What are the relative dimensions between the diameters and length of the heat exchanger.

These relationships are important to measure both the length and diameter have the same order of magnitude.

It is also important to know a priori the range of Reynolds that may occur in the simulation.

That is, any response based on the small amount of information is more speculation than anything.

Now, certainly you're using a good reference.

Thanks Rogerio for your reply.

heat exchanger spec is :  Shell dia : 0.5 m and tube dia = 10 cm.

Length of Heatex = 6* Shell dia . I am a novice in CFD. Hence, I cant understand why the size and shape of Heatx affects the Boundary conditions? Here I have simplified the case to be a laminar in nature to start with.

Maybe if you have any sources or reference can you please share it.

Thanks in advance 

I think the only why is to validate your result. Find similar experimental article. implement the geometrical structure of the paper to your HX. and of course same boundary conditions. when your result matched the one in the paper, your simulation is ok.

There are many papers published with the common boundary conditions for Heat Exchanger Analysis.  Check with the Process Heat Transfer book by Kern to get some boundary conditions