for help about cfx boundary condition settting

rainning139 · July 3, 2011, 10:11

Dear all,
I want to simulate the gas flow in a pipe using cfx ,gas at the inlet positon is
15000Pa(Guage pressure),velocity is 3m/s and i mean to know the outlet preesure.As a new learner ,I need your help now.I'm puzzled how to set the boundary conditons.Please give me the answer if you knows.Thanks a lot.

ghorrocks · July 3, 2011, 20:05

You have overspecified the inlet. You cannot define both velocity and pressure at an inlet.

I would convert the velocity to a flow rate, and apply the flow rate to the outlet. Then you can use 15kPa at the inlet and a mass/volume flow rate outlet boundary.

rainning139 · July 3, 2011, 22:26

Quote:

Originally Posted by ghorrocks

You have overspecified the inlet. You cannot define both velocity and pressure at an inlet.

I would convert the velocity to a flow rate, and apply the flow rate to the outlet. Then you can use 15kPa at the inlet and a mass/volume flow rate outlet boundary.

\Thank you for your answer.i will have a try .however I still have a question.NOW I want to use 2D model and the diameter of the pipe decreased in the middle, just like a Venturi tube.I guess the velocity at the outlet may be different from the velocity of the inlet,because the pressure change at the outlet,then the volumne flow rate will change.
I have used 15kPa at the inlet and the total mass flow rate of the 3D whole pipe as the mass flow rate of the 2D model,then I found the velocity is big abnormally,about 2900m/s. IS the mass flow rate improper?
HOPE for your answer.THANKS AGAIN!

ghorrocks · July 4, 2011, 07:25

If you are taking a 2D slice for a model then you need to reduce the flow rate according to your sector angle.

rainning139 · July 4, 2011, 21:45

Quote:

Originally Posted by ghorrocks

If you are taking a 2D slice for a model then you need to reduce the flow rate according to your sector angle.

THANKS.I will have a try

July 3, 2011, 10:11	for help about cfx boundary condition settting	#1
rainning139 New Member tao Join Date: Jul 2011 Posts: 5 Rep Power: 15	Dear all, I want to simulate the gas flow in a pipe using cfx ,gas at the inlet positon is 15000Pa(Guage pressure),velocity is 3m/s and i mean to know the outlet preesure.As a new learner ,I need your help now.I'm puzzled how to set the boundary conditons.Please give me the answer if you knows.Thanks a lot.

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July 3, 2011, 20:05		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You have overspecified the inlet. You cannot define both velocity and pressure at an inlet. I would convert the velocity to a flow rate, and apply the flow rate to the outlet. Then you can use 15kPa at the inlet and a mass/volume flow rate outlet boundary.

July 4, 2011, 07:25		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	If you are taking a 2D slice for a model then you need to reduce the flow rate according to your sector angle.