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-   -   Advice on Pump Outlet BC (https://www.cfd-online.com/Forums/fluent/113670-advice-pump-outlet-bc.html)

Gorz February 24, 2013 21:11

Advice on Pump Outlet BC
 
Hi,

I am performing a comparative analysis a regen pumps, modifying the inlet and outlet of it to observe the effect performance.

For a couple of weeks now I have been using the mass flow inlet or velocity inlet in conjunction with outflow BC to obtain my pressure rise. I realise now this is incorrect.

The model is set up using MRF, with the impeller fluid as the rotating zone, and the corresponding walls rotating with it. The annular fluid is stationary.

My geometry is based on previous research, for which there was a test rig. I do not have all the test data, however I do know the free surface height above inlet, the mass flow and diameter of the pipe, so can calculate the static pressure at the suction side. Using pressure-inlet BC I set total pressure to the static (above gauge) as calculated, operating pressure is zero (for incompressible fluids, water)

It is important to note that the model does not include all the length and height of the pipework as it would be in real life, the inlet and outlet are only a 50mm portion

My problem comes with defining the outlet side BC. Since mass is conserved, and the outlet dia = inlet, the fully developed velocity will be the same. I have tried umpteen variances on the pressure-outlet BC, such as setting a high outlet pressure, setting to zero gauge, target mass flow rate. Either I get a pressure drop across the pump, or the fluid is forced back through the pump to the inlet side.

I know that empirical data was previously collected, and the write up for that mentions that the pressure BCs where based on the measured values. In my case I do not have the opportunity to carry out experiments, but I MUST find the pressure rise across the pump.

Any help greatly appreciated.

delaneyluke February 26, 2013 16:20

Why do you say that using the outflow BC is incorrect?

Regards
Luke

Gorz February 26, 2013 16:38

Quote:

Originally Posted by delaneyluke (Post 410266)
Why do you say that using the outflow BC is incorrect?

Regards
Luke

Outflow BC cannot be used when there is a pressure inlet BC.

I have since run the problem using pressure inlet and target mass flow rate at pressure outlet from the command line approach and I get good results on the outlet pressure, however, it takes in excess of 1000 iterations for the outlet mass flow to become stable.

Is there a quicker method?

delaneyluke February 26, 2013 16:58

Quote:

Originally Posted by Gorz (Post 410270)
Outflow BC cannot be used when there is a pressure inlet BC.

I have since run the problem using pressure inlet and target mass flow rate at pressure outlet from the command line approach and I get good results on the outlet pressure, however, it takes in excess of 1000 iterations for the outlet mass flow to become stable.

Is there a quicker method?


Thought you said you used a mass flow/velocity inlet with the outflow.
That shud work the best

Regards
Luke

Gorz February 26, 2013 18:10

That's correct, but I don't get the right head rise. Pathlines are all good, fluid behaves as it should for this application. Only way that I can get the result is using the method I described above, but it is rather cumbersome.


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