The three plots you show have no labels on either the x or y axis. I can't follow how you are calculating Nusselt number.

Have you divided the pipe into an entrance length where the velocity profile is developing and a downstream length where the flow is fully developed?  Are you computing the heat transfer coefficient in the fully developed region, or including the entrance length?

I recommend you do an axisymmetric model for yourself, even if you don't show it to your advisor. What are your advisor's reasons for requiring a 3D model?  It is foolish to build a more complicated model when a simpler model will give you the same (or better) information.