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-   -   Inlet pressure vs mass flow diference (http://www.cfd-online.com/Forums/cfx/86287-inlet-pressure-vs-mass-flow-diference.html)

Miroslav March 18, 2011 12:47

Inlet pressure vs mass flow diference
 
Hello everybody,
I am trying to make calculation of flow in turbocharger, I have one rotary domain there, but I have discover that when I set up mass flow BC on inlet and in res file I calculate pressure on inlet and than i set this pressure as a total or static (tryed both ) pressure on inlet I get dífferent values of mass flow in second case. Can you help me dicover what is going on ?

ghorrocks March 20, 2011 22:10

Did you average the pressure, or keep the pressure distribution over the inlet patch?

Miroslav March 21, 2011 04:39

Hi
I calculated avagare pressure. One possibilities is that I/O pressure to pressure is no so strongly defined than the mass flow on inlet and it gives to solver some freedom to get equilibrium. My model is qiute complicated, 4 domain , 1 static interfaces and 2 frozen rotors.
What do you think about this assumption ?
Thanks

M.

ghorrocks March 21, 2011 20:29

Quote:

One possibilities is that I/O pressure to pressure is no so strongly defined than the mass flow on inlet and it gives to solver some freedom to get equilibrium.
I have no idea what this sentence means.

If you are applying a constant pressure across the boundary then that is an obvious difference to the mass flow inlet where pressure varies across the boundary.

Miroslav March 23, 2011 04:55

Yeah Glenn, you are right that the comparison of pressure from BC (constant value) and the pressures from .res with BC mass -> P2 when P1 is changing across the boundary brings some mistake, but difference was much bigger. I was wondering if somebody has some experience with this efect. (Big difference between inlet pressure if it is set as BC or if it is calculated as a result of mass flow on inlet BC, I have to doublecheck that all cases had been converged enought, but i do not think there can be a problem ). We have made some measurement in real condition, and results from calculation was much more close to it , when I used mass-> P2 BCs.

JACZ March 24, 2011 00:34

Quote:

Originally Posted by Miroslav (Post 300074)
...when I set up mass flow BC on inlet and in res file I calculate pressure on inlet and than i set this pressure as a total or static (tryed both ) pressure on inlet I get dífferent values of mass flow in second case. Can you help me dicover what is going on ?

Hi Miroslav, you are right!, if you set Total presure you are including the contribution of the flux, in concecuence the mass flow. But if you set the Static pressure as BC I/O, you will have "any" value as mass flow could be close the mass balance, so you need a initial condition in velocity calculated with the mass flow in your problem to have a better value en mass flow.
Hope to help
:D


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