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Which is the appropriate boundary type?

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Old   December 28, 2009, 10:20
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Panos
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Hello
I am Panos and i am working as researcher at the NTU Athens. I am new on cfd and im trying to simulate heat transfer phenomena and air flow in a heater. The geometry is quite simple. A cylinder that is heated by resistances. on the upper side of the cylinder i have the following boundaries. A velocity inlet, a mass inlet and wall. i have air flow from the top to the bottom and ore at the same time is imported from a hole near the air inlet hole. My question is about the appropriate outlet boundary type i have to choose. I am between pressure outlet and outflow. can anybody advice me or explain me the difference between those two types?
in addition, regardless of talking about the furnace exit it is possible to have some air coming in the furnace from the outlet of the system
Thank you in advance
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Old   December 29, 2009, 12:36
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The outflow is kind of boundary layer that you dont have any information about that surface or edge, but the pressure outlet you have the pressure of that part
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Old   December 29, 2009, 12:37
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so if you dont have any information about that area choose the OUTFLOW
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Old   December 29, 2009, 14:14
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Thank you arash for your answer.
I would choose outlet but im not sure about the Flow Rate Weighting i have to set. According to the mass concervation law it should be equal to the inlet flow rate which is known. But i think that setting the boundary as OUTFLOW, maybe fluent ignore the updrift and the air quantity that might enter the tube from the exit. ( the heated tube is vertical and the exit is located at the bottom).
So am i right?
As you can see i am amateur on that program and maybe my questions are silly but talking with people realy helps me

Thank you in advance
Panos
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Old   December 29, 2009, 16:23
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Honestly, i dont known the answer of your question, I am amateur too, just try it check if your result is reseonable
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Old   January 7, 2010, 10:19
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The OUTFLOW is the appropriate condition when the flow at these section is fully development.
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Old   January 8, 2010, 12:26
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If your outer exit is directly left out to atmospheric condition go for pressure outlet with atmospheric pressure. Outflow is used if you dont know the real exit condition.
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Old   January 9, 2010, 05:16
Default Which is the appropriate boundary type?
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Sivam,
i use the Pressure outlet as boundary type and the results seems to be good. Indeed the exit of the furnace is on atmospheric pressure. But due to the low air inlet velocity and the high temperature of the furnace walls i have some updrift from the exit. so a quantity of hot air left the furnace from the exit downward but at the same time cold updrift appears on the exit. So the Pressure outlet is exit and 'entry'at the same time. Will this affect the results of the simulation?
(forget the mass flow, i am running the simulation only with air)
Thank you
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