Total pressure and mass flow boundary condition at inlet

bscphil · May 6, 2013, 16:17

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, 00:44

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, 07:56

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, 14:39

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

April 19, 2015, 07:56	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

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July 9, 2017, 14:39		#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