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asal March 14, 2013 10:32

Volume flow rate (l/s) as boundary

I have two question:

1- Is it possible in FLUENT to set the boundary as volume flow rate (liter/Sec) for inlet and outlet?
2- Is it possible in Fluent to measure a surface area or curve length? for instance I want to measure the area of the inlet.

macfly March 14, 2013 12:22

Volume flow inlet is not available in the boundary conditions. Here is how you measure the inlet area:
1. Read mesh: File\Read\Mesh...
2. Initialize without setting anything: Solution\Solution Initialization\Initialize
3. Measure inlet area: Results\Reports\Surface Integrals => Report Type : Area => Surfaces : the inlet

asal March 14, 2013 12:49

Thanks for your reply!

I have the values just as Liter/Sec for my inlet ans outlets. If l/s is not available, since my fluid is ideal gas, how can I handle it?

macfly March 14, 2013 12:55

Divide volume flow rate by inlet area, what units do you get?

asal March 14, 2013 14:10

Sorry, I am new in the fluent, so sorry about my questions

I mean about the outlet, I have now the volume flow rate which I convert to velocity. which type of boundary condition available in Fluent I should choose?
Should I choose Velocity-int but assign opposite direction?

macfly March 14, 2013 14:16

typical outlet boundary condition : pressure outlet, leave Gauge Pressure to 0 Pascal (keep default settings)

after simulation, verify mass balance : Results\Reports\Mass Flow Rate => choose inlet and outlet under 'Boundaries' => Compute

asal March 14, 2013 15:33

I think you become misunderstood!!!!
I should assign a value for the outlet. Is is not a pressure-outlet type. a known amount of flow should extract from the outlet.

macfly March 14, 2013 15:48

Yes, exactly, I'm misunderstood.

You have 1 or multiple outlet(s)? Infiltration, exfiltration? If there is only 1 outlet, why impose a mass outflow, it's necessarily the same amount than the inflow in steady-state.

Steady-state or transient?

asal March 14, 2013 15:55

Yes, Multi outlet and Steady.

macfly March 14, 2013 16:12

Then use Outflow boundary condition and adjust the Flow Rate Weightings


Use Pressure Outlet or Outlet Vent boundary condition, turn on Target Mass Flow Rate and enter targeted values.

You should compare the results and the mass balance in order to choose the most appropriate for your problem.

asal March 16, 2013 09:44

Hello again.

I try two method in last couple of days, First Out-Vent and second Velocity outlet. Since for both, the same amount of flow extracted from the outlets, but I got totally different results! and I have no any experiment to compare. what should I do? Any suggestion?

Additionally, there is another problem!!
The whole amount of flow (kg/s) extracted from the all outlets are a bit smaller than the flow impose to the inlet (kg/s). this additional flow should be extracted from one leakage boundary (Pressure outlet). But fluent reported:
"reversed flow in ** faces on pressure-outlet **. "
I don't know what is the problem?!!!
Any idea/solution?


aminepoxnet March 17, 2013 19:07

hi dear,
I think you should learn some experimental problems that usually occur in modelling
1. usually we can not extract the exact data from modelling and ''a bit'' is not so important
2. back flow in fluent modeling is not so important and usually we have this note bye fluent, I had a piston pump modelling that we got the best results but every time got a note about back flow
In fluent help articles, you can read about this note
3. you should use mechanics fundamentals to know if your data extract by modelling is correct or not and don't forget every time they are not exactly correct but they should be near by correct.

macfly March 17, 2013 22:44

Asal, maybe you could try to refine the mesh and see if the different boundary conditions eventually produce the same results. You will find tons of information about how to perform grid independency study on the forum.

Here's some tips in order to avoid backflow:
And there is probably tons of other threads about backflow.

Aladinos05 April 30, 2015 09:49

Reversed flow at the pump outlet
Hi everybody!
I have a problem and I ' ll appreciate any help ! I am simulating the fluid flow through a centrifugal pump but I am having a reversed flow at the outlet !this problem appears Right after starting the calculation, ! My BC is velocity in-pressure out ! I am wondering if this can't affect the results in a way or another! Another question is about the pressure prediction at the inlet ! it is always negative (-) ! I think this can't allow me to calculate the total developed Head and all other head related parameters! Is there any method I can use to have positive values of pressure at the inlet ! I just started using this software so I do not know much about it !! thank u

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