FAO: Gabriel- Giant generator windmills aerodynamics

[Gabriel]

My idea of a joke.

One thought that springs to my non-engineers mind though is this.

Would a large diameter three bladed windmill, once it has got going in the wind "generate" a lot of inertia making it difficult/slow to stop when the wind drops so that it continues to generate power even when not being driven by the wind. Of course, it will eventually stop if the wind does not pick up again but not as quickly as a smaller multi bladed generator blade unit. ??

Discuss...............preferably in non-engineer English !

Yes, but at the same time the same inertia will make it slower to speed-up at the beginning.

Was this English non-engineer enough?

[brianw999]

Yup..... Perfectly understated to allow this dumb - ass to understand !

[Highkeas]

................ it would seem that lots of air sneaks by without encountering a blade to harvest energy...
.........................................

I have attended AIAA conferences on wind generators plus I also lived in the Coachella Valley downwind of a large wind farm and the Banning Pass (Not sure is this reference works but it might show the hundreds of wind generarors in the area https://www.google.com/maps/@33.9002...!3m1!1e3?hl=en)

Ideally wind generators are built in rows so the wind passing by the first row is picked up by the second row, and so on.

Blades are subject to damage by lightning and bird strikes so are built low cost (usually fiberglass) so they can be replaced. Blades are designed using aerodynamic principles.

This is far from my technical area but you can find papers and books on their design.

[3WE]

Tubojets? I'll take that you meant "high-bypass.

Turbojets was there deliberately

Let's blow tons of energy out the back, efficiency = crummy

Bring on the 1950s. Let's make some extra turbine and harvest energy and bypass the intake....efficiency gained...

Bring on the 70s and 80s bigger and bigger bypasses to save jet fuel

But I think a turboprop still wins the fuel efficiency battle.

Is some of that because the 10 zillion blades and vanes and ducts have so much more parasite drag?????

Or is it only because of the really fast airspeed ?

[Gabriel]

But I think a turboprop still wins the fuel efficiency battle.

Yes, they do, at equal powers and speeds. Now try to find a more efficient way than a turbofan to propel a 777 at M 0.85.

Is some of that because the 10 zillion blades and vanes and ducts have so much more parasite drag?????

Or is it only because of the really fast airspeed ?

In fact, no. It's because of the induced drag of the blades. Same reason why a wing of half the span and twice the chord has the capability to make the same lift but at much more induced (not parasite) drag.

However, the 777 fan is so large that I doubt that you'd manage to make a propeller of more diameter and, if the diameter is the same, the blade area must be the same to handle the same thrust.

In fact, what is the difference between a very-high bypass turbofan and a turboprop? Both make most of the thrust from the prop/fan and a bit from the exhaust gases.

I remember in the 80's GE experimenting with an MD-80 and a B-727 with a UHB/UDF (Ultra High Bypass / UnDucted Fan).

It got great fuel burn efficiencies, but:
- Noise was a big problem.
- Airspeed was limited (in the ducted fans, the cowling works as a difusor: The diameter of the intake is smaller than the diameter of the fan, so the air slows down to a manageable speed for the blade).
- What was "ultra-high bypass" back then in the 80's was "much more bypass than in the then current turbofans". I don't know if these UHB/UDF have much more bypass than today current high-bypass turbofans.
- Any blade failure is an uncontained engine failure (as it is the case with all turboprops).

[3WE]

In fact, what is the difference between a very-high bypass turbofan and a turboprop?

Numerous blades in a smaller disk versus a few blades in a bigger disk, and the prop is likely more fuel efficient....

Which is sort of what the question was about....

And it's not trying to say one is right....it's understanding why there are the differences.

I get the windmill.... Area ~ to blade length ^ 2 so theres big material efficiency counteracting the efficiency of harvesting more of the disk.

Ducted multiple blade fan probably has some sort of speed efficiency.

[Gabriel]

Numerous blades in a smaller disk versus a few blades in a bigger disk, and the prop is likely more fuel efficient....

Say that you want to make a 777 tuboprop. Do you think that you could fit props bigger than the fan disc the 777 already has?

The number of blades is not a matter of turbofan vs turboprop. It's the blade area you need to produce the design thrust. If you want to extract too much lift/thrust from too little wing/blade area, the wing/blade will stall.

If you want to extract as much thrust from a turboprop as from the fan of the 777, you'll need a similar blade area in the shape of length, chord, or number (in the same disc or in tandem discs), and I doubt you could go to the length way so much so you'd need to use one of the others and the efficiency would not improve, at least not because of the disc size that would be the same.

The MD-80 GE UHB/UDF of the photo above has a big size, maybe comparable with a 777 fan, but much less power than a 777 engine, so it can make the job with less blade area and hence fewer blades (it still has 16, which is not a small number I'd say).

I get the windmill.... Area ~ to blade length ^ 2 so theres big material efficiency counteracting the efficiency of harvesting more of the disk.

No, you don't get it. "harvesting more of the disc" is not an efficiency. Please go back and read my post #9 (more carefully this time!) where I said "I think that your flaw comes from what you think is "efficiency", but isn't." (which I still think is the case) and then used a good bunch of time to make some sketches to explain it with not so many words. Take a special look to the last sketch there.

Ducted multiple blade fan probably has some sort of speed efficiency.

Don't know what you mean here.

[elaw]

Ducted multiple blade fan probably has some sort of speed efficiency.

Don't know what you mean here.

I think he's asking whether a ducted fan is more efficient than an unducted one, and I think in general the answer is "yes".

Propellers (and presumably wind turbines are the same) suffer from the same losses of efficiency due to spanwise flow that wings do. With a wing, you can get an improvement by adding winglets but I think there are practical barriers to doing such a thing on a propeller - I suspect it relates to adding weight at the blade tips where centrifugal force is greatest, so the blade would require much more strength.

Anyhow, I think placing the "fan" in a duct reduces the spanwise flow and thus increases efficiency.

[Gabriel]

I think he's asking whether a ducted fan is more efficient than an unducted one, and I think in general the answer is "yes".

Propellers (and presumably wind turbines are the same) suffer from the same losses of efficiency due to spanwise flow that wings do. With a wing, you can get an improvement by adding winglets but I think there are practical barriers to doing such a thing on a propeller - I suspect it relates to adding weight at the blade tips where centrifugal force is greatest, so the blade would require much more strength.

Anyhow, I think placing the "fan" in a duct reduces the spanwise flow and thus increases efficiency.

That's correct. Of course the "duct" adds a lot of drag too, so much "wet" area (bathed outside and inside)

[3WE]

That's correct. Of course the "duct" adds a lot of drag too, so much "wet" area (bathed outside and inside)

Perhaps helping a turboprop to be more fuel efficient....

[3WE]

No, you don't get it. "harvesting more of the disc" is not an efficiency. Please go back and read my post #9 (more carefully this time!) where I said "I think that your flaw comes from what you think is "efficiency".

Efficiency means all sorts of things.

Your analogy of four 2 by 12 boards (for two 16 ft blades vs four 8 ft blades) is a ton more efficient for disk area per board ft..... And since it whirls it actually does harvest more of the disk than just 12" of chord.

The old aero motor windmill probably grabs more torque per unit area and maybe more horsepower per unit disk area.

As to the efficiency of this vs that vs cost efficiency vs maintenance vs time vs kilometers per pound of jet fuel.......

Anyway, your 2 by 12 analogy was glaringly big... If I missed a detail- I'll go back.

This thread also has the risk of becoming circular (oh the ironing).

But if we can call out bigger things that tip the scales between multi turbines and 2 to 5 blades.

Per's boats are generally the most fuel and cost efficient way to move tonnage across the ocean. The Concorde was more time efficient.

[Gabriel]

Efficiency means all sorts of things.

Yes, but not any thing.

In the windmill, the air is the source of energy, so it's ok to use the energy removed from the air as a denominator in an efficiency index, but it's the energy removed from the air, not the energy available in the air. The energy that was not used to begin with doesn't count, can't count.

It's like saying that car A is more efficient than car B because, in identical trips, used 50% of the fuel in the tank while B used 75%, not considering that the tank of B was smaller. (I'm not saying that this is the same than the windmill, but it's the same misuse of the word efficiency).

That's why I said, look the last sketch of that post. Oh, wait, let me re-post it here:

[3WE]

...it's ok to use the energy removed from the air as a denominator in an efficiency index, but it's the energy removed from the air, not the energy available in the air. The energy that was not used to begin with doesn't count...

I have no degree whatsoever in Aeronautical engineering.

But I promise you that I can design a windmill that extracts only the tinyest amount of energy from the passing wind, and that wastes the vast majority of available energy. Maybe I'll have 10 zilloin blades angled one way and 10 zillion and 10 angled the other way so 20 zillion fight each other, and only 10 left over "to provide torque"....that ought to do it!

EDIT: I have a new design: Two blades. ~100 feet long (just like the big generators)...Perfect planes/no-twist, with a 20-ft chord. I am going to cock them 0.1 degree off (opposite directions of course) from being perfectly feathered into the wind...Yeah, they will catch some wind and rotate....VERY slowly. This sucker is designed to allow the vast majority of wind to pass without being affected with any blade interaction. This design is very inefficient at harvest energy from it's disk because it lets so much of it pass- it really needs some better input from an aeronautical engineer so it can catch at least SOME of the passing wind...

...the ratio of what it catches versus the total of what's available (or the ratio of what it catches versus what it lets pass) has some meaning!

[/Edit]

I'm sorry, but my windmill would have low effeciency of harvesting available energy and you can't tell me I can't use "available energy" as the demonimator.

I really think they want to hire you to design a windmill that has a higher effeciency of harvesting energy from the available energy. (Yes, you will confer with a structural engineer and maybe even a finance guy, since cost effeciency is part of the equation too).

I have re-read your blade deal again...The take away I still get is that double-long blades work on a 4 times bigger area than double number blades (using your analogy of four boards). That markedy increases the effeciency of grabbing energy (over several different denominators, no less). Of course, it may be less effecient at gathering avalable energy- as you yourself said earlier that the few blades is going to waste more compared to the aeromotor windmill...

As Left Seat said- someone runs ALL the numbers and determines what set up is the most effecient (with a heavy cost emphasis)- but you don't want a super wastefull windmill.

That doesn't mean that there's little bits and pieces where effeciency is gained and lost (and with any number of logical denominators).

And I'm not saying that the big windmills are ineffecient- again- I'm breaking it down into bits and pieces and that there is a difference in the "harvest effeciency" of an aeromotor versus a modern generator.

The Concorde is probably the most time effecient way to get from Paris to New York. An ATR 72 is probably a lot more fuel effecient...and no, it's not all due to the arrangement of the propellors/turbine blades...but the Concorde'' still going to have more intake duct drag...Of course, a 787 has other ways that it's effecient.

[3WE]

By the way- I half suspect that in 1920 or so, Bubba and Goober took ag mechanization and never went on to airoknoticks at collage and just figgered you oughta have a bunch of blades to be sure you catch all that passing wind...

And lookey thar- it shore does pump water!

Let's patent it and call it Aeromotor!

[Gabriel]

I have no degree whatsoever in Aeronautical engineering.

But I promise you that I can design a windmill that extracts only the tinyest amount of energy from the passing wind, and that wastes the vast majority of available energy. Maybe I'll have 10 zilloin blades angled one way and 10 zillion and 10 angled the other way so 20 zillion fight each other, and only 10 left over "to provide torque"....that ought to do it!

I'm sorry, but my windmill would have low effeciency of harvesting available energy and you can't tell me I can't use "available energy" as the demonimator.

No, hu? Just watch me:
You can't use "available energy" as the denominator.
Ha! I did.

Yes, your windmill will be a piece of crap, but not because of the available energy, but because of the energy that you actually EXTRACTED, REMOVED from the wind (you slowed it down) by making lift in the blades in opposite directions, lift that had it share of induced drag, and lift that you used for nothing useful.

So you TOOK a lot of energy from the wind, and made almost nothing with it, and that's a very poor efficiency because I have small numerator (whatever you want to use to measure usefulness) and a very big denominator, which is the energy you TOOK from the wind, not the energy that was available, you didn't touch it, and it's still available for another windmill downwind.

I really think they want to hire you to design a windmill that has a higher effeciency of harvesting energy from the available energy. (Yes, you will confer with a structural engineer and maybe even a finance guy, since cost effeciency is part of the equation too).

Sorry for confusing you. No!!! I never meant to say "engineering efficiency". The engineering benefits are "just" a plus. Longer blades = more energy efficiency even if you had no engineering constrains.

Let me paraphrase you:
I promise you that I can design a windmill that extracts only the tinyest amount of energy from the wind that flows through a given disk and makes more mechanical energy that ANY other widmill design of whatever disk size smaller than mine that extracts the same amount of energy from their disk, EVEN IF they extract that same energy from a much smaller disk and are hence much more "efficient" at harvesting energy from the wind.

I have re-read your blade deal again...The take away I still get is that double-long blades work on a 4 times bigger area than double number blades (using your analogy of four boards). That markedy increases the effeciency of grabbing energy (over several different denominators, no less). Of course, it may be less effecient at gathering avalable energy- as you yourself said earlier that the few blades is going to waste more compared to the aeromotor windmill...

"Not use" is not the same than "waste". "Waste" is "use and extract no value".

It's like if you have fuel flowing through a big pipe, have a small derivation from that pipe that feeds an engine (which is the fuel that the engine consumes), the engine gives you some HP, and then you make HP/(fuel flowing through the big pipe) as measure of efficiency.

No! The fuel flowing through the pipe that you don't grab and let go can be used for another engine down the pipe. You can't use the fuel available in the denominator. You must divide the HP by the fuel EXTRACTED from the big pipe to feed the engine. (Note: in this analogy, "fuel" is analogous to "energy in the wind", not to "air". The fuel has energy just for being fuel, that the air lacks).

And sorry, no matter how you want to disguise it and how much you don't like absolute statements that are absolutely always absolutely wrong:

In the windmill, the air is the source of energy, so it's ok to use the energy removed from the air as a denominator in an efficiency index, but it's the energy removed from the air, not the energy available in the air. The energy that was not used to begin with doesn't count, can't count.