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Acieve Choking flow condition for incompressible fluid (Water)

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Old   April 17, 2013, 02:57
Default Acieve Choking flow condition for incompressible fluid (Water)
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Sudhakar P
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Hi Everyone
My intrest is to see the max flow rate(Choking condition) for the particular valve openinng.
I have an experimental result where they achived the choking conditions.

During my CFD simualtion (Similar experimental BCs values were used) ,I observed that mass flow rate is varrying by changing the down stream pressure.No symptoms of choking conditions)

Please find the attachedment for more details...

Kindly guide me how to achive the choking condition using numerical simulation
Attached Files
File Type: pptx Numerical setup1.pptx (88.3 KB, 13 views)
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Old   April 18, 2013, 13:18
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Stupid question here - are you sure the valve wasn't cavitating in the experimental setup?
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Old   April 18, 2013, 13:24
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try also different formulation than RANS ... what about the lenght scales?
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Old   April 19, 2013, 06:57
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any idea about CFD coding and Openfoam training?
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Old   April 22, 2013, 05:24
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Hi all.

Do you think it is possible to achieve choking in the simulation of an incompressible fluid? My first guess is no.

Acoustic waves propagate at an infinite speed in an (ideally) incompressible fluid so the mass flow rate will never become independent of the pressure downstream of the nozzle, since information will always be able to propagate upstream the flow.

In the real world water is slightly compressible but, does anyone know whether the code accounts for the compressibility of liquids? If no then I'd say you cannot achieve choking in your simulation.

Thanks.

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Old   April 22, 2013, 08:59
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Are you able to get the same mass flow rate at the given boundary conditions? Choking is phenomena occurs in compressible flow, but in incompressible flow we can have the maximum flow rate for given area !!!
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Old   April 23, 2013, 01:12
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Hi Everyone
thank you verymuch for your reply,

The overall objective is to find out the maximum mass flow rate for the corresponding opening of the valve .Please see below the quote which discuss mainly how to ensure the maximum mass flow rate of the Incompressible fluids.

"A valve passing a liquid begins to choke when the downward pressure swing toward the vena-contracta forces the liquid pressure to a value near or below its vapor pressure. This is the point where the liquid starts to form bubbles. The formation of bubbles causes the liquid molecules in the flow stream to become crowded or 'clustered' at the vena-contracta and this crowding prevents any further increase in flow with decrease in the outlet pressure. "
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Old   April 23, 2013, 10:01
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What you are describing is cavitation - which is why I asked about it. Cavitation is a multi-phase phenomenon, yet you describe your CFD approach as single-phase. As long as the assumptions in your CFD model don't match the actual physics, you won't get the same results numerically as the experiment did. If you want to model the choking behavior, you will need to add a multi-phase capability to your CFD effort.
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