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mireis July 12, 2010 05:49

Compressible flow, no data at the outlet
I’m a student and I would want to analyze a part of a pressure regulation chain. To do this I consider just a plenum with a pipe for the inlet and a pipe for the outlet. The latter narrows at the end to impose a mass flow rate of 5 mg/s at the outlet whereas the mass flow rate at the inlet is around 3000 mg/s. What I want to obtain is the difference in pressure between a point near the outlet and a point in the middle of the plenum.
For the inlet I use the “mass-flow-inlet” boundary condition as I can specify the mass flow rate, total temperature and pressure at this point. The problem comes when I have to enter the conditions at the outlet: I’m working with a compressible flow (Xenon at almost 3 bar and 70 degrees) so Fluent doesn’t allow me to use the ‘outflow’ boundary condition. If I use the “pressure-outlet” condition I’m obliged to impose a pressure there that I don’t know and that is what I would like to obtain from the analysis.
My attempts have been the following ones:
1) To lengthen the outlet pipe. Then the outlet is far from my point of interest and the pressure I impose there is not exactly the pressure I want to obtain. Anyway this is not a good solution since the results depend on the pressure at outlet.
2) To use the target-mass-flow-rate option as the only thing I know for sure at the outlet is the mass flow rate. This option permits specifying a target mass flow rate there. Fluent calculates a mean static pressure as a first value, then it calculates the corresponding mass flow rate and using the difference between the obtained mass flow rate and the target one it adjusts the static pressure. The problem is that, from the results, it seems that Fluent just imposes this static pressure at the outlet and the rest of the fluid does not perceive this change.

Is there any way to avoid imposing a pressure at the outlet when working with a compressible flow?
When I initialize a solution in fluent from the inlet conditions (you can choose between inlet, all zones, outlet …) it propagates the flow conditions at the inlet along all the model and when It arrives to the outlet it imposes the conditions you specified there?

mireis July 28, 2010 05:22

the use of a 'pressure-far-field' condition would be correct?

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