Input Boundary Condition Changed when converged
 

hi, im performing a simulation of double pipe heat exchanger...
the boundary conditions being
inlet temperatures and inlet mass flow rates, and outlet pressure

as the iteration was completed and i checked the values of area weighted static temperature at inlet and outlet, the inlet values were found to be cghanged by some decimals than what i gave. why is this happening and how can i control this?

also the mass flow rate being constant isnt it that we should get same velocity at both inlet and outlet side? here the value of velocity at outlet of inner tube also changed by some decimals...how can i control this and maintain the same velocity??

please help.....

Thanking in advance

It is not possible to get them exactly the same because of numerical error in the CFD solution. It is okay if the final values are less than 1% of the prescribed BC values. If not, you can improve the solution by refining the mesh on the boundaries but still you might never exactly get the initially defined values in Fluent. If your mass flow rate (Report->Fluxes->Mass Flow Rate) at the outlet and the inlet is the same to 5th decimal, I think you are good to go.

thanks...that was pretty usefull....