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-   -   shaft power versus RMP curve in wind turbine simulation (http://www.cfd-online.com/Forums/cfx/106329-shaft-power-versus-rmp-curve-wind-turbine-simulation.html)

zhenduan306 August 25, 2012 14:04

shaft power versus RMP curve in wind turbine simulation
 
Hi, Dear friends,
I am simulating HAWT now. My purpose is to obtain the curve between shaft power and RMP(or wind speed , TSR).

I used the way ghorrocks mentioned in
http://www.cfd-online.com/Forums/cfx...on-baldes.html

"Far easier, but a little less elegant is to sweep through a range of rotation speeds in a series of steady state runs. This will give you a torque/power versus speed curve. Then interpolate on this curve your intended torque/power and there you have it. If you like then run this point as well to improve the accuracy of the interpolation"

The weird thing is, when I fixed wind velocity, I increased rotational speed and torque increase too. The result is Cp is more than 0.59 (betz limit).
For example, wind speed=8m/s, angular velocity are 20 rad/s, 40 rad/s and 50 rad/s, respectively. The corresponding torque are 8.5, 22.35, 34.8 N.m. I think it is wrong. Because when wind speed is constant, the total wind power in the rot domain of the blades is fixed. Higher rotating speed means lower torque.



(my analysis type:steady)

Hence, I want to add CEL to my cfx-pre. This link gives me some clues:
http://www.cfd-online.com/Forums/cfx...imulation.html
from his CEL: he defined wind power as an expression. But I don't understand where he used it.

in my mind, I think the wind power and Betz limit should be as the constraint conditions. Otherwise, the results will be wrong.

These are my CFX pictures:
http://www.cfd-online.com/Forums/mem...419-cfxpre.jpg
http://www.cfd-online.com/Forums/mem...streamline.jpg

Any help would be greatly appreciated.

strobel August 25, 2012 20:22

I'm having this problem aswell. I'm using this expression to cp:
P_dot= torque_z()@blades*2*pi*Angular velocity [rev s^-1]
P= 0.5*ave(Density)@inlet*ave(Velocity)@inlet^3*area( )@in_rotor
cp=P_dot/P

But it results in cp>1.

ghorrocks August 25, 2012 20:56

Before worrying about getting the speed/torque correct, have you done all the basic checks to ensure your simulation is accurate?

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

zhenduan306 August 25, 2012 21:38

Quote:

Originally Posted by ghorrocks (Post 378702)
Before worrying about getting the speed/torque correct, have you done all the basic checks to ensure your simulation is accurate?

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Hi, ghorrocks, I think I can't include all relevant physics conditions in my simulation. I wonder I should set wind power in the rot domain as a constrain condition. But I don't know where can I add it and how to use it.

By the way, my blades' TSR=7.2. According to the rated wind speed, the corresponding rotating speed can be obtained. In that case, I can obtain the reasonable shaft power and Cp.

ghorrocks August 26, 2012 07:11

Quote:

I wonder I should set wind power in the rot domain as a constrain condition.
I have no idea what you mean by that.

The method I recommend is quite simple - do a series of steady state (well, frozen rotor or other simple RFR model) at various rotation speeds and get the torque versus rotation speed line. You can then match that to the load point (or line) to get the operating point.

I am not a wind turbine person so I have no idea what TSR=7.2 means. But you seem to imply that you already know the operating speed of this turbine.

zhenduan306 August 26, 2012 08:27

Hi, ghorrocks,Thank you for your reply. Actually, I know all the information of the blades in my wind turbine. So, I just use it to verify my simulation.

In fact, I found some guys have the same problem:
http://www.cfd-online.com/Forums/mai...orque-cfd.html

http://www.cfd-online.com/Forums/flu...imulation.html

Anyway, I will try to figure it out. After that, I will respond my method.

zhenduan306 August 26, 2012 08:29

Quote:

Originally Posted by strobel (Post 378701)
I'm having this problem aswell. I'm using this expression to cp:
P_dot= torque_z()@blades*2*pi*Angular velocity [rev s^-1]
P= 0.5*ave(Density)@inlet*ave(Velocity)@inlet^3*area( )@in_rotor
cp=P_dot/P

But it results in cp>1.

Hi, strobel, did you resolve it?

strobel August 26, 2012 20:17

Quote:

Originally Posted by zhenduan306 (Post 378742)
Hi, strobel, did you resolve it?

No, my problem is quite strange, because when i set a lower rotation i get cp<0.59, but if i increase the rotation, cp increase too and i get cp >1. I gonna try that way Glenn said, if it fix i post here the results.

ghorrocks August 26, 2012 21:00

If you already know the operating point, then you not simulate that point, and check the torque it produces is correct? No need to generate an operating curve if you do not need it.

zhenduan306 August 26, 2012 21:24

Quote:

Originally Posted by strobel (Post 378788)
No, my problem is quite strange, because when i set a lower rotation i get cp<0.59, but if i increase the rotation, cp increase too and i get cp >1. I gonna try that way Glenn said, if it fix i post here the results.

Good luck to you!

zhenduan306 August 26, 2012 21:27

Quote:

Originally Posted by ghorrocks (Post 378790)
If you already know the operating point, then you not simulate that point, and check the torque it produces is correct? No need to generate an operating curve if you do not need it.

I just know the information of the blades. I am designing a new wind turbine now. Hence, I need this curve to choose a generator.

ghorrocks August 27, 2012 06:24

Quote:

Hence, I need this curve to choose a generator.
Then you need to generate the turbine operating curve.

D.B August 27, 2012 07:18

Try areaAve or VolAve and see if it works out

zhenduan306 August 27, 2012 08:02

Quote:

Originally Posted by D.B (Post 378861)
Try areaAve or VolAve and see if it works out

Hi DB, can you tell me more details?

Thank you very much!:)

strobel August 28, 2012 01:23

Zhenduan, maybe when you increase too much your rotation your turbine don't work freely in this rotation and doesn't work anymore as Turbine. So, in these conditions, it is no longer more a turbine and become a pump.

ghorrocks August 28, 2012 07:25

I presume you are looking for the steady state operating condition - in that case, if the net torque is negative it simply means the rotor will decelerate, so the steady state speed is slower than the current speed.

zhenduan306 August 28, 2012 09:44

Quote:

Originally Posted by strobel (Post 379014)
Zhenduan, maybe when you increase too much your rotation your turbine don't work freely in this rotation and doesn't work anymore as Turbine. So, in these conditions, it is no longer more a turbine and become a pump.

Yes, you are right! CFX is just a tool. it can't identify the minimum or maximum condition the wind turbine is able to run.

zhenduan306 August 28, 2012 09:48

Quote:

Originally Posted by ghorrocks (Post 379087)
I presume you are looking for the steady state operating condition - in that case, if the net torque is negative it simply means the rotor will decelerate, so the steady state speed is slower than the current speed.

OK, I will check it. Thank you very much!

strobel October 31, 2012 16:33

Quote:

Originally Posted by zhenduan306 (Post 379113)
OK, I will check it. Thank you very much!

did you solve your problem?
i still getting high values of coefficient of shaft power(Cp). Have been strange, because in low rotation, i get Cp near of experimental values, but when i increase rotation Cp increase so fas and i get Cp higher than Beltz limit.

zhenduan306 November 1, 2012 15:24

Quote:

Originally Posted by strobel (Post 389567)
did you solve your problem?
i still getting high values of coefficient of shaft power(Cp). Have been strange, because in low rotation, i get Cp near of experimental values, but when i increase rotation Cp increase so fas and i get Cp higher than Beltz limit.

Unfortunately, I don't have time to continue it. If you just want to get Cp and Tip Speed Ratio curve, I suggest you use Qblade. It is a very good software. You can download from websit. Just google it.


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