report KD 696 about wind tunnel measurements on the CWD 2740 rotor available

[Adriaan Kragten]

Report KD 696 can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports. The tittle of this report is: "Summary of the most relevant wind tunnel measurements presented in report R 408 S from 1979, performed on a scale model of the CWD 2740 rotor".

Report R 408 S is more than 40 years old now and it is no longer available. But it contains valuable information about rotor characteristics of a 6-bladed rotor with a design tip speed ratio of 2. So I found it useful to copy the most relevant measurements in a report which is freely available.

[oneirondreamer]

Thanks so much Adriaan, this is a really lovely addition to your collection of work.   

It's useful for people today to see things like the adjustable blade twist system used, how it was simple and mechanical. 

I wish more young engineers looked over this kind of stuff.   

It's also amazing to see what it took to do real science and engineering back then, the instrumentation and data logging.   

We have such simple tools for so much of that now, we can easily collect realms of data, that sometimes I fear researchers get lost parsing the details.  I can.

My concern, (which is mine alone and may be entirely unfounded), about this research and this type of research, is that at the time this wind tunnel work was done the state of the art understanding of wind tunnel blockage effects, and higher solidity turbines was not aligned with reality, especialy at higher tip speeds and lower Reynolds numbers.   Even into 2010, correcting for wind tunnel blockage, especially for high solidity systems, was commonly done with formula's that produced erroneously high Cp's, even with the best intentioned researchers.
,
I wonder if there is anything similar that got real world testing?  The American Multiblade, looks to be it's closest relative with commonly available data.

[Adriaan Kragten]

Already in 1979 we had most of the knowledge which is now available for anyone in my report KD 35. So we knew that at a low design tip speed ratio, a lot of power is lost in wake rotation (see KD 35 figure 4.2). If you look at this figure, you can see that the maximum Cp is 0.416 for lambda = 1 and 0.513 for lambda is 2. These are the maximum values if the other three kind of losses being tip losses, airfoil drag and the fact that the real blade length k is shorter than R, are neglected. So the theoretical maximum Cp is much lower than the value which can be read from figure 4.2 of KD 35. The theoretical maximum Cp for a certain number of blades B and a certain Cd/Cl ratio is given in figures 4.6 up to 4.11 of KD 35. In figure 4.9 it can be read that the theoretical maximum Cp for a 6-bladed rotor with a design tip speed ratio of 2 is about 0.425 if the Cd/Cl ratio is 0.05. If you then correct this value for the real blade length using formula 6.3 of KD 35, you find a real maximum Cp of about 0.39 and that is just what we have measured in the wind tunnel. So our wind tunnel measurements were in very good accordance with the theory. But this is only the case if you use an open wind tunnel. Most wind tunnels are closed or half closed if the wind tunnel walls are removed at the measuring section and then you get the problem of tunnel blockage resulting in a Cp which is much too high if the swept area of the rotor is too large with respect to the cross sectional area of the wind tunnel.

The low maximum Cp at low design tip speed ratios was one of the reasons why we developed water pumping windmills with a design tip speed ratio of about 2 in stead of about 1.1 as used for traditional multi bladed water pumping windmills. The other reason was that the solidity decreases strongly if the design tip speed ratio is increased and this results in a much lighter rotor. However, the disadvantage of increasing the design tip speed ratio is a strong decrease of the starting torque coefficient and this gives starting problems if you use a single acting piston pump as load (see KD 294). But this problem was solved by using a piston pump with a floating valve (see KD 364). A floating valve makes that the pump load is zero at low rotational speeds and this allows a much higher design wind speed, and so a much bigger pump, than for a traditional water pumping windmill. At moderate wind speeds, we therefore got flows for the CWD 2000 windmill which were at least a factor four higher than for a traditional multi bladed windmill with the same rotor diameter and water height.

[Adriaan Kragten]

Report KD 696 explains wind tunnel tests performed on a scale model of the CWD 2740 rotor but I think that it is good to tell something more about the real CWD 2740 windmill.

We have built a prototype which was tested on the test field of the University of Technology Eindhoven for several years. Once there was a very big storm and many trees went down but the windmill survived without any problems. Three windmills were built in Tanzania near Lake Victoria. The CWD-2740 was also used in several other CWD-projects and the drawings and manuals were available at a low price at that time. So I think that at least ten of these windmills have been built. I have never heard of failure of one of them. But if someone would ask me if I would advise to build a windmill with such a rotor again, I would say no.

The reason is that the CWD 2740 rotor has some important disadvantage. The first is that the spoke assembly with six spokes and three ribs at each spoke is one welded construction. It is a lot of work to make this spoke assembly and transport of this component is rather difficult as it requires a big truck. The second is that the spokes are long and that they therefore create some extra airfoil drag. The position of the spoke is at the hollow side of the blade at 1/3 c from the airfoil nose. This is the position where a spoke has the least negative influence on the aerodynamic drag but a blade without spokes is better.

In my public report KD 319, the rotor of the VIRYA-2.8B4 is described. This rotor has four blades and each blade is made from a galvanized sheet size 0.333 * 1 m with a thickness of 1.5 mm. The 6-bladed CWD 2740 has the same blade geometry and so the total blade area of the VIRYA-2.8B4 is 2/3 of that of the CWD 2740 rotor. However, the VIRYA-2.8B4 has a blade thickness of 1.5 mm in stead of 1 mm and so the total weight of the blades of both rotors is the same. The VIRYA-2.8B4 rotor has a design tip speed ratio of 2.5, so somewhat higher than for the CWD 2740 rotor which has a design tip speed ratio of 2. Two opposite blades of the VIRYA-2.8B4 are connected to each other by a 1.5 m long twisted steel connecting strip and no ribs are used. The two connecting strips are connected to a square hub by four bolts. The outer 0.65 m of a blade is completely free so the drag of this part of the blade is very low. A big advantage is that the rotor needs no welding. Transport of the rotor takes only a little space even if two blades are already mounted to the connection strip in the workshop. The starting torque coefficient of this rotor is rather high (0.054). The maximum Cp was estimated rather pessimistically to be 0.38 so about the same as measured for the CWD 2740 rotor. The estimated Cp-lambda and Cq-lambda curves are given in figure 1 and 2 of KD 319. A sketch of the VIRYA-2.8B4 rotor is given in figure 7 of KD 319.

So if one needs a rotor with a rather low design tip speed ratio and a high starting torque coefficient, the VIRYA-2.8B4 rotor is a much better choice than the CWD 2740 rotor