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January 7, 2005, 13:21 
A Basic Question on ducted Flows!

#1 
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Hi
Here's a question I've been puzzling over today. If you place a concrete contracting duct on the sea bed which houses a water turbine within the contraction, what will be the primary limiting factor which dictates the maximum velocity which can be achieved through this duct, and hence the power which can be captured. Flow can obviously pass around the outside of the duct. There must be an upper limit, where the speed increases so much that the boundary layer becomes extremely large and the flow becomes backed up. Is there some kind of rule of thumb for working out this optimum? Any answers are welcome. Many Thanks CFD User 

January 7, 2005, 16:43 
Re: A Basic Question on ducted Flows!

#2 
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What is your primary energy source? Sorry for the simplistic question, but your description of the setup is not clear to me. Generally speaking the upper limit is going to be the maximum available energy, for example the potential energy of a waterfall through a hydroelectric turbine places an absolute maximum on the power that can be extracted.


January 7, 2005, 17:07 
Re: A Basic Question on ducted Flows!

#3 
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Sorry if I wasn't clear. The energy source is a tidal stream, i.e. there is no potential head it's all dynamic. What I'm trying to figure out is the maximum diameter ratio for my duct. Obviously the higher the ratio the higher the flow rate through the duct, but there must be an upper limit for duct which is placed within a tidal stream of fixed velocity.
Hopefully, I've explained my problem a bit more clearly. Thanks CFD User 

January 7, 2005, 17:51 
Re: A Basic Question on ducted Flows!

#4 
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I got it now  sorry about the confusion, it's been a hectic afternoon. I'll have to think about that a little. It's similar to the problem of an aircraft engine inlet, where you want to maximize the mass flow and minimize the spillage around the inlet, but it's been a while since I looked at such problems. But for reference you might try looking up references on inlet design for airbreathing engines, since the problem you're describing appears to be very similar.


January 8, 2005, 07:17 
Re: A Basic Question on ducted Flows!

#5 
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Yep, you've got it. I have a feeling that above some ratio like 2:1 water spillage will occur and the velocity through the duct will stop increasing. I'll defiantly have a search around for some ref's on inlet design. Let me know if you have any more ideas.
Thanks CFD User 

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