parameters for one of the components. Also as design target we can use 2/3 VO until models prove otherwise. Thanks again

[baeckmo]

Originally Posted by boatmon1955

No bad news at all. Thank you both very much for the info. We have a few other reasons for the outer ring, but that info gives us some better design parameters for one of the components. Also as design target we can use 2/3 VO until models prove otherwise. Thanks again

One known reason to use some kind of shrouding (either diffusing the flow after the turbine, or concentrating it before) is to increase the flow speed through the disc area, permitting higher tip speeds at the rotor. This is normally a bonus, due to lower speed ratios for the transmission, but so far, none of the many proposals along this line have been used commercially. But if you are using some stator for other purposes I suggest you include the diffusor effect into it. The "rollup" vortices of a delta-wing has the same effect, and might be easy to create as a part of the turbine fundament.......?!