Venturi Tube Wind Turbine Theory Discussion

[Generator]

As heading say, I'd like to discuss about Venturi tube and how it works in terms of power extraction for wind turbine. I haven't done any DIY experiments on this, so if this is the issue, then I understand it completely.

They say if wind speed increases then power increases by cube of velocity. So considering a Venturi tube with inlet diameter of 10 meter square and neck of 1 meter square with its length of about 1 meter at neck or maybe more. Here at blades wind speed is increased 10 times than inlet velocity and at exit, it might be 1 m/s or 1.5 m/s or 3 m/s (not sure, but they say 1/3 speed at back of propeller gives best output).

So in this case how much energy we can extract from the given setup as wind speed hitting blades is very high than inlet speed? Trying to understand the math before I build the model. I know inlet and outlet speed will be deciding factor but how we calculate those numbers?

Attaching 2 figures for visualization, we can add multiple propellers, if required.

[DamonHD]

I think it is fair to say that this has been done to death.

No one gets a special pass on the laws of physics because they didn't enjoy science at school or whatever, to be blunt.

If this really worked lots of money would be being thrown at it in production wind farms, given that it's not a new idea AFAIK.

Rgds

Damon

[SparWeb]

It's not a new idea. 
To be generous to you Generator, even university prof's get duped by this idea.  In a recent case, one fellow blew some university's wad of money doing wind-tunnel tests to prove that it actually gets a small increase - - until some smart soul pointed out that the amount of material required to build such a thing is tremendously expensive compared to making the turbine a bit bigger and accomplishing the same result.  I'm sure there were some red faces coming out of the provost's office that day.

[Mary B]

I remember the Popular Mechanics article where they setup rows of these on top of buildings facing prevailing winds...that article came out, bragged it up then ... SILENCE... A follow up article came out 3 years later saying it failed and the vibration just about destroyed the roof of the building. It was a paragraph buried in the back of the magazine in the middle of the ads...

[Generator]

Thank you all for the response. I know it doesn't work as I see many Windmill companies shutdown after implementing this idea.

My point was how we evaluate this design in Mathematical model. Is there any way to do it?

[Adriaan Kragten]

I have given comment on this idea earlier. The point is that increase of the mass flow is only caused by the part of the duct behind the rotor. It is useless to make a cone in front of the rotor and expect that the wind will go into this cone. The resistance of this cone will simply make that the wind will go around the whole structure. The part of the duct on front of the rotor must be small be must have a rounded nose to prevent that the flow loses contact with both the inner and the outer side of the duct. The part of the duct behind the rotor must be very big (about three times the rotor diameter) and this makes the whole duct very expensive. The big diameter at the end of the duct makes that an under pressure is created at this point and this under pressure sucks more air through the rotor. Testing of this idea in a wind tunnel can give much too optimistic results as a large duct causes tunnel blockage. So it must be tested in open air. It has been tried several times in the past and increase of the mass flow is possible for a big duct but is fails economically.

[Generator]

Thank you Very much Adrian for your insights.

So does that mean if the duct opening behind rotor, where air exits should be bigger than entry point, 3 times bigger? Then we will get better results?

In another word, if we ignore the economic aspect, would this wind turbine harness more power [within Betz limit]?

What happens when the inlet cone is 3 times bigger than outlet cone? [Would it perform better?]

Again, this is for my understanding to know what is possible technically (we are not talking about economical aspects at all).

I have given comment on this idea earlier. The point is that increase of the mass flow is only caused by the part of the duct behind the rotor. It is useless to make a cone in front of the rotor and expect that the wind will go into this cone. The resistance of this cone will simply make that the wind will go around the whole structure. The part of the duct on front of the rotor must be small be must have a rounded nose to prevent that the flow loses contact with both the inner and the outer side of the duct. The part of the duct behind the rotor must be very big (about three times the rotor diameter) and this makes the whole duct very expensive. The big diameter at the end of the duct makes that an under pressure is created at this point and this under pressure sucks more air through the rotor. Testing of this idea in a wind tunnel can give much too optimistic results as a large duct causes tunnel blockage. So it must be tested in open air. It has been tried several times in the past and increase of the mass flow is possible for a big duct but is fails economically.

[SparWeb]

My point was how we evaluate this design in Mathematical model. Is there any way to do it?

Sure.  You can model anything you want.  There were several scientific papers published in the 1970's that give a procedure to model a wind turbine's performance in great detail.  Fill yer boots.

[Adriaan Kragten]

Thank you Very much Adrian for your insights.

So does that mean if the duct opening behind rotor, where air exits should be bigger than entry point, 3 times bigger? Then we will get better results?

In another word, if we ignore the economic aspect, would this wind turbine harness more power [within Betz limit]?

[/quote]

The Betz limit is only valid for a wind turbine in open air and is the ratio in between the maximum possible power of the rotor and the power in undisturbed wind flowing through an area identical to the swept rotor area (see KD 35 page 15). If you have a duct with a good geometry, you can get a Cp value which is much higher than the Betz coefficient but only if you compare the generated power with the power in the undisturbed wind which is flowing through an area identical to the swept rotor area. But if you compare it with the power in the wind which is flowing through the cross sectional area of the duct at the maximum diameter, the Cp will be much lower than the Betz-coefficient.

Forget the idea of making only a cone in front of the rotor. The wind will simply go around the structure. Outside this duct the wind speed will reduce and this reduction of the wind speed outside the duct results in increase of the pressure. So at the rotor you get a higher pressure than the pressure in the undisturbed air and this pressure blocks an air flow through the rotor.

In November 2015 I have written report KD 600: "Diffusers for wind turbines" because at that moment a Dutch company was developing a wind turbine with a diffusor on the top of a building. This report wasn't made public because, to my opinion, development of diffusors is a waste of time. But if you send me a personal e-mail, I will send you a copy of this report and this may help you to get a better understanding. But that's the only thing you will get from me; I don't want to discuss this subject privately.
 

[kitestrings]

Generator,

Just to offer one additional thought -

On the one hand you are suggesting to ignore, for the moment, the "economical aspects"; let's look at the technical merits.  This is a not a bad approach.  Often times the costs of new things is very high, but over  time, with refinement and market influences, the price comes down.

Let's look then at the technical, and the practical aspects of what you are proposing.  We need a turbine that, if properly sited, is robust enough to withstand ferocious winds at times.  For all the time we spend looking to squeak out slightly more efficient blade profiles, or mechanical or electrical efficiency improvement, in the end we have days where we just want the thing to survive to the next storm.  So, now consider a turbine with a (2x to 3x diameter) cone structure - up-wind, down-wind, or both - and consider how much harder that task has just become.

I tend to over-simplify, but to me the proposal fails not due to economics, or theory; it fails because it is impractical.  ~ks

[Mary B]

Where I live that structure would have to survive 95mph gusts and sustained winds of 75mph. The forces are immense at those wind speeds! I have 10 square feet of antenna load at the top of one tower, in 75mph winds the forces at the bottom can exceed 50,000 pounds of down force or lifting force(upwind side lifts, downwind side compresses)