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-   -   Total pressure and mass flow boundary condition at inlet (http://www.cfd-online.com/Forums/openfoam-pre-processing/117323-total-pressure-mass-flow-boundary-condition-inlet.html)

bscphil May 6, 2013 16:17

Total pressure and mass flow boundary condition at inlet
 
1 Attachment(s)
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 (Post 425644)
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

change flow rate to static pressure
 
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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