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Ryan January 1, 2008 18:20

target flow rate: varying results
The system I am modeling has a 72" diameter pipe with a 12" branch pipe 45 degrees to the larger pipe (followed by a 90 degree bend and a small straight). So there is one inlet (velocity inlet with UDF specifying 1/nth power turbulent profile), and two exits (pressure outlets with UDF specifying the pressure gradient due to hydrostatic effects). Also, in the small branch pipe there is a butterfly valve.

The flow rates are known (measured) and approximate values for the pressures are found using energy balances with mechanical losses (I'm only modeling a small section of a much larger system).

I am using the "target mass flow rate" option for the pressure outlet of the branch only and leaving the pressure of the larger pipe constant. I've found that if the convergence criteria is too large, the target flow is not obtained. So I've reduced the mass flow criteria to 1.0e-05 and the target flow comes within 1-2%. However, after obtaining the centerline pressures at the outlets and running the model again without the target mass flow rate, the same flow rate is note obtained.

Does anyone have a possible explanation for this? Does this mean the results of the initial simulation are not valid? Any help will be much appreciated.

Thanks, Ryan

JSM January 2, 2008 02:05

Re: target flow rate: varying results

Are you using the UDF only for specifying the pressure gradient due to hydrostatic effects. I think it is not needed. Simply you can enable the gravity in operating conditions panel.

Try once without specifying the target mass flow rate at outlet. And cross check the mass flow rate obtained with your available data.

If i am understanding your words correctly, the procedure is correct.

with warm regards, JSM

Ryan January 3, 2008 01:20

Re: target flow rate: varying results
I am using the UDF for the hydrostatic effect and I will just use the gravity option as you have mentioned. I do have that turned on, but for some reason I thought the constant pressure value wouldn't allow the hydrostatic effect to be taken into account.

I have tried on several occasions to not use the target mass flow rate option, but the flow rates do not come near the desired value. Thanks in advance for any other suggestions.

Ryan January 9, 2008 14:54

Re: target flow rate: varying results
I've tried using just a constant pressure exit value (no UDF to define hydrostatic pressure effects), and it does not work. I have gravity enabled in the correct direction, but the pressure on the exit plane is a single value. Any suggestions?

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