| T O P I C R E V I E W |
| sacongo |
Posted - 02/03/2008 : 06:49:30
yello everyone, i have a question about a drum vawt and was wondering with the bottom and top of the drum removed will you get more or less rpm? I figure it would have more torque with them on but im interested if anyone every did any kind of test.
About bearings which i know little about i was going to use a 2" flange bearing for the main shaft which the whole thing would be riding on but will it be able to take the side to side load of a 200lbs or would it just fall apart? |
| 9 L A T E S T R E P L I E S (Newest First) |
| ghurd |
Posted - 03/03/2008 : 15:57:59
Testing has been done. They work better with the ends still on, and a bit more as BW stated.
Comparing a bucket VAWT and A Lenz2 is apples and wrenches.
Bucket VAWTs are drag based. The Lenz2 is lift based.
The Lenz2 is about 3 times more efficient than a savonius.
And a savonius is better than a bucket. |
| sacongo |
Posted - 03/03/2008 : 06:23:21
Ok thanks for all the info im going to go with a trust bearing as BW mentions.
If the buckets dont have there ends there will be less of a vacuum in the upwind bucket which should mean more rpm... but then again as you said your losing on the power stroke because of the missing ends. When its really spinning i wonder which is more important to get higher rpm. It would be a great thing to do a test with. |
| MattM |
Posted - 03/03/2008 : 02:08:18
Doesn't it strike you as odd how they say a two bucket savonius is better than a three bucket one? And then think how the Lenz progression of turbines work. The less impedance to the airflow the more energy they seem to realize from it.
Go Huskers! |
| BushWhacker |
Posted - 03/03/2008 : 00:24:40
At no point did I say the pressure differentials were large. They are not.
It all works together like a choir. Each voice on it's own has little volume but a bunch of little voices working in sync becomes quite powerful. The same can be said about a savonius. All of those minute forces working in unison can add up to a fair bit of power. One should harness all of those little forces that are practicable.
One of those forces I forgot to mention is, the inside 1/2 of the upwind bucket is also directing more air into the down wind bucket as well. It all adds up.
I wonder if anyone has gone to the trouble of actually measuring the pressure differences between the upwind and downwind buckets?
For some technical aspects that go over my head with the formulas try this link which Hillbilly posted some time ago.
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/1976/760131.pdf
If you go to page 31 the 'meat and potatoes of 2 bucket turbines starts there.
Everything should be made as simple as possible, but not one bit simpler. - Albert Einstein |
| MattM |
Posted - 02/03/2008 : 20:18:24
We had a discussion about this a little time back on otherpower, the pressure is not very significant. Its only considered a drop in pressure because the velocity is increased, but the pressure is nearly identical between the slow and fast moving streams of air. The high and low pressure delta is much less than one would suspect, the suction in a straw when you take a drink is a hundred magnitudes of difference.
The 'bucket' fighting against the progress of wind is plowing air, but because atmospheric pressure is filling the bucket there is no partial void. The 'bucket' moving downwind is trying to follow with the wind. In this respect the latter is doing all the work. You do not want to impede airflow as much as ride the flow. They create the gap in the savonius because it allows air to flow through the structure, not because its filling a void. If they did not create this relief for the airflow it would simply find another route around the impediment and the design would be less efficient as a result.
Go Huskers! |
| BushWhacker |
Posted - 02/03/2008 : 19:51:46
quote:
Originally posted by MattM
But pressure in the drum is non-existent, what he needs is flow which is why the space in the middle is left open. Without flow through the device the air will re-route around the structure.
Go Huskers!
What makes wind is air filling a partial void. As land heats up the warm (less dense) air rises and cooler (denser) air flows in to fill the (ever so slight) partial vacuum left by the rising air.
With a Savonius turbine the bucket traveling downwind has (very slightly) higher air pressure in it than the bucket traveling up wind(think of the vacuum behind a semi trailer truck going down the highway).
(In this top view diagram (top left) the lower bucket is traveling downwind and the upper bucket is traveling upwind)
The overlap of the inside edges of a bucket (from 10% for low wind situations and up to 20% for high wind situations) allows some of the higher pressure air in the bucket traveling downwind to cross over to the lower pressure (partial vacuum) area in and behind the bucket traveling up wind (relieving some of the vacuum effect).
If one were to build a savonius without the overlap gap looking like the letter S from the top, it would still spin but less efficiently. (think of the wind driven spinning advertising signs)
Back to the original question... capping the ends of the bucket helps 'trap' the air in the downwind bucket rather than allowing it to freely flow past the ends of the bucket.
I hope this helps,
BW
Everything should be made as simple as possible, but not one bit simpler. - Albert Einstein |
| MattM |
Posted - 02/03/2008 : 18:43:55
But pressure in the drum is non-existent, what he needs is flow which is why the space in the middle is left open. Without flow through the device the air will re-route around the structure.
Go Huskers! |
| gotwind |
Posted - 02/03/2008 : 18:37:52
What BushWhacker says - I concur.
Ben. |
| BushWhacker |
Posted - 02/03/2008 : 10:33:23
quote:
Originally posted by sacongo
yello everyone, i have a question about a drum vawt and was wondering with the bottom and top of the drum removed will you get more or less rpm? I figure it would have more torque with them on but im interested if anyone every did any kind of test.
While I have not done any testing it would stand to reason that failing to 'enclose' the tops and bottoms of the buckets of a savonius turbine would reduce the compression of air within the buckets. (thats what makes it spin) In fact my readings on the topic suggest that one should provide a 10% diameter flange beyond the bucket. I would assume that this is to reduce the 'spillage' of air from the bucket to keep the air pressure slightly higher within the bucket.
quote:
About bearings which i know little about i was going to use a 2" flange bearing for the main shaft which the whole thing would be riding on but will it be able to take the side to side load of a 200lbs or would it just fall apart?
I'm inclined to think a self aligning trust bearing...
http://en.wikipedia.org/wiki/Thrust_bearing
or a tapered roller bearing...
http://en.wikipedia.org/wiki/Tapered_roller_bearing
would better suit your needs assuming you are capturing the VAWT within a frame of some sort.
My two bits,
BW
Everything should be made as simple as possible, but not one bit simpler. - Albert Einstein |
