[Shomaz ul Haq]

Hi all. Hope everyone is well. I am modeling heat transfer in counter flow double pipe heat exchanger for nanofluids for the past three months. I am working without nanoparticles at the moment and just using heat exchanger with water. I have made the geometry, meshed it and ran the iterations which gave me good results. Now I am trying to plot some graphs of some variables along the length of the pipe. I am currently using constant normal speed boundary condition that is giving me constant Re number versus pipe length/axis in CFD-Post at inlet boundary condition (which I suppose it would). Now I want to use a velocity equation and not a constant normal speed boundary condition. I want to study how Re number changes and the different flow regimes (laminar, transitional, and turbulent) and boundary layer thickness and effect. I can't seem to find the relationship for velocity as a function of pipe length in counter flow double pipe heat exchanger. I have consulted numerous heat transfer articles and books. Does anybody know any relation of that kind? Would be grateful. Thanks.

[Opaque]

Not sure what your definition of Reynolds number is, but for a uniform cross section pipe, the Re number is defined as

Re = Density * Mean Velocity * Characteristic Length (Usually Hydraulic Diameter) / Dynamic Viscosity

If your inlet boundary condition is constant in time, your mass flow is also constant in time and so is the mean velocity.

For constant property fluid, there is nothing changing on the right hand side of the definition; therefore, what are you expecting to see ?

If the inlet velocity boundary condition is a profile, not uniform in the cross section, you will see the flow developing (with a constant mean velocity for every cross section along the pipe) and you could the growth of the boundary layer until the flow is fully developed (assume the pipe is long enough). In any case, the Re number remains constant along the pipe.

Hope the above helps you.