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Comparing theoretical calculations with ANSYS Fluent

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Old   May 21, 2015, 08:34
Default Comparing theoretical calculations with ANSYS Fluent
Mansoor Ahmed
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Hi everyone
I am trying to compare the outlet temperature obtained from ansys fluent simulation and theoretical calculations for 3D laminar, fully developed flow of water in a tube. I have used mass flow inlet and pressure outlet boundary conditions and a constant surface temperature at the wall. I have created a surface monitor at the outlet for mass-weighted average temperature. the results show a deviation of about 20 %. I am using a total inlet temperature of 300 K at inlet boundary in ANSYS Fluent. Outlet temperature obtained from ANSYS Fluent is greater than that obtained theoretically.
Is there a problem in defining the inlet temperature? should a UDF be used to describe the fully developed temperature profile at the inlet?
As in theoretical calculation mean inlet and outlet temperatures are considered.
Your expert opinion is required.
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Old   May 24, 2015, 12:26
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questions I would ask would be:
1. have you established grid independence,
2. what about your theoretical caluclations: are you using the same material properties, are you using a correlation for laminar flow etc.

then look at:
how long does the flow take to become an established flow profile - if this is too large a %age of the tube, you can set the inlet profile to be established (this is covered in tutorials from Ansys).
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Old   June 23, 2015, 08:53
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Check for velocity entry length is matches with the simuation for circular pipe the correlation is 0.05*Re*D. calculate the nusselt number using LMTD method across a section for constant wall temperature the nusselt number is 3.66. 5% error is acceptable .
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