# Comparing theoretical calculations with ANSYS Fluent

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May 21, 2015, 08:34
Comparing theoretical calculations with ANSYS Fluent
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Mansoor Ahmed
Join Date: Apr 2014
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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.
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 May 24, 2015, 12:26 #2 Member   Jim Join Date: May 2015 Posts: 47 Rep Power: 11 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).

 June 23, 2015, 08:53 #3 New Member   goku Join Date: Jun 2015 Posts: 12 Rep Power: 11 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 .