i want to simulate the two-phase laminar flow and heat transfer in the circular tube,and i firstly test the mesh and fluent calculating ability by using single phase(water).I have written udf of the inlet velocity for the laminar flow(that is U=2Um*(1-(r/R)^2)),and the final result of pressure drop is exactly in agreement with the theory.

But the problem is that the heat transfer Nu deviates from the theoretical value( Nu=4.366 for Constant Flux ,and Nu=3.67 for constant wall temperature,remember?)(by the way,i also have written udf for the inlet temperature ,and i surely got a fully-developed flow)?

what may be the reasons?any advices are much appreciated!

is it possible that Fluent does not solve the energy equation exactly?