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-   -   torque is 5 times more than expected why? (http://www.cfd-online.com/Forums/openfoam-solving/113388-torque-5-times-more-than-expected-why.html)

sivakumar February 19, 2013 02:06

torque is 5 times more than expected why?
 
Hi Foamers,
I am working on axial flow fan, I want to calculate the torque.
I am using the function, which is posted by linnenman.

The problem is I am always getting 5 times bigger values.
for the whole fan (8 blades) the torque is 72N-m,
since I am using quarter of the section the torque should be 18 N-m, but I am always getting above 80N-m.

The same way in fluent 10 times bigger value(170 N-m), that means I am doing something wrong in the settings, but I am not able to figure out that.

can you people give me some small hint, what could be the possible mistake?

I am running out of Idea.

any sort of idea or hint will be appreciated.

Thanks,
Sivakumar

jmf February 24, 2013 18:26

Default torque is 5 times more than expected why?
 
Hi

Please could you tell more about your case ?
Which solver ?
What are your boundaries ?
How is your mesh ?


Wishes

Jean-Michel

sivakumar February 25, 2013 05:47

Hi Jean-Michel,

Thanks for your reply, I am using MRFSimpleFoam and SRFSimpleFoam.

I got the information how to calculate torque from the output. I think in openFoam the problem is fixed. I am getting reasonable torque as I expected, but the pressure raise is 15% error.

But in fluent, I am using SRF Solver. I dont know how to read the torque out put

results --> Reports --> moment --> rotation axis (0 0 0) & moment (0 0 1)

I am bit confused now the out put is for the existing geometry (2blade) or for the whole Geometry (8Blades), I am using only 1/4 th of the geometry for the simulation.

the fluent out put is something like this (11.5 12.5 -78).

I have uploaded my openFoam case in sendspace, if you have some spare time please have a look and give me your commands.
http://www.sendspace.com/file/ywba19


Thanks for you time,
Sivakumar

sivakumar March 13, 2013 11:08

Hi,
I found the answer for my question,
The answer is my blade angle, it was wrong.
Now I have changed it and I got the perfect results.

Thanks,
Sivakumar

sam181087 March 20, 2013 07:57

Hi Sivakumar,
I am also doing the axial fan simulation in CFX and my torque prediction is more as compared to test value.
Can you please tell me about the boundary conditoin.



Thanks,
A.S.

sivakumar March 20, 2013 10:42

Hi Sam,
I am happy to give you the information, I have some questions,
i) how big is your torque?
ii) what is the static pressure raise?
if you are getting more torque and less static pressure raise, then you have problem with your blade angle. I will strongly suggest you to check the stagger angle.

Regarding BC's fan, hub and shroud --> wall, inlet --> mass flow rate, outlet --> Pressure outlet (0Pa) remaining all periodic (1/8th of the fan passage).

Thanks,
Sivakumar

sam181087 March 22, 2013 14:56

Hi Sivakumar,
Thanks for your reply.
1) Fan considered is ceiling fan and predicted torque value is 40-60% higher than test value.
2) RPM of fan is the only input, so mass flow rate cant be given as input.

So, can you help with this.

Thanks&Regards,
Sam

sivakumar March 23, 2013 05:19

Hi Sam,
I am not clear about your question, what are the boundary conditions your using at present (inlet and outlet)? how your case look like?

can you post some picture of your case, it may give some idea to the people those who want to help you.


It is difficult to give an idea with out seeing your case, try to post your mesh in cfx forum, some cfx experts can help you.

Thanks,
Sivakumar

sam181087 March 23, 2013 11:15

hi,
1) Its a simple case of ceiling fan rotating in a room at constant RPM.
2) So in rotating domain which contains whole fan and other is stationary domain.
The problem is prediction of fan torque.

Can anyone help with this.


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