Total pressure and mass flow boundary condition at inlet

bscphil · May 6, 2013, 17:17

Dear Foamer's,

i've some problems with my boundary conditions for a internal flow through a diffuser and the outflow in atmosphere.

For my case i must set the inlet total pressure + the inlet mass flow rate, because the diffuser is after an rotor/stator stage, which drops the total pressure to e.g. 101325 Pa + 680 Pa. The mass flow rate through the stage is constant e.g. 11.75 kg/s. The flow should be expanding to atmospheric static pressure e.g. 101325 Pa. Ok, this are the main restrictions...

My first attempt was for a incompressible RANS simulation with "simpleFoam":

inlet:
- p: zeroGradient
- U: fixedValue = 96.8m/s (plug profile)
  or U: flowRateInletVelocity = 11.75kg/s (plug profile)
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt calculates the right mass flow rate, but the total pressure was wrong (101325 Pa + ~5500 Pa).

My second attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: zeroGradient
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt calculates the right total pressure at inlet, but the wrong mass flow rate (~5.5 kg/s).

My third attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: flowRateInletVelocity = 11.75kg/s
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt crashes after some iteration.

My fourth attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: pressureInletVelocity
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This calculates very high mass flow rates.

Summary: i could not find out which boundary conditions are exactly right to set total pressure and mass flow rate (or velocity) at the inlet.

I hope somebody with some experience in a similar problem can help me, thanks and sorry for my poor english.

(the attached picture is a overview of the region)

Tushar@cfd · May 28, 2013, 01:44

Quote:

Originally Posted by bscphil

Dear Foamer's,

i've some problems with my boundary conditions for a internal flow through a diffuser and the outflow in atmosphere.

For my case i must set the inlet total pressure + the inlet mass flow rate, because the diffuser is after an rotor/stator stage, which drops the total pressure to e.g. 101325 Pa + 680 Pa. The mass flow rate through the stage is constant e.g. 11.75 kg/s. The flow should be expanding to atmospheric static pressure e.g. 101325 Pa. Ok, this are the main restrictions...

My first attempt was for a incompressible RANS simulation with "simpleFoam":

inlet:
- p: zeroGradient
- U: fixedValue = 96.8m/s (plug profile)
  or U: flowRateInletVelocity = 11.75kg/s (plug profile)
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt calculates the right mass flow rate, but the total pressure was wrong (101325 Pa + ~5500 Pa).

My second attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: zeroGradient
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt calculates the right total pressure at inlet, but the wrong mass flow rate (~5.5 kg/s).

My third attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: flowRateInletVelocity = 11.75kg/s
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This attempt crashes after some iteration.

My fourth attempt was:

inlet:
- p: totalPressure = 101325 Pa + 680 Pa
- U: pressureInletVelocity
walls:
- p: zeroGradient
- U: fixedValue = 0m/s
atmosphere:
- p: fixedValue = 101325 Pa
- U: zeroGradient

This calculates very high mass flow rates.

Summary: i could not find out which boundary conditions are exactly right to set total pressure and mass flow rate (or velocity) at the inlet.

I hope somebody with some experience in a similar problem can help me, thanks and sorry for my poor english.

(the attached picture is a overview of the region)

Good one... first attempt seems to be the best practice in case of diffusing problems, but don't know how come your Total pressure is varying a lot.. Are you able to reach convergence??

Ali hassan · April 19, 2015, 08:56

Hello everyone, I am doing validation for previous article about CNG mixer, my problem is using outlet static pressure instead of out flow because they ANSYS fluent don't allow to use out flow if I use inlet pressure, so, is there any one can help me to get static pressure from mass flow rate? Thanks

mrwan · July 9, 2017, 15:39

dear
i think the first attempt is the best but u v confusion about the area at inlet

April 19, 2015, 08:56	change flow rate to static pressure	#3
Ali hassan New Member Ali hassan Join Date: Apr 2015 Posts: 2 Rep Power: 0	Hello everyone, I am doing validation for previous article about CNG mixer, my problem is using outlet static pressure instead of out flow because they ANSYS fluent don't allow to use out flow if I use inlet pressure, so, is there any one can help me to get static pressure from mass flow rate? Thanks

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Wind turbine simulation	Saturn	CFX	60	July 17, 2024 06:45
mass flow in is not equal to mass flow out	saii	CFX	12	March 19, 2018 06:21
An error has occurred in cfx5solve:	volo87	CFX	5	June 14, 2013 18:44
Low Mixing time Problem	Mavier	CFX	5	April 29, 2013 01:00
RPM in Wind Turbine	Pankaj	CFX	9	November 23, 2009 05:05

July 9, 2017, 15:39		#4
mrwan New Member mrwan Join Date: Dec 2015 Location: egypt Posts: 27 Rep Power: 10	dear i think the first attempt is the best but u v confusion about the area at inlet