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-   -   Pipe flow with pressure-inlet (https://www.cfd-online.com/Forums/fluent/108035-pipe-flow-pressure-inlet.html)

 lummz October 12, 2012 16:40

Pipe flow with pressure-inlet

Hi

I'm trying to simulate the flow of water through different 4 mm pipe solutions. It's one inlet with different kind of outlets. The known conditions is that the pressure at the inlet is 3 MPa and that the flow at the outlets should be around 30 liters/minute. I'm not investigating exactly what the flow is, it's rather a comparison of the flow difference of the different pipe solutions.

My first thought was to assign both a pressure inlet and outlet. With the inlet boundary being 3 MPa and the outlet 0 Pa because it's flowing into open air. This lead to strange results; when looking at a surface integral of the inlet it only had a pressure of 1,4 MPa. A more complex pipe solution lead to the inlet pressure varying from -0,5 MPa to 5 MPa. It should be a constant 3 MPa?? And it seems really strange thinking about it, that it's correct to assume a 3 MPa pressure drop to the pipe, like i did.

The best way i can think of to simulate this is to lead the pipe into a big room of air, however i have no idea how to set this up with different kind of fluids mixing.

What's the best way of doing this?

 syavash October 12, 2012 22:05

Quote:
 Originally Posted by lummz (Post 386338) Hi I'm trying to simulate the flow of water through different 4 mm pipe solutions. It's one inlet with different kind of outlets. The known conditions is that the pressure at the inlet is 3 MPa and that the flow at the outlets should be around 30 liters/minute. I'm not investigating exactly what the flow is, it's rather a comparison of the flow difference of the different pipe solutions. My first thought was to assign both a pressure inlet and outlet. With the inlet boundary being 3 MPa and the outlet 0 Pa because it's flowing into open air. This lead to strange results; when looking at a surface integral of the inlet it only had a pressure of 1,4 MPa. A more complex pipe solution lead to the inlet pressure varying from -0,5 MPa to 5 MPa. It should be a constant 3 MPa?? And it seems really strange thinking about it, that it's correct to assume a 3 MPa pressure drop to the pipe, like i did. The best way i can think of to simulate this is to lead the pipe into a big room of air, however i have no idea how to set this up with different kind of fluids mixing. What's the best way of doing this?
The best way for simulating pipe flow is to implement velocity inlet B.C.
since you know flow rate at outlet, the inlet velocity can be calculated easily(by deviding flow rate by inlet area). By the way you can not expect fluent to preserve the inlet pressure you specified. Because the flow is subsonic and pressure is interpolated from interior.

Bests.

 lummz October 13, 2012 03:23

Okay i'll try that, should i set zero pressure in the outlet then? It feels like this would somewhat control the pressure drop, which should rather be calculated by the program.

 syavash October 13, 2012 13:29

Quote:
 Originally Posted by lummz (Post 386376) Okay i'll try that, should i set zero pressure in the outlet then? It feels like this would somewhat control the pressure drop, which should rather be calculated by the program.

It seems to be reasonable. Try it.

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