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-   -   Mass Flow Rate Conservation Problem (https://www.cfd-online.com/Forums/fluent/31404-mass-flow-rate-conservation-problem.html)

 philippe May 5, 2003 08:14

Mass Flow Rate Conservation Problem

Hello,

I simulate the flow of CO2 in a cylinder. After about 500 iterations, using Reports/Fluxes menu, I've got the good value of Mass-Flow rate (specified with mass-flow-inlet BC) i.e. 0.10 kg/s but I've got -0.10 kg/s at the outlet (defined as outflow BC) ! How can I solve this problem ?

Thanks.

Philippe.

 ap May 5, 2003 09:45

Re: Mass Flow Rate Conservation Problem

Maybe I don't understand the problem: you have a cylindrical pipe and 0.10 kg/s of C02 entering it. FLUENT tells you that 0.10 kg/s enter the tube and -0.10 kg/s exit it. Right?

FLUENT considers negative all flows which exit the domain and positive all fluxes entering it. So the value you have at the outlet seems good.

Hi.

 philippe May 5, 2003 10:03

Re: Mass Flow Rate Conservation Problem

Hi,

I think you understand my probleme: I have a negative mass flow rate on the surface which is the outlet of the whole 3D domain.

thank you.

 ap May 5, 2003 10:11

Re: Mass Flow Rate Conservation Problem

Ok. As I've already told you, negative values of the mass flow rate at the outlet are not a problem.

Good work

Hi

 philippe May 5, 2003 10:15

Re: Mass Flow Rate Conservation Problem

Hi I also have some negative values of static pressure at the outlet surface ?! I would like to be sure that the flow is computed in a good way.

Thanks.

philippe.

 ap May 5, 2003 10:26

Re: Mass Flow Rate Conservation Problem

Does FLUENT tells you that you have inverted flow on some cell at the outlet?

 philippe May 5, 2003 10:32

Re: Mass Flow Rate Conservation Problem

yes It tells that there is some reversed flows on 20/30 faces on the outlet surface. All the residuals are constant.

mass residuals less than 1.e-03 velocity residual : 1.e-03 energy (thermal model): a little more than 1e-03

 ap May 5, 2003 11:39

Re: Mass Flow Rate Conservation Problem

Try to do more iterations. If residuals of energy is over 1e-6, your solution is not considered as a converged one by FLUENT.

Check all boundary conditions.

If you can, try to replace the Outflow boundary condition with a Pressure outlet (you need to know gauge pressure at the outlet). Read the manual about Outflow BC use for more info.

 philippe May 5, 2003 11:43

Re: Mass Flow Rate Conservation Problem

thank you !!

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