Need help on fluid physics

rahulv · April 26, 2011, 04:44

Hi all,

I am designing a fan for a electrical machine. I have comparing two fans one is mixed flow type and another one is centrifugal fan..

i tasted the both fan for same conditions in cfx. My boundary conditions for both fans are

Inlet total pressure: 0 (because its open to air)
Outlet static pressure: 140 ( fin pressure drop..)
and rotation : 1500 rpm

The results of the both fans are quite confusing.
The approximate results are

Fan 1 (mixed flow) Fan2 (centrifugal fan)

mass flow rate 2.9 3.1 kg/s
total pressure out 640 340 Pa
torque 34 11 J

as i am interested in total efficiency , both fans are giving almost same efficiency.
So the my doubt is why the 2nd fan which is giving more mass flow rate giving less velocity pressure ( because static pressure at outlet is same for both of them) ...
Hint. as i went through the results. i found that in fan1 velocity is very high at some points but in fan 2 its is uniform. this may be the reason?.

and my 2nd question is that as fluid has to dissipate heat from fin, which fan i should prefer. The fan who is giving more mass flow rate or the fan who is giving more total pressure.

Please send me your suggestions
rv

juliom · April 26, 2011, 08:20

Good morning my dear friend, I suggest you to do an sensitive evaluation, I mean try to use other boundaries conditions, since your boundaries conditions are weak, maybe your are over defining the mometum law, since you are working in a subsonic problem. If were you I would use the mass flow defined for the solver as an boundary condition at the inlet and pressure known at the outlet (opening).
Then you will have other results, please do in that way and then share your results.
Greetings from Venezuela!!!
your sincerely JM

ghorrocks · April 26, 2011, 21:53

Have you done any checks that your simulations are accurate?

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

rahulv · April 27, 2011, 06:12

@juliom thanks for your suggestion friend.
indeed i have chosen previous boundary condition to estimate the mass flow rate given by the fan.
I will finish the simulation of two cases and will share the results with you.

@ghorrocks I think this is not the case of accuracy of the solver,but this is the case of understanding the fluid physics.

Thanks and regards Rv

April 26, 2011, 04:44	Need help on fluid physics	#1
rahulv Member Rahul Vaibhav Join Date: Nov 2009 Location: India Posts: 33 Rep Power: 16	Hi all, I am designing a fan for a electrical machine. I have comparing two fans one is mixed flow type and another one is centrifugal fan.. i tasted the both fan for same conditions in cfx. My boundary conditions for both fans are Inlet total pressure: 0 (because its open to air) Outlet static pressure: 140 ( fin pressure drop..) and rotation : 1500 rpm The results of the both fans are quite confusing. The approximate results are Fan 1 (mixed flow) Fan2 (centrifugal fan) mass flow rate 2.9 3.1 kg/s total pressure out 640 340 Pa torque 34 11 J as i am interested in total efficiency , both fans are giving almost same efficiency. So the my doubt is why the 2nd fan which is giving more mass flow rate giving less velocity pressure ( because static pressure at outlet is same for both of them) ... Hint. as i went through the results. i found that in fan1 velocity is very high at some points but in fan 2 its is uniform. this may be the reason?. and my 2nd question is that as fluid has to dissipate heat from fin, which fan i should prefer. The fan who is giving more mass flow rate or the fan who is giving more total pressure. Please send me your suggestions rv

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April 26, 2011, 08:20		#2
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Good morning my dear friend, I suggest you to do an sensitive evaluation, I mean try to use other boundaries conditions, since your boundaries conditions are weak, maybe your are over defining the mometum law, since you are working in a subsonic problem. If were you I would use the mass flow defined for the solver as an boundary condition at the inlet and pressure known at the outlet (opening). Then you will have other results, please do in that way and then share your results. Greetings from Venezuela!!! your sincerely JM

April 26, 2011, 21:53		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Have you done any checks that your simulations are accurate? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

April 27, 2011, 06:12		#4
rahulv Member Rahul Vaibhav Join Date: Nov 2009 Location: India Posts: 33 Rep Power: 16	@juliom thanks for your suggestion friend. indeed i have chosen previous boundary condition to estimate the mass flow rate given by the fan. I will finish the simulation of two cases and will share the results with you. @ghorrocks I think this is not the case of accuracy of the solver,but this is the case of understanding the fluid physics. Thanks and regards Rv