Entrance region. Entry region
A section of flow in a pipe over which the
field of the fundamental variable quantity
(velocity, temperature or concentration) depends
on the conditions at the entrance and
over which the boundary layer increases until
it fills the entire cross section of the pipe.
Comment. Depending on the nature of the transfer process, we distinguish between a "hydrodynamic entrance region", a "thermal entrance region" and a "diffusional entrance region."