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FAO: Gabriel- Giant generator windmills aerodynamics

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  • FAO: Gabriel- Giant generator windmills aerodynamics

    The question was sincere...

    Do those three thin blades really catch all the air they reasonably can?

    The old multi-blade water pump windmills would seem to really catch and harvest the energy from their disk/column much better.

    ....and are there aerodynamic implications to this that transfer between turboprops vs turbofans vs turbojets?

    Edit: Brian- I swear I was on Gen Aviation and not Aviation safety- please move or delete as needed.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  • #2
    Originally posted by 3WE View Post
    The question was sincere...

    Do those three thin blades really catch all the air they reasonably can?

    The old multi-blade water pump windmills would seem to really catch and harvest the energy from their disk/column much better.

    ....and are there aerodynamic implications to this that transfer between turboprops vs turbofans vs turbojets?
    Have you ever seen them up close? They are huge! We fly them to the U.S. from Holland and Germany. You can fit 3 or 4 or the blades end to end in a whole 74.

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    • #3
      I have passed many on the highway...80 or 100 feet long and maybe maybe 10 or 15 ft chords....

      So the outer arc would be something like 600 feet- but there's only 30 to 45 (3 times the chord) ft of blade so only 7% of the passing air is "covered" by a blade out towards the tips, thus over 90% passes.

      PS (I know there's some flaws there- but it illustrates my point that at first glance it would seem that lots of air sneaks by without encountering a blade to harvest energy...

      With an old-fashioned water pump windmill being a stark contrast with a turbojet like multi-multi-multi blade to catch and harvest "the whole entire disk" of passing air.
      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

      Comment


      • #4
        I know with airplane propellers, fewer blades are generally more efficient, although more blades can handle more power. So if some particular prop can translate 200 HP into propulsive force with x% efficiency, adding a blade to it would yield a prop that could translate more HP into propulsive force, but with less than x% efficiency. This is only a guess but I suspect it has to do with each blade's "wake" impinging on the following one.

        If the above applies to wind turbines too, that would mean you'll get more energy by deploying two three-bladed turbines vs. a single 6-bladed turbine. Of course that costs more, but maybe the increased efficiency offsets the cost in the long term?
        Be alert! America needs more lerts.

        Eric Law

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        • #5
          Wasn't my silence in the other forum enough?
          Just kidding

          I wrote a typical Gabrielian answer there and pressed "back" by accident, and I felt too troubled to do it again.

          So let me make it short this time:

          Yes: many blades of wide chords covering more area of the disk harvest a much larger fraction of the energy that flows through the disk.

          Now, why on Earth do you want to do that???
          You want to catch as much energy as possible. Why would you care about the energy that the air still has downwind of the windmill?

          Let's make a mental experiment:

          1- Take two identical wind generators, A and B, that have 3 blades 100ft long each (200ft diameter). The blades are not "optimized", but simple flat rectangular boards at an angle (very much like a ceiling fan).
          2- Put them side by side.
          3- Measure the energy each one is able to catch on one given wind condition. It will be the same for both windmills and let's say that it's 100 units of power.
          4- Take windmill A and add other 3 blades identical to the existing ones, so you've got a 6-blade rotor.
          5- Take windmill B and, take 3 other blades identical to the existing one, but add them at the tips of the existing blades to double their length.

          Obviously, the new A and B will generate more energy than the old ones. But will they generate the same increment?

          I think that by now the obvious answer is "no".

          There is a logical limit for how much the new A can generate. And that's the double than the old A: 200 units. It's the same disk than before, and you have doubled the blade area. But in fact it will produce less than that because of interference effect between the blades. The more blades you add, the more energy you "harvest", but the gain is smaller with each new blade.

          Now, for number B you haven't just doubled the blade area in the same disk, but you doubled the blade area in a disk that is double the diameter and 4 times the area than before.

          So, sure, maybe you'll harvest a smaller fraction of the energy of the air flowing through the disk, but that doesn't matter because the energy of the air that flowing through the disk is not something that you have to pay for (in windmill A, there is energy of the air flowing in a crown around the disk, the difference between the two disks, that you are not using either anyway).

          What does matter is that, with the same "material" usage (both have 6 of the original boards), windmill B produces much more energy.

          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
          --- Defend what you say with arguments, not by imposing your credentials ---

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          • #6
            Hmmmm.....

            I guess I'll have to trust "the hell better aeroengineers" that they run ratios of the cost of blades vs the cost of the generators + towers + the cost of wasted escaped wind since 90% of the air doesn't "see a blade" (which is a flawed estimate since you have to figure in blade speed versus wind speed.)

            Still seems like a lot of the disc is wasted- but I get it.

            Thanks.
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #7
              Originally posted by Gabriel View Post
              Wasn't my silence in the other forum enough?
              Just kidding
              I was concerned that your silence was because you were offended by the thread title where I was just kidding.
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment


              • #8
                Originally posted by BoeingBobby View Post
                Have you ever seen them up close? They are huge! We fly them to the U.S. from Holland and Germany. You can fit 3 or 4 or the blades end to end in a whole 74.
                I'm not sure they are as huge as the amount of money you charge to fly them.

                Amazing

                Thanks.
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                Comment


                • #9
                  Originally posted by 3WE View Post
                  Hmmmm.....

                  I guess I'll have to trust "the hell better aeroengineers" that they run ratios of the cost of blades vs the cost of the generators + towers + the cost of wasted escaped wind since 90% of the air doesn't "see a blade" (which is a flawed estimate since you have to figure in blade speed versus wind speed.)

                  Still seems like a lot of the disc is wasted- but I get it.
                  That's because a lot of the disc IS wasted!!!

                  The design input is never "take as much energy from the disk as possible".
                  It is "give me 1.21 gigawatt, Doc!".

                  I think that your flaw comes from what you think is "efficiency", but isn't.

                  Even leaving practical things at a side, like cost, engineering limiting factors, that we don't pay for the wind (so who cares if we use it inefficiently), etc.., the efficiency of the windmill to convert wind energy to mechanical energy (shaft power) is NOT the shaft power divided by the energy of the air that flows through the disc. It is shaft power divided by the energy that you EXTRACTED from the wind (which is the energy that the air flowing through the disc had before the disc minus what it still had after the disc).

                  They say that one image is worth a thousand words, so I guess that two images are worth two thousand words .

                  Key:
                  Pw1: power in the wind before the disc
                  Pw2: power in the wind after the disc
                  Psh: Power in the shaft




                  --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                  --- Defend what you say with arguments, not by imposing your credentials ---

                  Comment


                  • #10
                    The main reason for three blades on turbines ? You can't fit more on the hub.

                    If it 'ain't broken........ Don't try to mend it !

                    Comment


                    • #11
                      Originally posted by brianw999 View Post
                      The main reason for three blades on turbines ? You can't fit more on the hub.

                      http://www.rspb.org.uk/Images/wind_t...=488&crop=auto
                      Not sure if you are serious or if it was a joke.
                      Rather, the size of the hub and blade roots of this generator are the size they are because it has three blades!

                      Aerodynamically speaking, single-bladed is the most efficient design, with the efficiency diminishing as you add blades.

                      Of course, to generate a given amount of power, you need longer blades the fewer they are. So adding more blades reduces the size of the structure, particularly the mast, and also the load on said mast: With one blade, not only the mast is taller, but it needs to be stronger because the flexing moment in its root is higher too. So the structure is bigger, heavier and, of course, more expensive.

                      Another issue is dynamic stability. Any number of blades equal or greater than 3 are is dynamically stable. I could talk about the inertia tensor (the 3x3 matrix that defines the moment of inertia around any axis) and show how, from 3 blades on, the shaft is a principal axis of inertia, the one with higher moment of inertia, and any axis perpendicular to the shaft is also a principal axis of inertia, all of them of minimum moment of inertia. Or, you can use your imagination and realize that 2 blades don't define a disk by themselves until they are rotating (the prop is basically just one long pole) while 3 or more blades geometrically define the plane that will be the disc. It's the same reason why a straight boomerang would not work, they need either angled two wings or three or more wings so the plane of rotation is defined even before the thing starts to spin. That doesn't mean that 2-blades propellers don't work. Just look at most small planes and helicopters. But they has their drawbacks. Replacing 2-blades props with 3-blades prop has been known since long ago as a way to reduce prop vibrations.

                      So 3 blades looks like a good balance between between "blades as long as possible" (= "as few as possible") and "structure as simple and durable as possible".

                      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                      --- Defend what you say with arguments, not by imposing your credentials ---

                      Comment


                      • #12
                        Originally posted by 3WE View Post
                        ....and are there aerodynamic implications to this that transfer between turboprops vs turbofans vs turbojets?
                        Tubojets? I'll take that you meant "high-bypass turbofans".

                        It's the same thing.
                        You want to keep'em as long as possible, as long as possible.

                        There is a limit of thrust that you can obtain for each sq inch of blade, because the lift coefficient has a limit. Try to extract more, and the blade will stall in the same way like a wing stalls when you try to get from it more lift that it can give.

                        So to get the thrust you want you need a minimum blade area.

                        You can use a combination of length of blades, chord of blades, number of blades per disc and even number of discs (coaxial props) to get the needed blade area.

                        The longer (more diameter) the better, but you need to keep tips below the sonic speed and, at the same time, you want the blades to fly "sideways" (turn) at a good speed compared to the plane's speed (to have the blades lifting more "forward" than "sideways"), which again conspires with the speed of sound limit.

                        So the reason why blades of the fan in the GE GenX in the 777 have a lot of blades of a wide chord is not because it's very efficient to do so. It's because they can't make the blades longer and they need all that blade area to handle the thrust. And because other means to produce this much thrust (like a turbojet) are even more inefficient. You could probably make it more efficient if the plane was slower, and hence you could make the blades longer.

                        The reason why a small GE plane has 2 or 3 slender blades is because it's enough to handle the thrust. As you see more horsepower involved, you tend to see that the blade chord and number of blades clearly increase, but no so much the diameter.









                        And do you like this 16-blades prop?


                        I wonder if some day we'll see one engine moving many parallel-axis fans, each of a manageable diameter. That would be a way to involve more air mimicking a larger diameter but without the problems of the blade tip speeds.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

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                        • #13
                          The short answer is that people who went to school for a while have already thought about this at length.

                          Comment


                          • #14
                            Originally posted by Gabriel View Post
                            And do you like this 16-blades prop?
                            I've just realized that it's not 16 but 14 blades. The front disc has 8 and the aft disc has 6.

                            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                            --- Defend what you say with arguments, not by imposing your credentials ---

                            Comment


                            • #15
                              Originally posted by Gabriel View Post
                              Not sure if you are serious or if it was a joke.
                              Rather, the size of the hub and blade roots of this generator are the size they are because it has three blades!.........
                              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 !
                              If it 'ain't broken........ Don't try to mend it !

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