fan as a turbine

I think the area of the fan is too big for the flow in the small inlet pipe. The velocity past the blades are just too small.

I would look at 6" agricultural water meters for inspiration.

Can you give a picture or drawing?

From your description, your turbine seems to look like this:

I don't know this is accurate, but if it is, I think your problem is that the inlet is going directly to the outlet and not even impinging on the fan blades.

To make this work, I think your cylinder needs to be much longer and completely filled with water.

Alternatively, you could change your blades to curved vanes and inject the water tangentially with the outlet at the center.

Nice pic' Vulc' ... That looks like how it is described.

I think you are spot on.

Maybe just moving inlet and outlet to one side would do the trick.

Ideally the inlet outlet and turbine should be a similar cross section. Maybe the blades are too flat (insufficient pitch).

ok...if V is correct, as I think he is, the problem might be that the difference in area slows down the speed of the water significantly. Just look at the difference in area:

3in dia= 7 sq. in. area

17in dia= 227 sq. in.

therefore 32 times difference in area (and flow).

So, suppose the water is travelling 32 mph in the 3in pipe, it slows to 1 mph in the 17 in. pipe.

Also, Professor Cat is correct as far as pitch goes. Flattend pitches are good for driving blades and Higher pitches are better for driven blades.

Perhaps, but even if the water flow is low or the blade pitch is small, the fan should have turned (even just a little bit). However, he says that it doesn't turn at all.

Shankar, one thing I can suggest is that you divert your inlet towards the blades. Don't just change the inlet to one side, however, since the force of the water hitting one blade at a time might damage it.

Stop and think about it for a minute. the fan puts out air movement by turning the fan fast,flat pitch. To get the turbine to spin,you are going to need a much steeper pitch, then dirrect the water flow to the middle of those blades. The wider the stream of water the better (flatten your inlet nozzle to ovel shape to get a wider stream). If that doesn't work get hold of me off list I may have another idea for you to try.

Calculations are almost correct except for assumption of input velocity of 32 MPH - this is about 14.3 metres per second, I would love to see the back garden stream that flows that fast! 3 inch diameter pipe input velocity is more like 1 meter per second and hence the velocity past the fan blades is just a trickle - certainly not enough to turn it with any force.

Sounds like an interesting project. However, it will work better if you can find a water pump instead of an "air pump", since there are obscure (to me at least) mechanical reasons for the various blade shapes. thicknesses and attack angles. With a 30foot head an old centrifugal pump will work beautifully (and more efficiently). Jeff

Okay, coming out with an honest opinion.

I don't think an exhaust fan is going to make a very good water turbine but if the guy is willing to try it, I'm not going to prevent him. There are more efficient designs for water turbines but he obviously wants to use something that's on hand.

Shankar, I can't make any calculations or predictions as to whether the result will satisfy your needs but all you have to lose is a few hours of sweat. If it doesn't come up to spec, look for another, more efficient design.

I have to agree with you on this.

It basically comes down to this. A fan is designed to move air, which has a denisty orders of magnitude last than water. If you want to move water, or in this case, be moved by waer, you need an impeller that is designed for water.

Think of it this way.

A 100 HP outboard prop has a diameter of around 6 inches.

A 100 HP airplane prop has a diamter of around 6 feet.

My personal preference would be a waterwheel. Just because the technology is old, does not make it bad !!

IPG

I cannot quite visualize the setup yet.

A garden stream falls over a 30' plus waterfall (cascade). where it is diverted into a pipe.

My calculator greeted me - I am therefore guessing figures.

The 3" x 30' pipe should hold slightly less than 50litre.

The maximum velocity obtainable at the bottom of the pipe may be 1 m/s. (seems to be more stuff greeted me)

Some (most) energy will be lost by hitting the shaft of the fan and diverting it horizontally.

The flow velocity in the 17" section will only be 33 mm/s with almost no downward velocity vector available.

I still don't think it will work.

What could be done?

1 Introduce a spiral in the pipe to have the water rotating in the correct direction.

2 Place the inlet and outlet off centre in the big section.

3 Better Results may be obtained if the inlet can be on the side of the drum tangent to the blades on one side of the fan. better still if the blades are rotated by ±90deg to act as pedals.

4. Sell the water to the authority and buy electricity.

Guest - we need some more detail.

Is the 30' pipe is running vertically or only at a slope?

What is the available flow?

Please make some sketches.

I think you may possibly waste a lot of time re-inventing the wheel here.

(which is perfectly ok - if that's what you want to do, of course!)

But; this subject is, and has been, extensively researched on many sites.

Both commercially, and for home use. eg: http://www.microhydropower.com:80/products.htm and many others (incl. CR4)

I say this to save you covering the same route as me!

The best advice, before doing anything, is: research, research, research.

Hope this helps.

jt.

My Free Notice Board - http://www.mfnb.at

...re-inventing the wheel...

A waterwheel, perhaps?

I suspect the real issue is that the water flow has to be directed to the blades to do the work. Blade geometry and head will determine whether its ability to rotate and do work.

If not, use this little piece of New Zealand ingenuity. Put the fan back in the wall and use your washing machine. It is described in a number of different publications and has been around for a number of years.

How about angling the fan. Right now the fan and water are at 90deg to each other.

Also, forgive me, but are we certain the fan is 'pointing' the right direction. (had to ask)

cr3