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george November 6, 2008 18:03

boundary condition dilemma

i'm trying to model flow through a cylinder compressor completely governed by pressure difference between inlet and outlet openings. i tried two different boundary condition setting which finally resolves to be the same. but i'm getting two different results from the two different settings.

at first i tried to set the reference pressure as 101kpa, with inlet pressure as 201kpa and outlet pressure as 101kpa. i got very similar results as i expected for mass flow rate and temperature while the pressure was predicted atleast 1000kpa more than what i expected.

then i tried another setting with reference pressure as 101kpa, inlet pressure as 100kpa, and outlet pressure as 0kpa , while the setting resolves to the same as one defined above with exactly same resultant pressure. however in this case i got comparatively less mass flow rate, cfx predicted higher temperature generation (which i think due to less mass inside the cylinder), but very similar range of pressure as what i expected.

now i'm confused about which setting i can proceed with. i believe both the settings are essentially the same, but gives different results. i will be great if any of you can help me in this. thank you.


Pascal November 7, 2008 06:20

Re: boundary condition dilemma
Hi George,

I guess you havent noticed that for an outlet in CFX-Pre, the outlet pressure is relative to your reference pressure. So if you set Pref to 101kPa, and Pout to 0kPa, then you have actually fixed Pout at 101kPa.

Same thing for an inlet.

If you are not yet comfortable with CFX, I really recomend u to read you CFX manual! I know it's a lot of pages, but then you won't be stopped in your research for littles things.

Amod November 7, 2008 11:55

Re: boundary condition dilemma
(quote)"...cylinder compressor completely governed by pressure difference between inlet and outlet openings."

As fas as I know, compressor is a device which adds energy to the fluid it is working upon. Hence, in actual operation the pressure difference created across the inlet and outlet is function of compressor design. The better B.C. would be to use Mass Flow Inlet and Zero (or any other value depending upond downstream flow conditions) relative pressure at the outlet.

|A| Always positive!

george November 11, 2008 00:58

Re: boundary condition dilemma

thank you Pascal and Amod for your suggestions. I will try this way. I set the outlet pressure to 0kPa, however i don't understand why Amod suggested using mass flow at the inlet rather than pressure even if it is an energy adding problem. could you please clarify this. I also wish to know if anyone else have a different opinion about this.

Pascal November 11, 2008 10:07

Re: boundary condition dilemma
You just have to know exactly if your compressor is working as you said (governed by pressure difference) or as Amod said (amount of gas added to your tank).

You are the only one to know what kind of compressor you modelise, so then it's up to you!!

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