airfoil chord length for 6 bladed 2x3m VAWT turbine

[brandnewb]

I am designing a VAWT turbine with 6 blades. 3 x straight and 3 x helix.
The turbine it self will be 2 meters diameter and 3 meter high and stack-able to make it 2x6 or 2x9m etc etc

I have traced a supposedly optimized airfoil from this source
https://royalsocietypublishing.org/doi/10.1098/rsos.180540

I have attached an image of the turbine design.

Can someone please redirect me to a good source where I can learn how to scale the airfoil?
Suggestions regarding the turbine it self are also welcome

[taylorp035]

First of all, that's a really cool report.  We use ModeFrontier at work for optimization.

I don't understand the question for scaling the airfoil.  I would just measure it at a bunch of places and scale it proportionally to the size you want.

How do you plan on manufacturing the blades?  There is a thread on here that is fairly active that has gone through a few iterations of building the airfoils with some great pictures.

[brandnewb]

I meant to ask what chord length should I make the airfoils if the turbine it self has a diameter of 2 meter and a height of 3 meter.

I can not find the relationship between chord length and turbine size anywhere

[brandnewb]

During prototyping I plan on 3d printing the blades. And if I fail to print them strong and light enough I will go the fiber glass route.

[SparWeb]

Sadly, the authors ignored the body of work that defines the NACA airfoils.  Sigh.  Months in the lab could have been saved by a day in the library.
Genetic algorithm?  Get real.  The thermodynamics and fluid mechanics of air were well enough understood to produce a simple airfoil specification in the 1930's.

The authors compare a 15% thick airfoil with no camber to a 16.65% thick airfoil with camber, but they could easily have generated the NACA specification of their new airfoil if they'd just bothered to understand the NACA airfoil specification.  If they are going to build a fluid dynamics model in the computer, then they can use any NACA specification they want: "NACA2417" fits their arbitrary new shape rather well and it is as easy to plot as the NACA0015.  Once they have the new NACA spec, they can use the NACA fluid models to make predictions of airfoil performance (lift curve slope, angle of zero lift, etc.)

The chord isn't going to be pulled out of the air.  If it's too thin, the blade is so flexible that it ripples.  If it's too wide, the blades are very heavy and the supporting structure blocks the wind.  You'll have to work out the range that is acceptable, and stay away from extremes that are likely to fail.

[brandnewb]

OK, thanks for the heads up. I'll pick anoter airfoil then. Suggestions welcome.

But then the question still stands. How do I scale airfoils to the size of the turbine they will be used in?

[Adriaan Kragten]

In my public report KD 601, I have derived formula 4 with which you can calculate the chord for a certain rotor diameter, a certain number of blades and a certain airfoil for a H-Darrieus rotor. This report can be copied for free from my website www.kdwindturbines.nl at the menu KD-reports.

A point for Darrieus rotors is that you need a symmetrical airfoil because the angle of attack is positive if the blade is at the front side of the rotor and negative if the blade is a the back side. Such airfoils are sensible to stalling at low Reynolds values (see figure 1 KD 601). The number of blades must therefore be as small as possible. However, a 2-bladed Darrieus rotor will shake terribly and therefore three blades is the minimum. But if you take six blades, the chord will be half as for three blades and then the Reynolds number will only be large enough if the rotor has a very large diameter or if the wind speed is very high. The result of my calculations is that the chord must be about 0.2 m for a 3-bladed rotor with a diameter of 3 m and that the optimum tip speed ratio is about 4.2.

[brandnewb]

thank you for your contribution. But your paper kd-601 is too difficult for me to understand as I lack even basic math skills. Would you mind telling me what airfoil chord length I should use when the diameter is 2m? Local regulations do not allow a larger diameter.

[brandnewb]

Some more details to get to a chord length are;
planned rpm: 60
C = 6,28319m

makes for tip travel distance in 1 min = 376,9914m

I really tried but I can't get through your report. please advice

[Adriaan Kragten]

If KD 601 is too difficult, you should study first report KD 35 about the aerodynamic theory of VAWT's and answer the questions about each chapter as given in report KD 196. I will never say that designing a wind turbine is easy but KD 35 describes the theory in the most simple way and gives some examples of the calculation of the blade geometry. Formula 4 of the chord c of a H-Darrieus rotor as given in KD 601 is derived from the theory of HAWT's but it is taken into account that the lift coefficient varies during one revolution. To understand that the angle of attack and so the lift coefficient varies, you should study figure 2 of KD 601 in which I give the speed diagrams for twelve positions of the blade assuming that the tip speed ratio is 4.2 and that wind speed in the rotor plane is 2/3 V for every position.

[Mary B]

I remember seeing a flat bottomed airfoil used by wind gen kits(now defunct) that made mounting super easy. Would be easy to 3d print in sections that get glued over a spar... He claimed 500+ watts into a battery load and had good reviews. It really was a nice kit of parts with laser cut spars and mounting blocks for the airfoils.

These have also been tested and have some real world data(not snake oil data) http://www.thebackshed.com/windmill/Trade/AlBladeOrders.asp (yes I know, TheBackShed is a competitor for here)

[Bruce S]

(yes I know, TheBackShed is a competitor for here)

Naa, they're all good , most of them are fellow posters here :-). GHURD still posts there and here.

I even still have a login there.

Cheers
Bruce S

[taylorp035]

I have serious doubts you will be able to 3D print something this large and it be stiff enough without making the blades too heavy.  If money is less of an issue, a $500-$1500 would get you enough carbon fiber and glue to make some foam filled airfoils.  I've done a lot of C.F. work myself, but I would advise a new person to practice on some smaller samples before spending all the money or wasting the fabric and glue.

Otherwise, it's going to need a highly stressed skin design out of thin wood or metal in my opinion.  You might be able to get away with 3D printing the air foil cross sections for the supporting structure.

I'm guessing these airfoils are going to end up being ~8" long or so.  Less than 5-6" and it's going to be too floppy.  Longer than 12" and it's going to get heavy real fast and some serious engineering (on top of what you're already going to want to do) is going to be necessary.


Is there a particular reason why you want both straight and helix shaped blades? 

[Mary B]

I have serious doubts you will be able to 3D print something this large and it be stiff enough without making the blades too heavy.  If money is less of an issue, a $500-$1500 would get you enough carbon fiber and glue to make some foam filled airfoils.  I've done a lot of C.F. work myself, but I would advise a new person to practice on some smaller samples before spending all the money or wasting the fabric and glue.

Otherwise, it's going to need a highly stressed skin design out of thin wood or metal in my opinion.  You might be able to get away with 3D printing the air foil cross sections for the supporting structure.

I'm guessing these airfoils are going to end up being ~8" long or so.  Less than 5-6" and it's going to be too floppy.  Longer than 12" and it's going to get heavy real fast and some serious engineering (on top of what you're already going to want to do) is going to be necessary.


Is there a particular reason why you want both straight and helix shaped blades?

I would think in the 2 meter range you could print in sections to go over a central spar as a stiffener. with 2 levels of support from the central mast they should be plenty stiff

[brandnewb]

Is there a particular reason why you want both straight and helix shaped blades?

If I can believe what is stated in this abstract:
https://link.springer.com/chapter/10.1007/978-3-030-63591-6_35
Then having the helix chambered airfoils will help auto start in low wind speed scenario's and having straight symmetric airfoils will help increase torque which I am going to be needing plenty of.

I have not bought the full paper as I think it is way over priced so I might well be drawing the wrong conclusions.

As for 3d printing. I still think plenty stiff and light is doable if done correctly. I think I know how. And if I fail then I can always print molds

[Astro]

Is there a particular reason why you want both straight and helix shaped blades?

If I can believe what is stated in this abstract:
https://link.springer.com/chapter/10.1007/978-3-030-63591-6_35
Then having the helix chambered airfoils will help auto start in low wind speed scenario's and having straight symmetric airfoils will help increase torque which I am going to be needing plenty of.

I have not bought the full paper as I think it is way over priced so I might well be drawing the wrong conclusions.

As for 3d printing. I still think plenty stiff and light is doable if done correctly. I think I know how. And if I fail then I can always print molds

 In my research, I think you are correct. The helix seems to start at lower speeds. But it is not really doing anything or producing anything. With the blades being not as wide, I think you are also right that torque would be reduced. It is my thought and I may be wrong, that the helix and egg beater style blades would be great for a small turbine. Otherwise the project is going to become massive. I do not recall if you said what kind of output you are looking for. Just a guess, but if it were me, I do not think I would try the helix or egg beater style for anything over 3- 500 watts or so. They rely on speed and very little torque. So that when they hit charging (voltage) speed, they are already turning fast enough to overcome a light load (to much load and it will stall easy). I think the big corp and government funded ones, most likely use a gear box or some sort or transmission to allow them to spin slower. but you are going to have losses there and maybe that is why they are 10's of feet tall. A gear box on that style of mill for home use does not seem practical, because again, you are going to need some really tall (long) blades to overcome the gearing. So it seems to me that for home use, you would want it to just spin really fast and have a pretty high cut in speed (where it hits charging voltage).
So yeah if you want to make a mill around the 300 watt range, I think they would work well. Beyond that, I think you are going to need to harvest more wind then those blades allow.

[brandnewb]

Thank you for your input/

But No. 60 -120 RPM is tha max I dare to have turbines spinning near wildlife..... (i.e my family)

This has got to be cracked this code. This is why I combine helix and straight. One for start up and one for torque.

I really am not looking for optimal output. I am looking for something that is safe and output something worthwhile.

[brandnewb]

Also please remember that the turbines will need to become stack-able. So 2x3 as base or 2x6, 2x9 etc etc. This is going to become one syco beast if I ever get it built

[brandnewb]

Depending on local code regulations one could take this concept and make an even wider turbine. 3x3 to start with. 3x6,9 etc.

WOW. sooner or later these things are going to generate something insane whether or not we believe it or not yet

[brandnewb]

I meant if one makes them large enough and/or stacks up enough of them then physics dictate that something useful will happen

[brandnewb]

I have serious doubts you will be able to 3D print something this large and it be stiff enough without making the blades too heavy.  If money is less of an issue, a $500-$1500 would get you enough carbon fiber and glue to make some foam filled airfoils.  I've done a lot of C.F. work myself, but I would advise a new person to practice on some smaller samples before spending all the money or wasting the fabric and glue.

Otherwise, it's going to need a highly stressed skin design out of thin wood or metal in my opinion.  You might be able to get away with 3D printing the air foil cross sections for the supporting structure.

I'm guessing these airfoils are going to end up being ~8" long or so.  Less than 5-6" and it's going to be too floppy.  Longer than 12" and it's going to get heavy real fast and some serious engineering (on top of what you're already going to want to do) is going to be necessary.


Is there a particular reason why you want both straight and helix shaped blades?

I would think in the 2 meter range you could print in sections to go over a central spar as a stiffener. with 2 levels of support from the central mast they should be plenty stiff

yes my thoughts indeed. I have postponed my 1200x1200x2300 3d printer project for the time being and am now going to make due with a 350x350x400 printer. So I will be using a lot of segments to make all parts. But I no longer mind as that might even increase the stiffness of the whole blades in the end anyway

[brandnewb]

So I am going with 20cm chord lengths. this after a private conversation with one of the leading participants here..

About the airfoil shape I am still not sure about and if one knows I would soo much appreciate if one can share the knowledge.

[MattM]

If you're 3D printing then exotic is within your possibilities.  Nobody has tried the Lenz in the Darrieus afaik.  That's a proven VAWT design.

[Adriaan Kragten]

So I am going with 20cm chord lengths. this after a private conversation with one of the leading participants here..

About the airfoil shape I am still not sure about and if one knows I would soo much appreciate if one can share the knowledge.

In figure 1 of my report KD 601, the geometry and the aerodynamic characteristics of the symmetrical NACA 0015 airfoil are given. This airfoil has a maximum thickness of 15 % of the chord. You need a symmetrical airfoil for a H-Darrieus rotor as you have a positive angle of attack if the blade is at the front side of the rotor but a negative angle of attack if the blade is at the back side. Other symmetrical airfoils are given in report R443D of which you can find a copy on my website at the bottom of the menu KD-reports. Thicker airfoils have higher maximum lift coefficients but are more sensible to stalling at low Reynolds numbers.

[MattM]

Saw this video and immediately thought of your project.

https://www.youtube.com/watch?v=QJjhMan6T_E

[brandnewb]

sweet, that is a nice looking print.

Anyway, printing blades with a chord length of 40cm and 5 meters long is not recommended. as they needed to be post processed anyway I am going to give another method a try first.